WILEY Online Library I Unravelling the Potential of Microbial Carotenoids in Fish Health and Productivity
Tejas Jagannath Naik, Ramaballav Roy, Bhakti Balkrishna Salgaonkar
December 17, 2025
ABSTRACT
Aquaculture plays a pivotal role in meeting the escalating demand for seafood. However, the susceptibility of fish to diseases poses a substantial threat to the productivity and profitability of aquaculture operations. Disease outbreaks necessitate the development of strategies to improve fish health without relying on antibiotics. This review focuses on harnessing the potential of natural sources of carotenoids, specifically those derived from microbial sources, as supplements in aquaculture feed. Microbial carotenoids effectively bolster fish health, rendering them resistant to pathogens and also improving the quality of fish being farmed. Additionally, carotenoids contribute to increased pigmentation in fish, enhancing visual appeal, especially in ornamental fishes. The authors have also touched upon elucidating the mechanisms through which carotenoids enhance fish health. This work consolidates existing knowledge and underscores the potential for further research to broaden our understanding of microbial carotenoids in aquaculture.
1 Introduction
Aquaculture, the fastest-growing sector of the global food industry, plays a crucial role in meeting the increasing demand for seafood. Since 1961, the consumption of aquatic foods (apart from algae) has increased globally at an average annual rate of 3%, compared to a 1.6% population growth rate (FAO 2022; Mizuta et al. 2023). As traditional fisheries struggle to meet the ever-increasing demand, aquaculture has emerged as a viable alternative. However, the expansion of the aquaculture industry is not without drawbacks, and one of the major concerns is the health of farmed fish (Subasinghe et al. 2009).
Fish health is a critical factor that directly impacts the productivity and profitability of aquaculture operations. Disease outbreaks and mortality pose substantial risks to fish populations, leading to economic losses and scaling down social development in many countries (Bondad-Reantaso et al. 2005; Opiyo et al. 2018; Garza et al. 2019). Aquatic animal diseases result in annual industry losses exceeding six billion USD (Stentiford et al. 2017). The prevailing trajectory in the advancement of aquaculture entails a notable shift towards heightened intensification and commercialization of aquatic production. However, as aquaculture activities intensify and expand, the risk of encountering significant disease also rises. Consequently, the aquaculture industry finds itself confronted with a multitude of diseases and predicaments arising from various elusive and emerging pathogens such as bacteria, viruses, fungi and parasites. Numerous bacterial infections have been documented in various fish species within the aquaculture sector. Some of them are furunculosis (Baset 2022), aeromonas septicaemia (Semwal et al. 2023), edwardsiellosis (Oh et al. 2020), columnaris (Declercq et al. 2013), streptococcosis (Agnew and Barnes 2007) and vibriosis (Mougin et al. 2021). Table S1 lists bacterial pathogens responsible for disease outbreaks in various aquaculture fish species.
Antibiotics have been used to mitigate these risks, but the rise of antibiotic-resistant bacteria and the potential environmental impact of their usage has become a significant threat to both animal and human health (Schar et al. 2020). Alternative strategies are being explored, focusing on enhancing the immune system of farmed fish in order to minimize disease outbreaks and reduce reliance on antibiotics. Among the potential solutions, the application of carotenoids has gained considerable attention. Carotenoids, the pigments found naturally in various plants and microorganisms, have long been recognized for their beneficial effects on animal health (Maoka 2020). These compounds possess antioxidant and immunostimulatory properties, improving the overall immune response in fish and enhancing their resistance to pathogens (Nakano and Wiegertjes 2020). Additionally, carotenoids can enhance the visual appeal of fish, making them more desirable to consumers (Gupta et al. 2007; Patil and Thakare 2017).
This review article explores the promising applications of carotenoids in aquaculture feed as a means to boost the immune system of fish and take significant strides towards antibiotic-free practices, ensuring the long-term viability of fish farming operations. Through this comprehensive review, the authors aim to shed light on the current state of knowledge concerning the use of carotenoids from microorganisms as a supplement in fish feed, identify research gaps and inspire future research endeavours in this rapidly evolving field.
2 Carotenoids and Its Biosynthesis
Carotenoids are yellow, orange and red pigments responsible for the characteristic hues of many plants and animals. They are a class of naturally occurring lipid-soluble pigments and are biosynthesized by fungi, bacteria, algae and plants. The majority of animals are unable to biosynthesise carotenoids. However, they can be ingested through food and subsequently structurally altered. Certain invertebrates such as hemipteran (aphids, adelgids and phylloxerids), dipteran (gall midges) insect and mites have been found to synthesize carotenoids de novo (Park et al. 2018; Meléndez-Martínez et al. 2022). These compounds play essential roles in various biological processes including photosynthesis, photoprotection, alleviating photooxidative damage and reinforcing the cell membrane (Alcaíno et al. 2016; Calegari-Santos et al. 2016).
Carotenoids usually represent C40 tetraterpenoids, constructed from eight C5 isoprenoid units. These units are connected in a manner where the sequence is reversed at the centre. The fundamental linear and symmetrical framework, capable of cyclization at one or both ends, features lateral methyl groups separated by six C atoms at the centre and five C atoms in other locations (Rodriguez-Amaya 2001). The presence of alternate conjugated bonds is the most distinctive feature of carotenoids. Polyene structures with nine or more conjugated bond systems exhibit colour. The phenomenon of their absorption within the visible spectrum of 400–500 nm arises as a result of a pronounced electronic transition from the S0 to S2 state. This particular transition manifests as a vibrant colour spectrum encompassing shades of yellow, orange and red (Ram et al. 2020). A majority of carotenoids exhibit a 40-carbon skeleton and are commonly referred to as a C40 carotenoid. Conversely, higher carotenoids possess skeletons comprising 45 or 50 carbons. In contrast, apocarotenoids represent a category of carotenoids that are composed of carbon skeletons containing fewer than 40 carbons (Maoka 2020). As of 1 November 2020, the carotenoid database comprised a data set documenting 1204 carotenoids found in 722 source organisms (Yabuzaki 2017).
Carotenoids can be categorized into two distinct groups, namely carotenes and xanthophylls (Figure 1). Carotenes, including α-carotene, β-carotene, β,ψ-carotene (γ-carotene) and lycopene, consist solely of hydrocarbons comprising carbon and hydrogen atoms, that tend to be cyclized at either end of the compound. Conversely, xanthophylls such as β-cryptoxanthin, lutein, zeaxanthin, astaxanthin, fucoxanthin and peridinin contain oxygen atoms within their molecular structures, manifesting as hydroxy, carbonyl, aldehyde, carboxylic, epoxide and furanoxide groups (Maoka 2020; Olatunde et al. 2020). Carotenoids can also be categorized as primary and secondary based on the photosynthetic activity. Primary carotenoids encompass compounds essential for plant photosynthesis, such as β-carotene, violaxanthin and neoxanthin. In contrast, secondary carotenoids such as α-carotene, β-cryptoxanthin, zeaxanthin, antheraxanthin, capsanthin and capsorubin are localized in fruits and flowers (Delgado-Vargas et al. 2000). Concerning their nutritional properties, carotenoids can also be classified based on their ability to yield vitamin A upon breakdown. There are certain compounds, such as α-carotene, β-carotene and β-cryptoxanthin, referred to as vitamin A precursors. Alternatively, there are other compounds, such as lutein, zeaxanthin and lycopene, which do not serve as precursors for vitamin A production (Pasarin and Rovinaru 2018).
In addition to their role as pigments, carotenoids possess potent antioxidant properties due to their ability to scavenge reactive oxygen species (ROS) and protect cells from oxidative damage. These compounds have also been linked to various health benefits in humans, including the prevention of age-related macular degeneration, cardiovascular diseases and certain types of cancer. Furthermore, carotenoids are essential in animal nutrition, as they serve as precursors for vitamin A synthesis and are involved in vision, immune function and reproductive health (Fiedor and Burda 2014).
Isoprenoids and carotenoids both originate from common five-carbon (C5) building blocks and utilize common metabolic precursors in two distinct pathways for the synthesis of isopentenyl diphosphate (IPP) and dimethylallyl diphosphate (DMAPP) (Figure 2). These pathways encompass the mevalonate (MVA) pathway and the more recently discovered 2-C-methyl-d-erythritol 4-phosphate (MEP) pathway (Paniagua-Michel et al. 2012). Heterotrophic eukaryotes like animals and fungi utilize the MVA pathway. In contrast, photo-autotrophic plants with chloroplasts and most algal groups employ the MVA pathway in their cytosol, while their plastids use the MEP pathway for IPP production. Archaea exclusively rely on the MVA pathway (Lichtenthaler 2004). The initial step in carotenoid biosynthesis is the production of IPP and its isomer DMAPP. The interconversion between IPP and DMAPP is facilitated by an enzyme, IPP/DMAPP isomerase. IPP and DMAPP serve as the fundamental building blocks for the synthesis of carotenoids. These precursor molecules can be synthesized either through the MVA pathway or the non-MVA pathway. In the initial step of carotenoid biosynthesis, an IPP molecule and a DMAPP molecule undergo a head-to-tail condensation reaction, resulting in the formation of C10-geranyl diphosphate (GPP), catalysed by geranyl pyrophosphate synthase (GPPS). Subsequent addition of IPP to GPP by farnesyl pyrophosphate synthase (FPPS) produces C15-farnesyl pyrophosphate (FPP). Addition of one more molecule of IPP to FPP by geranylgeranyl pyrophosphate synthase (GGPPS)/CrtE yields C20-geranylgeranyl pyrophosphate (GGPP). FPP acts as a precursor for the synthesis of C30 carotenoids such as staphyloxanthin, while GGPP serves as the precursor for C40 carotenoids (Wang et al. 2019; Feng et al. 2020; López et al. 2023). Two GGPP molecules combine in a tail-to-tail manner rejecting two diphosphate groups to produce the first colourless C40 carotenoid, phytoene. This step is catalysed by phytoene synthase (CrtB) (Lee and Schmidt-Dannert 2002). Phytoene contains three conjugated double bonds and undergoes stepwise desaturation to form lycopene via phytofluene, ζ-carotene and neurosporene by a series of four consecutive reactions mediated by phytoene desaturase (CrtI). Lycopene is a linear molecule that serves as the foundational structure for subsequent modifications, giving rise to a wide range of carotenoids. Through cyclization reactions catalysed by Lycopene-α-cyclase or Lycopene-β-cyclase (CrtY), lycopene can be converted into carotenoids with cyclic terminal end groups, such as α-carotene and β-carotene, respectively (Britton et al. 1998; Molina-Márquez et al. 2019; Ram et al. 2020). Furthermore, a hydroxylation reaction by β-carotene hydroxylase (CrtZ) in one of the β-carotene rings leads to the formation of β-cryptoxanthin, another hydroxylation produces zeaxanthin, which can be further converted to astaxanthin by β-carotene ketolase (CrtW). CrtW also catalyses the conversion of β-carotene to canthaxanthin via echinenone (Maj et al. 2020; Stra et al. 2023). In halophilic archaea, C50-bacterioruberin is the main carotenoid synthesized from lycopene (Giani et al. 2020).
The base framework of carotenoids can undergo various alterations such as cyclization, hydrogenation, dehydrogenation, migration of double bonds, chain shortening/expansion, rearrangement, isomerization, introduction of oxygen function, methylation or a combination of these processes resulting in a diverse array of structural variations (Setyorini 2021).
3 Microbial Carotenoids as Feed Supplement in Aquaculture
Carotenoids are obtained from dietary sources such as fruits, vegetables and animal products (Martínez-Cámara et al. 2021). However, commercial carotenoids are mostly manufactured through chemical synthesis. These synthetic carotenoids may pose health risks and generate hazardous waste during the production process. On the other hand, carotenoids derived from plant sources are comparatively costly, and their availability is constrained by unpredictable climatic and geographical factors (Foong et al. 2021). It has been reported that microorganisms represent a promising alternative for its production (Mata-Gómez et al. 2014). Microorganisms including algae, cyanobacteria, fungi, yeast, archaea and bacteria are capable of synthesizing a wide array of carotenoids (Schweiggert and Carle 2016). These microorganisms offer several advantages, including the use of low-cost substrates like agricultural waste, which makes the process economically viable. Additionally, microbial fermentation under controlled cultivation conditions ensures optimal production rates and consistent quality. Furthermore, genetic and metabolic engineering can be applied to customize and enhance the production of specific carotenoids (Xie et al. 2015; Salgaonkar et al. 2019; Joshi et al. 2023). The microbial sources of carotenoids as feed supplements in aquaculture are summarized in
(A) Microorganisms used as a source of carotenoid in aquaculture feed, (B) growth and extraction of carotenoids from microorganisms and (C) supplementation of fish diet with carotenoids. [Color figure can be viewed at wileyonlinelibrary.com]
3.1 Carotenoids From Bacteria
Grassi et al. (2016) investigated the effects of Rubrivivax gelatinosus biomass as a carotenoid source on growth performance (feed consumption, weight gain and specific growth) of Nile tilapia (Oreochromis niloticus). The study revealed that the inclusion of carotenoids in the feed did not impact the performance of the fish, and no mortality was observed. However, it resulted in an increase of pigmentation and carotenoid content in the fish, indicating a successful enhancement of visual appeal.
Red tilapia (O. mossambicus × O. niloticus) experienced significant improvements when supplemented with Lycogen (a commercial carotenoid blend containing neurosporene, ξ-carotene, spheroidenone and methoxyneurosporene derived from Rhodobacter sphaeroides mutant strain WL-APD911) for 7 weeks. Increase in muscle weight, specific growth rate (SGR), weight gain and feed conversion ratio (FCR) in fish supplemented with 1.0% Lycogen was also documented. Moreover, the innate humoral responses, as reflected by lysozyme activity and alternative complement activity, were significantly increased indicating enhanced immune function (Chiu and Liu 2014). The alternative pathway of complement activity serves as a potent non-specific defence mechanism, protecting fish from a wide range of potentially invasive organisms such as bacteria, fungi, viruses and parasites (Momeni-Moghaddam et al. 2015).
In Atlantic salmon (Salmo salar), supplementation of Panaferd-AX (source of astaxanthin) derived from Paracoccus carotinifaciens led to a remarkable increase in the red colour of salmon flesh. These findings suggest the potential of astaxanthin as a natural pigment to enhance the visual appeal and market value of salmon fillets. In the same study, rainbow trout (Oncorhynchus mykiss) showed a dose-related trend towards a moderate increase in flesh colour when supplemented with Panaferd-AX. Although statistical significance was not reached, a positive correlation between carotenoid supplementation and flesh colouration in rainbow trout was observed (Bories et al. 2007).
3.2 Carotenoids From Algae
Zhu et al. (2022) incorporated powdered biomass of Haematococcus pluvialis as a source of astaxanthin into the diet of coral trout (Plectropomus leopardus). Astaxanthin supplementation did not significantly affect growth; however, it improved antioxidant activity, with higher glutathione peroxidase (GPx), catalase (CAT) and superoxide dismutase (SOD) levels, along with increased T-AOC in serum and liver. Subsequent to Vibrio harveyi exposure, the astaxanthin-fed group demonstrated enhanced resistance against V. harveyi. This protection was coupled with elevated serum and liver acid phosphatase, lysozyme activities and complement content, as well as upregulated gene expression for antioxidant enzymes, immune response and defence mechanisms. Similarly, in the case of rainbow trout (O. mykiss), supplementation with H. pluvialis biomass as a source of astaxanthin led to an increase in antioxidant activity and a reduction in lipid peroxidation product levels (Sheikhzadeh et al. 2012). In the study conducted by Li et al. (2018), the diet of blood parrot (Cichlasoma citrinellum × C. synspilum) was supplemented with astaxanthin sourced from H. pluvialis. The fish receiving carotenoids manifested a distinctive pink skin colour, while those in the control group exhibited a greyish skin tone. Additionally, the fish exhibited an increased weight, growth rate and elevated tissue total antioxidant capacity, accompanied by reduced SOD and CAT activity levels.
Hassaan et al. (2021) incorporated β-carotene from Spirulina platensis into the diet of O. niloticus, resulting in weight gain, improved FCR and better survival rate. The activities of intestinal digestive enzymes (chymotrypsin, trypsin, lipase and amylase) were higher, while alanine aminotransferase (ALT) and aspartate aminotransferase (AST) activities decreased. Elevated levels of haematological parameters, including serum total protein, albumin and globulin contents, were documented. Immune parameters such as phagocytic, lysozyme and IgM activities were elevated, and the supplementation also improved antioxidant activities (SOD, CAT, GPx and T-AOC) in the fish. Additionally, the study reported upregulation of interferon gamma and interleukin 1β transcripts, essential for immune response modulation. In contrast, the expression of the HP70 gene, a stress and inflammation marker, was downregulated.
3.2.1 Dunaliella salina
Albino oscar fish (Astronotus ocellatus) supplemented with lyophilized cells of D. salina containing β-carotene displayed increased serum and mucus lysozyme and bactericidal activity. Skin carotenoid content was also significantly higher compared to the control group. Furthermore, the fish exhibited a reduced mortality rate as a result of increased resistance against A. hydrophila accompanied with improvements in weight gain percentage, SGR, FCR and protein efficiency ratio (Alishahi et al. 2015).
Rainbow trout fed with β-carotene and astaxanthin sourced from D. salina (Probatenol EX) showed increased serum carotenoid concentration but no difference in growth and performance was observed. However, mortality rate was reduced in fish feed supplemented with D. salina after challenging the fish with infectious hematopoietic necrosis virus (Amar et al. 2012). Similarly, rainbow trout supplemented with β-carotene sourced from D. salina displayed increased serum alternative complement activity, higher serum lysozyme activity, higher phagocytic rate and index (Amar et al. 2004).
3.2.2 Chlorella vulgaris
Supplementation with C. vulgaris biomass as a carotenoid source has been evaluated for pigmentation. In rainbow trout (O. mykiss), this supplementation resulted in a higher pigment concentration in the muscle (Gouveia et al. 1998).
3.3 Carotenoids From Yeast
In a study conducted by Amar et al. (2012), on rainbow trout (O. mykiss), astaxanthin and total xanthophylls from P. rhodozyma in the form of powdered cells was supplemented in the fish diet. Increase in carotenoids in the tissue and serum of the trout, as well as an increase in humoral factors, was reported. Additionally, serum alternative complement activity was also reported. The researchers also observed an increase in the phagocytic rate and index, suggesting that carotenoids from P. rhodozyma can modulate some of the innate defence mechanisms in rainbow trout. Complementing these immune-related findings, other studies on O. mykiss supplemented with astaxanthin from P. rhodozyma reported a decrease in both serum transaminase and lipid peroxide levels, coupled with improvements in muscle pigmentation and enhanced phagocytic rate and index (Nakano 1999; Nakano et al. 1995; Amar et al. 2004). The supplementation further led to a decrease in serum lipid levels, including triglycerides, total cholesterol and phospholipids. Additionally, the studies demonstrated a decrease in the liver-to-body weight ratio (hepatosomatic index), indicating positive effects on liver function, and a decrease in serum glutamyl oxaloacetic transaminase (GOT) levels, suggesting no damage to the liver.
In another investigation conducted by Bjerkeng et al. (2007), Atlantic salmon (S. salar) was supplemented with astaxanthin as commercially available spray-dried cells of P. rhodozyma (Ecotone). The researchers found that the total carotenoid concentration in the muscle of the salmon increased as a result of the supplementation. The biomass of carotenogenic marine yeast Rhodotorula paludigena VA 242 has also been reported to enhance the pigmentation in the scales of Cyprinus carpio (Rekha et al. 2024). These findings highlight the potential benefits of carotenoids derived from yeast in modulating defence mechanisms, improving muscle pigmentation and positively affecting liver function and lipid peroxidation levels in various fish species. However, it is important to note that additional data and research are necessary to further elucidate the mechanisms and potential benefits of astaxanthin in marine organisms.
3.4 Carotenoids From Fungi
The effects of a carotenoid mixture derived from a fungus were investigated on the golden koi (C. carpio) by Patil and Thakare (2017). It was reported that the carotenoids from the fungi were nontoxic to the fish and exhibited positive effects such as an increase in weight, protein content and enhancement in colouration of the golden koi.
Rodrı́guez et al. (2004) also examined the effects of lycopene and β-carotene from fungi on the gilthead seabream (Sparus aurata). Lycopene produced by Mucor circinelloides MU224 and β-carotene produced by M. circinelloides T31 were investigated for their potential impact on the colouration and overall health of the gilthead seabream. The study findings demonstrated the positive influence of lycopene and β-carotene on the colouration of the fish, contributing to its desirable appearance. Both studies highlighted the significance of carotenoids derived from specific microorganisms in enhancing the colouration and visual appeal of different fish species, namely the C. carpio and the S. aurata.
3.5 Carotenoids From Archaea
Halophilic archaea represent a valuable reservoir of novel pigments, with bacterioruberin being the primary carotenoid, and its intermediates like bisanhydrobacterioruberin (BABR), monoanhydrobacterioruberin (MABR) and 2-isopentenyl-3,4-dehydrorhodopin (IDR) (Salgaonkar et al. 2012; Giani et al. 2020; Grivard et al. 2022). Bacterioruberin is a dipolar C50 isoprenoid featuring 13 conjugated ═
bonds and four terminal hydroxyl groups. This carotenoid is predominantly produced by members of halophilic archaea (Mani et al. 2012; Jehlička et al. 2013; Caimi et al. 2022). The antioxidant efficacy of carotenoids is generally linked to factors such as carbon chain length and the quantity of conjugated double bond (Stahl and Sies 2003; Han et al. 2012). In contrast to the nine pairs of conjugated double bonds found in C40 carotenoids such as β-carotene, bacterioruberin possesses 13 pairs, rendering it as a superior radical scavenger (Yatsunami et al. 2014). Despite these attributes, the application of archaeal pigments as supplements in fish feed remains relatively unexplored, with limited available reports on this subject.
In a study by Xie et al. (2022), the supplementation of bacterioruberin from the archaea Halorubrum resulted in an enhancement of pigmentation in golden trout. The supplementation was associated with a notable increase in the total antioxidant capacity in the serum, coupled with enhancements observed in liver GPx. Concurrently, there was a significant decrease in malondialdehyde (MDA) content. MDA serves as one of the prominent markers for lipid peroxidation in cells and tissues. Elevated levels of MDA are indicative of increased oxidative stress (Nuntapong et al. 2019). Furthermore, after exposure to ammonia and a challenge with A. hydrophilia, the golden trout exhibited increased survival rates. This increased survivability was linked to a reduction in the expression of pro-inflammatory cytokines il-1β, along with an increase in the expression of anti-inflammatory cytokines, such as transforming growth factor-beta (TGF-β) and heat shock protein 70 (HSP70). This interplay of molecular responses suggests a potential protective role of bacterioruberin supplementation against inflammatory challenges in golden trout (Xie et al. 2022). Additionally, bacterioruberin from Halorubrum has also been shown to increase survival rate and reduce ROS level and lipid peroxidation without increasing the activities of the antioxidant enzymes SOD and CAT in Litopenaeus vannamei postlarvae (Xie et al. 2021).
4 Importance of Carotenoids in Pisciculture
The utilization of carotenoids in fish feed formulations has gained significant attention in recent years, as fish, particularly those used in aquaculture, often require dietary supplementation to achieve optimal growth, pigmentation and overall health (Gouveia and Empis 2003; Pasarin and Rovinaru 2018). In fish, carotenoids serve a pivotal function as pigment, aggregating in the skin, tissues, muscles and organs, ultimately contributing to the striking vibrancy of their colour (Page and Davies 2003; Noori and Razi 2017). Additionally, carotenoids serve as precursors to vitamin A (Moren et al. 2002), act as antioxidants (Young and Lowe 2018) and contribute towards enhancing the immune system, growth and reproduction in fish (Rashidian et al. 2021). Therefore, the inclusion of carotenoids in the fish diet, especially in aquaculture, is essential (Nogueira et al. 2017).
4.1 Vitamin A Precursors
Vitamin A is an unsaturated monohydric alcohol possessing a β-ionone ring. It is an essential nutrient for fish and has the ability to scavenge peroxyl radicals, thereby effectively preventing lipid peroxidation in vitro (Wu et al. 2022). Provitamin A activity refers to the ability of natural carotenoids to form retinol (vitamin A) via the action of the dioxygenase enzyme. The primary precursors of vitamin A are α- and β-carotene. Any pigment that has at least one intact ionone ring in its structure can be classified as a provitamin A carotenoid (Khan et al. 2023). In salmonidae fish, canthaxanthin has been demonstrated to undergo conversion into retinol. Additionally, 3-Hydroxy carotenoids, namely lutein, zeaxanthin and astaxanthin, have been reported as precursors of 3,4-dehydroretinol (commonly referred to as vitamin A2) in some fish (Lara-Flores and García-Chavarría 2013). In a study conducted by Gross and Budowski (1966), it was demonstrated that β-carotene, astaxanthin, canthaxanthin and isozeaxanthin serve as precursors for vitamin A in both guppies (Poecilia reticulata) and platies (Xiphophorus maculatus). Various fish species have the ability to transform carotenoids; however, this capacity is contingent upon factors such as the species, its size, age and the vitamin A status (Hernandez and Hardy 2020). Vitamin A and Provitamin A (carotenoid) supplementation in fish has been shown to improve disease resistance and reduce mortality rate (Anbazahan et al. 2014; B. Liu et al. 2016). This highlights the critical role of carotenoids not only as pigments but also as essential contributors to fish health.
4.2 Fertility and Reproduction
The significance of colour in the selection of mates has been acknowledged across diverse taxa, predominantly in the context of female choice (Foote et al. 2004). Males raised on a high-carotenoid diet are preferred by females among fishes (Grether 2000). Aquatic animals have a noticeable accumulation of carotenoids in their gonads, which are considered to be necessary for reproduction, the successful development of their eggs and early larval stages. Astaxanthin administration has been found to improve ovary development, fertilization, hatching and larval growth in farmed salmon and red sea bream (de Carvalho and Caramujo 2017). Salmonid fish eggs are bright yellow, orange or red, due to the presence of carotenoids in the yolk (Craik 1985). In a study by Ando et al. (1986) on chum salmon (O. keta), it was observed that carotenoids are mobilized from muscle to ovaries during spawning migration, suggesting a potential role in reproduction. Salmonid eggs typically have high astaxanthin levels, which can improve egg quality during the initial feeding period of growth. Astaxanthin can also protect salmonid egg membranes from oxidative damage caused by UV radiation (Nakano and Wiegertjes 2020). Astaxanthin supplementation has been reported to enhance the egg survival rate and fertilization rate in goldfish (Carassius auratus) (Tizkar et al. 2013), and also promotes spermatocrit value, sperm concentration, motility, osmolality and fertilization rate (Tizkar et al. 2015). In a study conducted by Ahmadi et al. (2006), it was found that dietary astaxanthin supplementation in rainbow trout enhanced egg quality, improved fertilization, hatching and survival rate of the fish. Thus, carotenoids are crucial for improving reproductive outcomes and sustaining productivity in pisciculture.
4.3 Growth
The impact of carotenoids on fish growth is a subject of debate; some researchers have reported a positive influence while others did not observe any effect (Rashidian et al. 2021). Christiansen and Torrissen (1996) supplemented astaxanthin in the diet of Atlantic salmon (S. salar) leading to an increase in the average weight and survival of the fish compared to the groups that received the unsupplemented diet. Similar results were also observed upon the supplementation of astaxanthin in the diet of European sea bass (Saleh et al. 2018). Synthetic astaxanthin and canthaxanthin supplemented diet promoted the growth rate of S. salar during the early start-feeding period (Torrissen 1984). Piaractus mesopotamicus provided with β-carotene exhibited higher weight gain and a higher SGR (Bacchetta et al. 2019). However, growth of Atlantic salmon reared for 22 weeks remained unaffected by the dietary supplementation of astaxanthin (Bell et al. 2000). Similarly, in a study conducted by Amar et al. (2004) rainbow trout fed on marine alga D. salina and red yeast P. rhodozyma as a source of β-carotene and astaxanthin respectively showed no significant difference in growth rates between the treatment groups and the control group. The difference in growth observed in various studies could be due to factors such as rearing conditions, feed composition and feeding time.
Carotenoids are fat-soluble compounds, and the digestibility of carotenoids has been shown to increase with increasing dietary lipid content in fish feed (Barbosa et al. 1999). This suggests that factors such as feed composition, culture conditions, absorption and metabolism must be carefully considered to optimize carotenoid utilization by the fish and achieve maximum effect (Torrissen et al. 1990; Li et al. 2018). Furthermore, the absorption and transport of carotenoids in fish is significantly affected by factors such as age, physiological state, type of feed/carotenoid and dwelling environment. It is not solely the species of the fish but rather a combination of these elements that exerts a profound influence on the growth (Torrissen and Christiansen 1995; Das 2016; Grassi et al. 2016).
4.4 Pigmentation
The vibrant hues exhibited by ornamental fish possess not only an alluring aspect but also serve antioxidant and photoprotective purposes. These chromatic attributes stem from the presence of carotenoids within the skin of the fish (Pailan et al. 2019; Elbahnaswy and Elshopakey 2023). Specialized cells known as chromatophores, situated in the dermal layer of fish, are responsible for the pigmentation. Pigmentation is influenced by numerous internal and external factors (Price et al. 2008). These factors encompass the type of dietary carotenoid included in the diet, source and concentration of pigments, duration of carotenoid consumption, presence of other dietary components, body size and weight of the fish, life cycle, genetic makeup, carotenoid metabolism, environmental conditions and stress levels (Sathyaruban et al. 2021). Fish possess a variety of chromatophores, each contributing to specific colours. These chromatophores include melanophores responsible for black coloration, xanthophores responsible for yellow/orange hues, erythrophores responsible for red pigmentation, cyanophores contributing to blue coloration, light-reflecting iridophores responsible for silver colouring and leucophores responsible for the white colour (Irion and Nüsslein-Volhard 2019).
In the natural habitat, fish typically display vibrant coloration as they rely on dietary intake of carotenoids, which they cannot synthesize internally. Certain species possess the ability to transform one form of carotenoid into another. Fish are known to harbour a diverse array of carotenoids, each predominantly unique to their respective species. These carotenoids manifest in various colours within the fish species, including tunaxanthin (yellow), lutein (greenish−yellow), β-carotene (orange), α,β-doradexanthins (yellow), zeaxanthin (yellow−orange), canthaxanthin (orange−red), astaxanthin (red), eichinenone (red) and taraxanthin (yellow) (Kaur and Shah 2017). Under intensive artificial culture conditions, their colours appear faded as captive fish lack access to natural sources of carotenoids. The deficiency of pigmentation can be rectified by incorporating carotenoids into the artificial diet, which acts as a primary source of carotenoids (Tripathy et al. 2019). Synthetic carotenoids such as astaxanthin, canthaxanthin, lycopene and β-carotene have been utilized in fish diets to successfully enhance the pigmentation of fish (Pan and Chien 2009; Sun et al. 2012; Ebeneezar et al. 2020). Among these pigments, astaxanthin has demonstrated particularly promising results in augmenting the coloration of various species of ornamental fish, such as tetras, cichlids, gouramis, goldfish, koi, danios and several others (Gupta et al. 2007; Swain et al. 2020).
5 Mode of Action of Carotenoids on Fish Health
Carotenoids play an integral and diverse role in fish physiology, significantly affecting both direct health parameters and growth performance. Their mechanisms of action contribute directly to fish health by enhancing immune responses, improving haematological parameters and mitigating oxidative stress. These functions strengthen the fish’s immune system, thereby increasing resilience to diseases and improving survival rates (Yong Chow and Liong 2016; Zhu et al. 2022; Lim et al. 2023; Shastak and Pelletier 2023). Moreover, carotenoids are also associated with fish growth performance which serves as an important indirect indicator of overall health. These effects include improvements in growth and feed utilization efficiency (Alishahi et al. 2015; Lim et al. 2019a; Ritu et al. 2023). Figure 4 provides an overview of how carotenoids impact these key health parameters.
5.1 Effects on Fish Health
5.1.1 Antioxidant Activities
The physiological states of fish are dependent upon the temperature of their surroundings. Accordingly, the environmental temperature can bring about several changes in the fish’s body. An increase in temperature leads to heightened oxygen consumption, resulting in an elevated production of ROS (Nakano and Wiegertjes 2020). In aerobic environments, molecular oxygen (O2) can present a significant concern for all living organisms. This is due to its capacity to give rise to ROS including the superoxide anion radical (O2−), hydroxyl radical (OH) and hydrogen peroxide (H2O2). The escalated generation of these ROS within the organism is known to induce a state of heightened vulnerability known as oxidative stress (Parihar et al. 1997). Cellular oxidative stress occurs when the physiological antioxidant protection does not counteract the elevated ROS levels. When ROS exceeds that of the antioxidant system’s removal capability, adverse effects occur, such as an increase in lipid peroxidation (Rama and Manjabhat 2014). The generated ROS also targets cellular components like proteins and nucleic acids which trigger mitochondrial dysfunction and deoxyribonucleic acid (DNA) damage, ultimately culminating in programmed cell death (Zhao et al. 2013).
Modifications in the activity levels of antioxidant enzymes, namely SOD (EC 1. 15. 1.1), CAT (EC 1. 11. 1.6) and GPx (EC 1. 11. 1.9), along with their protective mechanisms, have been recognized as initial markers indicating cellular vulnerability to oxidative damage induced by O2−, OH and other ROS in mammalian systems (Parihar et al. 1997). SOD catalyses the transformation of ROS such as hydrogen peroxide anions into water and oxygen which are produced during various processes of the cells. Both SOD and GPx increase the antioxidant capacity. SOD serves as the primary defence against ROS by catalysing the conversion of superoxide anion radicals into oxygen and H2O2, while CAT subsequently transforms H2O2 into water and oxygen. GPx acts as a coenzyme and also plays an important role in the removal of free radicals (Tian et al. 2011; Arockiaraj et al. 2012; Kim et al. 2022). Supplementation of carotenoids in the diet of fish has been found to enhance their antioxidant activity by increasing the synthesis of GPx, SOD and CAT (Sheikhzadeh et al. 2012; Hassaan et al. 2021; Xie et al. 2022).
5.1.2 Immune Parameters
Lysozyme is considered a major marker of the immune response due to its close correlation with leucocytes. It is primarily produced by macrophages in response to microbial components and various other immune stimulants (Hassaan et al. 2021). Lysozyme operates as a cationic enzyme, breaking β-1,4 glycosidic bonds between N-acetylmuramic acid and N-acetylglucosamine found in the peptidoglycan of bacterial cell walls. This enzymatic action is primarily directed towards Gram-positive bacteria and it also complements the attack on certain Gram-negative bacteria when used in conjunction with complement proteins (Anbazahan et al. 2014). The complement system is important in linking innate and adaptive immune responses, allowing for integrated host defence against pathogenic challenge. This system performs a variety of functions, including inflammatory vasodilation, lysis of bacterial and infected cells, phagocytosis of foreign particles and apoptotic cell clearance. The complement system has been shown to have bactericidal activity in many fish (Mokhtar et al. 2023). Alternative pathway of complement activity serves as a potent non-specific defence mechanism, protecting fish from a wide range of potentially invasive organisms such as bacteria, fungi, viruses and parasites (Momeni-Moghaddam et al. 2015). Phagocytosis stands out as one of the most vital processes in poikilothermic animals. Among fish, the principal cells engaged in phagocytosis are neutrophils and macrophages. These cells effectively eliminate bacteria primarily through the generation of ROS during a respiratory burst (Uribe et al. 2011). The adaptive immune system is dependent on B and T cells, along with the diverse and specific antigen receptors they possess, namely the immunoglobulins (IG) or antibodies and the T cell receptors, respectively. The IG plays a crucial role in the immune response against pathogens. The mechanisms involved in humoral immunity mediated by IG include pathogen elimination through phagocytosis, neutralization of toxins and viruses, as well as activation of the complement cascade. In fish serum, secreted tetrameric IgM is recognized as the predominant IG, and it is worth noting that IgM can also be expressed on the surface of B cells (Mokhtar et al. 2023).
The innate humoral responses, as reflected by lysozyme activity and alternative complement activity, are found to be enhanced when carotenoids are supplemented in the diet of the fish indicating enhanced immune function (Amar et al. 2000, 2004; Chiu and Liu 2014). Studies have also shown that carotenoids enhance the serum total IG concentrations in fish such as rainbow trout, Asian seabass and yellow catfish (Amar et al. 2000; Lim et al. 2019b; Liu et al. 2019).
5.1.3 Haematology Parameters
Blood parameters serve as a reliable indicator for assessing the health status and physiological conditions of fish. The levels of mean GOT and glutamic-pyruvate transaminase (GPT) are used as indicators reflecting the physiological status of liver and hepatocytes in aquatic animals (Han et al. 2014; Kim et al. 2022). AST and ALT, formerly referred to as GOT and GPT, respectively, are enzymatic proteins predominantly located in the liver, though they can also be present in red blood cells. AST and ALT play crucial roles in cellular nitrogen metabolism, amino acid oxidation and liver gluconeogenesis. They can serve as valuable indicators for identifying liver damage or dysfunction resulting from toxic effects (Huang et al. 2006).
Carotenoids have been found to reduce the activity of ALT and AST which indicates that the carotenoids do not induce damage to liver and may produce cytokines which inturn protect the liver cells (Hassaan et al. 2021). Carotenoids such as β-carotene and lycopene supplemented in the diet of fish have been found to enhance the concentration of total serum proteins, serum albumin and globulin (Mahmoud et al. 2013; Hassaan et al. 2021; Kesbiç et al. 2022). The total protein level in the blood components of fish serves as an essential indicator of their overall health status. Higher serum protein and globulin levels, coupled with a reduced albumin globulin ratio are robust indicators of enhanced immunity, signifying a healthier state of the fish (Kumar and Singh 2019). Albumin plays a pivotal role in facilitating the transportation of bilirubin, hormones, metals, vitamins and drugs. Moreover, it holds significant importance in fat metabolism as it binds with fatty acids, maintaining their solubility in the plasma. Additionally, albumin’s interaction with hormones helps regulate the quantity of free hormones accessible at any given moment. Within the globulin fraction, there exists an extensive array of serum proteins. These encompass carrier proteins, enzymes, complement proteins and IG. The majority of these proteins are synthesized in the liver, with the exception of IG, which are produced by plasma cells (Kulkarni 2021).
5.2 Effects on Fish Growth Performance
Beyond direct health metrics, carotenoid supplementation has also been investigated for its impact on growth and feed efficiency, which are indirect indicators of health and well-being. Numerous investigations have addressed the potential impact on various growth parameters, yielding divergent findings. Some studies have highlighted notable enhancements in SGR, weight gain and reduced FCR among fish subjected to carotenoid supplementation (Liu et al. 2019). However, several studies have reported no change in growth indices when natural and synthetic carotenoid sources were orally administered (Choubert and Storebakken 1989; Amar et al. 2001; Amar et al. 2004).
In fish, the efficacy of carotenoid sources in terms of deposition and physiological functionality exhibits a species-specific nature. Furthermore, it is worth acknowledging that the metabolic pathways for carotenoid processing are not universally uniform across all fish species (Alishahi et al. 2015). Consequently, diverse outcomes in studies assessing the impact of carotenoid supplementation on growth parameters have been reported.
6 Future Prospects
Most research has focused on extracting carotenoids from microorganisms and supplementing them in the fish diet. While this approach has shown promise, it is accompanied with a limitation of downstream processing cost and feed expense. Moreover, carotenoids are susceptible to degradation by light, heat and pH (Borba et al. 2019). Therefore, carotenoids require special preservation conditions to maintain their efficacy. This can compromise feed effectiveness during long-term storage and large-scale applications. To address these challenges, supplementing biomass from carotenoid-producing microorganisms may offer a more effective and sustainable solution. Studies by Gouveia et al. (1998), Bjerkeng et al. (2007), Amar et al. (2012), Grassi et al. (2016), Kim et al. (2022) and Rekha et al. (2024) have demonstrated the potential of this approach. By using whole-cell biomass, the need for extraction is eliminated. Furthermore, the biomass of halophilic archaea may be particularly advantageous, as these organisms undergo lysis at lower salt concentrations, facilitating the availability of carotenoids for ingestion.
Various studies have shown the effect of carotenoids in improving the pigmentation, survival rate, immune parameters and disease resistance. However, there are limited reports on the ability of microbially derived carotenoids to promote fish weight gain. This limitation could be addressed by using microorganisms that accumulate/produce multiple types of bioactive molecules rather than carotenoids alone. In current studies, microorganisms are used exclusively as sources of carotenoids in fish feed. However, certain microorganisms produce carotenoids alongside other valuable metabolites such as lipids and exopolysaccharides (EPS) based on the culture conditions. These co-products could enhance the effects of carotenoids or provide additional benefits to fish health and development. For example, microalgae H. pluvialis co-synthesizes carotenoids and lipids under stress conditions, which potentially improve fat content in feed and improve carotenoid digestibility (Ren et al. 2021). Similarly, the yeast Rhodosporidium toruloides synthesizes carotenoids and lipids using inexpensive cane molasses as a carbon source (Jiang et al. 2023). EPS extracted from the mycelium of Ganoderma lucidum have been shown to significantly increase the weight of red hybrid tilapia (Oreochromis sp.) (Wan-Mohtar et al. 2021). Moreover, certain halophilic archaea, such as Haloterrigena turkmenica, Halorubrum sp. TBZ112 and Haloferax mediterranei, have been found to co-produce C50 carotenoids and EPS (Squillaci et al. 2016; Poli et al. 2018; Hamidi et al. 2019). The use of these microorganisms in fish feed could have dual benefits, not only enhancing fish weight but also extending the shelf-life of carotenoid-enriched feed as EPS could prevent oxidation (Aparici-Carratalá et al. 2023). Furthermore, haloarchaeal biomass offers additional advantages, including the non-toxicity of bacterioruberin and good stability at high temperature conditions and exposure to light (Palanisamy and Ramalingam 2024).
So far, the research has primarily focused on carotenoids from bacteria and algae for enriching fish feed. However, a noteworthy research gap exists, particularly in exploring carotenoids from archaea. This is especially significant considering that archaeal C50-carotenoids have demonstrated superior antioxidant capacity compared to their C40 counterparts (astaxanthin and β-carotene) and have been shown to mitigate ROS without even increasing the levels of antioxidant enzymes in shrimp. This highlights the need for future investigations to broaden our understanding of the diverse microbial carotenoid sources and their potential contributions to aquaculture.
7 Conclusion
To meet the growing demand for seafood while reducing the significant threat posed by pathogens, carotenoids have emerged as valuable feed supplements in aquaculture. Beyond the fundamental health benefits, microbial sources of carotenoids play a dual role by enhancing the pigmentation of fish, thereby increasing their visual appeal and market value. The multifaceted advantages of carotenoid supplementation in fish diet include improvement in growth parameters like weight gain, FCR and SGR, as well as immune parameters such as improved antioxidant capacity, increased total serum proteins and IG production. Additionally, carotenoids contribute to improved fertility and reproductive performance, and reduce disease susceptibility, thereby enhancing survival rates. The effectiveness of carotenoid supplementation depends on various factors, including fish species and age, feed composition, carotenoid type and dosage and duration of administration. These insights are essential for optimizing aquaculture practices. In summary, microbial carotenoids are promising natural additives that support fish health, immunity, growth and visual appeal.
https://onlinelibrary.wiley.com/doi/10.1111/jpn.70037
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Lafeber Company I Winter Ready: A Look at Birds’ Feet
by Laura Doering
December 11, 2025
Winter can make even the most resilient birdwatchers stop and wonder: how do birds perch on snow or ice without their feet freezing? Birds have incredible natural adaptations that allow them to survive in cold weather, from heat-conserving circulatory systems to specialized foot structures. Understanding these features not only explains some of their quirky behaviors but can also help pet bird stewards keep their companions cozy indoors.
Nature’s Heat Recycling System
Birds have a remarkable circulatory adaptation called counter-current heat exchange. Warm blood traveling down the leg runs adjacent to cooler blood returning from the foot. As the two bloodstreams pass each other, heat is exchanged, thus warming the returning blood and cooling the blood moving toward the toes. This helps birds conserve energy, while keeping their core warm and their feet from freezing.
This fascinating study shows how birds regulate heat by adjusting leg temperatures, making them cooler than their feathers in winter to conserve warmth, and warmer than their feathers in summer to expel excess heat.
Bird’s Feet Have Better Cold Tolerance
A bird’s foot, parrots included, is made mostly of bone, tendons, keratin scales, and very little soft tissue. Compared to human feet, there’s less fluid to freeze. Birds’ feet also have tough skin, which helps make their feet more cold-resistant. Cold-tolerant parrot species like the kea of New Zealand famously thrive in alpine conditions thanks to this same foot structure.
Smart Foot-Warming Behaviors
Birds also rely on instinctive behaviors to conserve heat. You’ve probably noticed your feathered companion standing on one foot, tucking their feet into their feathers, especially when they’re relaxed or sleepy—a parrot’s way of being snuggly. These behaviors are natural ways birds preserve warmth, and they demonstrate just how well-designed avian bodies are for seasonal changes, even if your parrot lives in a cozy home year-round.
Climate Adaptations
Birds’ feet are remarkably adapted to their environments, with toe length, thickness, and vascular structure playing a key role in thermoregulation. Birds living in cold climates typically have shorter, thicker toes to minimize heat loss. The lesser the surface area exposed to frigid air or snow, the more body heat conserved. Species like penguins also combine short toes with insulating scales or feathers, which help maintain warmth in extreme cold. Conversely, birds in hot climates tend to have longer, thinner toes, which increases the surface area relative to volume, allowing excess heat to dissipate more efficiently.
Cozy Corners for Feathered Feet
When it’s cold inside, we might reach for a cozy pair of slippers. But our feathered friends need their own version of comfort. Providing cozy roosting spots, like natural wood perches, helps birds retain body heat far better than plastic or stainless steel perches. Textured or branch-like perches give birds a secure grip, while rope perches offer gentle insulation and are soft on their feet. Offering a variety of perching options helps keep your bird’s feet warm and comfortable, giving them true “happy feet.”
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Veterinary Practice News – A look at what’s new in reptile analgesia
This article will review multiple recent analgesic studies in reptiles, including which drugs are currently recommended and which should be avoided due to their lack of efficacy.
September 27, 2025
By Olivia A. Petritz, DVM, DACZM
There have been substantial advances in the field of reptile analgesia over the last decade, which is critically important for the welfare of these species. However, due to the extreme diversity and numbers (more than 7000 species) within the Class Reptilia, there are still significant gaps in knowledge.
This article will review multiple recent analgesic studies in reptiles, including which drugs are currently recommended and which should be avoided due to their lack of efficacy. It is important to note that most of the referenced studies were performed in healthy reptiles, and the results may be different in a compromised or ill animal.
Pain recognition
Most veterinarians are in agreement reptiles can feel pain, but difficulty recognizing pain in reptiles was cited as a major impediment for providing appropriate analgesia by a recent survey of veterinarians.1 Only ~33 percent of veterinarians in that survey believed they had adequate knowledge of analgesia in reptiles, but 82 percent of respondents reported providing analgesia to their reptile patients.1
In another published survey, 76 percent of reptile veterinarians stated they diagnosed pain in reptiles based on extrapolation from other species, and 66 percent reported they diagnosed pain from “behavioral changes.”2
While grimace scores and other objective measures of pain have become more commonplace for many mammalian pet species, no such scores exist for any reptile species to date.
Reptiles have limited muscles of facial expression compared with most mammals, and most species do not vocalize. Therefore, objective measurements of pain are difficult to evaluate in most reptile species.
According to an informal survey of reptile veterinarians, the most commonly reported pain behaviors across reptile families included decreased interaction with the environment, decreased appetite, decreased movement, and inability to bear weight on an affected limb.3
The anatomic diversity amongst reptiles also poses a challenge for pain assessment, including the lack of limbs in snakes and some lizard species, and the presence of a shell (plastron and carapace) in chelonians.
The term “pain” is often used to imply a higher level of neural processing, encompassing both a physical and emotional response, such as what occurs in the neocortex of mammals. However, since both birds and reptiles lack a neocortex, the terms nociception and antinociception are likely more appropriate to use to describe “pain” and “analgesia” in most non-mammalian species.4 However, many veterinarians and publications still use these terms interchangeably.
The presence of scaled skin and certain species’ proclivity for self-induced thermal burns (such as from a heating element) pose challenges for designing accurate analgesic efficacy research methods, specifically those that utilize thermal stimuli.
Alternative models have been evaluated in reptiles, and another study found that a decrease in feeding behavior showed promise to assess antinociception in ball pythons.5
For additional information on pain models and comparisons of the pain pathway between mammals and reptiles, the reader is encouraged to consult a recently published review article on this subject.3
Opioids
Full mu opioids have been proven to be efficacious in numerous reptile species to date. However, based on several previous studies, it is still unclear whether mu opioids are efficacious in any snake species.6,7
Morphine
Morphine provided antinociception in bearded dragons (1 and 5 mg/kg SC) and red-eared sliders (1.5 and 6.5 mg/kg SC), but there was no clear efficacy proven in corn snakes, even when administered at an extremely high dose of 40 mg/kg.8,9,10 Antinociception was also shown in black and white tegus following single intramuscular injections of morphine at 5 and 10 mg/kg.11 Similar to mammals, respiratory depression has also been seen in higher doses of morphine in reptiles, and clinicians should be aware of this possible side effect if morphine is administered on an outpatient basis or when a respiratory rate cannot be monitored.8
Hydromorphone
Hydromorphone is another mu opioid receptor agonist that has been evaluated in several reptile species, with good analgesic efficacy. In red-eared sliders and bearded dragons, hydromorphone was found to provide analgesia at 0.5 mg/kg SC for up to 24 hours.12,13 Respiratory depression was seen at higher doses, similar to morphine, and clinicians should plan appropriately for monitoring respiration.
Injections of any drug (analgesic or other) should be performed in the forelimbs (Figure 1) or cranial portions of a reptile’s body, as drugs that undergo hepatic metabolism/excretion or renal tubular excretion can have reduced efficacy and/or enhanced toxicity if injected in the caudal aspects of the body/hindlimbs due to the presence of the renal portal system.
Fentanyl
There are several previous studies that have evaluated fentanyl patches in reptile species. Anecdotally, fentanyl patches have shown promise for providing analgesia in several snake species, particularly for alleviating pain associated with chronic conditions such as spinal osteoarthritis.
As there are limited studies demonstrating any effective analgesics in snakes, this was a promising finding. A 2017 study found that ball pythons had similar concentrations of mu-opioid receptors in their brain and spinal cord tissues as turtles and showed respiratory depression after placement of transdermal fentanyl patches, suggesting the physiologic efficacy and route of this drug.14
In addition, plasma concentrations were high following transdermal administration, suggesting positive transdermal absorption. However, there were no differences seen in response to a thermal stimulus, suggesting a lack of analgesia, at least according to this pain model.14
A subsequent study has recently been published by this same research group, evaluating serum fentanyl concentrations and behaviors after the placement of transdermal fentanyl patches in healthy corn snakes.15 The plasma concentrations of fentanyl in those snakes remained above therapeutic concentrations in mammals for approximately four weeks. In addition, the behavioral changes suggested this dose provided analgesia without negative side effects.15
Buprenorphine
Buprenorphine, a partial mu opioid receptor agonist and kappa receptor antagonist, is frequently used for analgesia in mammals. However, according to studies in green iguanas16 and red-eared slider turtles,17 effective analgesia has not been demonstrated in reptiles. Therefore, this drug is not currently recommended for analgesia in any reptile species until proven otherwise by additional research.
Butorphanol
Butorphanol is a kappa receptor agonist/mu receptor antagonist that is commonly used in many avian species for analgesia and was previously used in reptiles for the same purpose. According to several publications, this drug does not provide analgesia in bearded dragons, corn snakes, ball pythons, black and white tegus, and red-eared slider turtles at a wide range of dosages.6,7 Therefore, butorphanol is also not currently recommended for analgesia in any reptile species.
NSAIDs
Non-steroidal anti-inflammatory drugs (NSAIDs) are commonly used in many species for analgesia and anti-inflammatory properties, including reptiles. They act primarily by inhibiting cyclooxygenase enzymes (COX), which prevents the production of prostaglandins and several other inflammatory mediators.7
The main COX enzymes are COX-1, which are expressed in most tissues, and COX-2, which are elevated secondary inflammatory processes. Consequently, most NSAIDs that are classified according to their COX selectivity: COX-1, COX-2, or both (non-selective).
In addition, NSAID efficacy is difficult to quantify, as plasma concentrations of these drugs often do not reflect the tissue concentrations at the site of inflammation, which further complicates NSAID pharmacokinetic and pharmacodynamic research.7
Several studies have found increased concentrations of COX-1 enzymes in inflamed tissues of box turtles18 and ball pythons19 compared with COX-2 enzymes, suggesting COX-1 selective NSAIDs may be more appropriate for these reptile species.
Anatomic diversity in reptiles—from limbless snakes to shell-covered chelonians—adds complexity to pain assessment across species.
Meloxicam
Meloxicam is a primarily COX-2 inhibitor, and widely used in exotic animal medicine, including in reptiles. However, there is conflicting evidence of its efficacy in several reptile species, despite numerous pharmacokinetic studies in this Class. Ball pythons administered meloxicam 0.3 mg/kg intramuscularly showed no indications of analgesia for a surgical catheterization procedure.20 Conversely, after administration of meloxicam (0.4 mg/kg IM) to bearded dragons, signs of successful antinociception were noted.21 Intramuscular meloxicam administered at 0.2 mg/kg in red-eared slider turtles reached plasma concentrations sufficient to provide analgesia in mammalian species for ~48 hours, but the oral bioavailability was poor.22
Ketoprofen
Ketoprofen is a non-selective inhibitor of both COX-1 and COX-2 enzymes and has been used with increasing frequency in reptiles. In green iguanas, ketoprofen had a long half-life when administered at 2 mg/kg IV, but was slightly shorter when administered intramuscularly (~8 hours).23
The safety of repeated doses of ketoprofen (2 and 20 mg/kg IM for 14 days) were recently evaluated in bearded dragons.24 No adverse systemic effects were noted on biochemical panels or histopathologic examination post-mortem, but there was severe muscle necrosis present at the injection sites of the 20 mg/kg group.
Alpha-2 agonists
Despite the lack of experimental antinociception for many analgesics in snakes, several recent studies have demonstrated positive effects with the alpha-two agonist dexmedetomidine in ball pythons.
Dexmedetomidine is often used in combination with other sedatives for anesthesia in reptiles, at much higher doses than what is typical in most mammals. However, a recent study evaluated the response of ball pythons to a thermal noxious stimulus after administration of 0.1 – 0.2 mg/kg SC dexmedetomidine and found it did produce antinociception at those doses.25 Excessive sedation was not noted, but respiration was depressed, without apnea.
In a follow-up paper by the same investigators, dexmedetomidine was administered at 0.1 mg/kg SC, and antinociception was confirmed.26 In addition, concurrent administration of doxapram (10 mg/kg) helped mitigate the reduced respirations without changing the thermal antinociception.26
While doxapram is not routinely used concurrently with dexmedetomidine in reptiles in clinical practice, the use of dexmedetomidine as an analgesic is becoming more common, as few other proven analgesic drugs for snakes exist.
Olivia A. Petritz, DVM, DACZM, graduated from Purdue University and then completed several internships and a residency in the field of zoo and exotic animal medicine. Dr. Petritz became a diplomate in the American College of Zoological Medicine in 2013, specializing in zoological companion animals (exotic pets). Petritz started an exotics service at a specialty hospital in Los Angeles, Calif., following her residency, and is currently an associate professor of Avian and Exotic Animal Medicine at North Carolina State University.
References
Gris, Vanessa N., et al. “Attitudes of Brazilian veterinarians towards anesthesia and pain management in reptiles.” Journal of Herpetological Medicine and Surgery 32.3 (2022): 208-218.
Read, Matt R. “Evaluation of the use of anesthesia and analgesia in reptiles.” Journal of the American Veterinary Medical Association 224.4 (2004): 547-552.
La’Toya, V. Latney. “Pain recognition in reptiles.” Veterinary Clinics: Exotic Animal Practice 26.1 (2023): 27-41.
Perry, Sean M., and Javier G. Nevarez. “Pain and its control in reptiles.” Veterinary Clinics: Exotic Animal Practice 21.1 (2018): 1-16.
James, Lauren E., et al. “Evaluation of feeding behavior as an indicator of pain in snakes.” Journal of Zoo and Wildlife Medicine 48.1 (2017): 196-199.
Sladky KK. Reptile and Amphibian Analgesia. In: Miller ER, Calle PP, Lamberski N, editors. Fowler’s zoo and wild animal medicine, vol. 9. St Louis (MO): Elsevier-Saunders; 2019. p. 421–39
Sladky, Kurt K. “Treatment of pain in reptiles.” Veterinary Clinics: Exotic Animal Practice 26.1 (2023): 43-64.
Sladky KK, Miletic V, Paul-Murphy J, et al. Analgesic efficacy and respiratory effects of butorphanol and morphine in turtles. J Am Vet Med Assoc 2007;230: 1356–62
Kinney M, Johnson SM, Sladky KK. Behavioral evaluation of red-eared slider turtles (Trachemys scripta) administered either morphine or butorphanol following unilateral gonadectomy. J Herp Med Surg 2011;21:54–62.
Sladky KK, Kinney ME, Johnson SM. Analgesic efficacy of butorphanol and morphine in bearded dragons and corn snakes. J Am Vet Med Assoc 2008; 233:267–73.
Leal WP, Carregaro AB, Bressan TF, et al. Antinociceptive efficacy of intramuscular administration of morphine sulfate and butorphanol tartrate in tegus (Salvator merianae). Am J Vet Res 2016;78:1019–24.
Mans C, Lahner LL, Baker BB, et al. Antinociceptive efficacy of buprenorphine and hydromorphone in red-eared slider turtles (Trachemys scripta elegans). J Zoo Wildl Med 2012;43:662–5.
Hawkins SJ, Cox S, Yaw TJ, et al. Pharmacokinetics of subcutaneous administered hydromorphone in bearded dragons (Pogona vitticeps) and red-eared slider turtles (Trachemys scripta elegans). Vet Anaesth Analg 2019;46(3):352–9.
Kharbush R, Gutwillig A, Hartzler K, et al. Antinociceptive and respiratory effects following application of transdermal fentanyl patches and assessment of brain mu opioid receptor mRNA expression in ball pythons. Am J Vet Res 2017;78:785–95.
Walter, Benjamin, et al. “Serum fentanyl concentrations and behavior associated with transdermal fentanyl application on healthy corn snakes (Pantherophis guttatus).” Journal of Zoo and Wildlife Medicine 54.4 (2024): 738-745.
Greenacre CB, Schumacher JP, Tacke G, et al. Comparative antinociception of morphine, butorphanol, and buprenorphine versus saline in the green iguana, Iguana iguana, using electrostimulation. J Herp Med Surg 2006;16:88–92.
Mans C, Lahner LL, Baker BB, et al. Antinociceptive efficacy of buprenorphine and hydromorphone in red-eared slider turtles (Trachemys scripta elegans). J Zoo Wildl Med 2012;43:662–5.
Royal, Lillian W., et al. “Evaluation of cyclooxygenase protein expression in traumatized versus normal tissues from eastern box turtles (Terrapene carolina carolina).” Journal of Zoo and Wildlife Medicine 43.2 (2012): 289-295.
Sadler, Ryan A., et al. “Evaluation of the role of the cyclooxygenase signaling pathway during inflammation in skin and muscle tissues of ball pythons (Python regius).” American journal of veterinary research 77.5 (2016): 487-494.
Olesen, Mette G., et al. “Effects of preoperative administration of butorphanol or meloxicam on physiologic responses to surgery in ball pythons.” Journal of the American Veterinary Medical Association 233.12 (2008): 1883-1888.
Greenacre CB, Massi K, Schumacher JP, et al. Comparative antinociception of various opioids and non-steroidal anti-inflammatory medications versus saline in the bearded dragon (Pogona vitticeps) using electrostimulation. Proc Annu Conf Rept Amph Vet 2008;87–8.
Di Salvo A, Giorgi M, Catanzaro A, et al. Pharmacokinetic profiles of meloxicam in turtles (Trachemys scripta scripta) after single oral, intracoelomic and intramuscular administrations. Vet Pharmacol Therap 2015;39:102–5.
Tuttle AD, Papich M, Lewbart GA, et al. Pharmacokinetics of ketoprofen in the green iguana (Iguana iguana) following single intravenous and intramuscular injections. J Zoo Wildl Med 2006;37:567–70.
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