1. Introduction
Exotic animals are typically non-domesticated and non-indigenous to the geographical region they inhabit [
1]. Fungal diseases are widespread and have led to the decline of multifarious exotic animals. Fatal dermatomycoses caused by
Nannizziopsis spp. have been reported in captive reptiles, despite treatment with antifungals [
2]. Dermatomycoses such as those caused by the fungal complex
Chrysosporium anamorph of Nannizziopsis vriesii (CANV) are emerging infectious diseases in reptiles, with increasing prevalence and spread across geographical regions and, unfortunately, increasing antifungal resistance patterns [
3]. Likewise, the most common fungal disease in avians, aspergillosis, has a high mortality rate [
4]. Other than the avian anatomical and physiological characteristics which predisposes them to aspergillosis, azole resistance, especially in
Aspergillus fumigatus, is a rising global concern requiring stricter epidemiological surveillance to reduce treatment failures [
5]. Limited studies were collated for other fungal diseases, such as candidiasis and cryptococcosis, in avians.
Candida albicans is the most notorious pathogen for candidiasis, while the pathogenic agents for cryptococcosis are
Cryptococcus neoformans and
Cryptococcus gattii [
6]. Although some antifungals were studied for candidiasis and cryptococcosis, the sample sizes were not statistically significant for recommendations to be made. With rising mortality and morbidity rates in these exotic animals, there is a need to refine the currently inadequate armamentaria of antifungals used in the treatment of fungal diseases.
Antifungals are often used in the treatment of fungal diseases in exotic animals to eliminate fungal pathogens and resolve clinical symptoms. Different classes of antifungals such as azoles, allylamines, echinocandins, nucleoside analogues and polyenes with different mechanism(s) of actions have been used [
7]. Veterinarians commonly adopt recommendations from formularies such as the Exotic Animal Formulary (EAF) to guide dosing regimens of antifungals. Due to the lack of pharmacological data in some exotic animals, such as those from the crocodylia or phoenicopteriformes order, veterinarians often extrapolate doses from closely-related or sometimes even distantly related species. This may not be ideal due to marked interspecies differences in metabolic rates and physiological features, even within closely-related species, which may affect the pharmacological profile of antifungals and ultimately therapeutic outcomes. When inappropriate dose extrapolations are made, suboptimal treatment outcomes may arise in the form of increased risk of toxicity, subtherapeutic dosing or treatment failure, subsequently contributing to the decline of exotic animals [
8].
2. Pharmacodynamic Variabilities in Avians and Reptiles
Seventy studies or case reports on the efficacy and/or safety of antifungals in avians were found, with itraconazole (n = 20) and voriconazole (n = 17) being the most commonly used antifungal, especially in Psittaciformes and Sphenisciformes. In reptiles, 30 such reports were found, with itraconazole (n = 13), terbinafine (n = 5) and voriconazole (n = 5) being the most commonly used antifungal, especially in Squamata.
2.1. Evaluation of Dosing Regimens in Relation to Efficacy
Clinical resolution of lesions or symptoms are often used to assess the efficacy of antifungals in treating fungal diseases. Objective measures of efficacy include the use of MIC data to determine the susceptibility of fungal pathogens to antifungals, or to maintain therapeutic plasma concentrations above MIC. Determination of MIC of fungal isolates were limited and often relied upon comparisons with reported values in human studies or other related species. AST is recommended, when possible, to attain specific MIC values, especially for causative pathogens with MIC yet to be determined and also to look out for emerging resistance patterns.
2.1.1. Type of Fungal Pathogens/Disease
The type of fungal pathogens is pivotal in the selection of antifungals to optimise treatment efficacy in exotic animals. The evaluation on the use of different antifungals for the more common fungal diseases in avian and reptiles are summarised in Table 2 and Table 3.
Aspergillosis commonly involves the respiratory tract, with
Aspergillus fumigatus being the most common causative pathogen. Voriconazole appears to be efficacious in treating
Aspergillus fumigatus infections in Japanese quails and racing pigeons (
Table 2), and the large sample size provides further credibility. AST conducted on racing pigeons revealed an MIC of 0.25µg/mL for
Aspergillus fumigatus, showing susceptibility to voriconazole, which could explain the efficacy observed. However, not all studies performed AST, and many used MIC data from other species to evaluate the efficacy of antifungals [
32]. Additionally,
Aspergillus fumigatus can be easily confused with more than 11 other
Aspergillus spp. based on their conidial morphology [
33], including
Aspergillus lentulus, which is typically known to be resistant to triazoles and amphotericin B [
34]. Accurate identification of the causative pathogen and its susceptibility to different antifungals is crucial to ensure efficacy in treatment. AST is recommended, especially for severely ill exotic animals, where time is critical and using antifungals that are resistant to the causative fungal isolates may result in poor clinical outcomes.
For Aspergillus flavus infections, itraconazole proved efficacious in turkeys by reducing the lung lesion score (Table 2). In Okinawa rails, however, though MIC revealed susceptibility of Aspergillus flavus to itraconazole, inefficacy was observed, possibly due to low concentrations in the air sacs. Voriconazole might also be efficacious for Aspergillus flavus infections, but this is inconclusive due to the small sample sizes of studies.
CANV infections in reptiles are the rising cause of mortality, especially in bearded dragons (n = 7) [
35], which also have the most studies/case reports collated (
Table 3). Numerous pathogens under the CANV denomination were identified and have been subsequently renamed to include three genera:
Nannizziopsis spp.,
Ophidiomyces spp. and
Paranannizziopsis spp. Formerly termed “yellow fungus disease” has been replaced by “nannizziomycosis” and “paranannizziomycosis”; thus, careful interpretation of older literature is warranted [
3]. With the limited reports of antifungal therapy in CANV-related infections, itraconazole did not show efficacy in most reptiles, except the tuataras infected by
Paranannizziopsis spp. (
Table 3). In a direct comparison study, voriconazole demonstrated better efficacy in clearing CANV infection and had a higher survival rate (
Table 3). In another giant girdled lizard, voriconazole used at the same dose cleared CANV infection and AST revealed the lowest MIC (0.25µg/mL) for voriconazole amongst the other antifungals tested [
36]. Hence, voriconazole shows promising potential for the treatment of CANV infections, but further studies should be conducted in other reptile species to ensure the efficacy and safety of the dosing regimens [
36].
Ophidiomycosis, previously termed “snake fungal disease” appears to only cause disease in snakes, though the reasons are unclear [
3]. Studies on antifungal therapy for ophidiomycosis are scarce, with inconclusive efficacy (
Table 3). Unconventional methods with terbinafine using subcutaneous implants and nebulization have been described to be possibly efficacious based on plasma-concentration studies, but further studies on actual diseased snakes are required to confirm these findings. Such methods are preferred, especially when venomous snakes are involved and also due to their bimonthly feeding, which makes oral administration of antifungals unfavourable [
37]. Though voriconazole may also be a possible option, efficacy is unclear.
While MIC data is important for assessing efficacy, efficacy is not guaranteed, even when the pathogen is susceptible to the antifungal in vitro. Other reasons for the lack of efficacy may be insufficient concentrations at the site of infection, acquired resistance, interspecies differences affecting the disposition of antifungals, or poor condition of species at initial presentation. Such factors should also be considered when assessing the efficacy of antifungals.
5.1.2. Site of Infection
The site of infection plays a crucial role in the selection of appropriate antifungals. Fungal diseases in reptiles commonly occur in the cutaneous layers, and their much thinner hypodermis can predispose them to subsequent invasion of deeper tissues, such as the bones or muscles, requiring systemic antifungals [
38]. The common sites of infection in avians preferentially involve the respiratory tract, possibly because the primary route of fungal invasion is via inhalation [
39].
Tissue concentrations of antifungals are more clinically relevant and reflective of the efficacy of the antifungal rather than the plasma concentrations, though they are not commonly measured in PK studies. A typical dose of itraconazole 10 mg/kg seems to be efficacious in different species, such as the African grey parrots and red-tailed hawks, but not in Okinawa rail or Pesquet’s parrots (
Table 2). A possible reason for this could be that the tissue concentrations were not high enough to eliminate the fungal pathogens. In mallard ducks and racing pigeons, itraconazole concentrations in lung and brain tissues were the lowest, and it was posited that a significantly higher dose would be required to have a fungicidal effect in lung tissues [
39,
40]. Voriconazole tissue concentrations were not measured in any studies during our search. In reptiles, the keratinophilic nature of terbinafine makes it ideal for the treatment of dermatomycoses, due to accumulation in keratinized tissues [
41].
Tissue concentration studies were scarce, and future studies should focus on methods to predict tissue concentrations with plasma concentrations in specific species to better assess the therapeutic efficacy of antifungals.
5.2. Evaluation of Dosing Regimens in Relation to Safety
Species-specific data are crucial when assessing the safety of antifungals. Itraconazole, voriconazole and terbinafine had more extensive safety data reported in avians and reptiles. Variable manifestations of neurotoxicity were observed for voriconazole in the form of lethargy, anorexia, ataxia and seizure-like symptoms in multiple penguin species such as Humboldt, African or Macaroni penguins, hind limb paresis in red-eared slider turtles and torticollis in cottonmouths, but not in African grey parrots or Japanese quails, despite using significantly higher doses. Possible hepatotoxicity from voriconazole administration was seen in the form of mild increases in aspartate aminotransferase in African grey parrots and a few inland bearded dragons. Similar adverse events have been observed in humans, and high trough concentrations are reported to increase the risk of neurotoxicity and hepatotoxicity [
42], though the target trough concentrations in specific exotic species are unclear. Adverse events in itraconazole were observed as hepatotoxicity mainly in the form of increases in aspartate aminotransferase in bearded dragons and a Parson’s chameleon, but not in tuataras, despite being given similar doses. Terbinafine was generally found to be safe in both avians and reptiles. For the other antifungals, there were either no adverse events reported or insufficient sample sizes to evaluate safety accurately.
Though many single-dose studies reported no adverse events, safety cannot be accurately concluded as toxicity may sometimes only be observed upon chronic administration. This was evident in the numerous penguin species which showed neurotoxicity in multiple-dosing regimens but not in single-dosing regimens. Furthermore, the deranged physiology in immunocompromised animals may further predispose them to adverse events that healthy animals are not subjected to. Adverse drug reactions may be unpredictable and therefore hard to predict in specific species [
43]. Hence, close therapeutic drug monitoring is recommended, especially for the azoles, to ensure safe dosing regimens.
Table 2. Summary of efficacy and safety data in avian species accompanied by a critical analysis of the study findings.
Table 3. Summary of efficacy and safety data in reptile species accompanied by a critical analysis of the study findings.
Site of Infection |
Antifungal Used |
Dosing Regimen |
Species |
Evaluation of Dosing Regimens in Relation to Efficacy and Safety |
References |
Chrysosporium anamorph of Nannizziopsis vriesii (CANV and related spp.) infections a |
Chrysosporium anamorph of Nannizziopsis vriesii (CANV) |
Skin |
Itraconazole |
PO 5 mg/kg q24 h + 1% topical silver sulfadiazine cream q12 h |
Boa constrictor (n = 1) |
- •
-
Lack of efficacy as snake still died after 3 weeks of itraconazole treatment.
- •
-
Safety was not reported.
|
[72] |
Skin |
Itraconazole |
PO 5 mg/kg q24 h |
Coastal bearded dragon (n = 1) |
- •
-
Lack of efficacy as bearded dragon was euthanised due to persistent systemic mycoses, with several granulomas indicating the presence of fungal infections.
- •
-
Safety was not reported.
|
[73] |
Skin |
Itraconazole |
PO 5 mg/kg q24 h |
Inland bearded dragon (n = 7) |
- •
-
Possibly efficacious as the fungus was not re-isolated from the dermal lesions after approximately 4 weeks in 2/7 of the bearded dragons.
- •
-
5/7 of the other bearded dragons did not survive, and upon post-mortem culture, CANV was cultured in 2/5 of these bearded dragons.
- •
-
Possible drug toxicity was apparent.
|
[74] |
Skin |
Itraconazole |
PO 5 mg/kg q48 h × 14 days + 20 mg/kg SC ceftazidime q72 h × 12 days |
Inland bearded dragon (n = 1) |
- •
-
Efficacy was observed.
- •
-
Safety was not reported.
|
[35] |
Skin |
Itraconazole |
PO 10 mg/kg q24 h × 6 weeks + 0.125% chlorhexidine topical solution |
Inland bearded dragon (n = 1) |
- •
-
Lack of efficacy was observed.
- •
-
Possible drug toxicity was apparent.
|
[35] |
Skin |
Itraconazole |
PO 10 mg/kg q24 h × 10 weeks + daily baths in dilute povidone-iodine solution |
Inland bearded dragon (n = 1) |
- •
-
Lack of efficacy was observed.
- •
-
Post-mortem examination revealed an extensive ulcerative dermatitis of the ventral abdomen and a focal hepatic granuloma. Bacterial rods and fungal hyphae were found within the granuloma.
|
[35] |
Systemic |
Itraconazole |
PO 10 mg/kg q24 h × 21 days |
Jewel chameleon (n = 1) |
- •
-
Lack of efficacy was observed.
- •
-
Safety was observed during the course of treatment, but the duration of therapy was too short for toxicity to be observed.
|
[75] |
Skin |
Itraconazole |
PO 10 mg/kg q24 h × 21 days |
Parson’s chameleon (n = 1) |
- •
-
Possibly efficacious despite the general loss of condition, but the chameleon’s overall clinical condition improved months later. However, the chameleon still died a year later. Autopsy indicated that death was due to cholecystitis and septicaemia, and no evidence was mycosis was seen.
- •
-
Possible drug toxicity was apparent.
|
[75] |
Skin |
Voriconazole |
PO 10 mg/kg q24 h |
Giant girdled lizard (n = 1) |
- •
-
Efficacy was observed.
- •
-
Voriconazole had the lowest MIC upon antifungal susceptibility testing (0.25µg/mL) amongst the other antifungals tested.
- •
-
Safety was observed.
|
[36] |
Skin |
Voriconazole |
PO 10 mg/kg q24 h |
Inland bearded dragon (n = 7) |
- •
-
Efficacy was observed.
- •
-
Possible drug toxicity was apparent.
|
[74] |
Chrysosporium spp. |
Skin |
Ketoconazole |
PO 20 mg/kg q24 h + 2% chlorhexidine solution + topical terbinafine |
Green Iguana (n = 1) |
- •
-
Efficacy was observed.
- •
-
Safety was not reported.
|
[76] |
Skin |
Ketoconazole |
PO 20 mg/kg q24 h × 14 weeks + 2% chlorhexidine solution + topical terbinafine |
Green Iguana (n = 1) |
- •
-
Efficacy was observed.
- •
-
Safety not reported.
|
[76] |
Nannizziopsis guarroi |
Skin |
Terbinafine |
PO 20 mg/kg SD |
Inland bearded dragons (n = 8) |
- •
-
Efficacy was inconclusive.
- •
-
Multiple-dosing studies on actual diseased bearded dragons are required to demonstrate efficacy.
- •
-
Safety was observed.
|
[77] |
Paranannizziopsis australasiensis |
Skin |
Itraconazole |
PO 5 mg/kg q24 h × 28 days + topical 1% terbinafine ointment × 21 days |
Tuatara (n = 1) |
- •
-
Efficacy was inconclusive.
- •
-
Safety was not reported.
|
[78] |
Skin |
Itraconazole |
PO 5 mg/kg q24 h × 29 days |
Tuatara (n = 1) |
- •
-
Efficacy was observed.
- •
-
Safety not reported.
|
[78] |
a Evaluation of antifungal therapy:
- •
-
Interpretations are not statistically significant due to small sample sizes and scarce reports of the outcomes of antifungal therapy in different species. More studies on a larger sample size are required to assess the efficacy of such dosing regimens more accurately in different reptile species.
- •
-
Complications arise in the evaluation of therapeutic efficacy due to the changing nomenclature of CANV spp. Molecular testing has revealed numerous subgroups within CANV that are yet to be accurately identified [ 73]. In the past, identification of CANV-related species has also been confused with morphologically similar looking species, such as the Trichophyton spp. [ 73]. Hence, even with case reports showing efficacy for CANV-related infections, it is unclear if the fungal pathogens involved were indeed CANV-related spp.
- •
-
Itraconazole has been widely used, with a typical dose ranging from 5 to 10 mg/kg in different reptile species. Lack of efficacy was observed for most of the species, and possible itraconazole-induced hepatoxicity was observed in bearded dragons and a Parson’s chameleon. However, it did seem to show efficacy in another distinctly related species, the tuatara, though the sample size was small.
- •
-
Voriconazole seems to be much more potent in vitro (based on antifungal susceptibility testing) than itraconazole, and it also shows better therapeutic efficacy in bearded dragons and a giant girdled lizard. Though these findings are preliminary, voriconazole seems to have a better efficacy and safety profile than itraconazole. However, species-specific voriconazole toxicity has been observed and caution should be taken through close monitoring upon initiation of therapy.
- •
-
Although ketoconazole seems efficacious, the small sample size (n = 1) in a single species, the Green Iguanas, the superior pharmacokinetics, and potency of voriconazole is still likely to make it the more favourable antifungal for the treatment of CANV-related infections.
|
Ophidiomyces ophiodiicola b |
Skin |
Itraconazole |
10 mg/kg SD per cloaca |
Cottonmouth (n = 7) |
- •
-
Efficacy is inconclusive but predicted to be inefficacious as the therapeutic concentrations of itraconazole and hydroxyitraconazole (bioactive metabolite) in both plasma and tissues were not reached.
- •
-
Multiple-dosing studies on actual diseased snakes are required to demonstrate efficacy.
- •
-
No adverse events were observed.
|
[79] |
Skin |
Voriconazole |
SC 5 mg/kg SD |
Cottonmouth (n = 6) |
- •
-
Efficacy is inconclusive. Multiple-dosing studies on actual diseased snakes are required to demonstrate efficacy.
- •
-
Even though two of the six the cottonmouths survived with no adverse events, the other four died and presented with a range of symptoms—lethargy, depression, loss of righting reflex, and even torticollis in one of the cottonmouths. This could be suggestive of potential voriconazole-induced neurotoxicity.
|
[79] |
Skin |
Voriconazole |
SC 10 mg/kg SD |
Cottonmouth (n = 1) |
- •
-
Efficacy is inconclusive. Multiple-dosing studies on actual diseased snakes are required to accurately predict efficacy in cottonmouths.
- •
-
Safety was observed despite given a higher dose of voriconazole, but sample size (n = 1) is too small to make conclusions on the safety profile.
|
[79] |
Skin |
Voriconazole |
SC osmotic pump 22.2 mg/mL (1.02–1.6 mg/kg/h) |
Eastern massasauga (n = 2) |
- •
-
Efficacy was not observed.
- •
-
However, sample size is too small to make any generalised conclusions on efficacy.
- •
-
Safety was not reported.
|
[79] |
Skin |
Voriconazole |
SC osmotic pump 10 mg/mL (12.1–17.5 mg/kg/h) |
Timber rattlesnake (n = 1) |
- •
-
Efficacy is inconclusive. Therapeutic efficacy cannot be concluded due to small sample size.
- •
-
Safety was not reported.
|
[79] |
Skin |
Terbinafine |
2 mg/mL (18 mg total dose) × 30 min via nebulisation |
Cottonmouth (n = 7) |
- •
-
Efficacy was inconclusive.
- •
-
In vitro susceptibility concentration (0.015 µg/mL) was maintained for at least 12 h in four of the nebulized cottonmouths. The author recommends using a dose of 2 mg/mL (16 mg total dose) × 30 min via nebulisation once a day for the treatment of snake fungal disease. However, further studies need to be carried out to ensure that this recommendation is clinically relevant in actual diseased cottonmouths.
- •
-
No adverse effects were observed, but all snakes non-significantly gained weight.
|
[37] |
Skin |
Terbinafine |
24.5 mg (75–190 mg/kg) SC implant cranial to midbody point |
Cottonmouth (n = 7) |
- •
-
Efficacy was inconclusive.
- •
-
In vitro susceptibility concentration (0.015 µg/mL) was maintained for > 5 weeks in implanted snakes (peak concentration of 100 ng/mL at 3 weeks). The author recommends giving a 24.5 mg subcutaneous implant every 5–6 weeks for the treatment of snake fungal disease. However, further studies need to be carried out to ensure that this recommendation is clinically relevant in actual diseased cottonmouths.
- •
-
No adverse effects were observed, but all snakes non-significantly gained weight.
|
[37] |
Mandible and eye |
Ketoconazole |
PO 50 mg/kg q24 h |
Black ratsnake (n = 1) |
- •
-
Reported as Chrysosporium ophiodiicola.
- •
-
Efficacy was not observed.
- •
-
Safety was not reported.
|
[80] |
b Evaluation of antifungal therapy:
- •
-
Interpretations are not statistically significant due to small sample sizes and scarce reports of the outcomes of antifungal therapy in different species. More studies on a larger sample size are required to assess the efficacy of such dosing regimens more accurately in different reptile species.
- •
-
Therapeutic recommendations are unclear given the limited studies. Given that some of the studies were single-dose studies, efficacy cannot be concluded as the disposition of the drug may vary in multiple-dose studies and clinical disease state of the snake.
- •
-
Voriconazole and terbinafine appear to be favourable options given that therapeutic plasma concentrations were reached when using different routes of administrations such as SC osmotic pumps or implants and nebulisation. These are especially critical in the treatment of venomous snakes as oral administration is not favourable given that they feed twice a month and the danger posed to the administrator (s10). SC implants are safer and more practical as the snake can be released back into the wild and be captured back weeks later to monitor the outcomes of the treatment. However, multiple-dose studies are required to supplement the results of the single-dose studies and determine if they are indeed appropriate for the treatment of snake fungal disease.
- •
-
Voriconazole also appears to exhibit species-specific neurotoxicity, in this case the cottonmouths but not the Eastern massasaugas or Timber rattlesnake. Hence, caution needs to be made when using voriconazole in snakes as the mechanism of this toxicity is still unclear. Terbinafine might be favoured given its better safety profile, but the only studies available were in cottonmouths, and it is unclear if the same safety profile will be observed in other snake species. Nevertheless, it is recommended to closely monitor the snake upon drug administration, especially if voriconazole is being used.
|
This entry is adapted from the peer-reviewed paper 10.3390/jof9080810