I have worked with a small handful of amphibian species in captive collections, including White's Tree Frogs, Cane Toads, Common Toads (one cohort sharing an enclosure with Scottish Wildcats), and Mountain Chicken Frogs. Because Mountain Chicken Frogs are of such high conservation importance, procedures like swabbing for Batrachochytrium dendrobatidis (Bd) and ultrasounding individuals are essential to their captive management and safe transport between collections. What follows is a review of the disease those safeguards exist to contain.

Introduction

Chytridiomycosis has led to the extinction and rapid decline of amphibian species (>200 species) globally, requiring various species to be managed in captivity. This has led to extensive research being undertaken on the chytrid fungus Batrachochytrium dendrobatidis (Bd), which causes chytridiomycosis (1,2,3).

Causes and susceptibility

An amphibian's range and how each species is specialised to its niche impacts the likelihood of contracting Bd. For example, Dixie Valley Toads live in geothermal springs, which produce temperatures upwards of 27°C that inhibit Bd growth (4). Similarly, light intensity affects the risk of Bd, as seen in rainforest frogs, where less canopy cover led to a reduction in Bd-positive cases because of the increase in temperature (5). Species that hibernate are also at higher risk of Bd, because during this period their immune system is compromised (4).

These factors are exacerbated by anthropogenic ones, including international trade, pollution, hunting, and the introduction of invasive species. The Green and Golden Bell Frog (Ranoidea aurea) has seen population declines due to Bd, but its decline has been compounded by predation from invasive fish species consuming its larvae and eggs (3).

Bd develops rapidly. In one study, this rapid development contributed to the virulence of the disease: amphibians have a weak humoral immune response and instead rely on the antifungal properties of skin peptides, so the simpler these peptides are, the higher the risk of contracting Bd (6).

Transmission

The transmission of Bd occurs through direct contact with infected individuals or surfaces. The spores swim using their flagella, and they have also been found on the feathers and feet of waterfowl, which can spread them (2,5). This fungus reproduces using thalli that are spherical in shape with a papilla to discharge the zoospores that penetrate the pores of the amphibian's skin, and these spores accumulate in shed skin and deceased infected individuals (7,8).

A handful of amphibian species (e.g. the American Bullfrog and the Cane Toad) can become infected with Bd but do not develop chytridiomycosis; instead they function as vectors for the disease (sub-clinically infected) (2,9). This is likely explained by the fact that in the fungus's native range in Asia, Bd coexists with its hosts without causing outbreaks, and in Northern Leopard Frogs, Bd infection has been associated with reduced predation pressure (5).

Bd spores can survive for as long as seven weeks after release, which is a lengthy period given that bodies of water are essential to most amphibian species (10).

Signs and symptoms

In tadpoles — which are more susceptible to the disease due to their underdeveloped immunity — morphological symptoms appear in the mouthparts (5). Bd attacks the jaw sheaths and the cells composed of keratin (which is speculated to be Bd's food source), characterised by a lack of black pigmentation, which compromises filter feeding and limits growth capacity (7,8).

In adults, ulcers and epidermal inflammation lead to disruption of osmoregulation and respiration, and can cause cardiac arrest (5,11). Other symptoms of Bd include weight loss caused by anorexia, seizures, abnormal bone development, lethargy, and excessive skin shedding (this shed can be tested for Bd) (12).

Diagnosis

For an individual to be tested for the fungus, the amphibian must be swabbed along its digits and ventral side with sterile swabs to prevent cross-contamination and false results. The swabs are then tested through polymerase chain reaction (PCR) (6). The two types of PCR are real-time PCR (typically more sensitive because it targets ribosomal DNA) and conventional. Both can be used to diagnose the disease primarily and to re-evaluate post-treatment (2,13).

Sterile gloves must be used during swab handling to prevent the spread of the fungus and other pathogens, especially due to the highly absorbent capacities of amphibian skin (14). Veterinarians should monitor patients carefully because fungal infections make the host more susceptible to secondary infections, as Bd reduces cutaneous resistance to other pathogens and toxins (5).

Treatment

In adult frogs, a 0.01% itraconazole bath (formed from 1% itraconazole oral solution diluted with Amphibian Ringer's or saline solution at 0.6%) is provided for 11 days to successfully treat the disease (2).

In some species, an increase in temperature can effectively kill Bd, because it stimulates epidermal growth, preventing the Bd life cycle from completing before the skin is shed; the recommendation is to expose the patient to 37°C for 16 hours. All treatments must be paired with suitable husbandry procedures, including thorough cleaning and disinfection of potentially contaminated surfaces (2,14). Elevated temperatures also improve the immune response, and repeated exposure and treatment can build immunity to the disease (4).

In some species in their larval state, Bd can be successfully treated with UVB radiation, and salinity increases survival rates in infected cases (5). Alternatively, Bd cannot withstand freezing; however, this treatment would only be successful for a small number of amphibian species (8).

To prevent further spread of Bd, the vivarium in which an infected patient is housed must be thoroughly cleaned, including waste water, which should be treated with 4 ml/l of F10 or another concentrated quaternary ammonium disinfectant before being disposed of appropriately, away from water bodies (11).

Areas for research

Further exploration into the symbiotic relationship between bacteria and amphibian skin, which prevents the development of Bd, is needed; these studies need to occur in the field, not just in the lab (2). Other fungi from the chytrid lineage have not been studied or identified, and doing so could provide further information on the evolution of these diseases and help prevent future outbreaks (5).

The limitations of Bd also need to be researched: for example, the knowledge gaps that exist in the West African range of Bd, where the largest proportion of studies take place in Ghana (14,15). Alongside this research, relocation of invasive and sub-clinically infected species should take place, because these reservoirs contribute to Bd prevalence (5).

In the area of metamorphosis research, work should focus on how larvae and tadpoles contract Bd across a wide variety of amphibian species, particularly those that have immunological resistance and do not develop lesions and ulcers (16).

In the fungus's native ranges, continual testing and surveillance of wild amphibian habitats should be carried out to ensure that current management plans (including water management) are best suited to the current threat risk to each ecosystem — particularly in Key Biodiversity Areas — and to support the potential development of amphibian vaccines (17). Further research should also take place into other environmental parameters such as pH, salinity, and dissolved oxygen (5).

Evaluation of husbandry procedures should occur frequently, because research is constantly broadening our understanding of this fungus: examples include the implementation of automatic waste-water disposal systems in more collections, which reduce staff time and prevent chemicals from leaching into the environment (10). Recent husbandry improvements also include raising the temperature of enclosures by painting masonry bricks black (which absorb heat from sunlight or UV bulbs), providing shelter for the targeted species (18).

Conclusion

This article has reviewed the already detrimental effect Bd has had on amphibian species, and has also evaluated the future challenges this fungus will pose to ecosystems — particularly as amphibians are vital habitat-health indicators. It has put forward potential husbandry procedures for both captive and wild populations, such as the disinfection of footwear for individuals on-site and the associated transportation vehicles (17). Across the world, zoos and in situ organisations (linking to the One Plan Approach) are collaborating to conserve the species most threatened by Bd, providing a variety of training programmes, workshops, and funding for facilities to set up breeding programmes.

References

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  2. Pessier, A.P. Management of disease as a threat to amphibian conservation. International Zoo Yearbook 42, 30–39 (2008). https://doi.org/10.1111/j.1748-1090.2008.00047.x

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  10. Robertson, H., Eden, P., Gaikhorst, G., Matson, P., Slattery, T., and Vitali, S. An automatic waste-water disinfection system for an amphibian captive-breeding and research facility. International Zoo Yearbook 42, 53–57 (2008). https://doi.org/10.1111/j.1748-1090.2008.00048.x

  11. Naguib, M. Veterinary approach to the amphibian patient. In Practice 44(2), 91–99 (2022). https://doi.org/10.1002/inpr.176

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  13. Boyle, D.G., Boyle, D.B., Olsen, V., Morgan, J.A.T., and Hyatt, A.D. Rapid quantitative detection of chytridiomycosis (Batrachochytrium dendrobatidis) in amphibian samples using real-time Taqman PCR assay. Diseases of Aquatic Organisms 60(2), 141–148 (2004). https://doi.org/10.3354/dao060141

  14. Benício, R.A., Carvalho, T., Barbosa, M.D.R., Costa, J. de M., da Silva, F.C.C., and Fonseca, M.G. Worrying news for Brazilian Caatinga: prevalence of Batrachochytrium dendrobatidis in amphibians. Tropical Conservation Science 12, 1–6 (2019). https://doi.org/10.1177/1940082919892626

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