An Update on Amphibian Chytridiomycosis: In the Field, Diagnostic Laboratory, and Clinic
Allan P. Pessier, DVM, DACVP
Amphibian Disease Laboratory, Wildlife Disease Laboratories, Institute for Conservation Research, San Diego Zoo Global, San Diego, CA, USA
The skin disease chytridiomycosis, caused by infection with the chytrid fungus Batrachochytrium dendrobatidis (Bd), is now a well-recognized contributor to the global amphibian extinction crisis. In addition to the consequences of Bd infection for susceptible wild amphibian populations, there are also challenges for veterinarians that treat captive amphibians in settings that vary from rescue populations on front line of disease outbreaks in the tropics to well-equipped clinics supporting modern cosmopolitan zoo collections.
In the 15 years since the first description of chytridiomycosis there have been significant advances in the understanding of Bd biology, genetics, host defense and pathogenesis. Although Bd is widely distributed and reported from every continent with amphibians, research suggests that some strains or genotypes are more damaging than others and provides support for efforts that aim to minimize movement of Bd infected animals to new locations (e.g., listing of Bd by the OIE under the Aquatic Animal Health Code).3,7 Bd has a very wide host range with reports of infection in over 300 species of frogs, salamanders and caecilians to date; however, there are significant differences in host susceptibility to the disease chytridiomycosis. The range of species that can become subclinically infected with Bd provides ample reservoir sources of infection for highly susceptible species. Elements of host defense against infection can include cutaneous antimicrobial peptides, composition of cutaneous microbial flora, acquired immunity, differences in MHC IIB alleles and host thermoregulatory behavior.6,8
The methods used to diagnose and control Bd infections will vary depending on factors such as the species affected, the number of animals in the group, and resource availability. Rapid diagnosis of clinically significant infections can be accomplished in minutes by wet mount or cytologic examination, whereas subclinical infections will only be reliably diagnosed by molecular methods such as PCR.5 Although PCR is very sensitive, low-level subclinical infections can sometimes require multiple tests before detection. Treatment methods for infected animals have received a lot of recent attention including some controlled experimental trials of safety and efficacy. Despite this treatment, outcomes are variable depending on the species, life stage (e.g., tadpole vs. metamorph vs. adult), and the laboratory or institution administering treatment. The most widely used treatment methods are itraconazole baths and elevation of environmental temperature. Recent reports suggest that lower concentrations of itraconazole (25 to 50 mg/L) than has been previously used (e.g., 100 mg/L) can be effective and in some situations minimize treatment-associated side effects.1,4 Heat treatment (30°C or more) is useful, but only if tolerated by the species.2 Use of chloramphenicol baths has been successful but requires more experience in a wider range of species.9 Regardless of the method chosen, successful treatment requires good husbandry and hygiene. Very sick animals can benefit from supplemental electrolytes as lethal infection is associated with hyponatremia and hypokalemia.
1. Brannelly, L.A., Richards-Zawacki, C.L., and Pessier, A.P. 2012. Clinical trials with itraconazole as a treatment for chytrid fungal infections in amphibians. Dis. Aquat. Org. 101:95–104.
2. Chatfield, M.W.H. and Richards-Zawacki, C.L. 2011. Elevated temperature as a treatment for Batrachochytrium dendrobatidis infection in captive frogs. Dis. Aquat. Org. 94:235–238.
3. Farrer, R.A., Weinert, L.A., Bielby, J., et al. 2011. Multiple emergences of genetically diverse amphibian-infecting chytrids include a globalized hypervirulent recombinant lineage. Proc. Natl. Acad. Sci. USA 108:18732–18736.
4. Jones, M.E.B., Paddock, D., Bender, L., Allen, J.L., Schrenzel, M.S. and Pessier, A.P. 2012.Treatment of chytridiomycosis with reduced-dose itraconazole. Dis. Aquat. Org 99:243–249.
5. Pessier, A.P. and Mendelson, J.R. (eds). 2010. A manual for control of infectious diseases in amphibian survival assurance colonies and reintroduction programs. Apple Valley, MN: IUCN/SSC Conservation Breeding Specialist Group http://www.cbsg.org/cbsg/workshopreports/26/amphibian_disease_manual.pdf. (VIN editor: Link could not be accessed as of 12-11-20.)
6. Savage, A.E. and Zamudio, K.R. 2011. MHC genotypes associate with resistance to a frog-killing fungus. Proc. Natl. Acad. Sci. USA. 108:16705–16710.
7. Schloegel, L.M., Daszak, P., Cunningham A.A. 2010. Two amphibian diseases, chytridiomycosis and ranaviral disease, are now globally notifiable to the World Organization for Animal Health (OIE): an assessment. Dis. Aquat. Org. 92:101–108.
8. Voyles, J., Rosenblum, E.B., and Berger, L. 2011. Interactions between Batrachochytrium dendrobatidis and its amphibian hosts: a review of pathogenesis and immunity. Microbes and Infection. 3:25–32.
9. Young, S., Speare, R., Berger, L. and Skerratt, L.F. 2012. Chloramphenicol with fluid and electrolyte therapy cures terminally ill green tree frogs (Litoria caerulea) with chytridiomycosis. J. Zoo Wildl. Med. 43: 330–337.