Amphibian disease- Can anti-fungal treatment in the field save frogs during a disease epidemic?
Significant research focus has been dedicated recently to amphibian diseases and yet, despite a much better understanding of the mechanisms of infection, the genetics of the pathogens and the impacts on the populations of various species, we are still some distance away from finding a solution that can be applied more widely to groups of species in order to rescue them in the wild and to be able to maintain their populations in the face of disease epidemics caused by the chytrid fungus Batrachochytrium dendrobatidis (Bd). An estimated 200 amphibian species are at risk of extinction due to this fungus worldwide making this an extremely urgent situation. The current proposed scenario of captive breeding and collections of some of the most threatened amphibian species remains only a crisis and short-term solution with so far little realistic prospect of releases back into the wild on a large scale. Even more, as revealed in a previous Croaking science article, zoos are currently failing to prioritise amphibian species that are likely to require captive breeding programmes to prevent their extinction and amphibians remain a comparatively low priority for most zoo conservation programmes.
A treatment in the wild remains elusive and so far the few success stories such as the eradication of chytrid infection from the island of Mallorca (Bosch et al. 2015) would be immensely difficult to apply more widely and certainly in the complex high-altitude tropical forest where many of the critically endangered amphibians live. Anti-fungal solutions, especially itraconazole, have proved effective at treatments for chytrid infections in captivity but remain difficult to use in the wild as they require repeated applications. In this context a new publication from the Institute of Zoology in London and Durrell and the University of Kent looking at in-situ treatments of the mountain chicken frog (Leptodactylus fallax) represents an important step forward. Populations of this charismatic frog species, native only to the islands of Montserrat and Dominica in the Caribbean, have been devastated by the chytrid and despite the establishment of several captive breeding collections the situation in the wild remains critical. The study describes an experiment where over the course of 24 weeks frogs were captured and marked using pit tags, swabbed for the presence chytrid on the skin and then some were treated in the field using itraconazole for 15 weeks. Of 228 frogs only 13 tested negative for chytrid infections and by the end of the study 50 frogs had been found dead. Disappointingly, treatment did not prevent frogs from dying from the infection but it significantly increased their chance to live longer, with models suggesting that treating the entire population would have extended their survival by 75 weeks. The antifungal treatment also reduced the infection rate during the treatment period, suggesting there might be a short-term prophylactic effect. While the treatment effects were short-lived and did not prevent the individuals affected from eventually dying from the infection, it is important to note that the treatment period was short in itself and impacts might have been considerably more positive if it would have been further extended. Overall though, while encouraging, this study shows that a field-based solution for mitigating the impacts of Bd for amphibian populations remains elusive for the moment and also indicates that such efforts in the wild would require highly intensive and expensive treatment solutions. As such, captive collections and captive breeding by zoos and other research facilities remain a critical safety net for a wide diversity of amphibian species in the face of the chytrid epidemic.
References
Bosch et al., 2015 Successful elimination of a lethal wildlife infectious disease in nature Biol. Lett. (2015) http://dx.doi.org/10.1098/rsbl.2015.0874
Hudson et al. (2016) In-situ itraconazole treatment improves survival rate during an amphibian chytridiomycosis epidemic Biological Conservation 195, pp 37-45 doi:10.1016/j.biocon.2015.12.041
Further information about the Mountain Chicken Frog
https://www.zsl.org/conservation/regions/americas/mountain-chicken-frog-conservation