BY ANNELIESE MOLL
For the UAS Whalesong
Eariler this semester I wrote an article about frogs. More specifically about some of the challenges frogs are currently facing. One of those challenges had to do with a disease called chytridiomycosis, a disease largely caused by the aquatic chytrid fungus Batrachochytrium dendrobatidis.
The first case of chytridomycosis recorded was in 1938 in the African clawed frog (Xenopus laevis) and for two decades it remained in Africa with no cases reported elsewhere. However, it did not remain this way, and has gone on to heavily impact many species of frogs around the world. Currently there are 700 amphibian species who have been infected.
In order to understand a disease, it is important to know how it works and have a time frame of survival. Chytrid begins by infecting the skin of its frog host. This heavily impairs or completely stops the transport of electrolytes and can result in cardiac arrest. Depending on the levels of infection a frog’s skin can be thickened, if the infection is heavy enough the skin will fall off. In an effort to understand this disease further researchers supplied infected frogs with electrolytes. They found the disease to still be fatal, but that frogs could were able to survive around 20 hours longer than those who did not receive electrolytes (Voyles et al. 2009). It has been noted that there are some species of frogs who are resistant to this disease, but unfortunately for other frogs, the resistant ones turn into carriers.
Chytrid has become such a major killer of frogs that it was suggested in 2014 that a solution may be to “vaccinate” frogs against this disease, however it may not be that simple. Evolutionary geneticist, Anna Savage, from the University of Central Florida, Orlando states that this approach could easily fail. She and a team of researches currently working with a species of leopard frogs (Lithobates yavapaiensis) have found that frogs who are exposed to the fungus who had an immune response fared the worst because the fungus killed off their white blood cells. She commented that an immune stimulant that causes the production of more white blood cells would not work and that an immune suppressant may be a better idea.
Other limiting factors that must be taken into account when attempting to handle wide spread diseases such as this are cost, efficiency, and environmental impacts. This is especially difficult because of how remote the habitats of infected frogs can be. Environmental impacts are also an important piece to keep in mind because in order to stop the infections we may be introducing chemicals into the environment that may be effective for the frogs, but could have negative impacts on other organisms inhabiting those areas.
While this disease is a global problem there has been some progress. On November 18, 2015 the first-ever successful elimination of chytrid in a wild frog was published. The study, over the span of five years, looked at the application of an antifungal treatment with an environmental disinfection on the Mallorcan midwife toad (Alytes muletensis) tadpoles.
Their study consisted of biannual surveys of five permanent ponds located in Spain. The researchers gathered tadpoles from various locations around the study sites to bring back to the laboratory for treatment. Swab samples were taken from around the mouth of the tadpoles and were then processed and PCR methods were used to duplicate and un against positive and negative controls for the zoospore genomic equivalents. The tadpoles were then treated with an antifungal, itraconazole, and washed daily for a week with tap water combined with 1.0 mg l−1 itraconazole (Bosch,et al 2015). The water in their aquariums was replaced daily during their time in the laboratory. Once the treatment was completed the collected tadpoles were returned to their collection sites.
As for the environmental disinfection portion of their study, the researchers used Virkon S, a multipurpose disinfectant. At the sites Virkon S was applied to all rocks, gravel, crevice, and vegetated areas that surrounded the ponds.
The researchers then waited and observed. Samples taken two years after environmental treatment combined with treated tadpoles showed no evidence of infection. However, tadpoles who had just been treated and did not have their ponds treated with environmental disinfectant still had evidence of infection. The reasons for this are currently unknown, however, researchers believe that it is linked with when the tadpole metamorphosis. Tadpoles have also been known to consume the bodies of other tadpoles that have died. If the dead tadpole was infected the fungus can be transmitted.
While this may have proven successful this study still leaves us with many questions and concerns. Virkon S is an inexpensive and is readily available, unfortunately is also a controversial chemical and to have to spread it in and around ponds could hold heavy repercussions in the long run. However, this is a major step in the direction of being able to save frogs from chytridiomycosis.
References:
Bosch, J., Sanchez-Tomé, E., Fernández-Loras, A., Oliver, J. A., Fisher, M. C., & Garner, T. W. (2015). Successful elimination of a lethal wildlife infectious disease in nature. Biology Letters, 11(11), 20150874.
Voyles, J., Young, S., Berger, L., Campbell, C., Voyles, W. F., Dinudom, A., … & Speare, R. (2009). Pathogenesis of chytridiomycosis, a cause of catastrophic amphibian declines. Science, 326(5952), 582-585.
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