Frogs: Dropping Like Flies

BY ANNELIESE MOLL
For the UAS Whalesong
Frogs are a surprisingly important part of many ecosystems. Their impact begins in their tadpole stage because of their ability to keep algae growth in check and by being a food source for other organisms. They are also beneficial for humans in a variety of ways. Many species of frogs eat insects such as mosquitoes, ticks, and flies, which can carry diseases such as malaria, West Nile virus, and Lyme disease. Frogs have also played an important part in research of human medicine. Since their skin is so easily affected by chemicals they are amazing bioindicators.
Today the existence of frogs around the world is being threatened. There are 6,565 recorded species of frogs and toads, but that number is steadily falling. There are several major factors that play a part in their decline: climate change/loss of habitat, pollution, and especially Chytridiomycosis. One of the reasons that frogs are being hit particularly hard by these factors comes down to their physiology.
In amphibians, skin is highly important because of its involvement in respiration, hydration, osmoregulation, and thermoregulation (Duellman and Trueb 1986). This is important to understand because chemicals within their water supply are readily absorbed through their skin and if their skin becomes compromised it can, and more often than not does, have deadly consequences.
Unfortunately, the decline of frogs is not necessarily new information. Since the 1980’s, up to 133 species of frogs have become extinct, and 435 species have shown a rapid decline. Pollutants from agricultural runoff, livestock, and industrial and human wastes have been shown to cause developmental mutations and have lethal affects (Rouse et al 1999). However, when it comes to Chytridiomycosis, it seems that hundreds of species have met their match. Chytridiomycosis is a disease caused by the fungus Batrachochytrium dendrobatidi, a highly transmissible and lethal fungus that has been one of the leading causes for either the decline or extinction of at least 200 species of frogs around the globe (Skerratt et al 2007).
The first case of Chytridiomycosis recorded was in 1938 in Xenopus laevis, more commonly known as the African clawed frog. The origin of this lethal disease remains unknown, however Africa has been proposed as the origin site. This disease remained prevalent within only Africa for about 23 years before any cases were reported elsewhere (Weldon et al 2004).
The way this disease works is 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 the issues there is that they become carriers for it.
This 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, but 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 a immune stimulant that causes the production of more white blood cells would not work and that an immune suppressant may be a better idea.
As of right now the future is looking dark for many species of frogs since there is nothing that we can do for frogs. It’s also very likely that even if there were a “cure” it would be impossible or too impractical to be able to administer on a large scale.

Further Reading:

AmphibiaWeb: Information on amphibian biology and conservation. 2015. Berkeley, California.
Duellman, W. E., & Trueb, L. (1986). Biology of amphibians.
Rouse, J. D., Bishop, C. A., & Struger, J. (1999). Nitrogen pollution: an assessment of its threat to amphibian survival. Environmental health perspectives.
Skerratt, L. F., Berger, L., Speare, R., Cashins, S., McDonald, K. R., Phillott, A. D., … & Kenyon, N. (2007). Spread of chytridiomycosis has caused the rapid global decline and extinction of frogs. EcoHealth.
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.
Weldon, C., du Preez, L. H., Hyatt, A. D., Muller, R., & Speare, R. (2004). Origin of the Amphibian Chytrid Fungus. Emerging Infectious Diseases.

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