The Disease

That evening we had several discussions about the disease and the decline and loss of frog species. This was fascinating to hear because Karen is a world expert on it and Heidi and Egardo are local experts on methods of culturing and modes of disease. Both have dedicated their professional lives to different aspects of the disease.

Not much is known about how the disease propagates and why it kills some species and not others. The thought is that the fungus is centered on the streams and when adult frogs come in contact with the stream or infected animals they contract the disease. Some tadpoles can withstand the disease, but others loose their mouthparts and become unable to feed. The mode of disease transmission explains why there were still some tadpoles in Rio Maria. Adult frogs can live for many years and do not necessarily breed every year. If a frog stays out of the stream and has no contact with infected individuals, it may live through the first wave of disease. Eventually, however, the frog mates or contacts another frog with the disease and contracts the disease.

The frog-to-frog transmission also makes sense with respect to the rate of spread of the disease. The disease spreads through the lowland tropics without killing most species. Apparently at the higher temperatures, the fungus is less deadly to amphibians found Central America. Models of the rate of the disease spread based on animal-to-anima contact are consistent with the observed rates of spread. The wave-like movement of the disease front is also consistent with an amphibian contact route. If the disease was spread by wind or carried around by birds or flowing water, the rate of spread and geographic pattern of disease spread would be very different.

Karen also mentioned recent research indicating that the cause of the spread of the disease is not well known. Recent results suggest that the African Clawed Frogs are not the source of the disease because genetic analysis indicates the strain of fungus originated in New England.

However, the disease was reported in samples of Clawed Frogs taken in Africa as early as 1938, the approximate time when international trade of the species began. The presence of the disease was determined in museum specimens. Nobody knew about this disease in the 1930’s when the sample was collected. However, modern techniques can isolate DNA from preserved samples, amplify it with the polymerase chain reaction method, and analyze the sample for genes that are only found in the chytrid fungus that causes the disease. This is the same procedure used to exonerate criminals in rape or murder cases that are many years old; the biological samples from the crime are analyzed for the DNA left in them. Biologists trying to reconstruct genetics of the past now analyze tiny bits of samples preserved in museums.

The difficulty with pinning down an exact source of the frog disease is that many species of frogs are moved around the world all the time, and other species can carry the disease as well. Indiscriminate movement of species can have very negative unforeseen consequences. The end result is the same regardless of exactly where the disease came from; the disease is in Central America and causing numerous extinctions. The moral for the future is that we should be very careful about moving biological materials around the globe.

Regardless of the exact original cause, the disease has spread around the world and is endangering species in many places. The disease was first recognized in Australia, where it has caused many frog deaths and spread to New Zealand and Tasmania. Only a few realized the problems the disease caused in the US because it spread in the mid 1900’s when people were unaware of the issues or and frog populations were not carefully documented. Now people are becoming aware of the disease in the US. For example, it is harming salamanders in the Southeastern part of the country.

With so many people and animals moving around, the spread of the disease is inexorable. In South America, separate infections were initiated in several distinct areas and now it has spread through much of the northern part of the continent.