The big picture: Some other emerging animal pathogens

While the amphibian chytrid fungus often gets the most press as a pathogen responsible for the decline of amphibians, it is not the only  new disease on the block. Some eastern massasauga rattlesnakes (Sistrurus catenatus catenatus) in Illinois were recently found to  be infected with a  fungus of the genus Chrysosporium. This fungus, similarly to Bd, also keratinizes the skin, resulting in swollen abscesses and wounds on the jaws of the rattlesnakes. Similar fungi have been recorded in the captive reptile pet trade, and some Chrysosporium species can even infect people with immune system deficiencies. Further research is needed to determine if this infection of rattlesnakes will result in further population declines of these already endangered snakes, and if this Chrysosporium strain poses any risk to human health.

Another fungal pathogen that has gotten a lot of recent attention is Geomyces destructans, the fungus responsible for White-nose Syndrome in bats. Cave-dwelling bats of the eastern United States have been experiencing rapid population declines due to this pathogen (it is estimated that over a million bats have already succumbed to it since 2008). Emaciation and the presence of a white fungal growth around the nose are both signs of infection in hibernating bats. Typically G. destructans tends to infect bats during hibernation, causing them to use up all their reserves of fat to battle the infection (thus the emaciation). White-nose syndrome is currently believed to be spreading westward across the United States, with far-reaching ecological effects predicted should population declines continue. Small bats are an essential link in the ecosystem through their consumption of large numbers of flying insects.

Finally, getting back to amphibians, Ranaviruses are believed to be a second pathogen besides Bd that is responsible for amphibian declines. However, unlike the amphibian chytrid fungus, ranaviruses (family Iridoviridae) can infect a variety of ectothermic, or cold-blooded, vertebrate species. Ranavirus strains have been reported from reptiles, fish and amphibians, where it typically results in tissue death and lesions. In Montgomery County, Maryland, researchers recently found a number of eastern box turtles (Terrapene carolina carolina) that were dead, presumably due to a ranavirus. It is believed that almost all salamander larvae and tadpoles in the area were also killed by a ranavirus. Adult eastern box turtles are already at risk of being hit by cars on roads in Maryland, so with the addition of a pathogen such as the ranavirus, their adult populations could plunge to levels that can not be recovered.

With such a plethora of emerging infectious diseases, the future of all wildlife seems uncertain. One thing that should be remembered is that sometimes there is an element of sensationalism in the reporting of a new pathogen. This is not to say that we should not be concerned – when a new pathogen is found in an already endangered species, such as in the eastern massasauga rattlesnake (Sistrurus catenatus catenatus), it is a major source of concern because it is easier for a rare species to be completely eradicated by a new disease than a common species that might have the potential to bounce back in population. It is important to understand that the decline of wildlife populations typically is not explained by just one disease or pollutant. While the amphibian chytrid fungus is certainly one of the most well known agents of decline for frogs around the world, conservationists should not forget about the ranavirus and other factors responsible for the loss of global biodiversity.

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