Zoonotic Chytrid Fungus in Amphibians

The infectious disease chytridiomycosis is caused by the fungal pathogen Batrachochytrium dendrobatidis. These motile zoospores form colonies in amphibians’ epidermal layer. The epidemiology of the disease is different for pre- and postmetamorphic amphibians; zoospores invade and colonize areas around larval mouthparts, reducing their ability to intake food. In adults they grow in the dermis, resulting in hyperkeratinization of the skin. In this case, mortality is caused by disruption of osmoregulation across the skin. B. dendrobatidis does not cause direct mortality in tadpoles, but as metamorphosis sets in, the frog will display the same conditions as infected adult frogs.

The chytrid fungus is the cause of worldwide amphibian decline and, in some cases, species extinction. It affects about 350 species of amphibian, and the extinction of 200 species is attributed to the disease. It originated in South Africa in the African clawed frog and for a while was localized to this region and species. In the 1960s, the disease was spread globally when this species began to be distributed for scientific research and for pets; the fungus is now found on every continent that amphibians inhabit. The African clawed frog, the Northern leopard frog, and other species serve as reservoirs for the disease.

The stream community is heavily affected once frog species have been extirpated. Algae growth increases, frog predators die off, and the entire stream foodweb is affected.

Chytridiomycosis spreads very rapidly, and it has the potential to crash an amphibian population in 4-6 months. The most highly susceptible species are those that live in higher elevations and breed and/or live in streams. The zoospores thrive at cooler temperatures, and their aquatic lifestyle allows them to be easily distributed through moving water. The most effective way for humans to prevent spreading the disease is to never put captive amphibians in the wild for release or to use as bait.

Chytridiomycosis is easily diagnosed in larval amphibians by the presence of deformed or missing mouthparts. In adults, diagnosis is done by histological examination of the digits and ventral side, the areas where the zoospores most commonly colonize. Sheddings of keratinized dermis can also be analyzed for the presence of zoospores or zoosporangia.

Unfortunately, there are no known treatments for chytridiomycosis in the wild. In the lab however, there are a few ways to treat the disease. Chloramphenicol can be applied topically, but this is absorbed into the skin so it cannot be used on animals meant for consumption. If the animal can tolerate temperatures higher than 30°C, exposure will kill the zoospores. Amphibians also have an inhibitory bacteria associated with their skin that can ward off B. dendrobatidis. This bacteria can be added to captive populations as a preventative measure against the fungus.

An interesting fact about the chytrid fungus is that it does not affect salamanders at nearly as high a rate as frogs. What I had not known was that a reason for this is may be that larval salamanders do not have the keratinized mouthparts that tadpoles do, so the fungus is not attracted to it. In the case of adult salamanders, many have terrestrial development rather than aquatic.