Crawfish frogs (Litheobates areolatus) are rare North American frogs that are endangered in the state of Indiana. They live inside crayfish burrows during the summer and winter months, leaving their burrows to breed in the spring. Some researchers from Indiana State University recently revealed that these endangered frogs are in danger of chytridiomycosis as well as habitat loss. They found that crawfish frogs exhibited a seasonal pattern of chytrid infection, with most infections occurring after breeding in the spring. Also, while the frogs were in crayfish burrows during the summer, all individuals sampled were negative for Bd, but after overwintering, some frogs had chytrid. There were a number of chytrid-related crawfish frog deaths following breeding season, suggesting that when crawfish frogs congregate for breeding, Bd is more easily transmitted from frog to frog. Stress hormones during the breeding season may also be higher, resulting in a reduced immune response to the chytrid infection. This study also had evidence corroborating Vredenberg’s 10,000 zoospore rule – that infection loads on frogs of more than 10,000 zoospores will result in mortalities.
Despite increased numbers of zoospores on the adults during the breeding season, most crawfish frog tadpoles and metamorphs were not infected with Bd. The role of environmental transmission on the spread of Bd is still not fully understood, and how Bd infects the amphibian life cycle is likewise still under investigation. Chytrid is not randomly distributed across the environment, however, so it is almost certain that the environment does contribute to the spread of chytridiomycosis. One study found that frogs breeding in permanently flowing streams in Australia were far more likely to become infected by chytrid than those breeding in ephemeral, or temporary, wetland sites. Given Bd‘s preference for cold and moist situations, this makes sense since permanently flowing streams provide permanent sites for the fungus to thrive. Ephemeral ponds or wetlands, on the other hand, are likely to reach higher temperatures or dry out during the dry season so that chytrid would most likely be eliminated for at least part of the year. Cycles like this may be one of the factors that contributes to the ability of some species of frogs to escape the effects of the chytrid fungus.