Chytridiomycosis is a disease that afflicts amphibians, caused by the amphibian chytrid fungus (scientific name: Batrachochytrium dendrobatidis). Chytrid fungi have been on the Earth for millions of years, but most of them do not negatively affect living animals. In fact, B. dendrobatidis appears to be unique among other chytrids in its deadliness to amphibians. The amphibian chytrid fungus may have originated in Africa on the African Clawed Frog (Xenopus laevis) (for more details, go here), although there is a lot of debate about its origins. Xenopus laevis is commonly used in research and was transported throughout the globe for this purpose as well as for the pet trade. It is presumed that the spread of the chytrid fungus was directly related to the global transport of X. laevis, american bullfrogs (Lithobates catesbeianus) or other frog species. Batrachochytrium dendrobatidis is now thought to be the leading cause behind amphibian population declines across the world, especially in tropical regions.
The amphibian chytrid fungus lifecycle consists of two parts: the mature zoosporangia (singular: zoosporangium) and the mobile zoospores. Zoospores are the “offspring” of the zoosporangia, and are the part that infect new amphibian hosts. When viewed live under a light microscope, zoosporangia appear as large, transparent round blobs with filaments extending out from them, while zoospores resemble tiny moving dots with a single flagellum. Zoospores infect the skin of a frog and develop into zoosporangium. Chytridiomycosis results in the keratinization and hardening of the infected amphibian’s skin, making it impermeable to water, air and salts, all of which most healthy amphibians need to absorb through their skin. By this stage, mortality, or death, usually occurs in the infected individual. However, it is thought that some animals like the North American Bullfrog (Lithobates catesbeianus) may be carriers of Bd that are relatively unaffected by the fungus. If we can understand the reasons for the bullfrog and other species’ apparent immunity, we will be one step closer to finding a way to defeat this deadly pathogen.