Batrachochytrium dendrobatidis : Frog Killer

Zoosporangia of Batrachochytrium dendrobatidis...

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Batrachochytrium dendrobatidis (Bd) is a fungus, also known as the chytrid fungus. The scientific classification is as follows:

It causes a condition known as chytridiomycosis which is a fungal skin infection. This infection attacks the top layers of skin and causes thickening. B. dendrobatidis was discovered in 1998 in Central America and Australia in connection with their amphibian deaths. It has recently spread to Africa, Europe, North America and South America and has had a hand in amphibian decline. (B. dendrobatidis Database). See Holocene extinction. If you read the Scientific American article about frog leg consumption and extinction, you’ll know that some frogs, such as American bullfrogs and the African clawed frog, can withstand higher infection rates allowing them to be effective vectors of the disease. The fungus has a wide temperature range, 4-25˚C, allowing it to overwinter in hosts but shows halted growth at temperatures above 28˚C. At lower temperatures, the fungus becomes more pathogenic. In terms of recovery based on temperature, “Infected red-eye tree frogs (Litoria chloris) recovered from their infections when incubated at a temperature of 37˚C.” [^ Woodhams DC, Alford RA, Marantelli G (June 2003). “Emerging disease of amphibians cured by elevated body temperature”. Dis. Aquat. Org. 55 (1): 65–7.doi:10.3354/dao055065PMID 12887256.]

Via The World of Frogs

There are two primary life cycle stages of Bd, one sessile and one motile. The stationary form is basically just a sexual structure, the zoosporangium, from which the motile form emerges. The motile form, the zoospore, has a single flagellum that allows it to move short distances (1-2 cm) with the help of chemotaxis. In order to digest amphibian cells and use the skin as a nutrient source, Bd uses esterases and proteolytic enzymes. Upon reaching the host, the zoospore forms a cyst beneath the skin before beginning its reproductive cycle. Reinfection can occur within the host if the encysted zoospores develop into zoosporangia and release more zoospores. Cause of death after infection is cardiac arrest.

The issue with this fungus seems to be one of many cases where human intervention has increased the spread. There are anti-fungals that are successful in captive populations but no good method for wild amphibian treatment. (Amphibian Ark). There has been some resistance within previously susceptible species, but will any evolution or mutations be rapid enough to prevent extinction of many amphibians?

For information on yet another disease affecting amphibian populations, check out “From chytrid to ranavirus: Another disease is devastating frog populations” by Scientific American.