LAUSR

What is the Batrachochytrium dendrobatidis (Bd) fungus? Batrachochytrium dendrobatidis (Bd) is a water-borne chytrid fungus that infects keratinized tissues of its amphibian hosts [1]. Specifically, Bd infects the skin of subadult and adult amphibians and the mouthparts of larval amphibians, and often causes host mortality via its effects on osmotic balance [2]. Bd has a two-part lifecycle. First, the aquatic, flagellated zoospore is the infective stage, which encounters the amphibian’s keratinized skin and encysts within epithelial cells. Once embedded, the zoospore develops into the reproductive stage, known as the zoosporangium, which produces more zoospores and releases them onto the skin of the amphibian and into the environment within days to weeks [3–5]. Bd transmission occurs through direct contact with infected individuals [6] and indirect contact with infected materials [7]. To date, there are no confirmed environmental reservoirs of Bd with documented signs of saprophytic growth outside the amphibian host; however, Bd DNA has been detected on a number of non-amphibian species and materials (e.g., [8]). An interesting aspect of Bd biology is that it contains characteristics of both a microparasite and macroparasite [9]. Although Bd is small-like microparasites, host infection intensity (i.e., number of zoospores) is predictive of disease outcome and infectivity, which is common of macroparasites. However, unlike many microparasites, Bd lacks an efficient way to transmit between cells within an infected host (although transmission between cells has been documented [10,11]). Increasing infection intensity generally depends on external reinfection, which is a general characteristic of macroparasites. In the case of Bd, zoospores are released from within the skin and onto the skin surface to reinfect a host.