Batrachochytrium dendrobatidis


Batrachochytrium dendrobatidis, also known as Bd or the amphibian chytrid fungus, is a fungus that causes the disease chytridiomycosis in amphibians.
In the decade after it was first discovered in 1998 by Lee Berger, the disease devastated amphibian populations around the world, in a global decline towards multiple extinctions, part of the Holocene extinction. A recently described second species, B. salamandrivorans, also cause chytridiomycosis and death in salamanders.
Some amphibian species appear to have an innate capacity to withstand chytridiomycosis infection due to symbiosis with Janthinobacterium lividum. Even within species that generally succumb, some populations survive, possibly demonstrating that these traits or alleles of species are being subjected to evolutionary selection.

Etymology

The generic name is derived from the Greek words batrachos and chytra, while the specific epithet is derived from the genus of frogs from which the original confirmation of pathogenicity was made, dendrobatidis is from the Greek dendron, "tree" and bates, "one who climbs", referring to a genus of poison dart frogs.

Systematics

Batrachochytrium dendrobatidis was until recently considered the single species of the genus Batrachochytrium. The initial classification of the pathogen as a chytrid was based on zoospore ultrastructure. DNA analysis of the SSU-rDNA has corroborated the view, with the closest match to Chytridium confervae. A second species of Batrachochytrium was discovered in 2013: B. salamandrivorans, which mainly affects salamanders and also causes chytridiomycosis. B. salamandrivorans differs from B. dendrobatidis primarily in the formation of germ tubes in vitro, the formation of colonial thalli with multiple sporangia in vivo, and a lower thermal preference.

Morphology

B. dendrobatidis infects the keratinized skin of amphibians. The fungus in the epidermis has a thallus bearing a network of rhizoids and smooth-walled, roughly spherical, inoperculate sporangia. Each sporangium produces a single tube to discharge spores.

Zoospore structure

of B. dendrobatidis, which are typically 3–5 µm in size, have an elongate–ovoidal body with a single, posterior flagellum, and possess a core area of ribosomes often with membrane-bound spheres of ribosomes within the main ribosomal mass. A small spur has been observed, located at the posterior of the cell body, adjacent to the flagellum, but this may be an artifact in the formalin-fixed specimens. The core area of ribosomes is surrounded by a single cisterna of endoplasmic reticulum, two to three mitochondria, and an extensive microbody–lipid globule complex. The microbodies closely appose and almost surround four to six lipid globules, some of which appear bound by a cisterna. Some zoospores appear to contain more lipid globules. A rumposome has not been observed.

Flagellum structure

A nonfunctioning centriole lies adjacent to the kinetosome. Nine interconnected props attach the kinetosome to the plasmalemma, and a terminal plate is present in the transitional zone. An inner ring-like structure attached to the tubules of the flagellar doublets within the transitional zone has been observed in transverse section. No roots associated with the kinetosome have been observed. In many zoospores, the nucleus lies partially within the aggregation of ribosomes and was invariably situated laterally. Small vacuoles and a Golgi body with stacked cisternae occurred within the cytoplasm outside the ribosomal area. Mitochondria, which often contain a small number of ribosomes, are densely staining with discoidal cristae.

Life cycle

B. dendrobatidis has two primary life stages: a sessile, reproductive zoosporangium and a motile, uniflagellated zoospore released from the zoosporangium. The zoospores are known to be active only for a short period of time, and can travel short distances of one to two centimeters. However, the zoospores are capable of chemotaxis, and can move towards a variety of molecules that are present on the amphibian surface, such as sugars, proteins and amino acids. B. dendrobatidis also contains a variety of proteolytic enzymes and esterases that help it digest amphibian cells and use amphibian skin as a nutrient source. Once the zoospore reaches its host, it forms a cyst underneath the surface of the skin, and initiates the reproductive portion of its life cycle. The encysted zoospores develop into zoosporangia, which may produce more zoospores that can reinfect the host, or be released into the surrounding aquatic environment. The amphibians infected with these zoospores are shown to die from cardiac arrest.

Physiology

B. dendrobatidis can grow within a wide temperature range, with optimal temperatures being between 17-25 °C. The wide temperature range for growth, including the ability to survive at 4 °C gives the fungus the ability to overwinter in its hosts, even where temperatures in the aquatic environments are low. The species does not grow well above temperatures of 25 °C, and growth is halted above 28 °C. Infected red-eyed treefrogs recovered from their infections when incubated at a temperature of 37 °C.

Varying forms

B. dendrobatidis has occasionally been found in forms distinct from its traditional zoospore and sporangia stages. For example, before the 2003 European heat wave that decimated populations of the water frog Rana lessonae through chytridiomycosis, the fungus existed on the amphibians as spherical, unicellular organisms, confined to minute patches. These organisms, unknown at the time, were subsequently identified as B. dendrobatidis. Characteristics of the organisms were suggestive of encysted zoospores; they may have embodied a resting spore, a saprobe, or a parasitic form of the fungus that is non-pathogenic.

Habitat and relationship to amphibians

The fungus grows on amphibian skin and produces aquatic zoospores. It is widespread and ranges from lowland forests to cold mountain tops. It is sometimes a non-lethal parasite and possibly a saprophyte. The fungus is associated with host mortality in highlands or during winter, and becomes more pathogenic at lower temperatures.

Geographic distribution

It has been suggested that B. dendrobatidis originated in Africa and subsequently spread to other parts of the world by trade in African clawed frogs. In this study, 697 archived specimens of three species of Xenopus, previously collected from 1879 to 1999 in southern Africa, were examined. The earliest case of chytridiomycosis was found in a X. laevis specimen from 1938. The study also suggests that chytridiomycosis had been a stable infection in southern Africa from 23 years prior to finding any infected outside of Africa. There is more recent information that the species originated on the Korean peninsular and was spread by the trade in frogs.
American bullfrogs, also widely distributed, are also thought to be carriers of the disease due to their inherent low susceptibility to B. dendrobatidis infection. The bullfrog often escapes captivity and can establish feral populations where it may introduce the disease to new areas. It has also been shown that B. dendrobatidis can survive and grow in moist soil and on bird feathers, suggesting that B. dendrobatidis may also be spread in the environment by birds and transportation of soils.
Infections have been linked to mass mortalities of amphibians in North America, South America, Central America, Europe and Australia. B. dendrobatidis has been implicated in the extinction of the sharp-snouted day frog in Australia.
A wide variety of amphibian hosts have been identified as being susceptible to infection by B. dendrobatidis, including wood frogs, the mountain yellow-legged frog, the southern two-lined salamander, San Marcos Salamander, Texas Salamander, Blanco River Springs Salamander, Barton Springs Salamander, Jollyville Plateau Salamander, Ambystoma jeffersonianum, the western chorus frog, the southern cricket frog, the eastern spadefoot toad, the southern leopard frog, the Rio Grande Leopard frog, and the Sardinian newt.

Southeast Asia

While most studies concerning B. dendrobatidis have been performed in various locations across the world, the presence of the fungus in Southeast Asia remains a relatively recent development. The exact process through which the fungus was introduced to Asia is not known, however, as mentioned above, it has been suggested transportation of asymptomatic carrier species may be a key component in the dissemination of the fungus, at least in China. Initial studies demonstrated the presence of the fungus on island states/countries such as Hong Kong, Indonesia, Taiwan, and Japan. Soon thereafter, mainland Asian countries such as Thailand, South Korea, and China reported incidences of B. dendrobatidis among their amphibian populations. Much effort has been put into classifying herpetofauna in countries like Cambodia, Vietnam, and Laos where new species of frogs, toads, and other amphibians and reptiles are being discovered on a frequent basis. Scientists simultaneously are swabbing herpetofauna in order to determine if these newly discovered animals possess traces of the fungus.
In Cambodia, a study showed B. dendrobatidis to be prevalent throughout the country in areas near Phnom Penh, Sihanoukville, Kratie, and Siem Reap. Another study in Cambodia questioned the potential anthropological impact in the dissemination of B. dendrobatidis on local amphibian populations in 3 different areas in relation to human interaction: low, medium, and high. Using quantitative PCR, evidence of B. dendrobatidis was found in all 3 sites with the highest percentage of amphibians positive for the fungus from the forest road, followed by the mountain forest and village. Human influence most likely explains detection of the fungus in the medium and high areas, however it does not provide an adequate explanation why even isolated amphibians were positive for B. dendrobatidis. This may go unanswered until more research is performed on transmission of the fungus across landscapes. However, recent evidence suggests mosquitoes may be a possible vector which may help spread B. dendrobatidis. Another study in French Guiana reports widespread infection, with 8 of 11 sites sampled being positive for B. dendrobatidis infection for at least one species. This study suggests that Bd is more widespread than previously thought.

Effect on amphibians

Worldwide amphibian populations have been on a steady decline due to an increase in the disease Chytridiomycosis, caused by this Bd fungus. Bd can be introduced to an amphibian primarily through water exposure, colonizing the digits and ventral surfaces of the animal's body most heavily and spreading throughout the body as the animal matures. Potential effects of this pathogen are hyperkeratosis, epidermal hyperplasia, ulcers, and most prominently the change in osmotic regulation often leading to cardiac arrest. The death toll on amphibians is dependent on a variety of factors but most crucially on the intensity of infection. Some amphibian species have been found to adapt to infection after an initial die-off with survival rates of infected and non-infected individuals being equal.
According to a study by the Australian National University estimates that the Bd fungus has caused the decline of 501 amphibian species—about 6.5 percent of the world known total. Of these, 90 have been entirely wiped out and another 124 species have declined by more than 90 percent, and their odds of the effected species recovering to a healthy population are doubtful. However, these conclusions were criticized by later studies, which proposed that Bd was not as primary a driver of amphibian declines as found by the previous study.