Batrachochytrium dendrobatidis

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Batrachochytrium dendrobatidis
Zoosporangia of B. dendrobatidis growing on a freshwater arthropod (a) and algae (b); scale bars = 30 μm
Scientific classification Edit this classification
Domain: Eukaryota
Kingdom: Fungi
Division: Chytridiomycota
Class: Chytridiomycetes
Order: Rhizophydiales
Family: Batrachochytriaceae
Genus: Batrachochytrium
Species:
B. dendrobatidis
Binomial name
Batrachochytrium dendrobatidis
Longcore, Pessier & D.K. Nichols (1999)

Batrachochytrium dendrobatidis (

amphibians
.

Since its discovery in 1998 by

salamanders
.

The fungal pathogens that cause the disease chytridiomycosis ravage the skin of frogs, toads, and other amphibians, throwing off their balance of water and salt and eventually causing heart failure,

selection
.

Etymology

The generic name is derived from the Greek words batrachos (frog) and chytra (earthen pot), while the

specific epithet is derived from the genus of frogs from which the original confirmation of pathogenicity was made (Dendrobates),[2] dendrobatidis is from the Greek dendron, "tree" and bates, "one who climbs", referring to a genus of poison dart frogs.[3]

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.

salamanders and also causes chytridiomycosis.[4] 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.[4]

Morphology

Scanning electron micrograph of a frozen intact zoospore and sporangia of the chytrid fungus (Batrachochytrium dendrobatidis), CSIRO

B. dendrobatidis infects the

sporangia
. Each sporangium produces a single tube to discharge spores.

Zoospore structure

rumposome has not been observed.[2]

Flagellum structure

A nonfunctioning

cristae.[2]

Life cycle

B. dendrobatidis sporangia in the skin of an Atelopus varius. The arrows indicate discharge tubes through which zoospores exit the host cell. Scale bar = 35 μm.

B. dendrobatidis has two primary life stages: a sessile, reproductive

proteolytic enzymes and esterases that help it digest amphibian cells and use amphibian skin as a nutrient source.[7] 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.[8] The amphibians infected with these zoospores are shown to die from cardiac arrest.[9]

Besides amphibians B. dendrobatidis also infects crayfish (

Gambusia holbrooki).[10]

Physiology

B. dendrobatidis can grow within a wide temperature range (4-25 °C), with optimal temperatures being between 17 and 25 °C.

Litoria chloris) recovered from their infections when incubated at a temperature of 37 °C.[12]

Varying forms

B. dendrobatidis has occasionally been found in forms distinct from its traditional zoospore and sporangia stages. For example, before the

saprobe, or a parasitic form of the fungus that is non-pathogenic.[13]

Habitat and relationship to amphibians

The fungus grows on amphibian skin and produces aquatic zoospores.

saprophyte. The fungus is associated with host mortality in highlands or during winter, and becomes more pathogenic at lower temperatures.[15]

Geographic distribution

Main article: Chytridiomycosis

It has been suggested that B. dendrobatidis originated in Africa or Asia and subsequently spread to other parts of the world by trade in African clawed frogs (Xenopus laevis).[16] 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.[16] There is more recent information that the species originated on the Korean peninsula and was spread by the trade in frogs.[17]

American bullfrogs (

Lithobates catesbeianus), also widely distributed, are also thought to be carriers of the disease due to their inherent low susceptibility to B. dendrobatidis infection.[18][19] The bullfrog often escapes captivity and can establish feral populations where it may introduce the disease to new areas.[5] 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.[20]
Infections have been linked to mass mortalities of amphibians in
Taudactylus acutirostris) in Australia.[24]

A wide variety of amphibian hosts have been identified as being susceptible to infection by B. dendrobatidis, including wood frogs (

Euproctus platycephalus).[32] and endemic frog species, the Beysehir frog in Turkey (Pelophylax caralitanus).[33]

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

Lithobates catesbeianus, the American Bullfrog) may be a key component in the dissemination of the fungus, at least in China.[34] Initial studies demonstrated the presence of the fungus on island states/countries such as Hong Kong,[35] Indonesia,[36] Taiwan,[30] and Japan.[37] Soon thereafter, mainland Asian countries such as Thailand,[38] South Korea,[39] and China[40] reported incidences[spelling?] 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

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 (medium impact; 50%), followed by the mountain forest (low impact; 44%) and village (high impact; 36%).[42] 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.[43]
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.[44] The death toll on amphibians is dependent on a variety of factors but most crucially on the intensity of infection. Certain frogs adopt skin sloughing as a defense mechanism for B. dendrobatidis; however, this is not always effective, as mortality fluctuates between species. For example, the Fletcher frog, despite practising skin sloughing, suffers from a particularly high mortality rate when infected with the disease compared to similar species like Lim. peronii and Lim. tasmaniensis. 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.[45]

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.[46] 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.[47]

One amphibian in particular that Batrachochytrium dendrobatidis (Bd) has affected greatly was the Lithobates clamitans. Bd kills this frog by interfering with external water exchange thereby causing an imbalance with ion exchange which leads to heart failure.

Immunity

Some amphibian species are actually immune to Bd, or have biological protections against the fungus.[48] One such salamander is the alpine salamander, or S. atra. These salamanders have several subspecies, but they share a common trait: toxicity. A 2012 study demonstrated that no alpine salamanders in the area had the disease, despite its prevalence in the area.[49] Alpine salamanders can produce alkaloid products[50][49] or other toxic peptides[50] that may be protective against microbes.[51]

See also

References

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Further reading

External links

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