Chytridiomycosis

Chytridiomycosis (/kaɪˌtrɪdiəmaɪˈkoʊsɪs/ ky-TRID-ee-ə-my-KOH-sis) is an infectious disease in amphibians, caused by the chytrid fungi Batrachochytrium dendrobatidis and Batrachochytrium salamandrivorans. Chytridiomycosis has been linked to dramatic population declines or extinctions of amphibian species in western North America, Central America, South America, eastern Australia, east Africa (Tanzania), and Dominica and Montserrat in the Caribbean. Much of the New World is also at risk of the disease arriving within the coming years. The fungus is capable of causing sporadic deaths in some amphibian populations and 100% mortality in others. No effective measure is known for control of the disease in wild populations. Various clinical signs are seen by individuals affected by the disease. A number of options are possible for controlling this disease-causing fungus, though none has proved to be feasible on a large scale. The disease has been proposed as a contributing factor to a global decline in amphibian populations that apparently has affected about 30% of the amphibian species of the world. Some research found evidence insufficient for linking chytrid fungi and chytridiomycosis to global amphibian declines, but more recent research establishes a connection and attributes the spread of the disease to its transmission through international trade routes into native ecosystems.

Whether chytridiomycosis is a new, emergent pathogen or an extant pathogen with recently increased virulence is unclear.

The disease in its epizootic form was first discovered in 1993 in dead and dying frogs in Queensland, Australia. It had been present in the country since at least 1978 and is widespread across Australia. It is also found in Africa, the Americas, Europe, New Zealand, and Oceania. In Australia, Panama, and New Zealand, the fungus seemed to have suddenly 'appeared' and expanded its range at the same time frog numbers declined. In the Americas, it originated in Venezuela in 1987, where it swept up the continent into Central America. It was also found in the lower part of Central America in 1987, where it spread down to meet the upward sweep from South America. However, it may simply be that the fungus occurs naturally and was only identified recently because it has become more virulent or more prevalent in the environment, or because host populations have become less resistant to the disease. The fungus has been detected in four areas of Australia—the east coast, Adelaide, south-west Western Australia and the Kimberley—and is probably present elsewhere. Lately, the genomes of 234 Batrachochytrium dendrobatidis isolates were phylogenetically compared and the results strongly suggest that a lineage found in the Korean peninsula likely seeded the panzootic.

Among frogs, the oldest documented occurrence of Batrachochytrium is from a specimen of a Titicaca water frog collected in 1863, and among salamanders the oldest was a Japanese giant salamander collected in 1902. However, both these involved strains of the fungus that have not been implicated in mass-mortality events. A later instance of a Bd-infected amphibian was a specimen of an African clawed frog (Xenopus laevis) collected in 1938, and this species also appears to be essentially unaffected by the disease, making it a suitable vector. The first well-documented method of human pregnancy testing, known as the frog test, involved this species, and as a result, large-scale international trade in living African clawed frogs began more than 60 years ago. If Batrachochytrium originated in Africa, the African clawed frog is thought to have been the vector of the initial spread out of the continent. The earliest documented case of the disease chytridiomycosis was an American bullfrog (Rana catesbeiana) collected in 1978.

The geographic range of chytridiomycosis is difficult to ascertain. If it occurs, the disease is only present where the fungus B. dendrobatidis is present. However, the disease is not always present where the fungus is. Reasons for amphibian declines are often termed 'enigmatic' because the cause is unknown. Why some areas are affected by the fungus while others are not is not fully understood. Oscillating factors such as climate, habitat suitability, and population density may be factors which cause the fungus to infect amphibians of a given area. Therefore, when considering the geographic range of chytridiomycosis, the range of B. dendrobatidis occurrence must be considered. The geographic range of B. dendrobatidis has recently been mapped, and spans much of the world. B. dendrobatidis has been detected in 56 of 82 countries, and in 516 of 1240 (42%) species using a data set of more than 36,000 individuals. It is widely distributed in the Americas, and detected sporadically in Africa, Asia, and Europe. Asia, for example, has only 2.35% prevalence.

The range suitable for B. dendrobatidis in the New World is vast. Regions with its highest suitability include habitats that contain the world's most diverse amphibian fauna. Areas at risk are the Sierra Madre Pine Oak Occidental Forest, the Sonoran and Sinaloan dry forest, the Veracruz moist forest, Central America east from the Isthmus of Tehuantepec, the Caribbean Islands, the temperate forest in Chile and western Argentina south of 30°S, the Andes above 1000 m above sea level in Venezuela, Colombia, and Ecuador, eastern slopes of the Andes in Peru and Bolivia, the Brazilian Atlantic forest, Uruguay, Paraguay, and northeastern Argentina, as well as the southwestern and Madeira–Tapajós Amazonian rainforests.

Currently, the effects of chytridiomycosis are seen most readily in Central America, eastern Australia, South America, and western North America.

A study suggests that changing global temperatures may be responsible for increased proliferation of chytridiomycosis. The rise in temperature has increased evaporation in certain forest environments that as a result has promoted cloud formation. Experts propose that increased cloud cover might actually be decreasing the daytime temperature by blocking the sun, while at night the cloud cover serves as insulation to raise the nighttime temperature from its normal range. The combination of decreased daytime temperature and increased nighttime temperatures may be providing optimal growth and reproduction for Chytrid fungus which has preferred temperature range between 17° and 25 °C (63° and 77 °F). The fungus dies at temperatures at and above 30 °C, which without the cloud cover from increased evaporation is more easily reached by the environment and can, therefore, more easily keep the fungus population in check.