Chytridiomycosis ( ) 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 Old 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.
History
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.Weldon; du Preez; Hyatt; Muller; and Speare (2004). Origin of the Amphibian Chytrid Fungus. Emerging Infectious Diseases 10(12). 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.
Range 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.
Climate change 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.
Causative agents Chytridiomycosis caused by the fungus B. dendrobatidis predominantly affects the outermost layers of skin containing keratin. When most species reach a B. dendrobatidis threshold of 10,000 zoospores, they are not able to breathe, hydrate, osmoregulate, or thermoregulate correctly. This is proven by blood samples that show a lack of certain electrolytes, such as sodium, magnesium, and potassium. B. dendrobatidis is currently known to have two life stages. The first is the asexual zoosporangial stage. When a host first contracts the disease, spores penetrate the skin and attach themselves using microtubule roots. The second stage takes place when the initial asexual zoosporangia produce motile zoospores. To disperse and infect epidermal cells, a wet surface is needed. A second species of Batrachochytrium, B. salamandrivorans, was discovered in 2013 and is known to cause chytridiomycosis in salamanders.
Disease transmission and progression B. dendrobatidis, a waterborne pathogen, disperses zoospores into the environment. The zoospores use flagella for locomotion through water systems until they reach a new host and enter cutaneously. The B. dendrobatidis' lifecycle continues until new zoospores are produced from the zoosporangium and exit to the environment or reinfect the same host. Once the host is infected with B. dendrobatidis, it can potentially develop chytridiomycosis, but not all infected hosts develop it. Other forms of transmission are currently unknown; however, chytridiomycosis is postulated to be transmitted through direct contact of hosts or through an intermediate host.
Much of how B. dendrobatidis is successfully transmitted from one host to the next is largely unknown. Once released into the aquatic environment, zoospores travel less than within 24 hours before they encyst. The limited range of B. dendrobatidis zoospores suggest some unknown mechanism exists by which they transmit from one host to the next, which can involve the pet trade, and especially the American bullfrog. Abiotic factors such as temperature, pH level, and nutrient levels affect the success of B. dendrobatidis zoospores. The fungus zoospores can survive within a temperature range of and a pH range of 6–7.
Chytridiomycosis is believed to follow this course: zoospores first encounter amphibian skin and quickly give rise to sporangia, which produce new zoospores. The disease then progresses as these new zoospores reinfect the host. Morphological changes in amphibians infected with the fungus include a reddening of the ventral skin, convulsions with extension of hind limbs, accumulations of sloughed skin over the body, sloughing of the superficial epidermis of the feet and other areas, slight roughening of the surface with minute skin tags, and occasional small ulcers or hemorrhage. Behavioral changes can include lethargy, a failure to seek shelter, a failure to flee, a loss of righting reflex, and abnormal posture (e.g., sitting with the hind legs away from the body).
Besides amphibians Chytridiomycosis also infects crayfish (Procambarus alleni, P. clarkii, Orconectes virilis, and O. immunis) but not mosquitofish (Gambusia holbrooki).
Clinical signs Amphibians infected with B. dendrobatidis have been known to show many different clinical signs. Perhaps the earliest sign of infection is anorexia, occurring as quickly as eight days after being exposed. Individuals infected are also commonly found in a lethargic state, characterized by slow movements, and refuse to move when stimulated. Excessive shedding of skin is seen in most frog species affected by B. dendrobatidis. These pieces of shed skin are described as opaque, gray-white, and tan. Some of these patches of skin are also found adhered to the skin of the amphibians. These signs of infection are often seen 12–15 days following exposure. The most typical symptom of chytridiomycosis is thickening of skin, which promptly leads to the death of the infected individuals because those individuals cannot take in the proper nutrients, release toxins, or, in some cases, breathe. Other common signs are reddening of the skin, convulsions, and a loss of righting reflex. A meta-analysis showed skin disruption, hormonal changes, and osmoregulation can occur with light infection, while higher pathogen loads are required to influence reproduction. In tadpoles, B. dendrobatidis affects the mouthparts, where keratin is present, leading to abnormal feeding behaviors or discoloration of the mouth.
Research and impact