Chytridiomycosis

Cause

Chytridiomycosis is an infectious disease of amphibians caused by the fungus Batrachochytrium dendrobatidis (Bd). It was rst discovered in 1993 in Australia after a massive mortality event involving several species of frog. Further studies conducted on preserved amphibian specimens have shown that Bd has been present in Australia since 1978 and that the disease may have originated in Africa in as early as 1938.

Significance

Chytridiomycosis is an emerging disease that is signicantly impacting amphibian populations across the globe. In the past few decades, the disease has caused the decline or complete extinction of over 200 species of frogs and other amphibians. Chytrid disease is believed to be responsible for one of the most signicant disease-caused losses of biodiversity in recorded history.

Species Affected

Currently, chytrid disease is known to aect over 350 species of amphibians, though it appears to be aecting frog species most severely. However, the disease does not aect all frog species. The American bullfrog and the African clawed frog appear to be resistant to the disease, but may still act as carriers. The disease is not known to aect humans.

Distribution

Chytridiomycosis is present on every continent except for Antarctica, though the disease is having the biggest impact in South and Central America, Australia, and North America. In the United States, chytrid disease has been conrmed in 46 out of the 50 states. The highest incidence of disease is occurring in the Western part of the United States.

Transmission

Bd is a waterborne fungus that disperses zoospores (a agellum used by the fungus for movement) into the environment in order to search for a new host. The fungus travels through water sources until it nds a new host, which it then enters cutaneously (through the skin). Once the host is infected with Bd, chytridiomycosis may or may not develop. The disease is also believed to be transmitted through direct contact with diseased amphibians, though this has not yet been conrmed. Research has shown that Bd grows best in water that is between 17-25°C (62-77°F) and that in the wild, most disease outbreaks occur at higher elevations during cooler

Clinical Signs

Clinical signs of chytridiomycosis vary by species. The earliest signs of chytrid disease tend to be anorexia and lethargy. Most frogs experience excessive shedding of skin, which appears opaque and gray-white or tan in color. Many frogs also experience a thickening of the skin, which may prevent breathing, thermoregulation, nutrient intake, hydration, and/or the release of harmful toxins. Other common signs include red skin, convulsions, lack of the righting reex (a reex that corrects the orientation of the body after it has been taken out of its normal upright position), abnormal feeding behavior, and discoloration near the mouth.

Diagnosis

Laboratory tests are used to isolate the Bd from the skin (either direct skin samples or a skin swab) of infected animals.

Treatment

Captive animals may be treated for chytridiomycosis with antifungal medications and heat therapy. However, it is very dicult to treat amphibians in the wild due to the inability to regulate the temperature of natural bodies of water and the diculty of dispersing antifungal treatments into the environment. There is no vaccine.

Management

Chytridiomycosis is easily spread during anthropogenic activities. Boots, clothes, and all eld equipment should be cleaned with a 10% bleach-water mixture before moving between sites. Wild amphibians should not be moved between habitats, and captive amphibians should not be released into the environment or used as shing bait. All newly acquired captive amphibians should be initially quarantined from other amphibians until it has been conrmed that they are disease free. Chytridiomycosis is a reportable disease and any detection of the disease should be reported to the appropriate wildlife authorities.

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