Chytridiomycosis (Bd)

This fact sheet was updated on March 21st, 2020. 

Cause

Chytridiomycosis is an infectious disease of amphibians caused by the fungus Batrachochytrium dendrobatidis (Bd). It was first discovered in 1993 in Australia after a massive mortality event involving several species of frog. A retrospective study investigating archived specimen from Africa found the earliest case of chytridiomycosis in a Xenopus laevis frog from 1938. This study further found the prevalence of chytridiomycosis in 3 species of Xenopus to be 2.7%, with no difference in the species, regions, season, or time period. These data suggest Bd, and chytridiomycosis was endemic in Africa for 23 years before a positive case was found outside of Africa (Weldon et al., 2004). Another study points to East Asia (Korea) as the point source for Bd and dates the origin of expansion to the early 20th century, which also coincides with the global expansion of the commercial trade in amphibians (O’Hanlon et al., 2018).

Phylogenetic studies of Bd have revealed rather extensive genetic diversity. One strain, considered to be hypervirulent, is globally dispersed (Bd-GPL) and thought to be the primary lineage responsible for population declines and extinctions (Miller et al., 2018). Numerous lineages of Bd have been discovered, and it is hypothesized that the hybridization of different strains may lead to the emergence of more virulent, pathogenic strains of Bd. 

Significance

Chytridiomycosis is considered an emerging disease that significantly impacts amphibian populations across the globe. Over 700 species of amphibian are affected by Bd, and in the past few decades the disease has contributed to the threatened status of over 501 amphibian species, including 90 possible extinctions (Scheele et al., 2019). Chytrid fungus (Bd) is believed to be responsible for one of the most significant disease-associated losses of biodiversity in recorded history.

Chytridiomycosis is a reportable disease and any detection of the disease should be reported to the appropriate wildlife authorities.

Species Affected

Currently, chytrid disease is known to affect over 700 species of amphibians. It appears to impact amphibian species associated with permanent water (i.e., streams, moist bogs, ponds); the disease impact may be strongly mitigated by high, or extreme temperatures. However, the disease does not affect all frog species the same. For example, the American bullfrog and the African clawed frog appear to be resistant to the disease, and may act as carriers of the fungus. The fungus is not known to infect humans.

Distribution

The fungus, Bd, and associated disease, chytridiomycosis, is present on every continent where amphibians are found. The disease appears to be having the biggest impact in South and Central America, Australia, and North America. In general, the presence of Bd is associated with relatively cool to moderate temperatures (4-28oC) and Bd infection increases with cooler conditions, high altitude and mountainous regions. Bd infections and mortality vary significantly by season.

Transmission

Bd is a waterborne fungus that disperses zoospores, which have a flagellum 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 finds 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 between diseased amphibians. Research has shown that Bd grows best in water that is between 17-25oC (62-77oF) and that in the wild, most disease outbreaks occur seasonally, at higher elevations, and during wet, cooler months. 

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 reflex (a reflex 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

Typically, diagnosis of chytridiomycosis requires a combination of molecular, fungal culture, and histopathology. Such tests are used to isolate and/or identify Bd from the skin (either direct skin samples or a skin swab) of suspected infected animals. It should be noted that many infected animals may show no clinical signs while sick individuals may be infected with something other than Bd. There are no symptoms of chytridiomycosis that are considered pathognomonic.

Treatment

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

Management

The causative agent (Bd) of chytridiomycosis can easily be spread during anthropogenic activities, especially during field-work and animal handling. Boots, clothes, and all field equipment should be cleaned with an appropriate cleaner followed by a disinfectant, according to product label(s). Gray et al., 2017 provides an extremely useful overview and table of commonly used, researched disinfectants. Wild amphibians should not be moved between sites, and captive amphibians should not be released into the environment or used as bait. All newly acquired captive amphibians should be initially quarantined from other amphibians until it has been confirmed that they are disease free. Appropriate testing should be done for pathogens of concern (i.e., Bd, Bsal, and Ranavirus).  

Suggested Reading

  • Whittaker, K and Vredenburg. V. 2011. An Overview of Chytridiomycosis. Amphibiaweb.
  • The Merck Veterinary Manual. 2013. Infectious Diseases of Amphibians- Fungal Diseases
  • The Amphibian Ark. 2014. Chytrid Fungus.
  • Weldon, C. et al. 2004. Origin of the Amphibian Chytrid Fungus. Emerging Infectious Diseases. 10 (12). 2100-2105
  • O’Hanlon et al. 2018. Recent Asian Origin of Chytrid Fungi Causing Global Amphibian Declines. Science 360(6389): 621-627
  • Miller et al. 2018. Distribution Modeling and Lineage Diversity of the Chytrid Fungus Batrachochytrium dendrobatidis (Bd) in a central African Amphibian Hotspot. PLoS ONE 13(6): e0199288.
  • Scheele et al. 2019. Amphibian Fungal Panzootic Causes Catastrophic and Ongoing Loss of Biodiversity. Science 636(6434): 1459-1463.
  • Rosenblum EB, Stajich JE, Maddox N, Eisen MB. 2008. Global gene expression profiles for life stages of the deadly amphibian pathogen Batrachochytrium dendrabatidis. PNAS 105(44): 17034-17093.

Images

Map showing hot spots of species considered most vulnerable to Bd across the globeMap showing hot spots of species considered most vulnerable to Bd across the globe
Dead mountain yellow-legged frogs (Rana muscosa) killed by chytrid fungus in 2008. Mountain yellow-legged frogs (Rana muscosa) killed by chytrid fungus in 2008
Bd LifecycleBd Lifecycle