Chytrid Fungus and Frog Extinctions

Strawberry Poison-dart Frog on lichen
Strawberry poison-dart frog. This family of frogs is particularly vulnerable to the chytrid fungus. Joe McDonald / Getty Images

In 1998 a paper published in the Proceedings of the National Academy of Sciences caused a stir in the world of biodiversity conservation. Titled “Chytridiomycosis causes amphibian mortality associated with population declines in the rain forests of Australia and Central America”, the article introduced to the conservation community a devastating disease affecting frogs all around the world. The news, however, did not surprise field biologists working in Central America. For years they had been flummoxed by the mysterious disappearance of entire frog populations from their study areas. These biologists were not observing the gradual declines typical of habitat loss and fragmentation, the usual scapegoats, but instead they were witnessing populations vanishing from one year to the next.

An Unusual Foe

Chytridiomycosis is a condition resulting from an infection from a fungus, Batrachochytrium dendrobatidis, or Bd for short. It is from a diverse family of fungi that had never before been observed in vertebrates. Bd attacks the skin of frogs, hardening it to the point where it impedes respiration (frogs breathe through their skin) and affects water and ion balance. The lesions end up killing the frog within a few weeks after exposure. Once established in a frog’s skin, the fungus releases spores into the water, which will infect other individuals. Tadpoles can carry the fungus cells but will not die from the disease. Bd needs to remain in moist environments, and will die when exposed to temperatures above 30 degrees Celsius (86 degrees Fahrenheit). The moist, thick rainforests of Central America offer an ideal environment for the fungus. 

A Fast Moving Disease

The El Cope area in Panama has hosted herpetologists (scientists studying amphibians and reptiles) for a long time, and starting in 2000 biologists started carefully monitoring frogs. Bd had been moving south across South American countries, and it was anticipated to hit El Cope sooner or later. In September 2004, the number and diversity of frogs suddenly dropped, and on the 23rd of that month the first Bd infected frog was found. Four to six months later, half the local amphibian species had disappeared. Those species still present were 80% less abundant than they had been before.

How Bad Is It, Really?

The emergence of chytridiomycosis is extremely worrisome for anyone concerned with biodiversity. It is estimated that 150 to 200 species of frogs have already gone extinct because of it, with about 500 more species at an extreme risk of disappearing. The International Union for Conservation of Nature (IUCN) called chytridiomycosis “the worst infectious disease ever recorded among vertebrates in terms of the number of species impacted, and its propensity to drive them to extinction.”

Where Did Bd Come From?

It is not yet clear where the fungus responsible for chytridiomycosis comes from, but it is likely not native to the Americas, Australia, or Europe. Based on the study of museum specimens collected over decades, some scientists put its origins somewhere in Asia from where it spread worldwide. One possible vector for the spread of Bd might be the African clawed frog. This frog species has the unfortunate characteristics of being a carrier of Bd while suffering no ill effects from it, and of being shipped and sold worldwide. African clawed frogs are sold as pets, as food, and for medical purposes. Surprisingly, these frogs were once held in hospitals and clinics to be used as part of a type of pregnancy test. It is possible that the heavy trade for these frogs has helped disseminate the Bd fungus.

Pregnancy tests have come a long way from African clawed frogs, but another species now replaced them as an effective vector of Bd. The North America bullfrog too has been found to be a resistant carrier of Bd, which is unfortunate since that species has been widely introduced outside of its natural range. Moreover, bullfrog farms have been established in South and Central America, as well as in Asia, from where they are shipped as food. Recent analyses have found a high proportion of these farm-raised bullfrogs to carry Bd.

What Can Be Done?

Disinfectants and antibiotics have been shown to cure individual frogs from a Bd infection, but these treatments are not applicable in the wild to protect populations. Some promising avenues of research include figuring out how some frog species can mount an effective resistance to the fungus.

A lot of efforts are currently deployed to provide shelter to some individuals of the most at-risk species. They are taken out of the wild and kept in facilities free from the fungus, as an insurance against the possibility that the wild population gets wiped out. The project Amphibian Ark helps organizations establish such captive populations in hard-hit regions. Currently zoos have captive populations of only a handful of the most threatened frogs, and Amphibian Ark assists them in broadening the scope of their protective efforts. There are now facilities in Central America wholly dedicated to protecting frogs threatened by Bd.

Next, Salamanders?

Recently, further mysterious declines have alarmed herpetologists, this time affecting salamanders. Conservationists’ fears were confirmed in September 2013 when the discovery of a new disease was announced in the scientific press. The disease agent is another fungus of the chytrid family, Batrachochytrium salamandrivorans (or Bsal). It appears to have originated from China, and was first detected in the West in a salamander population in the Netherlands. Since then, Bsal has decimated populations of fire salamanders in Europe, threatening a once common animal with extinction. As of 2016, Bsal has spread to Belgium and Germany. The very rich diversity of salamanders in North America is vulnerable to Bsal, and the U.S. Fish & Wildlife Service has taken steps to keep at bay the infectious disease.  In January 2016, a total of 201 salamander species were listed as injurious by the Fish & Wildlife Service, in effect prohibiting their importation and transportation across state lines.