The Deadly Chytrid Fungus

As if amphibians don’t have enough to worry about, what with a host of adversaries. Now, they are faced with the deadly chytrid fungus. Amphibian populations, worldwide have been in a 40% decline! Due, in part, to habitat loss and destruction, pollution, climate change, disease, and over-collection for the pet trade. Now enter Batrachochytrium dendrobatidis, which develops into chytridiomycosis. This leads to certain death, in susceptible species. The first documented outbreaks of chytrid fungus happened in the late 1990s in Central America and Australia simultaneously. Post discovery, the pathogen has been detected in approximately 100 different amphibian species.

What is it? What does it do?

Batrachochytrium dendrobatidis (Bd.) is an external based pathogen that attaches to keratinized parts of amphibians, like the mouth-parts of tadpoles and also the skin of adult specimens. The pathogen reproduces via sporangia, and can be spread, not only by movement of flagellated zoospores, but direct contact between host specimens and even between a host’s developmental stages. As the fungus grows, it leads to damage of the keratin (skin) layer, which eventually causes their skin to peal, weight loss, weakness, and possible death. The ultimate reason for death isn’t known, but it seems to come from the disruption of skin function – like water absorption and oxygen exchange.

Some are Seemingly Immune

While some amphibians fall to chytrid, others seem to be able to cope with it, unharmed, or relatively so. Tadpole phases of affected amphibians don’t seem too affected by it. Yet, as they metamorphose into adults, the pathogen blossoms and takes its deadly toll. It is presumed that the rapid growth of new tissue during metamorphosis coincides with the growth of chytrid and leads, ultimately, in their demise.

Chytrid’s History

The oldest recorded discovery, in frogs was from a species of Lake Titicaca frog, in 1863. The oldest sample from a salamander was from a Japanese giant salamander, in 1902. Later, in 1938, Bd. was found in an African clawed frog. However, the African frog did not seem phased by chytrid.

Did you know…?
The first well-known means of human pregnancy tests utilized this species, and as such, wide-scale international trade in living African clawed frogs began over 60 years ago.

If Batrachochytrium did, in fact, originate in Africa, the African clawed frog is widely thought to have been the primary vector of the initial spread out of the continent to countries around the world. The earliest recorded case of chytridiomycosis was an American bullfrog, which was collected back in 1978.

The Environment

It is widely accepted that climate change has had a huge impact on the fungus. As global temperatures rise, the fungus proliferates due to increased cloud cover. The cloud cover prohibits the drying effects of the sun during the day which lowers temperatures. Then the evening cloud cover insulates the air, increasing nighttime temperatures; thus enabling Bd. to spread.

The Impact

The amphibian chytrid fungus seems to fare best within the temperatures of 62°F – 77°F. This explains why it tends to flourish at moderately higher elevations, where the temperatures are cooler. Warmer temperatures tend to weaken and/or kill the pathogen. This has been proven by exposing amphibians to higher temperatures. Some feel that amphibian research and conservation efforts may also be actually helping to spread the fungus. People tracking it in from other locales and such. The generalized consensus is that chytrid has been responsible for the decline, and in some cases, outright extinction of up to 40% or more of the world’s amphibian species. However, there still isn’t enough concrete evidence to make this claim.

Hope Looms on the Horizon

Due to this fungus’ incredible and dramatic impact on global amphibian populations, massive research efforts have been made to develop means to fight its spread in the wild. One of the most incredible discoveries is that amphibians in colonies that survive the epidemic tend to carry higher levels of the bacterium Janthinobacterium lividum. This particular bacterium creates antifungal compounds, like violacein and indole-3-carboxaldehyde, that stop the growth of Bd. even at low amounts.

The use of heat-induced therapy as well as antifungals, like itraconazole, have been suggested as a treatment. However, some amphibian species are adversely affected by the various available antifungal agents. Plus, the treated animals don’t always make a full recovery. Another cool treatment method is that of bioaugmentation. This is where the host and also the environment can be adjusted with probiotic bacteria that expresses anti-fungal metabolites that can then fight Bd.