Rapid rise of highly contagious killer fungus poses new threat to amphibians across Africa

Mass fungal infections that drive populations worldwide to near collapse don’t just appear in science fiction. Chytridiomycosis, the worst vertebrate disease in recorded history, has already wiped out hundreds of amphibian species worldwide. Due in large part to this fungal disease, 41% of amphibians are currently threatened with extinction. Only species living in Africa seem to have been relatively spared the scourge of chytridiomycosis — at least so far.

Now, a study at Frontiers in Conservation Science showed that this inhibition was likely temporary: The results show that the fungus Batrachochytrium dendrobatidis (Bd), the pathogen that causes chytridiomycosis, is now firmly established across Africa. It may have so far been overlooked there, but it is likely that the pathogen will spread further and cause epidemics across Africa in the near future.

“We show that Bd has become more prevalent and widespread across the African continent since the year 2000,” said Dr. Vance Vredenburg, a professor at San Francisco State University and the University of California, Berkeley’s Museum of Vertebrate Zoology, and the study’s department. corresponding author. “This rapid increase may signal that declining amphibian diseases and extinctions may already be happening in Africa without anyone knowing.”

Highly contagious

Bd is a chytridiomycid fungus, a key group of fungi that produces asexual “swarm spores,” which use a whip-like flagellum to swim. Bd spores thrive in cool, moist habitats and embed and multiply in the keratinized mouthparts and skin of salamanders, newts, and lizards, but especially frogs and toads. Chytridiomycosis is highly contagious, as it does not need an animal carrier to spread, and the spores can infect at least 1,000 related species. The disease rarely kills tadpoles, but often kills adults, in which it causes peeling skin, lethargy, weight loss and eventually cardiac arrest.

The first known epizootics of chytridiomycosis occurred in the late 1970s and early 1980s in western North America, in the late 1990s in Central America and Australia, and in the early 2000s in South America. Genetic analysis showed that in addition to the Bd-GPL (“Global Pandemic Lineage”) strain, which is responsible for most chytridiomycosis epizootics, at least four other (probably less virulent) strains from South Korea, Switzerland, South America and South Africa exist today.

Some species, for example the sea toad and the American bullfrog, appear to be relatively immune, and there is also evidence that previously devastated populations may be currently developing a degree of resistance to the pathogen.

A study covering 171 years

Here, Vredenburg and colleagues used quantitative real-time PCR for the first time to determine whether 2,972 museum specimens were infected with any known strain of Bd. These had been collected from Cameroon, Ethiopia, Kenya, Lesotho, Tanzania and Uganda between 1908 and 2013 and were held at the California Academy of Sciences, the Museum of Vertebrate Zoology at Berkeley and the Museum of Comparative Zoology at Harvard University .

The researchers also examined skin swabs from 1,651 live amphibians caught between 2011 and 2013 in Burundi, Equatorial Guinea, Cameroon and the Democratic Republic of Congo for Bd infection. Finally, they reviewed the scientific literature between 1852 and 2017 for records of the presence or absence of visible chytridiomycosis infection in amphibians from across Africa. In total, the study analyzed patterns of pathogen presence in more than 16,900 amphibians.

In this study, they present estimates of the prevalence of Bd infection over time for Africa as a whole and for individual countries and regions.

Error of the millennium

The results show that the earliest Bd-positive amphibian in the dataset dates from 1933 in Cameroon. The prevalence of the infection across Africa remained below 5% for every decade between 1930 and 2000. But in the same year, the prevalence of the infection suddenly jumped to 17.2% and further increased to 21.6% during during the 2010s. In countries for which more data are available, such as Cameroon, Kenya, Equatorial Guinea and Burundi, this jump in prevalence after 2000 was even more pronounced: for example, up to 73.7 % of samples were infected in Burundi.

An exception was South Africa, where positive samples were found as early as 1943 and the prevalence of infection was relatively stable (about 23.3%, including the local Bd-CAPE strain) from the 1970s until the end of the study.

The authors conclude that since 2000, there has been a largely overlooked but significant increase in the prevalence of Bd, which poses a new threat to amphibians across Africa. The regions most at risk are eastern, central and western Africa.

“We don’t have a good idea why the change seems more pronounced around 2000 in Africa, which is later than in other continents. This may be due to chance,” said Vredenburg.

“But there is also a hypothesis that the stress caused by climate change could make amphibians more vulnerable to pathogens, or that the climate may be changing so that the climate for the pathogen is becoming more common. It is possible that increased aerial people travel and Cargo is also guilty.’

“The first thing that needs to be done is that we need to know whether the amphibians in the areas that we identified as having high Bd prevalence are experiencing epizootics. What we need is a dynamic picture of the pathogen-host interaction,” said Vredenburg.

“Eliminating this tiny pathogen in the wild is not possible, but we know that given the chance, many host species can survive. Given that this pathogen appears to be moved globally by humans, we have a moral imperative to get involved and to try to manage and mitigate whenever possible.”