The deadly, highly contagious fungus is likely to spread and cause zoonotic epidemics across Africa

The researchers studied the prevalence of the fungus Batrachochytrium dendrobatidis (Bd) in a total of 16,900 historical and modern museum specimens, wild-living individuals, and literature records of amphibians from Africa. Bd causes mycosis fungoides, an infectious and fatal disease of amphibians that has driven hundreds of species worldwide to extinction. Only amphibian species in Africa have so far been spared epidemics of chytridiomycosis, but the current study shows that this postponement is now over. The oldest positive Bd sample was from 1933, but prevalence across Africa remained less than 5% per decade until 2000. In that year, prevalence jumped to 17.2%, and increased thereafter. The authors conclude that Bd began spreading throughout Africa in the year 2000 and may currently cause overlooked zoonotic epidemics. It is likely that Bd will spread further across Africa in the near future.

Mass fungal infections that drive populations around the world into a tailspin don’t happen nearly in science fiction. Mycosis fungoides, the worst vertebrate disease in recorded history, has already wiped out hundreds of species of amphibians worldwide. Due in large part to this fungal disease, 41% of amphibians are currently threatened with extinction. Only species that live in Africa seem to have been relatively spared the scourge of mycosis fungoides — at least for now.

Now, study in Frontiers in conservation science Show that this reprieve was likely temporary: results show that mushrooms Batrachochytrium dendrobatidis (Bd), the pathogen that causes mycosis fungoides, is now well established across Africa. It may have been overlooked there so far, but the pathogen is likely to spread further and cause zoonotic epidemics across Africa in the near future.

“We show that Bd has become more prevalent and widespread across the African continent since 2000,” said Dr. Vance Vrydenberg, a professor at San Francisco State University and the Museum of Vertebrate Zoology at the University of California, Berkeley, and corresponding author of the study.

“This rapid increase may indicate that disease-caused extinctions and extinctions of amphibians may already be occurring in Africa without anyone knowing about them.”

Exceptionally contagious

Bd is a chytridiomycid fungus, an essential group of fungi that produces asexual ‘swarm spores’, which use flagella like flagella to swim. Bd spores thrive in cool, moist habitats, becoming fused and reproducing in the horny mouthparts and skin of salamanders, newts, caecilians, and especially frogs and toads. Mycosis fungoides is exceptionally contagious, as it does not require an animal vector to spread, while the spores can infect at least 1,000 distantly related species. The disease rarely kills tadpoles, but often kills adults, causing peeling skin, lethargy, weight loss, and eventually cardiac arrest.

The first known epidemics of mycosis fungoides 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 analyzes showed that besides the Bd-GPL (“global pandemic strain”), which is believed to be responsible for most mycobacterial epidemics, there are at least four other strains, possibly less virulent from South Korea, Switzerland, South America and South Africa. today.

Some species, for example the sea frog and bullfrog, appear to be relatively immune, while there is also evidence that previously devastated populations may now be evolving some degree of resistance to the pathogens.

Study covering 171 years

Here, Frydenberg and colleagues used for the first time quantitative real-time PCR to determine whether 2,972 museum samples were infected with any known strain of Bd. It was collected from Cameroon, Ethiopia, Kenya, Lesotho, Tanzania, and Uganda between 1908 and 2013 and preserved at the California Academy of Sciences, Berkeley Museum of Vertebrate Zoology, and Harvard University’s Museum of Comparative Zoology. They also tested skin swabs from 1,651 live amphibians caught between 2011 and 2013 in Burundi, Equatorial Guinea, Cameroon and the Democratic Republic of the Congo for infection with Bd. Finally, they searched the scientific literature between 1852 and 2017 looking for records of the presence or absence of visible mycosis fungoides infections in amphibians from across Africa. Finally, the study analyzed patterns of presence of the pathogen in more than 16,900 amphibians.

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

Millennium bug

The results show that the oldest Bd-positive amphibian in the data set dates back to 1933 in Cameroon. Infection prevalence across Africa as a whole remained less than 5% per decade between the 1930s to 2000. But in the same year, infection prevalence suddenly jumped to 17.2%, and increased further to 21.6% during 2010. In countries for which more data are available, Like Cameroon, Kenya, Equatorial Guinea and Burundi, this jump in prevalence after 2000 was more pronounced: for example, in Burundi up to 73.7% of samples were infected.

The exception was South Africa, where positive samples were found as far back as 1943, and the prevalence of infection was relatively constant (about 23.3%, including the local strain Bd-CAPE) from the 1970s to the end of the study.

The authors conclude that since 2000, there has been a largely ignored but significant increase in the prevalence of Bd, posing a new threat to amphibians across Africa. The areas most at risk are East, Central and West Africa.

We don’t have a good idea why the change was most evident around the year 2000 in Africa, later than on other continents. This may be due to chance.

But there is also a hypothesis that stress from climate change could make amphibians more susceptible to pathogens, or that the climate might change so that the microclimate of the pathogen becomes more common. Increased air travel for people and cargo is also likely to be to blame.

The first thing to do is we need to find out if the amphibians in the areas we have identified as high Bd prevalence are experiencing zoonotic epidemics. What we need is a dynamic picture of the host-pathogen interaction.

This microscopic pathogen cannot be eliminated in the wild, but we know that given the opportunity, many host species can survive. Because this pathogen appears to be universally transmitted by humans, we have a moral duty to get involved and try to manage and mitigate it whenever possible.”

Dr. Vance Vrydenberg, Professor, San Francisco State University and Museum of Vertebrate Zoology, University of California, Berkeley


Journal reference:

Ghose, S.L., et al. (2023) Recent continent-wide occurrence of a cosmopolitan pathogen in African amphibians. Frontiers in conservation science.

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