Science Snippets: Central American Amphibians Affect Human Health

Draft script:

From the Anthropocene e-zine on 23 December 2020 comes a story titled Study ties amphibian collapses with increased malaria outbreaks. The subhead: “If you remove frogs and other ‘mosquito-reducers’ from the landscape, what happens to malaria rates?” This was the year after COVID-19 began spreading throughout the world. Here’s the lede, followed by another sentence that introduces COVID-19 and the disease it causes, SARS-CoV-2: “As we’ve learned the hard way this year, our health is intertwined with the health of other animals. SARS-CoV-2, the coronavirus that causes Covid-19, probably jumped to humans from bats, potentially by way of a secondary host and perhaps via a wildlife market.”

Skipping ahead a paragraph, we find a description of a deadly fungus: “Batrachochytrium dendrobatidis, or Bd, is a highly contagious fungus that infects the skin of amphibians, blocking them from breathing and eventually killing them. Ponds hit by Bd are quickly choked by belly-up frogs. It is so deadly and so easily spread that it has caused the extinction of an estimated 90 species, and reduced the populations of another 124 species by over 90%.” The embedded link referring to the cause of extinction of an estimated 90 species leads to a peer-reviewed paper in the renowned journal, Science. This paper is not open-access.

We find this information about malaria in the following paragraph: “Malaria, meanwhile, is a disease caused by a single-celled Plasmodium parasite and transmitted by mosquitos. It kills hundreds of thousands of people every year and sickens millions more, mostly in equatorial countries. Vaccines for it are few and not particularly effective, and preventative medicines are expensive and can come with difficult side effects, so the best anti-malarial strategies involve reducing contact between people and mosquitos, often through nets and insecticides.”

The next paragraph describes species that target mosquito populations, and the consequences of reducing these species: “Frogs and other amphibians are great mosquito-reducers. If you remove a huge number of them from the landscape, what happens to malaria rates? To investigate, researchers cross-referenced dates of Bd-driven amphibian decline in different parts of Costa Rica and Panama with changes in malaria incidence in those same places. Bd swept across these two countries over the course of about twenty-five years, starting in northwest Costa Rica and progressing south and east. They found that, generally, around a year after Bd entered a county, malaria cases began to increase. They continue to rise for two years, then stay at that higher level for six more, before beginning to attenuate 9 years after the fungus arrived.”

The story at the Anthropocene refers to a paper delivered to the American Geophysical Union Fall Meeting in 2025. It draws extensively from the paper delivered at this professional meeting: “For the six years our estimated effect of amphibian decline is at its highest, the annual expected increase in malaria ranges from 0.76-1.0 additional cases per 1,000 population, a substantial share of cases overall.” As an example, this rate of increase in Costa Rica’s capital, San Jose, would translate to 1,000 more cases there.

The authors of the paper delivered at the 2025 American Geophysical Union Fall Meeting concluded that anything other than a causal relationship to explain the relationship between amphibians and malaria would be “extremely unlikely.” In response, the article at the Anthropocene e-zine includes this paragraph: “The notion that all species are connected can seem abstract or even fanciful. Knowing that a fungus kills an amphibian that would otherwise eat an insect that harbors a single-celled parasite that sickens humans makes it all much more concrete — and maybe more urgent, too, especially when considered alongside this year’s events.”

For you, of course, “the notion that all species are connected” does not “seem abstract or even fanciful.” Rather, it makes perfect sense.

The lead author of the paper at the professional meeting is a Ph.D. candidate in environmental economics at the University of California-Davis. He provides the bottom line of the paper in the Anthropocene e-zine: “The results in our paper suggest that some policies, such as amphibian conservation policies or the regulation of wildlife trade, could have benefits for human health which are not currently accounted for. I expect that more people will start looking into these questions, not necessarily because of our paper, but because of COVID-19.”

I now turn to the paper delivered at the 2025 American Geophysical Union Fall Meeting. Written by five scholars, it includes a Significance Statement, which reads thusly: “Despite substantial multicontinental collapses in amphibian populations from spread of the fungal pathogen Batrachochytrium dendrobatidis (Bd), the implications for humans have not been systematically studied. Amphibians are known to affect food webs, including mosquitoes that serve as a vector for the spread of disease. However, little is known about how their loss erodes ecosystem services, including the regulation of the transmission of 30 infectious diseases. Using Central America as a case study, this study shows that Bd-driven amphibian loss led to a substantial increase in malaria incidence. The results highlight the often underappreciated social costs of biodiversity loss, including the potential stakes of ecosystem disruption from failing to stop spread of future novel pathogens.”

Imagine my surprise when I found that those five scholars produced a peer-reviewed, open-access paper published before the paper was delivered at the 2025 American Geophysical Union Fall Meeting. Published in the peer-reviewed, open-access Environmental Research Letters, the paper is titled Amphibian collapses increased malaria in Central America. It was published 20 September 2022. The Abstract provides this information: “Biodiversity in ecosystems plays an important role in supporting human welfare, including regulating the transmission of infectious diseases. Many of these services are not fully-appreciated due to complex environmental dynamics and lack of baseline data. Multicontinental amphibian decline due to the fungal pathogen Batrachochytrium dendrobatidis (Bd) provides a stark example. Even though amphibians are known to affect natural food webs—including mosquitoes that transmit human diseases—the human health impacts connected to their massive decline have received little attention. Here we leverage a unique ensemble of ecological surveys, satellite data, and newly digitized public health records to show an empirical link between a wave of Bd-driven collapse of amphibians in Costa Rica and Panama and increased human malaria incidence.”