Scientifically Speaking: A bizarre link between pregnancy tests, frogs & malaria
This planet has always been part of an interconnected network. Take a pregnancy test in the 1940s-1960s, a loss of frogs and salamanders decades later and a rise in malaria cases. These seemingly random events are linked in a cautionary tale of unintended consequences
Covid-19 is one of many microbial infections that have shaped the course of human history. But life on this planet has always been part of a complex, interconnected network. The effects of infections that spread across the planet due to human activities are not limited to humans. And damages that result may linger on in plants and animals for decades, or even centuries. Connecting the dots between seemingly disparate events is a painstaking process.
Take a pregnancy test used from the 1940s through the 1960s, a major loss of types of frogs and salamanders that followed decades later, and a rise in malaria cases in one part of the world that has only recently come to light. These seemingly random events are linked in a cautionary tale of unintended consequences — a theme that is repeated in biology over and over.
Before the development of modern tests, certain frogs known as African clawed frogs (Xenopus laevis) were used in pregnancy tests. What we did not know then was that these frogs and others shipped around the world harboured a kind of fungus for which frogs and salamanders in other parts of the world species had no resistance. Exotic frogs were carriers of chytrid fungus, which over decades contributed to one of the greatest biodiversity losses ever documented.
A research article published in Science in 2019 tried to estimate the toll. In the article, scientists estimated that over 400 species of frogs and salamanders in over 60 countries have suffered due to the recent spread of chytrid fungus. Sadly, around 90 species had been wiped out forever.
A loss of this magnitude is difficult to imagine. Indeed, chytrid fungus may be the deadliest microbe ever described. One of the lead authors of the study, Jonathan Kirby, noted that “we’ve never before had a single disease that had the power to make multiple species extinct on multiple continents, at the same time… Hundreds, if not thousands of frog species could go extinct.”
How did this lethal fungus spread?
We don’t know the very early history, but some of the earliest cases of chytrid fungus were found in South Africa in African clawed frogs. These frogs were exported to different parts of the world for at least two decades in the early 20th century as a live pregnancy test.
It is a bizarre story that is now mostly forgotten. The story begins with scientist named Lancelot Hogben who discovered that female African clawed frogs could determine if a woman was pregnant. Urine from a woman was injected into a female frog. If the woman was pregnant, hormones in her urine would stimulate the frog to lay eggs. Apparently, this test was remarkably accurate, and thousands of frogs were shipped around the world for this pregnancy test.
Much later, descendants of those frogs were found to harbor chytrid fungus, which they had acquitted some resistance to. While not the sole cause of chytrid fungus infection, these and other frogs that were traded certainly contributed to the spread of the disease globally.
The chytrid fungus has since decimated a large number of frog species on all continents, except in Asia. In 2018, a report in Science found that Asian amphibians were probably the original source of the fungus, explaining why some frogs developed some resistance over time. But what really cause widespread devastation was a mutational event in the 1980s after which chytrid fungi became more virulent.
What has been clear since is that the loss of frogs impacts entire ecosystems. Now, a research article published in Environmental Research Letters has found that the collapse of amphibian populations in Central America led to a surge in cases of malaria in Central America.
There’s a throughline from the loss of frogs because of chytrid fungus to an increase in malaria in humans. Take away amphibians that prey on insects such as mosquitoes, and presumably, their numbers explode leading to detrimental effects on human health.
The new research is a stunning reminder that our health is inextricably linked to global trade animals and the environment. Disruptions in animal populations can shift the balance towards the spread of diseases in humans, even though the effect is not always immediate or direct.
Anirban Mahapatra is a scientist by training and the author of a book on COVID-19. He’s writing a second popular-science book
The views expressed are personal
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