The African clawed frog is an evolutionary marvel — an entirely aquatic species with powerful hind legs, flattened bodies, and an ability to regenerate damaged tissue. It’s native to sub-Saharan Africa, where it thrives in murky ponds and stagnant pools, and can withstand extreme drought by burrowing into mud and entering a state of suspended animation.

But that’s not what the species is most known for. It’s known for its ability to show whether a woman is pregnant. How? Well, by injecting a woman’s urine into the frog’s skin, of course. If the woman is pregnant, it would cause the amphibian to produce eggs. That, of course, and the devastating disease it spread because of us humans.

A surprising discovery

British zoologist Lancelot Hogben was studying African clawed frogs (Xenopus laevis) when he noticed that their skin color would change depending on the growing environment. He hypothesized that this has something to do with the pituitary gland. To test it, he took the gland out of several frogs. He was right, and the frogs lost color when the gland was taken out. During some experiments in the 1930s, he also injected some frogs with ox pituitary gland extracts, finding that this triggered the female frogs to ovulate.

It was an unusual finding and Hogben continued researching the frogs. He realized that the frogs were very sensitive to any hormonal changes, particularly pituitary hormones. At some point, something clicked. The researcher realized that pregnant women also have pituitary hormones. This triggered a new direction of research where, with colleagues, Hogben developed a pregnancy test using the frog.

It was a groundbreaking find. This wasn’t the first pregnancy test where an animal would be injected with urine, but it was the first time that you could reuse the same animal.

A Sad but Familiar Story

It may sound shocking now, but such tests had been developed using mice and rabbits. In these tests, animals had to be dissected to confirm the result. The frogs, however, survived the process and could be reused multiple times. Hospitals and pharmacies imported thousands of Xenopus frogs from Africa throughout the 1940s, ’50s, and ’60s, establishing breeding colonies across the world.

Demand for the frog surged. People took more and more frogs from the wild, without much thought for conservation. Several populations collapsed as, for decades, this was the most reliable and affordable pregnancy test.

Then, by the late 1960s, immunological tests rendered the frog method obsolete. Hospitals no longer needed their amphibian test subjects. Demand still continued from communist countries, where access to advanced medical technology was often limited and resources were scarce.

But our story doesn’t end here.

What happened to the frogs?

After demand dropped, the population of African clawed frogs rebounded. Nowadays, we no longer consider the species to be at risk of extinction. But in the 1960s and 70s, there were still lots of frogs in labs.

Some were euthanized, but many were simply released into the wild. And with them, the devastating chytrid fungus spread.

Batrachochytrium dendrobatidis was first described in 1998 by Australian scientist Dr. Lee Berger, who tracked its spread as part of her PhD research. Berger realized something was strange with the native frogs in Australia.

“In the 1980s, people just didn’t think frogs were disappearing,” Berger recalls. “Scientists said, ‘Don’t panic, we need the stats to prove it.’” But by the early 1990s, at the World Congress of Herpetology, researchers could no longer deny the crisis — frogs had vanished en masse, and no one understood why.

The culprit was this fungus.

Chytrid fungus is unlike other amphibian diseases. It invades the skin of frogs, disrupting their ability to absorb water and electrolytes. Infected animals become lethargic, their skin thickens, and within days they suffer cardiac arrest. Some populations disappear in a matter of months. In Panama, once-deafening rainforests fell eerily silent as entire species vanished because of this fungus.

The numbers are staggering. The fungus has affected over 700 species and is responsible for at least 200 amphibian extinctions. Australia’s gastric-brooding frogs — extraordinary creatures that gestated their young in their stomachs — are gone. The vibrant Costa Rican golden toad has not been seen since 1989. Once-common species like the Corroboree frog have reduced to tiny captive populations.

How could such a devastating disease start wreaking havoc all of a sudden? The answer, of course, lied with the African clawed frog.

A global mistake

The ethics of injecting another species with urine to see if you’re pregnant is grueling. But the damage that this practice caused was way worse than anyone could have anticipated.

Scientists now believe the disease originated in Korea, alongside Xenopus frogs moved from their original habitat. These amphibians carried the pathogen without suffering its lethal effects. When they were exported for pregnancy tests, they became unwitting super-spreaders, introducing chytrid to ecosystems where native amphibians had no defenses.

Because many of the frogs used in laboratories were later released into the wild, Xenopus laevis became an invasive species in several countries. These hardy, adaptable amphibians outcompeted native species and disrupted local ecosystems all while spreading this pathogen.

“It was shocking to watch the spread of the disease happen in real time 15 years ago,” says Jamie Voyles, assistant professor at the University of Nevada, for BBC. “In Panama, spots in the rainforest were deafeningly loud with amphibian calls. Their abundance was incredible; you couldn’t even walk through the forest without worrying about stepping on them. The next summer they were just gone.”

The global trade in amphibians made matters worse. Even today, an estimated 100 million frogs are transported across international borders each year for the pet trade and culinary markets. These movements continue to spread chytrid and its deadly cousin, Batrachochytrium salamandrivorans, now devastating salamander populations.

It’s hard to emphasize just how devastating these bacteria have been.

A study conducted by the Australian National University found that B. dendrobatidis (Bd) has contributed to the decline of 501 amphibian species, accounting for approximately 6.5% of all known amphibians worldwide. Among these, 90 species have gone completely extinct, while 124 have suffered population declines of over 90%, with little hope of full recovery. This study positioned Bd as one of the most devastating wildlife pathogens ever recorded. However, subsequent research challenged these findings, suggesting that while Bd is a significant factor, it may not be the primary driver of amphibian declines, as habitat destruction, climate change, and pollution also play important roles.

A lesson in unintended consequences

Doctors didn’t realize what this would lead to. It seemed like a breakthrough that could save mice and rabbits from sacrifice. Yet its unintended consequences have reshaped the amphibian world.

Chytrid still lurks. It has spread to all sorts of habitats and continues to be one of the major sources of extinction, alongside climate change and habitat destruction.

Despite the grim toll, scientists find glimmers of hope. Captive-breeding programs, slight changes in water salinity, and new forms of assisted reproduction could preserve vulnerable frogs until better solutions emerge. Some amphibian populations seem to be evolving resistance, but the fungus remains a formidable foe.

The story is a reminder that medical breakthroughs can ripple outward in unexpected ways. A small breakthrough in medical testing led to vast global consequences for amphibians everywhere. Our world is more connected than it seems. And as scientific knowledge continues to advance, the ripple effects spread across forests, marshes, and streams in ways no one ever imagined.