Frog Pregnancy Test and the Deadly Fungus Threat to Amphibians

For decades in the mid-twentieth century, the standard way to confirm a human pregnancy involved injecting a woman’s urine into an African clawed frog. If the frog ovulated within hours, the test was positive. Hundreds of thousands of these frogs were shipped from South Africa to hospitals and laboratories on every continent, and they carried a hidden passenger: a skin fungus now responsible for the decline of at least 501 amphibian species worldwide and roughly 90 presumed extinctions. The connection between a routine medical test and one of the worst wildlife disease events in recorded history is stranger and more consequential than most people realize.

How a Frog Became a Pregnancy Test

The African clawed frog, Xenopus laevis, has an unusual sensitivity to hormones in its environment. In the late 1930s, a British scientist named Lancelot Hogben discovered that injecting urine from a pregnant woman beneath the frog’s skin caused it to lay eggs within about 8 to 12 hours. The active ingredient was human chorionic gonadotropin (hCG), a hormone the placenta produces in early pregnancy. No other reliable rapid pregnancy test existed at the time, and the frog method, known as the Hogben test, became the global standard. It was cheap, reusable (the same frog could be tested again after a rest period), and remarkably accurate.

Before the frog test, doctors relied on injecting urine into mice or rabbits and then dissecting the animals to check their ovaries, which killed the test animal every time. Xenopus offered a live readout: eggs or no eggs, visible in the tank. That practical advantage drove enormous demand. The test remained in widespread clinical use until immunoassay-based tests replaced it in the 1960s and 1970s. By then, clawed frogs were already entrenched in laboratories worldwide for entirely different reasons, including embryology, genetics, and neuroscience research.

The Scale of Global Shipments

The volume of frogs shipped out of South Africa was staggering. A single government facility at Jonkershoek filled more than 2,100 orders totaling over 400,000 Xenopus between 1941 and 1969, sending them to at least 104 destinations around the world. During the same period, at least 254 laboratory locations globally were publishing research using these frogs, meaning the actual number of institutions keeping live colonies was far higher than the shipping records alone suggest.¹

Many of these frogs escaped or were deliberately released once pregnancy testing moved to chemical kits. Feral populations of Xenopus established themselves in California, Chile, the United Kingdom, France, Portugal, and elsewhere. The frogs are hardy, tolerant of poor water quality, and voracious predators of local aquatic invertebrates and small vertebrates. They thrived in their new habitats. And they brought something with them.

A Fungus Riding on Frog Skin

Xenopus laevis is an asymptomatic carrier of Batrachochytrium dendrobatidis, universally known as Bd, the chytrid fungus responsible for the disease chytridiomycosis. “Asymptomatic” is the key word: clawed frogs can harbor the pathogen on their skin with minimal or no ill effects. Surveys of wild Xenopus populations in southern California found about 10% of individuals infected, all with extremely low fungal loads.² Testing of archived museum specimens has confirmed Bd on Xenopus collected in Africa as far back as 1934, decades before the mass export era reached its peak.³

The implication is clear: the international pregnancy-test trade moved not just frogs but a fungal pathogen into ecosystems whose native amphibians had never encountered it and had no evolutionary defenses against it. Xenopus was not the sole vector. The broader commercial trade in amphibians for food, pets, and bait also played a role. But the sheer volume and geographic reach of Xenopus shipments made it a particularly effective vehicle for spreading Bd to new continents.

Where Did the Fungus Originate?

For years, researchers assumed Bd was native to southern Africa because Xenopus carried it so efficiently. Genomic work published in 2018 overturned that assumption. Whole-genome sequencing of Bd strains from around the world traced the pathogen’s origin not to Africa but to the Korean peninsula, where one ancestral lineage shows the genetic hallmarks of the founding population. The researchers dated the emergence of the panzootic strain to the early twentieth century, coinciding with the rapid expansion of global amphibian trade.⁴

The finding reframed the story. Bd likely infected African frogs via trade routes well before the Hogben test existed, and those frogs then became long-distance couriers when medical demand shipped them worldwide. East Asia remains a hotspot for Bd genetic diversity, and intercontinental transmission of the fungus is still ongoing. The pathogen did not stop spreading when pregnancy testing stopped.

How Chytridiomycosis Kills

Amphibian skin is not just a barrier. It is an active organ that regulates water and electrolyte balance, much the way kidneys function in mammals. Bd colonizes the outer layers of the skin, specifically the keratin-rich cells, and disrupts this regulatory function. In diseased frogs, electrolyte transport across the skin drops by more than half. Blood sodium levels fall by roughly a fifth, and potassium levels crash by about half. The resulting electrolyte imbalance leads to cardiac arrest.⁵

Research on specific frog species has shown that Bd infection also slows the heart directly. In susceptible species, the fungus appears to impair calcium transport in heart muscle cells, weakening contractions and reducing heart rate. The combined effect of slower, weaker heartbeats and reduced blood volume from electrolyte loss produces a dramatic drop in the heart’s ability to pump blood. For sensitive species, the outcome is rapid death.⁶

This mechanism explains why the disease is so broadly lethal across distantly related frog families. Almost all amphibians depend on their skin for electrolyte balance, so a pathogen that attacks that system threatens a huge range of species regardless of their evolutionary history.

The Toll on Global Amphibian Biodiversity

A comprehensive assessment published in 2019 put numbers on the disaster. Chytridiomycosis has driven the decline of at least 501 amphibian species over the past half-century, with 90 of those species now presumed extinct. The hardest-hit species tend to be larger-bodied frogs with restricted geographic ranges living in wet climates, particularly in Central and South America and in Australia. Declines peaked during the 1980s, and the recovery picture is grim: only about 12% of species that declined have shown any signs of bouncing back, while 39% continue to decline.⁷

Those numbers make chytridiomycosis one of the most destructive disease events ever documented in wildlife, comparable in scale to the impact of introduced predators on island bird species but concentrated on a single class of vertebrates. Some entire genera of colorful, ecologically important frogs have vanished from mountain streams in the neotropics within a few years of Bd’s arrival.

Why the Fungus Persists Even After Frogs Disappear

One of the more troubling discoveries about Bd is that it does not need amphibians to survive. Field surveys and laboratory experiments have shown that crayfish can become infected with the fungus, maintain the infection for at least 12 weeks, and transmit it to amphibians they share habitat with. In some wetlands, crayfish Bd prevalence reached nearly 30%, and the presence of crayfish was a positive predictor of Bd infection in co-occurring amphibians.⁸

This means that even after Bd has wiped out local frog populations, the fungus can linger in crayfish and potentially other organisms, lying in wait if amphibians ever return. It also complicates conservation efforts: you cannot simply treat the frogs and expect the environment to be clean.

How Climate Change Makes It Worse

Temperature shapes how Bd interacts with its hosts in ways that are not always intuitive. Research on the critically endangered Panamanian golden frog showed that frogs experienced the highest pathogen loads and highest mortality when exposed to Bd at unusually warm temperatures, consistent with a concept called the thermal mismatch hypothesis. Species adapted to cool environments become more vulnerable to disease when their habitat warms, because their immune systems perform poorly outside their preferred temperature range while the pathogen may thrive.⁹

This interaction helps explain some of the geographic patterns of amphibian declines. Cloud-forest species in tropical mountains, living at the edge of their thermal comfort zone, have been disproportionately devastated. As temperatures creep upward, the window of conditions favorable to the pathogen overlaps more and more with the conditions these frogs experience daily.

What Keeps Some Species Alive

Not all amphibians exposed to Bd die. Some species coexist with the fungus, and understanding why has become a major focus of conservation biology. One layer of defense comes from the skin itself. Many frogs secrete antimicrobial peptides that can inhibit Bd growth. Studies of Andean frog species found that natural peptide mixtures from their skin significantly reduced Bd growth in laboratory tests, though the specific peptide profiles varied considerably between species.¹⁰

Another layer involves the community of bacteria living on amphibian skin. Certain bacterial species produce antifungal metabolites that suppress Bd. Frogs whose skin microbiome includes these protective bacteria tend to fare better when exposed to the fungus. This has led researchers to explore whether you could deliberately add protective bacteria to vulnerable frogs, an approach called bioaugmentation or probiotic treatment.

Probiotics, Antifungals, and the Limits of Treatment

The idea of treating wild frogs with beneficial bacteria has shown promise in the lab. One bacterium in particular, Janthinobacterium lividum, produces a compound called violacein that inhibits Bd. Inoculating frogs with this bacterium in captivity increased survival by about 40% during experimental Bd exposure.¹¹ Laboratory trials and at least one field trial have demonstrated that bioaugmentation can work in principle, and that it may help reintroduce susceptible species into areas where Bd is already established.¹²

The reality in wild populations, however, has been far more sobering. Field experiments on endangered frogs using both the antifungal drug itraconazole and J. lividum probiotic treatments showed that while itraconazole reduced fungal loads immediately, the effects on early life stages faded within a year, and treated populations eventually declined to near-extirpation anyway. In adults, antifungal treatment produced longer-lasting benefits over a three-year period, possibly because adult immune systems could mount an adaptive response once the fungal burden was reduced. But recruitment of young frogs into the adult population was too low to sustain the population. Probiotic concentrations on treated frogs dropped to baseline within a month and provided no detectable protection against Bd.¹³

The pattern is frustratingly consistent: short-term treatments can knock Bd back, but the fungus returns because the environment remains contaminated and young frogs remain vulnerable. A lasting solution likely requires the frogs themselves to evolve or be bred for resistance, rather than relying on repeated pharmaceutical intervention.

Breeding for Resistance

Some conservation programs have started looking at the genetic basis of disease resistance. Research on the critically endangered southern corroboree frog found meaningful variation in immune-related genes among captive individuals, variation that could potentially be selected for through managed breeding. The idea is to identify frogs whose immune profiles give them an edge against Bd, breed those individuals preferentially, and eventually release their offspring into the wild.¹⁴

Early results from reintroduction programs suggest that releasing frogs with enhanced resistance can help populations recover at a landscape scale, though this approach requires careful genetic management to avoid inbreeding and maintain the adaptive capacity of the population over time.¹⁵ The timeline is long. Breeding programs for small-bodied amphibians with complex life histories are expensive, technically demanding, and vulnerable to the same captivity-related microbiome changes that weaken disease resistance in the first place.

A Second Chytrid Fungus on the Horizon

As if Bd were not enough, a second chytrid species, Batrachochytrium salamandrivorans (Bsal), has emerged in Europe, where it has devastated fire salamander populations in the Netherlands and Belgium. Bsal appears to have originated in East Asia, just as Bd did, and was likely introduced to Europe through the pet trade in Asian newts. It has not yet been detected in North America, but laboratory testing paints a grim picture of what could happen if it arrives.

Experiments exposing 35 North American amphibian species to Bsal found that nearly three-quarters became infected and over a third died. Both salamanders and frogs were susceptible. Modeling based on these results, combined with environmental suitability maps, suggests that salamander losses could exceed 80 species in the United States alone and 140 across North America, with the greatest predicted damage in the Appalachian region and along the West Coast, areas that are global hotspots for salamander diversity.¹⁶

Several countries have imposed import restrictions on live salamanders in an attempt to prevent Bsal from crossing borders, but enforcement is uneven and the international amphibian pet trade remains large and loosely regulated. The lesson of Bd and the pregnancy test frog, that moving animals across continents moves their pathogens too, has been learned slowly and at enormous cost.

Why Amphibian Losses Ripple Through Ecosystems

Frogs and salamanders are not just charismatic animals that people enjoy seeing in the wild. They occupy critical positions in food webs, serving as both predators of insects and prey for birds, snakes, and mammals. Tadpoles graze algae in streams, regulating primary productivity. Adult frogs consume vast quantities of mosquitoes and other pest insects. When amphibians vanish from a stream or a forest floor, insect populations can surge, algal dynamics shift, and the animals that depended on amphibians as food lose a significant calorie source.

In tropical streams where chytridiomycosis eliminated most frog species, researchers have documented increases in algal biomass, changes in invertebrate communities, and cascading effects on stream nutrient cycling. The full ecological consequences are still being measured decades after the initial die-offs, in part because tropical stream ecosystems are complex and understudied. What is already clear is that losing hundreds of amphibian species is not a cosmetic problem confined to conservationists. It reshapes the ecosystems those species inhabited.

The Pregnancy Test Connection in Retrospect

The Hogben test was a genuine medical breakthrough in its time. It gave doctors a reliable, fast pregnancy test decades before the hormone-detecting strips that sit on pharmacy shelves today. No one in the 1940s or 1950s had the tools to detect a microscopic skin fungus on an apparently healthy frog, or to predict what would happen when that frog’s descendants colonized streams on other continents. The concept of wildlife disease ecology barely existed.

Still, the story is a sharp illustration of how moving organisms outside their native range, even for perfectly reasonable purposes, can trigger consequences that take decades to become visible. The frogs shipped from Jonkershoek in the 1950s for pregnancy testing contributed to a disease event whose full scope was not appreciated until the 2000s. Amphibian species that went extinct in the intervening decades had no obvious connection to a midcentury medical procedure. It took genetic detective work, museum specimen analysis, and global epidemiological surveys to stitch the timeline together. The connection between a frog in a hospital laboratory and a silent extinction in a Panamanian cloud forest turns out to be direct, traceable, and devastating.