Frog Pregnancy Test and the Deadly Fungus Threat to Amphibians

Decades ago, before modern pregnancy tests, scientists used an unusual method: injecting a woman’s urine into certain species of frogs. If the woman was pregnant, hormonal changes would trigger a response in the frog, providing a biological indicator. This technique was a key medical diagnostic tool in the mid-20th century.

However, the global transport of these frogs may have contributed to the spread of deadly fungal pathogens now threatening amphibians worldwide. Understanding this historical practice and its unintended ecological consequences highlights the complex interactions between human activity and wildlife health.

Biological Process Of Testing

The frog pregnancy test relied on the amphibian response to human chorionic gonadotropin (hCG), a hormone produced during pregnancy. When a woman’s urine containing hCG was injected into a live frog, the hormone triggered ovulation in certain species. This provided a rapid and relatively reliable pregnancy test before modern immunoassays. Unlike earlier methods requiring the sacrifice of laboratory animals, this approach allowed repeated testing with the same frog.

The most widely used species was the African clawed frog (Xenopus laevis), which exhibited a distinct reproductive response to hCG. Within 8 to 12 hours of injection, a gravid female Xenopus would release eggs if the urine sample contained sufficient hormone levels. This reaction was highly specific, as amphibian reproductive physiology is particularly sensitive to gonadotropins. The test’s reliability was comparable to early rodent bioassays but offered faster results without euthanizing the test subject, making it a preferred tool in hospitals and research labs from the 1930s to the 1960s.

In Xenopus laevis, hCG mimics the natural luteinizing hormone (LH) surge that triggers ovulation. The frog’s pituitary gland responds by stimulating the ovaries, leading to egg release. This process mirrors the hormonal cascade in human reproduction, making Xenopus an ideal model for pregnancy detection. The test’s accuracy was estimated at around 98%, making it one of the most dependable biological assays of its time.

Species Typically Used

The African clawed frog (Xenopus laevis) was the primary species used for pregnancy testing due to its reproductive physiology and adaptability to laboratory conditions. Native to sub-Saharan Africa, these aquatic frogs thrive in diverse environments, making them easy to maintain for research. Their year-round fertility allowed continuous testing, unlike amphibians with seasonal breeding cycles. Additionally, their resilience and tolerance to captivity ensured they could be transported globally and housed in medical facilities with minimal specialized care.

Beyond their availability, Xenopus laevis exhibited a rapid and reliable response to hCG. Female specimens injected with hCG-containing urine would begin ovulating within hours, expelling eggs into the surrounding water. Their smooth, scaleless skin facilitated easy handling and injection, streamlining the testing process. The widespread use of Xenopus for pregnancy detection also established it as a model organism in developmental biology, a role it continues to play today in studies of embryogenesis and genetics.

While Xenopus laevis dominated the field, other amphibians were occasionally used. The common European frog (Rana temporaria) and the American leopard frog (Lithobates pipiens) were explored as alternatives, particularly where Xenopus was less accessible. These species also exhibited ovulatory responses to hCG, but their seasonal breeding patterns and delicate care requirements made them less practical. Additionally, variability in their hormonal responses led to inconsistent results, reinforcing the preference for Xenopus laevis in clinical practice.

Spread Of Fungal Pathogens

As African clawed frogs (Xenopus laevis) were exported for pregnancy testing and biological research, they carried more than their reproductive traits. These amphibians became unwitting vectors for Batrachochytrium dendrobatidis (Bd), a chytrid fungus that has decimated amphibian populations worldwide. Originally endemic to parts of Africa, Bd remained relatively contained until the global trade of Xenopus accelerated its spread. By the mid-20th century, shipments of these frogs to Europe, North America, and beyond introduced Bd into ecosystems where amphibians had no prior exposure.

Bd spreads through waterborne zoospores that infect amphibians via skin contact. Once introduced to a new environment, the fungus persists in aquatic habitats, infecting native species lacking evolutionary defenses. The expansion of Xenopus populations in laboratories and medical facilities facilitated the fungus’s proliferation. Since Xenopus laevis can carry Bd asymptomatically, infected individuals were often released or escaped into the wild, seeding outbreaks in previously unexposed amphibian communities.

The consequences have been devastating. Since the late 20th century, Bd has caused mass die-offs and population declines across multiple continents, affecting over 500 amphibian species. A 2019 study in Science estimated Bd-driven declines in nearly 90 countries, marking it as one of the most destructive wildlife pathogens ever recorded. The fungus disrupts amphibian skin function, impairing electrolyte balance and leading to cardiac arrest in severe cases. Particularly vulnerable species, such as the Panamanian golden frog (Atelopus zeteki) and the mountain yellow-legged frog (Rana muscosa), have been pushed to the brink of extinction.

Effects On Amphibians

The emergence of Batrachochytrium dendrobatidis (Bd) has triggered widespread amphibian declines, with some populations collapsing within years of infection. This fungal pathogen disrupts amphibian skin physiology, which plays a vital role in respiration, hydration, and electrolyte regulation. Unlike mammals, many amphibians rely on their skin for gas exchange, making them especially vulnerable to Bd’s invasive growth. As the fungus proliferates, it thickens the outer skin layers, impairing oxygen absorption and ion balance. Severe infections cause lethargy, loss of coordination, and, ultimately, cardiac failure.

Species with specialized ecological niches have been particularly hard-hit. High-altitude amphibians, such as the harlequin frogs (Atelopus spp.) of Central and South America, have suffered catastrophic losses, with some species disappearing entirely. These frogs, adapted to stable, cool environments, lack the physiological flexibility to cope with sudden disease pressure. Similarly, amphibians with limited geographic ranges, such as the Kihansi spray toad (Nectophrynoides asperginis), have struggled against Bd. Once thriving in the misty spray zones of Tanzania’s Kihansi Falls, the toad was declared extinct in the wild due to Bd but survives in captive populations.