Long before lateral flow pregnancy tests became a routine purchase in pharmacies or supermarkets, confirming a pregnancy was a slow, uncertain, and often expensive process. During the early twentieth century, physicians relied on indirect signs and symptoms or laboratory tests that were both cumbersome and ethically problematic. One remarkable breakthrough came from an unlikely source: a fully aquatic frog from southern Africa. The African clawed frog (Xenopus laevis) became one of the most important laboratory animals of the twentieth century, transforming pregnancy diagnosis while simultaneously reshaping biological research. Yet this success carried unintended consequences. As millions of frogs were exported across the globe, they became vectors for one of the most devastating wildlife diseases ever recorded, contributing to declines of amphibian populations on almost every continent. The story of the Hogben test is therefore much more than a chapter in medical history. It is a tale of scientific ingenuity, colonial trade, globalisation, and unforeseen ecological disaster. It is one that I want to share with you all as it is so strange and bizarre, it can only be true.

For centuries, pregnancy diagnosis relied largely on observation. Physicians looked for the cessation of menstruation, enlargement of the abdomen, breast changes, or foetal movement, but these signs often appeared relatively late in pregnancy. Women frequently waited months before receiving confirmation, and false diagnoses were common. The discovery of hormones during the early twentieth century transformed reproductive medicine. Researchers realised that pregnant women produced a hormone now known as human chorionic gonadotropin (hCG), secreted by cells of the developing placenta shortly after implantation. If this hormone could be detected, pregnancy could be confirmed far earlier than ever before. The first biological pregnancy tests emerged during the 1920s. These involved injecting urine from women into immature mice or rabbits. If hCG was present, it stimulated changes in the animals’ reproductive organs that could be identified during post-mortem examination. The best known was the Aschheim-Zondek test, developed in Germany in 1927 using mice. Shortly afterwards, Maurice Friedman introduced a rabbit-based method that became widely adopted. Although revolutionary for their time, these tests had serious limitations. They were labour intensive, required several days to complete, and the animals had to be euthanised to examine their ovaries. Contrary to the enduring myth, rabbits did not die because the test was positive, they died because examination required dissection regardless of the result. Scientists therefore continued searching for a faster, cheaper, and more humane alternative.

The breakthrough came through the work of British zoologist and physiologist Lancelot Thomas Hogben (1895–1975), what a name! Hogben was an exceptionally gifted experimental biologist whose interests ranged from endocrinology and genetics, to statistics and philosophy of science. During the 1930s he worked in South Africa, where he became interested in the remarkable biology of the African clawed frog (Xenopus laevis). Unlike most frogs, Xenopus spends its entire life in water. They possess flattened bodies, powerful hind legs, and distinctive black claws on their rear feet that are used to tear apart food and stir up sediment. These frogs are remarkably hardy, tolerating a wide range of environmental conditions and surviving extended periods without food. Their resilience would later prove invaluable for laboratory use. Hogben’s research focused on hormones controlling reproduction. He discovered that injections of extracts from the anterior pituitary gland caused female Xenopus to ovulate rapidly, laying hundreds of eggs within hours. Crucially, the frogs survived the procedure and could be used repeatedly. Soon afterwards, researchers realised that hCG from pregnant women’s urine acted in much the same way as pituitary hormones. Injecting urine from pregnant women into sexually mature female Xenopus reliably induced egg laying within eight to twelve hours. If no eggs appeared, the test was negative. The implications of this discovery were enormous.

The procedure itself was elegantly simple. Female Xenopus were maintained in aquaria within hospital laboratories. A technician collected a urine sample from the patient and injected a small volume into the frog’s dorsal lymph sac. The frog was then returned to its tank and monitored overnight. If hCG was present, the frog typically released dozens to hundreds of eggs into the water. Their appearance provided a clear positive result that required no specialised equipment beyond maintaining healthy frogs. Unlike previous pregnancy tests, the animal remained alive and could be reused many times over several years. The speed and simplicity of the Hogben test revolutionised pregnancy diagnosis. Hospitals could obtain reliable results within a day rather than waiting several days for rodent-based methods. The test was also considerably less expensive, making it accessible to a wider range of medical institutions. By the late 1930s and throughout the 1940s, Xenopus colonies had been established across Europe, North America, Australia, and elsewhere. The frogs became standard equipment in pathology laboratories, with thousands being imported annually from South Africa.

Several biological characteristics made Xenopus laevis uniquely suited to laboratory life. Unlike many amphibians, Xenopus breeds readily in captivity, particularly when stimulated by hormones. Females produce hundreds or even thousands of eggs in a single spawning event, providing abundant material for developmental studies. The frogs are exceptionally robust. They tolerate handling well, require relatively simple aquatic housing, and possess long lifespans, often surviving for more than fifteen years in captivity. Individual females could therefore perform many pregnancy tests over their lifetime. Their embryos also develop externally and are transparent during early development, allowing researchers to observe cell division, tissue formation, and organ development without invasive procedures. These characteristics would eventually establish Xenopus as one of biology’s most important model organisms. Although the Hogben test brought Xenopus into laboratories, its scientific importance soon expanded far beyond pregnancy diagnosis. Developmental biologists quickly recognised the value of the frog’s large eggs. Fertilised embryos could be manipulated with remarkable ease, enabling researchers to study fertilisation, embryonic pattern formation, and the earliest stages of vertebrate development. Throughout the latter half of the twentieth century, Xenopus became indispensable in research on genetics, molecular biology, neuroscience, toxicology, regenerative medicine, and cell biology. Discoveries made using Xenopus contributed to our understanding of how genes regulate embryonic development, how cells divide, and how organs form. Perhaps most notably, studies using Xenopus eggs helped uncover fundamental mechanisms controlling the cell cycle. This work ultimately contributed to Nobel Prize-winning research and profoundly influenced modern cancer biology. Even after pregnancy testing moved on to newer technologies, Xenopus retained its place as one of the world’s premier laboratory vertebrates.

By the 1960s, advances in immunology transformed diagnostic medicine once again. Scientists developed immunoassays capable of detecting hCG directly using antibodies rather than living animals. These tests were faster, required less specialised care, and produced highly accurate results. Eventually, advances in biotechnology led to home pregnancy tests during the late 1970s and early 1980s. These immunological tests detect hCG within minutes using antibody-coated strips, eliminating the need for laboratory animals altogether. As hospitals adopted these new technologies, the Hogben test rapidly disappeared from routine clinical practice. Yet by then, hundreds of thousands (perhaps millions) of African clawed frogs had already been distributed worldwide. Their greatest impact was still to come.

For decades, Xenopus appeared to be an ideal export species. Few people suspected the frogs carried a microscopic fungus capable of infecting amphibians. This pathogen, now known as Batrachochytrium dendrobatidis (Bd), causes chytridiomycosis, a disease affecting the skin of amphibians. Amphibian skin is far more than a protective covering: it plays essential roles in respiration, water balance, and electrolyte regulation. Infection disrupts these physiological processes, eventually leading to cardiac arrest in susceptible species. Remarkably, Xenopus laevis generally shows little or no illness when infected. The fungus lives on the frogs’ skin without causing significant disease, allowing infected individuals to survive while shedding infectious spores into surrounding water. In modern terms, Xenopus functions as an asymptomatic reservoir host. During the mid-twentieth century, enormous numbers of African clawed frogs were shipped internationally for pregnancy testing, biomedical research, and eventually the pet trade. Many escaped captivity, while others were deliberately released after laboratory use. Today, feral populations of Xenopus laevis occur in numerous countries including France, Portugal, Italy, Chile, the United States, and Japan. Those that were present in the United Kingdom have since been eradicated. Some populations have persisted for decades, demonstrating the frog’s extraordinary adaptability. Although several pathways probably contributed to the worldwide spread of chytrid fungus (including the international trade in North American bullfrogs) the global movement of Xenopus is widely regarded as one of the earliest and most significant mechanisms by which Bd reached new continents. As infected frogs often appeared completely healthy, there was little reason for exporters or researchers to suspect they carried a dangerous pathogen.

The emergence of chytridiomycosis represents one of the greatest disease-driven biodiversity crises ever documented. Since the late twentieth century, the disease has been implicated in the decline of more than 500 amphibian species, with dozens believed to have become extinct (current estimates put this number at around 90 species). Entire frog communities have disappeared within only a few breeding seasons after the fungus arrived. Mountain streams once filled with colourful harlequin frogs, torrent frogs, and other specialised amphibians fell silent across Central and South America. Similar declines occurred in Australia, Europe, and elsewhere. Not every species is equally susceptible. Some frogs coexist with the fungus, while others succumb rapidly. Environmental conditions, temperature, immune responses, and microbial communities living on amphibian skin all influence disease severity. Nevertheless, chytridiomycosis has become one of the clearest demonstrations of how global trade can unintentionally transport emerging infectious diseases across natural barriers. The disappearance of amphibians has effects far beyond the loss of individual species. Frogs occupy central positions within food webs, consuming enormous numbers of insects while serving as prey for birds, mammals, reptiles, and fish. Tadpoles graze algae, recycle nutrients, and influence freshwater ecosystem dynamics. When amphibians decline, insect populations may increase, aquatic nutrient cycling changes, and predators lose important food resources. In some tropical forests, amphibian losses have altered decomposition rates and stream ecology in measurable ways. The chytrid pandemic therefore illustrates how disease affecting a single animal group can cascade through entire ecosystems.

The history of the Hogben test illustrates both the extraordinary power and the unforeseen risks of scientific innovation. On one hand, Xenopus laevis transformed medicine. The frog provided millions of women with reliable pregnancy diagnoses at a time when no comparable alternatives existed. It also became one of the most influential model organisms in modern biology, contributing to countless discoveries in developmental biology, physiology, genetics, and medicine. On the other hand, the international movement of living animals occurred decades before modern biosecurity protocols. Scientists had no knowledge of chytrid fungi or their potential to devastate amphibian populations. What seemed like a harmless laboratory animal became part of a global pathway for an emerging wildlife disease whose consequences continue to unfold. Today, researchers transporting amphibians follow strict quarantine procedures, pathogen screening, and international regulations designed to minimise disease transmission. The lessons learned from Xenopus have influenced biosecurity practices across zoological collections, research institutions, and wildlife conservation programmes worldwide.

Few animals have influenced both medicine and conservation as profoundly as the African clawed frog. Through the Hogben test, Xenopus laevis revolutionised pregnancy diagnosis, advancing reproductive healthcare and helping establish modern endocrinology. As a research organism, it continues to underpin discoveries that improve our understanding of vertebrate biology and human disease. Yet its story also serves as a cautionary tale. Scientific advances rarely occur in isolation. Every movement of organisms across the globe carries ecological risks that may not become apparent for decades. The worldwide spread of chytrid fungus reminds us that even the most beneficial scientific innovations can have unintended environmental consequences when biosecurity is overlooked. The legacy of the Hogben test is therefore one of remarkable achievement tempered by humility. It celebrates human curiosity and ingenuity while highlighting our responsibility to consider the broader ecological consequences of scientific progress. In the history of medicine, few laboratory animals have left such a lasting imprint, not only on human health, but on the fate of wildlife across the planet.

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