Frogs and toads and pregnancy tests

It has been suggested that the worldwide decline in amphibian numbers, most recently highlighted in the plight of the rare midwife toad (Alytes obstetricans) of the Pyrenees, could be a legacy of pregnancy testing techniques employed decades ago.

It was the discovery of human chorionic gonadotropin (hCG) in samples of urine from pregnant women in 1927, and the finding in 1930 that it was produced by the trophoblast cells of the fertilised ovum, that allowed the development of accurate scientific pregnancy testing.

An early test, called the Aschheim-Zondek test after its inventors, involved injecting urine samples into immature female rabbits, when positive samples (containing hCG) were found to stimulate ovulation in the rabbits. A drawback of the test was the need to kill and dissect the animal to determine the result.

Lancelot Hogben further developed the test, using frogs instead of rabbits. The advantage was that ovulation could be detected without having to kill the animal, which could be reused. The test was still in use in the 1950s, and led to the export from southern Africa of thousands of African clawed frogs, Xenopus laevis,  for use in Europe, Australia and North America.

It has now been suggested that the amphibian disease chytridiomycosis could be attributed to these early pregnancy testing techniques. The disease has been identified as a major cause of the decline in amphibian populations. Up to 30 per cent of species worldwide are under threat of extinction.

Specimens of Xenopus laevis examined in museums in Africa were found to contain the fungus responsible for the disease, Batrachochytrium dendrobatidis, in samples dating back to 1938.

However, the incidence of the disease has not altered since that time in Africa, which suggests that the local frog population had developed a resistance to the disease, but that when they were transported abroad, amphibians with no previous exposure were vulnerable to infection, resulting in the disease and death.