Among frogs, the oldest documented occurrence of "Batrachochytrium" is from a specimen of a Titicaca water frog collected in 1863, and among salamanders the oldest was a Japanese giant salamander collected in 1902. However, both these involved strains of the fungus that have not been implicated in mass-mortality events. A later instance of a Bd-infected amphibian was a specimen of an African clawed frog ("Xenopus laevis") collected in 1938, and this species also appears to be essentially unaffected by the disease, making it a suitable vector. The first well-documented method of human pregnancy testing involved this species, and as a result, large-scale international trade in living African clawed frogs began more than 60 years ago. If "Batrachochytrium" originated in Africa, the African clawed frog is thought to have been the vector of the initial spread out of the continent. The earliest documented case of the disease chytridiomycosis was an American bullfrog ("Rana catesbeiana") collected in 1978.