Isolation of poxvirus from an African Rodent

A poxvirus was isolated from a wild gerbil (Tatera kempii) caught in northern Dahomey, Africa at the time of an epidemic of human smallpox. Electron microscopic appearance and serologic reactions placed it in the vaccinia subgroup of poxviruses. The isolate differed from ectromelia, rabbitpox, vaccinia, monkeypox, and cowpox viruses in pock morphology on chorioallantoic membrane, ceiling temperature, relative innocuity for mice, and cytopathic effect in tissue culture. Like variola minor virus, it had a ceiling temperature of 38 C, produced small hypertrophic foci in tissue culture, and failed to grow in rabbit skin. Inoculated into a rhesus monkey, it caused fever but no skin eruption and produced seroconversion and protection from subsequent challenge with monkeypox virus. The growing list of animal viruses that differ only slightly from smallpox virus suggests the hypothesis that long-term survival of variola virus may be based on inapparent infection in animals as well as virulent spread among humans.