LETTERS Adventitious Viruses and Smallpox Vaccine

and Osburn (1) strongly argued for testing old smallpox vaccine stocks made in animal skin for adventitious infectious agents such as viruses, mycoplasmas, and eventually, prions. Their argument appears clearly justi-fied after unexpected cases of myopericarditis occurred during recent campaigns of smallpox vacci-nations in the United States (2). To the long list of bovine viruses cited in this paper, it seems necessary to add another, the pseudocowpox virus, a widespread parapoxvirus that may infect humans. During the 1960s, this virus was identified in vaccine lymph from a heifer at the Institut Pasteur, Paris (3). In humans, this virus is responsible for limited skin lesions, more fre-quently in immunocompromised patients. Mainly farmers and butchers are affected. Pseudocowpox virus is easily differentiated from orthopox-viruses such as vaccinia virus by the virus’s peculiar form on transmission electron microscopy scan, but poly-merase chain reaction is probably the best detection method (4). In fact, many other more hazardous viruses may be found in the oldest stocks of smallpox vaccine and deserve more attention than previously considered