Evaluation of virological laboratory methods for smallpox diagnosis.

Between July 1966 and May 1972 the Vesicular Disease Laboratory, Center for Disease Control, Atlanta, Ga., USA, tested specimens from 849 suspected smallpox cases by at least 2 methods, electron microscopy and chick embryo chorioallantoic membrane (CAM) cultures. A smaller number of specimens was tested by each of 4 methods: electron microscopy, CAM culture, agar gel precipitation, and tissue culture. For specimens handled in the field the CAM culture method was less sensitive than electron microscopy because the adverse conditions often inactivated the virus. CAM cultures were valuable for identifying members of the poxvirus subgroups, however, particularly when supplemented by tissue culture. The agar gel precipitation test was the least sensitive but was of value in confirming the results of electron microscopy. The latter was highly effective for the diagnosis of varicella, but dependably identified only about half of the vaccinia infections; for vaccinia, the CAM technique was essential. The occurrence of human monkeypox cases in West Africa emphasized that the usual smallpox diagnostic methods were inadequate. More sophisticated tests, such as the rabbit dermal sensitivity test, are necessary for accurate diagnosis of these cases as monkeypox.