Both variola and vaccinia viruses grow in the chick embryos. They produce pocks on chorioallantoic membrane (CAM) of 11–13 days’ old chick embryo within 48–72 hours.
Variola virus can be grown in chick embryo cells, monkey kidney, and HeLa cells. Unlike the vaccinia virus, which produces cytopathic effects (CPEs) rapidly within 24–48 hours, the variola virus produces CPEs very slowly. The CPEs consist of Guarnieri bodies, which are eosinophilic inclusion bodies that consist of aggregation of virus parti-cles in matrix. These can be demonstrated in stained preparations.
Variola virus causes experimental infection only in monkeys. Intranasal infection of monkeys by variola virus causes smallpox in monkeys with generalized skin lesions. Scarification of cornea of rabbit with variola virus causes keratitis with demonstration of characteristic Guarnieri bodies in the stained smears of cornea.