Laboratory investigation of two "whitepox" viruses and comparison with two variola strains from southern India.

Two variola-like viruses were isolated in Bilthoven in 1964 from monkey kidney tissue cultures. These viruses, coded 64/7255 and 64/7275, have been considered as two of the six "whitepox" viruses isolated from animal tissues, all of which are indistinguishable from variola virus by laboratory tests.Two specimens from suspect smallpox cases in India were examined in the Bilthoven laboratory at about the same time as the "whitepox" viruses and two strains of variola virus were isolated from them. A detailed comparison of certain biological markers of these four viruses, and of their DNAs, shows that the two "whitepox" viruses could not be distinguished from each other or from one of the two variola isolates. In view of this, and since there was a possibility of cross-contamination at the time of isolation, it is concluded that 64/7255 and 64/7275 must be regarded as genuine variola viruses and be deleted from the list of variola-like viruses isolated from animal tissues.