Amplification of 'variola virus-specific' sequences in German cowpox virus isolates

In 1995 a polymerase chain reaction (PCR) protocol describing the specific detection of variola virus, the causative agent of smallpox, was published by Knight and others. Virulent variola major strains could be differentiated from less virulent variola minor strains because of the distinct amplicon sizes. Here, we applied this PCR protocol to DNA from various orthopoxvirus isolates. There was no amplification with the orthopoxvirus species vaccinia, monkeypox, mousepox, or camelpox viruses. However, amplification was observed in six out of 15 cowpox virus strains investigated. The size of the amplicons corresponded exactly with the size described for variola minor strains and the nucleotide sequence identity accounted for 97%. Findings are discussed with respect to the evolution of orthopoxvirus species assuming that variola virus most probably stems from a rodent-transmitted cowpox virus-like progenitor.