Practice question

Edward Jenner utilized material from cowpox, a mild zoonotic orthopoxvirus infection naturally occurring in cattle, to immunize against smallpox. Cowpox virus causes vesicular lesions on teats and udders of cows, transmitted via hands of milkers producing localized pustules on hands, mild fever and regional lymphadenopathy but self-limited disease due to limited replication in human skin. Jenner reasoned that cross-reactive antigens shared within Orthopoxvirus genus provide cross protection, since orthopoxviruses possess conserved envelope proteins like A27, H3, D8 and B5. In May 1796 he collected exudate from cowpox lesion on dairymaid Sarah Nelmes whose infection traced to cow Blossom, inoculated via two superficial incisions into arm of eight-year-old James Phipps, producing mild local lesion and fever resolving. Six weeks later challenged with smallpox pus via variolation and observed no disease, repeating challenge months later. This heterologous immunization approach safer than variolation because cowpox rarely caused systemic disease. Observations derived from folklore among milkmaids stating they could not take smallpox after cowpox. Cowpox remains basis of vaccinia vaccine strain used today although modern vaccinia may be derived from horsepox lineage distinct.