RAPID MOLECULAR EVOLUTION OF WILD TYPE-3 POLIOVIRUS DURING INFECTION IN INDIVIDUAL HOSTS

A panel of nine neutralizing monoclonal antibodies was used to analyse the antigenic properties of 188 plaque-purified type 3 poliovirus strains from 17 faecal specimens, derived from eight people during a 2 month observation period. Most poliovirus specimens consisted of a mixture of antigenically distinct variants and the composition of the mixture was found to change between sequential specimens in many individuals, indicating antigenic evolution. Thirty-five strains representing different antigenic patterns were selected for partial sequencing of genomic RNA. Mutations leading to amino acid substitutions, as well as silent mutations, were seen at and close to the known antigenic sites. The frequency of silent mutations was used to estimate the evolutionary potential of the virus. The largest difference in silent changes between strains isolated from one person was 0.8, which corresponds to a minimum of about 60 mutations per genome within a period of 3 weeks. The observed incidence of silent mutations between isolates from different persons was usually between 0.8% and 2%. These figures agree with the previously reported overall mutation rates of poliovirus, determined by other methods.