GENETIC-DIVERGENCE OF POLIOVIRUS STRAINS ISOLATED IN THE KARACHI REGION OF PAKISTAN

Seventy-seven wild poliovirus strains isolated from poliomyelitis case s in the Civil Hospital of Karachi in Pakistan in 1989-1993 were selec ted for partial sequence analysis covering the VP1/2A junction region of the viral genome to study the genetic relationships and epidemiolog ical links between strains. Viral RNA was partially amplified by RT-PC R and sequenced by a solid phase method. Computer analysis revealed ge netic divergence of the strains within each serotype. Most of the nucl eotide differences between strains were silent: only a few specific am ino acid substitutions were seen in the sequenced region. Three genoty pes of poliovirus type 1 and two of poliovirus type 3 were co-circulat ing, while type 2 strains were represented by a single genotype. Repre sentatives of all the genotypes present have been found among previous ly or concurrently characterized strains isolated elsewhere, but direc t epidemiological links were found only in the case of serotype 1. Man y of the epidemics caused by poliovirus type 1 in other countries were genetically linked to Pakistan. This study clearly shows the endemic circulation and wide variation of all three poliovirus serotypes in so uthern Pakistan and indicates the need for more effective vaccination programmes to prevent the further spread of these wild viruses.