Possible selection of virulent poliovirus strains in Third-World countries.

In Kenya for more than twenty years a remarkable periodicity has been observed in the circulation of type-1 poliovirus and in epidemics of poliomyelitis caused by this virus type. Periodicity was less obvious for poliovirus types 2 and 3. Cases of poliomyelitis increased, and this trend was only partly controlled by vaccination. It is postulated that virulent strains of poliovirus became dominant because of selection pressure. It is suggested that this selection pressure occurred earlier in economically developed countries and was partly responsible for the pronounced changes in the epidemiology of poliomyelitis seen in such countries.