Research Laboratory for Infectious Diseases (World Health Organization Collaborating Center for Reference and Research on Poliomyelitis), National Institute of Public Health and the Environment (RIVM), Bilthoven, The Netherlands.
Wild polioviruses were isolated a number of times in The Netherlands outside the epidemic periods (1978 and 1992-1993) from patients infected abroad, from subclinically infected persons, and from river water. Sequence comparisons revealed discrete sources of importation: the Mediterranean, India, and Indonesia. The observed wide genetic variation is indicative of repeated importation and not of indigenous circulation. Isolates identical or closely related to the epidemic type 1 strain of 1978 were found in clinical and environmental specimens until 1983, probably due to repeated importation from Turkey. Viruses related to the 1992-1993 epidemic type 3 virus had already been isolated six times before the epidemic. Of particular importance are two documented isolations of prototype wild poliovirus indistinguishable from that used to produce the inactivated vaccine. These data underscore the continued risk to the unvaccinated religious population of exposure to wild poliovirus.