GENETIC-ANALYSIS OF WILD-TYPE POLIOVIRUS IMPORTATION INTO THE NETHERLANDS (1979-1995)

Wild polioviruses were isolated a number of times in The Netherlands o utside the epidemic periods (1978 and 1992-1993) from patients infecte d abroad, from subclinically infected persons, and from river water. S equence comparisons revealed discrete sources of importation: the Medi terranean, India, and Indonesia. The observed wide genetic variation i s 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 time s before the epidemic. Of particular importance are two documented iso lations of prototype wild poliovirus indistinguishable from that used to produce the inactivated vaccine. These data underscore the continue d risk to the unvaccinated religious population of exposure to wild po liovirus.