Characterisation of vaccine-derived polioviruses isolated from sewage and river water in Japan

Background A nucleotide change from U to C at position 472 in the 5' non-co ding region of the type 3 poliovirus is associated with increased neuroviru lence. Moreover, the proportion of type 3 polioviruses containing this muta tion (472-C revertants) correlates with the neurovirulence of a particular sample. We used mutant analysis by PCR and restriction-enzyme cleavage (MAP REC) to estimate the neurovirulence of environmental samples obtained from Toyama prefecture, Japan. Methods Sewage and river water were collected between October, 1993, and Se ptember, 1995, and concentrated samples were inoculated into three differen t cell types. Isolated type 3 viruses were analysed to determine whether th ey were derived from the live oral poliovirus vaccine strain; they were the n tested for neurovirulence by MAPREC. Results 29 type 3 strains were isolated-all of which were vaccine-derived. 16 (55%) comprised between 2% and 91% 472-C revertants by MAPREC and were e xpected to have high neurovirulence. The remaining strains included less th an 0.25% revertants, and were regarded as attenuated viruses. Both types we re isolated about 3 months after routine oral poliovirus vaccine administra tions in May and October. Three strains isolated from river water were of t he virulent type. Interpretation Our results emphasise that there is an environmental risk of vaccine-associated paralytic poliomyelitis as long as live oral poliovirus vaccine is not replaced by inactivated polio vaccine.