[Epidemiological surveillance for viral pollution of rivers in Toyama Prefecture]

Virus pollution of three rivers in Toyama Prefecture was surveyed over a long period in order to predict and to prevent water-born infection.

Water samples were collected from three rivers (Itachi, Sembo, Oyabe), then concentrated and inoculated into cultured cells to isolate viruses. The survey at Itachi River was carried out 4 times: in 1979-1981, 1983-1985, 1993-1995, and 2002-2003. The surveys at Sembo and Oyabe Rivers were carried out twice (together with the 3rd and 4th surveys of Itachi River).

1) Various species of enteric viruses, i.e., poliovirus, human enteroviruses B (HEV-B), and reovirus were isolated from Itachi River. Since polioviruses were isolated at the same time as the oral vaccination of babies, these isolates appeared to be derived from vaccine strains. Consistently, isolates in the 3rd and the 4th surveys had only 0-2 base differences in the 480 or 474 nucleotide sequences of the VP3-VP1 region, compared with vaccine strains. The poliovirus detection rates, defined as the ratios of times viruses were detected to the total investigation times, were 33.3%, 41.7%, 2.1% and 0% for the Itachi River from the 1st to 4th surveys, respectively. The lowering between the 2nd and the 3rd surveys was significant (P< 0.001), this being associated with the widespread introduction of paper diapers for babies. The types of HEV-B were various and coincided well with those of prevalent clinical isolates. Reoviruses were frequently detected throughout the year in the 1st and the 2nd surveys, but fell in spring and summer in the 3rd and 4th surveys. 2) The types and the rates of viruses detected from Sembo and Oyabe Rivers were similar to those from Itachi River.

The detection rates of poliovirus and reovirus in the river water during 2002 to 2003 were lower than during 1979 to 1981. This may be due to the improvement of sewerage system or sewerage and the increase in use of paper diapers for babies. The reason that the detection rate of HEV-B did not similarly decrease, and the fact that various types of HEV-B were isolated every year seemed to reflect the epidemic status of HEV-B among inhabitants. Thus, while virus pollution of river water has generally decreased, there is still a possibility of outbreak of water-born infections, since rivers continue to be contaminated with various species of viruses.