Detection of poliovirus circulation by environmental surveillance in the absence of clinical cases in Israel and the Palestinian Authority

The global eradication of poliomyelitis, believed to be achievable around t he year 2000, relies on strategies which include high routine immunization coverage and mass vaccination campaigns, along with continuous monitoring o f wild-type virus circulation by using the laboratory-based acute flaccid p aralysis (AFP) surveillance. Israel and the Palestinian Authority are locat ed in a geographical region in which poliovirus is still endemic but have b een free of poliomyelitis since 1988 as a result of intensive immunization programs and mass vaccination campaigns. To monitor the wild-type virus cir culation, environmental surveillance of sewage samples collected monthly fr om 25 to 30 sites across the country was implemented in 1989 and AFP survei llance began in 1994. The sewage samples were processed in tie laboratory w ith a double-selective tissue culture system, which enabled economical proc essing of large number of samples. Between 1989 and 1997, 2,294 samples wer e processed, and wild-type poliovirus was isolated from 17 of them in four clusters, termed "silent outbreaks," in September 1990 (type 3), between Ma y and September 1991 (type 1), between October 1994 and June 1995 (type 1), and in December 1996 (type 1). Fifteen of the 17 positive samples were col lected in the Gaza Strip, 1 was collected in the West Bank, and 1 was colle cted in the Israeli city of Ashdod, located close to the Gaza Strip. The AF P surveillance system failed to detect the circulating wild-type viruses. T hese findings further emphasize the important role that environmental surve illance ran play in monitoring the eradication of polioviruses.