Resolution of the pathways of poliovirus type 1 transmission during an outbreak

An outbreak of poliomyelitis with 20 eases occurred in Israel, Gaza, and th e West Bank from October 1987 to October 1988. The wild type 1 poliovirus a ssociated with the outbreak was most closely related to viruses found in th e Nile Delta. The epidemiologic links among patients involved in the outbre ak and patients with community-acquired Infections during the outbreak were inferred from the evolutionary relationships among isolates of the outbrea k virus. Complete VP1 sequences (906 nucleotides) were determined for 12 cl inical and 4 sewage isolates. A total of 58 nucleotide differences were fou nd among the 16 isolates; 74% of all substitutions were synonymous third-po sition transitions. An evolutionary tree, representing both the pathways of VP1 sequence evolution and the inferred chains of virus transmission durin g the outbreak, was constructed under the assumption that each substitution had occurred only once. The combined epidemiologic and molecular data sugg est that a single founder strain was introduced into Israel from the vicini ty of Gaza in the fall of 1987. Poliovirus circulation was apparently local ized to southern communities during the winter and spread north by the foll owing summer into the Hadera subdistrict of Israel, where it radiated via m ultiple chains of transmission into other communities in northern Israel an d the West Bank The close sequence matches (>99%) between clinical and sewa ge isolates from the same communities confirm the utility of environmental sampling as a tool for monitoring wild poliovirus circulation.