Molecular and Antigenic Characterization of a Highly Evolved Derivative of the Type 2 Oral Poliovaccine Strain Isolated from Sewage in Israel

Rapid evolution is characteristic of both wild and vaccine-derived polioviruses (4, 5, 11, 12, 15; A. Heim, A. Bellmunt, G. May, P. Pring-Akerblom, and W. Verhagen, Abstr. Eur. Soc. Clin. Virol. Prog. Clin. Virol. IV, abstr. 350, p. 72, 1998). For wild polioviruses, nucleotide substitutions accumulate at a rate of approximately 1% per year and consist primarily of changes at synonymous codon positions (10; L. De, J. Jorba, J. Boshell, R. Salas, and O. Kew, Abstr. 17th Annu. Meet. Am. Soc. Virol., abstr. W36-3, p. 123, 1998), i.e., do not result in amino acid changes at those loci. In contrast, the mutations initially appearing and fixed into the genomes of the Sabin vaccine strains upon administration of oral poliovaccine (OPV) are frequently associated with reversion of the attenuated phenotype and alteration of the neutralizing antigenic (NAg) sites of the OPV strains (1, 12). Reversion of the OPV strains to increased neurovirulence is one key factor for the occurrence of cases of vaccine-associated paralytic poliomyelitis (VAPP), which occur at a rate of ∼1 per 500,000 first doses of OPV in immunocompetent individuals (23) and at a ∼1,000-fold-higher rate for immunodeficient patients (24). Poliovirus replication is restricted to about 2 months in immunocompetent persons (1, 2) but may be prolonged up to 10 years in patients with deficiencies in antibody production (11). Because poliovirus genomes evolve rapidly, the duration of replication of an OPV-derived virus may be estimated by the extent of its nucleotide sequence divergence from its respective prototype OPV strain (11; De et al., Abstr. 17th Annu. Meet. Am. Soc. Virol.). Routine environmental surveillance of wastewater for polioviruses was initiated in Israel and the Palestinian Authority following the last poliomyelitis outbreak in 1988 (14, 22). This approach has proven to be a powerful method to detect wild poliovirus circulation in communities where no poliomyelitis cases have been reported (14). Concentrated environmental samples are cultured using a double-selective cultivation technique (13) that selects against the growth of most OPV-derived strains by use of a supraoptimal temperature of incubation (40°C). Polioviruses that grow at 40°C are said to be positive for the RCT (reproductive capacity at supraoptimal temperature) marker. A small number (3 to 25) of RCT-positive isolates are recovered each year from the environment. Most of these isolates have been otherwise typical OPV-derived viruses that have lost their temperature-sensitive phenotypes; some have been imported wild polioviruses (14; L. M. Shulman, Y. Manor, R. Handsher, A. Vonsover, O. M. Kew, E. Mendelson, et al., Abstr. Xth Int. Cong. Virol., abstr. W54-6, p. 79, 1996); and one, PV2/4568-1/ISR98, isolated in 1998, proved to be a highly divergent, neurovirulent derivative of the Sabin type 2 OPV strain. In this report, we describe the molecular, antigenic, and neurovirulence properties of this unusual isolate. We estimate from the high degree of nucleotide divergence of 4568-1 from Sabin 2 that the initiating OPV dose was given approximately 6 years before recovery of the virus from the environment. The observations cannot distinguish between chronic infection of a single individual and person-to-person transmission of an OPV-derived poliovirus. In either event, the detection in the environment of a highly evolved derivative of Sabin 2 has important implications for development of the optimal strategy for cessation of immunization with OPV following the eradication of all wild poliovirus transmission (4).