Poliovirus vaccine strains in sewage and river water in South Africa

Since the initiation of the global poliomyelitis eradication program in 1988, the number of wild-type polio cases decreased from 350 000 to fewer than 500, and the number of polio endemic countries declined from more than 125 to 10. The last case of polio in South Africa caused by a wild-type poliovirus (PV) occurred in 1989. The live attenuated oral poliovirus vaccine (OPV) has been effectively used in the reduction and control of poliomyelitis. However, as OPV strains are excreted in stools after vaccination, this vaccine could become a source of dissemination of PVs in the environment and the potential cause of poliomyelitis. Therefore, the aim of the study was to determine the occurrence of OPV strains in selected sewage and river water samples. During the period between 2001 and 2003, 138 samples of river water and 213 samples of settled sewage were collected from selected areas of South Africa. A total of 860 plaques were analysed, which consisted of 703 plaques from the sewage and 157 plaques from the river water samples. Using a reverse transcriptase (RT)-multiplex PCR, 49 PVs were successfully distinguished from 176 non-polio enteroviruses (NPEVs). The 176 NPEVs consisted of 50 coxsackie B2 viruses (CBV2), followed by 39 echoviruses 11 (ECV11), 25 CBV5, 21 CBV3, 15 CBV4, 14 coxsackie A6 viruses (CAV6), 7 CBV6, 2 CAV5, 2 CBV1, and 1 ECV19, which was in agreement with the prevalence of these EVs in other parts of the world. The Sabin-specific RT-triplex PCR revealed the presence of 29 Sabin PV type 1, 8 Sabin PV type 2, and 12 Sabin PV type 3 isolates. Buffalo green monkey kidney and primary liver carcinoma cell cultures allowed the amplification of a broad spectrum of EVs, whereas human epidermoid carcinoma cells were more selective for PVs. This study addressed some of the issues regarding the prevalence of OPV strains in the environment. The identification of 49 viable OPV isolates confirmed the presence and circulation of PV vaccine strains in sewage and river water. The extent of the potential health risk constituted by these OPV isolates remains to be investigated.Key words: poliovirus vaccine strains, enteroviruses, circulation, sewage, river water, molecular techniques.