INFERRING PREVALENCE OF POLIOVIRUS INFECTIONS FROM QUANTITATIVE RECOVERY OF POLIOVIRUS RNA FROM SEWAGE

Polioviruses infections can cause irreversible acute flaccid paralysis (AFP) and/or death in < 1% of naïve infected individuals, while >90% are asymptomatic. The Global Polio Eradication Initiative (GPEI), started in 1988 has succeeding in eradicating wild type 2 poliovirus and greatly reduced morbidity caused by serotypes 1 and 3. When GPEI enters its final stage and there are no more AFP cases worldwide, it is very important to demonstrate that there is no silent circulation of poliovirus before considering stopping polio vaccination. The recent (2013) silent outbreak of type 1 wild poliovirus in Israel demonstrated the importance of active environmental surveillance.

In order to efficiently allocate resources during outbreaks, estimation of infected individuals in the affected areas is vital. The current method, stool survey, is very laborious and time consuming to perform. By the time the results become available (weeks to months) they are not relevant for planning and evaluating intervention.

We built and calibrated an algorithm inferring the number of poliovirus excreting individuals based on quantitative RT-RTPCR analysis of poliovirus RNA extracted directly from sewage, conversion of molecular results to plaque forming units (PFUs) divided by the PFU per person per 24 hrs (after averaging actual PFUs in one gram from poliovirus-positive stool samples) and including a measured population size-dependent dilution factor from quantitative recovery of poliovirus virus at sewage system sites downstream from sites at which polio vaccine strains were introduced. Recovery of Sabin vaccine strains from known numbers of vaccinated individuals validated the algorithm. Our innovative algorithm provides accurate quantitative results in 3 to 5 days, enables intervention planning, and measurement of intervention efficacy in real time during poliovirus outbreaks.