Keeping an eye on polio

A new method of quantitative surveillance of poliovirus shed into sewage systems can help detect polio outbreaks early or verify its absence in a population, says a new study in Science Translational Medicine¹.

Polio causes paralysis, even death and is highly contagious. Although now endemic only in Afghanistan, Pakistan and Nigeria, “failure to eradicate polio from these last remaining strongholds could result in as many as 200,000 new cases every year, within 10 years, all over the world,” according to the World Health Organisation. 

The outbreak in Syria in 2013 showed that polio can be easily reintroduced into a previously polio-free setting. It’s why effective immunization and surveillance programs are always needed.

The WHO primarily depends on monitoring cases of acute flaccid paralysis. However, it only affects one in 100 infected individuals, which explains how outbreaks can spike silently until a tipping point is reached. 

Environmental surveillance, says the paper, which detects the presence of viruses in sewage, is “10³ to 10⁴ times more sensitive than acute flaccid paralysis surveillance.” 

The only downside? Measurement of zero viruses cannot be statistically interpreted. 

Now, Yakir Berchenko and colleagues developed an approach that allows the extrapolation of more precise estimates of the number of infected individuals, even when the virus is circulating at low levels. They invented a new algorithm to screen sewage for polio particles, using data from over 63,000 people during an outbreak in Israel in 2013. 

Although the work was conducted using data on oral polio vaccine strains, and not the more prolific wild-type virus, Berchenko stated that “the point estimates in the study could be used as a lower bound for the number of infected individuals.” This can potentially help track future reappearances, certify elimination, or validate the need for more containment efforts.

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