Can Environmental Surveillance Data Estimate Polio Infection?

The fight to end polio has been challenging, with success as evasive as the disease. The virus is highly transmissible between people through the fecal-oral route and frequently takes advantage of contaminated water and poor sanitation. In the fight to end the spread of polio, public health efforts have struggled against not only vaccine-derived polio, but also the surveillance of such a disease.

Vaccination practices against polio utilize two methods—oral polio vaccine (OPV) and inactivated poliovirus vaccine (IPV). OPV is more common in less-developed countries where vaccination efforts are strained, but it also comes with a risk. Since OPV contains a weakened virus, it triggers an immune response to build up antibodies; however, during this phase, the individual excretes the virus. This makes transmission possible in an area with poor sanitation.

OPV poses this risk for non-immune patients and any potential contacts that lack immunity. Poor immunization levels within a community allow the excreted vaccine-virus to spread, even changing genetically, providing the potential to cause paralysis. And so, although OPV is an effective vaccination strategy, it does come with a circulating vaccine-derived poliovirus (cVDPV) if poor sanitation allows for the excreted virus to come into contact with people. According to the World Health Organization (WHO), “on rare occasions, if a population is seriously under-immunized, an excreted vaccine-virus can continue to circulate for an extended period of time. The longer it is allowed to survive, the more genetic changes it undergoes. In very rare instances, the vaccine-virus can genetically change into a form that can paralyze—this is what is known as a cVDPV.”

The recent Centers for Disease Control and Prevention’s (CDC) Morbidity and Mortality Weekly Report, notes that “the primary means of detecting polio virus transmission is surveillance for acute flaccid paralysis (AFP)” which is supplemented by environmental surveillance (ES). The second strategy utilizes sewage or wastewater samples to detect poliovirus excreted by infected individuals.

ES is a newer tool in the public health toolbox to fight disease. Traditional surveillance efforts rely on hospital and laboratory reporting, field work, etc. In the absence of AFP cases, environmental sampling is a strong method, which has increased in use in Afghanistan, Nigeria, and Pakistan. Taking environmental samples allows investigators to test for the presence (or absence) of a disease, particularly those that are transmitted due to poor sanitation and water quality.

Unlike passive surveillance, which relies on a lab or healthcare provider notification, ES can let investigators know immediately if the disease is present in the sewage system or environment. ES is even used in healthcare settings to see if hospital room disinfection is effective or if there is environmental contamination with methicillin-resistant Staphylococcus aureus (MRSA) or Clostridium difficile (C. diff).

Now, researchers are looking at ES as an under-utilized, but potentially valuable tool in polio prevalence analysis and eradication efforts. Using data from a 2013 outbreak in Israel, they looked at ES data and immunization rates utilizing the live vaccine (OPV). The analysis looked at seven cities and considered, daily sewage production, population size, and vaccination levels in order to perform a mixed-effects linear regression analysis and quantitatively address the potential sensitivity of ES and confidence levels of poliovirus circulation following an outbreak.

Traditionally, ES data has been difficult to find, which required public health investigators to model and use simulations to determine its sensitivity parameters. By using actual ES data, the researchers conducting this study uncovered several interesting findings during their work.

The environmental presence of viral particles tends to follow vaccination by around 7-14 days. Although, ES calculations can be crude, they aid in a more “formal” eradication verification method. Perhaps one of the most helpful components to ES is that it can more quickly detect the presence of poliovirus, whereas surveillance that relies on AFP diagnosis can be delayed. Global polio eradication efforts utilized by WHO do not distinguish detection by transmission between ES or AFP surveillance, despite the drastic difference in sensitivity between the two. “For example, at vaccine coverage rates higher than 90% (in most countries that administer IPV, the coverage is very high), we find that ES is 10³ to 10⁴ more sensitive than AFP surveillance, given a monthly sampling frequency,” the authors stated in their study. Based off the success of ES sensitivity, the Global Polio Eradication Initiative has made it a crucial component.

ES is not without challenges, though. It is vital to know the population, daily sewage production volume, etc. Determining the estimation of shedders based off the number of particles detected and the number of people meant that the researchers had to switch the independent and dependent variables in their study. Their analysis incorporated the log of the number of particles detected, log of the number of people vaccinated before, etc.

Without going too heavily into the data components of the study, it should be noted that the investigators did try to use ES data to provide some kind of estimate of infection prevalence, finding that there is indeed a dose-dependent relationship between the number of shedders and the mount of poliovirus found in the sewage. Through this analysis, the researchers were able to develop quantitative tools that highlighted the volume of infected persons, sensitivity of ES, and a refined tool for determining the end of a poliovirus outbreak. While no effort is without flaws, this points to ES as a strong tool in the fight against polio and efforts to eradicate this historically horrific virus. Environmental sampling provides a strong supplement to AFP surveillance and can help improve detection of the circulating virus, especially in areas where AFP surveillance may be poor.