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Polio persists 3 years after last case
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Poliovirus transmission could continue for 3 years or more after the last reported case, suggesting that public health officials should remain vigilant in surveillance of the disease, according to research published in PLoS Biology.
“Using transmission models, we show that you can have sustained chains of silent transmission in populations for more than 3 years, without a single person ever showing up as a reported polio case,” Micaela Martinez-Bakker, a doctoral candidate in the University of Michigan department of ecology and evolutionary biology, said in a press release. “Once we’ve eradicated polio — or think we’ve eradicated polio — we should intensify environmental surveillance to make sure the virus is not lurking under the hood at very low levels. Polio eradication is about eradicating the virus. It’s not about eradicating the disease paralytic polio.”
Martinez-Bakker and colleagues examined polio case reports from the pre-polio vaccine era (January 1931 to December 1954) compiled by the CDC for the 48 contiguous states and Washington, D.C. State births and population sizes also were analyzed for use in a disease transmission model.
Macaela Martinez-Bakker
“Reaching eradication and preventing re-emergence of polio requires intimate knowledge of how the virus persists,” Martinez-Bakker said in the release. “Historical epidemics that predate the use of vaccines can be used to disentangle the epidemiology of disease from vaccine effects. They allow us to establish a baseline by studying the system in the absence of intervention.”
The analysis led Martinez-Bakker and colleagues to determine that a long-held theory — that better sanitation and hygiene were behind the polio surges of the 1930s to 1950s — is likely inaccurate. Instead, the researchers suggested that the increase in polio during the mid-1940s and later was most likely associated with a postwar surge in births that increased the susceptible pool for transmission, according to the release.
“If you have more kindling, you can have a much larger forest fire,” Martinez-Bakker said. “The baby boom provided more kindling for polio epidemics — young children and infants over 6 months of age — so much more explosive outbreaks were now possible.”
The researchers suggested that in the peak year of 1952 more than 3 million Americans were likely infected with poliovirus, which may serve to further illustrate the need to eradicate the virus as well as the disease.
“Detailed data on clinical symptoms are extremely useful when modeling pathogen transmission,” Martinez-Bakker told Infectious Disease News. Polio data from the US were informative because they included monthly reports from clinicians of both paralytic polio cases and non-paralytic cases. The inclusion of the more mild non-paralytic cases allowed us to have more data to work with while reconstructing epidemics and studying transmission. Individuals with mild symptoms, as well as asymptomatic individuals, are key because they contribute to transmission and help maintain the pathogen in the population.”
Jonas Salk’s polio vaccine was introduced in 1955, and the disease is currently endemic only in Afghanistan, Nigeria and Pakistan. – by David Jwanier
Disclosure: The researchers report no relevant financial disclosures.
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