Only a few hundred cases of paralytic polio still exist worldwide, thanks to massive vaccination campaigns. But a new study concludes that so-called silent cases of polio can infect populations for years, threatening to reignite disease epidemics.
The World Health Organization designates a region polio-free if there have been no active infections for three or more years. However, people can be infected with the polio virus for years and not develop any symptoms. It's estimated that in only one out of every 400 cases does the virus cause paralysis.
Writing in the journal PLoS Biology, researchers from the University of Michigan say continued surveillance is necessary because these silent infections can spark a resurgence of active polio.
Micaela Martinez-Bakker led a six-year polio study in which she and her colleagues looked at data on the incidence of polio in the United States before introduction of the Salk vaccine in 1955.
Before widespread vaccination, Martinez-Bakker said, it was not uncommon for children around the world to have antibodies to polio by age 15, a sign that they had been exposed to the disease but hadn't become sick.
Today, Martinez-Bakker said, there are pockets of unvaccinated children living in Afghanistan and Pakistan, and in some African countries. Many of them could be asymptomatic carriers of the viral illness.
“Individuals who are infected, whether or not they show signs of the disease, still shed that virus through their feces," she said. "So, in places where you actually have sewage infrastructures, sanitation infrastructure [that are poor], that sewage can be checked on a regular basis for polio virus.”
Martinez-Bakker said the Global Polio Eradication Initiative and the WHO have been checking sewage samples for evidence of silent infections, but not in every country where these asymptomatic infections could trigger the recurrence of active polio cases.
“And even if the virus would go locally extinct in one part of the country ... the fact that it was maintained in other states would allow it to be reintroduced,” she said.
Using a mathematical model, Martinez-Bakker found that 3 million people in the United States were infected with asymptomatic polio at the height of the epidemic in 1952, despite the fact that there were only 57,000 cases of paralytic polio in the country.
She said extensive, global surveillance for asymptomatic polio should continue long after it looks as if the battle has been won.