First U.S. polio case in nearly a decade.
Last month, an unvaccinated 20-year-old in Rockland County, New York, was diagnosed with paralytic polio. He had not traveled in the previous month; he was infected by an (unknown) individual who had travelled and acquired the infection abroad.
According to the CDC, poliovirus was also found in wastewater from Rockland and a neighboring county—coincidentally sampled in May, June, and July as part of SARS-CoV-2 surveillance—and some of the positive samples were genetically linked to virus from the patient. This reflects ongoing community transmission and ongoing risk to unvaccinated residents. Because most poliovirus infections are asymptomatic, the virus spreads “under the radar” until it causes another life-changing infection.
A single case of paralytic polio in the U.S. is a public health emergency. It is estimated that there could be 100 to more than 1,000 infections for every paralytic case diagnosed.
Polio infection is most common in infants and young children, and (in temperate climates) in summer and fall months. The virus is transmitted primarily via the fecal–oral route, entering through the mouth and multiplying in the oropharynx and GI tract. It can be shed in stool for several weeks after infection, even in asymptomatic cases. It is present in nasopharyngeal secretions for one to two weeks, so droplet transmission also is possible.
About 24 percent of infected people will have brief flu-like symptoms that resolve without intervention. About one in 25 people will develop aseptic meningitis. Fewer than one percent of people infected with polio will go on to have weakness or paralysis in their arms, legs, or both. The incubation period for non-paralytic symptoms is three to six days; for paralysis, seven to 21 days.
Polio should be considered when a person presents with an acute-onset flaccid paralysis of one or more limbs, with diminished or absent tendon reflexes in those limbs. Poliovirus can also cause meningitis, and a negative cerebrospinal fluid (CSF) test for poliovirus doesn’t rule out the disease. Polio is a more likely diagnosis when the individual is unvaccinated or not fully vaccinated, has recently traveled to a country where polio still occurs, or who was exposed to someone who recently traveled to these areas.
When polio is suspected, begin contact precautions and practice excellent hand hygiene. Stool and throat specimens should be sent to the lab. Note that the CDC classifies polio as an “immediately notifiable” disease. When paralytic polio is suspected, the case should be reported to your local health department and the CDC within four hours. Non-paralytic polio should be reported within 24 hours.. This will be done either by your laboratory, infection control practitioner, or hospital epidemiologist.
Types of poliovirus.
Polioviruses are described as either “wild type” or vaccine-derived. Wild polioviruses are naturally occurring. Vaccine-derived polioviruses (VDPV) are strains that come from the weakened viruses used in oral polio vaccine (OPV) that have replicated and changed to be more like wild type virus. VDPV strains can emerge when oral poliovirus vaccine is given in a community where there is low vaccination coverage.
The Rockland County case was caused by a strain of VDPV. Oral polio vaccine has not been used in the U.S. since 2000, so this was the epidemiological clue that this case was contracted from someone who had traveled to a country where polio is present and oral polio vaccines are used.
There are two types of poliovirus vaccines.
- Inactivated polio vaccine (IPV) (the “Salk” vaccine) is used in the U.S. It’s highly effective in preventing polio infection. However, it results in relatively low levels of antibodies in the intestinal mucosa, where the virus multiplies. People who have been vaccinated with IPV and are later exposed to wild-type poliovirus will be protected from infection, but the virus can multiply in their intestines and be shed in stool, presenting a risk to unvaccinated people.
- Oral polio vaccine (OPV) (the “Sabin” vaccine) is made using live, weakened versions of all three polio serotypes. It, too, is highly effective. Unlike IPV, OPV results in a good local immune response in intestinal mucosa. It is able to inhibit viral replication after exposure to wild-type poliovirus, arresting further transmission. However, as OPV replicates in a newly vaccinated person, it may revert to its stronger, neurovirulent form. These VDPV strains are more like wild-type poliovirus and can infect non-immune people.
OPV is considerably less expensive than IPV, and eliminates the need for sterile equipment and trained personnel to administer the vaccine. The U.S. uses IPV instead because it’s thought that the risk of paralytic polio from OPV is greater than the risk of exposure to any imported cases of wild virus in the U.S. But even in countries where IPV is routinely given, OPV is used to contain outbreaks because it will stop transmission and more quickly contain spread.
Vaccination of children.
Children receive routine vaccination for the poliovirus. The most current CDC recommendation for vaccination can be viewed here. If you encounter a child who has not been routinely vaccinated, the CDC also provides catch-up recommendations. They can be accessed here.
Resources for nurses.
Nurses can learn about all the vaccine-preventable illness by using this CDC resource: You Call the Shots. It is an interactive, web-based immunization training course, that consists of a series of modules that discuss vaccine-preventable diseases and explain the latest recommendations for vaccine use. Each module provides learning opportunities, self-test practice questions, reference and resource materials, and an extensive glossary.
Poliovirus remains a global problem, and until all countries are polio-free, the U.S. will see imported cases of both wild type and VDPV infections.