Poliomyelitis (Polio)

Poliomyelitis, or polio, is a vaccine-preventable disease caused by the highly infectious poliovirus, which is a member of the enterovirus subgroup, family Picornaviridae. Enteroviruses are transient inhabitants of the gastrointestinal tract. There are three poliovirus serotypes (P1, P2, and P3).

Humans are the only known reservoir of poliovirus, which is transmitted most frequently by persons with inapparent infections. There is no asymptomatic carrier state except in immune deficient persons. Person-to-person spread of poliovirus via the fecal-oral route is the most important route of transmission, although the oral-oral route may account for some cases. The virus enters through the mouth and primary multiplication of the virus occurs at the site of implantation in the pharynx and gastrointestinal tract. The virus is usually present in the throat and in the stool before the onset of illness. One week after onset there is little virus in the throat, but virus continues to be excreted in the stool for several weeks. The virus invades local lymphoid tissue, enters the blood stream, and then may infect cells of the central nervous system. Replication of poliovirus in motor neurons of the anterior horn and brain stem results in cell destruction and causes the typical manifestations of poliomyelitis.

Polio can be a very serious disease, causing permanent paralysis and even death. However, up to 95 percent of all polio infections are inapparent or asymptomatic. Infected persons without symptoms shed virus in the stool and are able to transmit the virus to others. The incubation period for poliomyelitis is commonly 6-20 days with a range from 3 to 35 days. The response to poliovirus infection is categorized according to severity.

Approximately 4-8 percent of polio infections consist of a minor, nonspecific illness without clinical or laboratory evidence of central nervous system invasion. Complete recovery usually occurs in less than a week. This form of the disease is usually charactarized by upper respiratory infection such as sore throat and fever; gastrointestinal disturbances such as nausea, vomiting, abdominal pain, constipation, and, rarely, diarrhea; and influenza-like illness.

Nonparalytic aseptic meningitis (symptoms of stiffness of the neck, back, and/or legs), usually following several days after a prodrome similar to that of minor illness, occurs in 1 percent–2 percent of polio infections. Increased or abnormal sensations can also occur. Typically these symptoms will last from 2 to 10 days, followed by complete recovery.

Less than 1 percent of all polio infections result in flaccid paralysis. Paralytic symptoms generally begin 1 to 10 days after the first symptoms and progress for 2 to 3 days. Generally, no further paralysis occurs after the temperature returns to normal. Many persons with paralytic poliomyelitis recover completely and, in most, muscle function returns to some degree. Weakness or paralysis still present 12 months after onset is usually permanent.

Paralytic polio is classified into three types, depending on the level of involvement. Spinal polio is most common, and accounted for 79 percent of paralytic cases from 1969 to 1979. It is characterized by asymmetric paralysis that most often involves the legs. Bulbar polio accounted for 2 percent of cases and led to weakness of muscles innervated by cranial nerves. Bulbospinal polio accounted for 19 percent of cases and was a combination of bulbar and spinal paralysis.

The death rate is 2–5 percent for children and up to 15–30 percent in adults (depending on age). It increases to 25–75 percent with bulbar polio.

There are no medications that can cure the infection, but symptoms can be relieved while the disease runs it course.

Polio occurs worldwide, however, no cases of wild polio virus have been reported in the United States since 1979. In 2003 only 784 confirmed cases of polio were reported globally and polio was endemic in 6 countries. The World Health Organization (WHO) and the Centers for Disease Control and Prevention (CDC) have a goal of eradicating polio throughout the world by 2010.

To prevent poliomyelitis, polio vaccine should be administered. Two types of polio vaccine are available: oral polio vaccine (OPV) and inactivated polio vaccine (IPV). OPV, which is made with a live, but weakened, virus, is no longer used in the U.S. because in rare instances it causes vaccine-associated paralytic polio (VAPP). From 1980 through 1999, there were 144 confirmed cases of VAPP. In order to eliminate VAPP from the United States, ACIP recommended in 2000 that IPV be used exclusively in the United States. The last case of VAPP was reported in 1999.

On September 29, 2005, the Minnesota Department of Health (MDH) identified poliovirus type 1 in an unvaccinated, immunocompromised infant girl aged 7 months (the index patient) in an Amish community whose members predominantly were unvaccinated for polio. The patient has no paralysis; the source of the patient's infection is unknown. Subsequently, poliovirus infections in three other children within the index patient's community have been documented. The following report summarizes the ongoing investigation, provides information regarding poliovirus exposure risks and prevention measures in the United States, and offers recommendations to state health departments and clinicians .

Centers for Disease Control and Prevention. MMWR Dispatch October 14, 2005 / 54(Dispatch); 1-3. Poliovirus Infections in Four Unvaccinated Children - Minnesota, August - October 2005. http://www.cdc.gov/mmwr/preview/mmwrhtml/mm54d1014a1.htm

(Since the publication of this article one additional child within the community has been identified, thus resulting in poliovirus infections in five unvaccinated children – Minnesota as of 11/3/05.)

The Missouri State Public Health Laboratory (SPHL) can perform virus culture on stool and throat swab specimens (Stools are the preferred specimen; culture results can be available within two days, but may take as long as 16 days). A PCR test for poliovirus is available from CDC, however arrangements have to be made by the SPHL to obtain the reagents from CDC, for testing here in Missouri. CDC can perform serological testing, but requests for such testing must be made through the SPHL before specimens can be submitted to CDC. (If a blood specimen is collected, it should be obtained before any polio immunizations are given.) The SPHL will supply a virus isolation kit with instructions, which must be used when collecting and transporting any specimens for polio testing (i.e. stool, throat swabs, blood/serum). See http://www.dhss.mo.gov/Lab/Virology/TestsAvailable.html for more information. Specific questions on laboratory testing should be directed to the SPHL at (573) 751-3334 or 751-0633, or (800) 392-0272 (24 hours a day – 7 days a week).

If individuals have specific questions on polio they should contact their physician, local health department or the Missouri Department of Health and Senior Services (866-628-9891).