Content Information
Disease Information
Polio is caused by poliovirus (genus Enterovirus), which has three serotypes.
Overview
Report Immediately by Phone
Also known as: Polio, Polioviral Fever and Infantile Paralysis
Responsibilities
- Hospital: Report immediately by phone
- Lab: Report immediately by phone, send isolate to University Hygienic Lab (ULH)
- Physician: Report immediately by phone
- Local Public Health Agency (LPHA): Follow-up required. Iowa Department of Public Health will lead the follow-up investigation
Iowa Department of Public Health
Disease Reporting Hotline: (800) 362-2736
Secure Fax: (515) 281-5698
A. Agent
Polio is caused by poliovirus (genus Enterovirus), which has three serotypes. Type 1 virus is most frequently involved in epidemics and is most often isolated from paralytic cases of poliomyelitis. Type 3 and, to a lesser degree, types 1 and 2 can also cause paralysis. All three are contained in the vaccine.
B. Clinical Description
Infection with poliovirus results in a spectrum of manifestations. The overwhelming majority of infections (95%) are asymptomatic. About 4 - 8% of infected individuals will experience non-specific viral symptoms, such as a low-grade fever, headache, sore throat, nausea, abdominal pain, constipation, diarrhea, and/or vomiting (referred to as abortive disease). About 1-2% of infections will result in aseptic meningitis, involving stiffness of the back, neck and/or legs, at times with paresthesias, a few days after the minor illness has resolved. Less than 1% of infections will progress to acute flaccid paralysis (AFP) with loss of reflexes in the involved limbs, usually with fever present (paralytic poliomyelitis). Please note, today in the U.S., the most common cause of AFP is Guillain-Barré syndrome (GBS). Polio has been eradicated from the western hemisphere.
Progression to paralytic poliomyelitis usually occurs within 2 - 4 days and rarely continues after the fever subsides. Spinal paralysis is typically asymmetric and more severe proximally than distally. Paralysis may compromise respiration and swallowing. After the acute episode, many patients recover at least some muscle function and prognosis for recovery can usually be established within 6 months after onset of paralytic disease. Between 2 – 5% of paralytic infections in children are fatal, in adults it is up to 15 – 30%. Risk factors for paralytic disease include larger inoculum of poliovirus, increasing age, pregnancy, strenuous exercise, tonsillectomy, and intramuscular injections administered while the patient is infected with poliovirus.
C. Reservoirs
Humans are the only known host.
D. Modes of Transmission
The principal mode of transmission is person-to-person by the fecal-oral route. Transmission via oral secretions, such as saliva, is very uncommon but may account for some cases. In rare instances, the virus may be transmitted by contaminated sewage or water. Asymptomatic individuals, especially children, comprise a significant source of infections. No reliable evidence of spread by insects exists. No long-term carrier state is known. In temperate climates, poliovirus infections are most common in the summer and fall.
E. Incubation period
Paralytic polio: The incubation period is usually 7 - 14 days, with a range of 3 - 35 days.
F. Period of Communicability or Infectious Period
The period of communicability is not precisely defined. It appears to be greatest 7 - 10 days before and after onset of clinical symptoms, when poliovirus is present in the throat and excreted in the highest quantities in the feces. Poliovirus can continue to be shed in the feces for 3 - 6 weeks. Poliovirus can be found in throat secretions as early as 36 hours and in the feces 72 hours after exposure to infection in both symptomatic and asymptomatic cases.
G. Epidemiology
Prior to the widespread use of polio vaccine, poliomyelitis occurred worldwide. Polio was epidemic in the U.S. for the first half of the 20th century with over 20,000 cases of paralytic disease in 1952. The first inactivated poliovirus vaccine (IPV) was introduced in 1955, monovalent oral poliovirus vaccine (OPV) in 1961, trivalent (TOPV) in 1963, and enhanced inactivated poliovirus vaccine (eIPV) in 1987. After the introduction of vaccination, the reported number of cases of poliomyelitis in the U.S. dropped to <100 in 1965 and <10 cases in 1973. The last cases of indigenously-transmitted wild-type poliovirus in the U.S. were in 1979. The last case of wild-type polio disease in the Western Hemisphere was detected in Peru in 1991. The Western Hemisphere was declared free from indigenous wild-type poliovirus transmission in 1994. Poliomyelitis is now on the verge of worldwide eradication.
Almost the entire world is now considered polio-free. Worldwide efforts to eradicate polio in the few countries where the disease is still endemic are underway. Strategies include: (1) achieving and maintaining high vaccination coverage among infants < 1 year old; (2) developing sensitive surveillance systems for AFP including a laboratory network; (3) conducting National Immunization Days; (4) and conducting “mopping-up” campaigns to directly target geographic areas known to be high risk for polio transmission. The number of countries where poliovirus continues to be isolated has decreased substantially, with Afghanistan, Egypt, India, Niger, Nigeria, and Pakistan the major areas of wild-type virus circulation.
Due to the success of global efforts towards eradication and the elimination of indigenously transmitted disease in the Western Hemisphere, cases of paralytic poliomyelitis in the industrialized countries have become almost non-existent. During the period 1980-94, there was an average of 8-9 cases of paralytic polio reported annually in the U.S. Most of these cases were vaccine-associated paralytic poliomyelitis (VAPP), which can occur after receipt of OPV. This very rare disease accounted for an average of 8 reported cases per year in the US, during the period 1980-94 (or 1 case for every 2.4 million doses of OPV distributed). The risk for VAPP is highest after receipt of the first dose of poliovirus vaccine, occurring at one case per 750,000 doses distributed. Since 1986, the only cases of paralytic poliomyelitis occurring in the US have been vaccine-associated.
In January 1997, in an effort to reduce the risk of VAPP, a sequential polio vaccination schedule (IPV for doses 1 and 2, OPV for doses 3 and 4) was recommended in the US. With the continued success of worldwide efforts to eradicate poliovirus and in the interest of completely eliminating the occurrence of VAPP, an all-IPV immunization schedule was initiated on January 1, 2000 in the US.
Despite the great achievement in polio eradication in the U.S., vigilance is needed because of the possibility of importation of wild poliovirus from areas of the world where it is endemic. The importation of wild poliovirus from polio-endemic regions of the world may occur among under-immunized (1) tourists, (2) immigrants revisiting their countries of origin, or (3) members of religious groups opposed to vaccination, regardless of travel history. In 1992-93 an outbreak occurred in the Netherlands among members of a religious group that refuse immunization. Poliovirus has also been isolated from members of a similar religious group in Canada, although no cases of disease occurred. Polio remains endemic in three countries: Afghanistan, Nigeria and Pakistan. In addition, Cameroon and Somalia are considered infected but not currently exporting Polio.
H. Bioterrorism Potential
None.
I. Additional Information
The Council of State and Territorial Epidemiologists (CSTE) surveillance case definitions should not affect the investigation or reporting of a case that fulfills the criteria in this chapter. (CSTE case definitions are used by the state health department and the CDC to maintain uniform standards for national reporting.)
Fact Sheets and Forms
References
American Academy of Pediatrics. Red Book 2006: Report of the Committee on Infectious Diseases, 27th Edition. Illinois, Academy of Pediatrics, 2006.
CDC. Epidemiology & Prevention of Vaccine-Preventable Diseases: The Pink Book, 8th Edition. CDC, January 2004.
CDC. Vaccine-Preventable Disease Surveillance Manual, 3rd Edition, 2002. CDC, 1999.
Heymann, David L., ed., Control of Communicable Diseases Manual, 20th Edition. Washington, DC, American Public Health Association, 2015.
IDPH. Public Health (641) Chapter 1, Notification and Surveillance of Reportable Communicable and Infectious Diseases, Poisonings and Conditions (Printed April 2004).