Epidemiology of Infectious Diseases: Poliomyelitis
Poliovirus (genus Enterovirus), types 1, 2 and 3.
Type 1 is commonly associated with paralytic poliomyelitis and epidemics1.
Common clinical features
- Up to 95% of polio infections are asymptomatic. Estimates of the ratio of inapparent to paralytic illness varies from 50:1 to 1000:1 (usually 200:1)1,2.
- Approximately 10% of polio infections are characterized by minor, non-specific illness, including upper respiratory tract infection, gastrointestinal disturbances and vomiting.
- Fewer than 1% of polio infections result in flaccid paralysis.
- A further 5-10% of polio infections result in aseptic meningitis.
- Paralytic polio is classified into 3 types, depending on the level of involvement; Spinal polio (most common), bulbar polio and bulbospinal polio.
- The case-fatality rate for paralytic polio is approximately 2-5% among children and up to 15-30% in adults.
- 90% of all cases occur in children <5 years, and 50% under the age of 3.
- According to the WHO, global cases of poliomyelitis have declined by over 99% since 1988 from a reported 350,000 to 1919 cases in 2002.
- Between April 2005 and April 2006 WHO reported 1815 cases worldwide3.
- According to the CDC global eradication programme only 4 countries classified as polio endemic (where indigenous polio transmission has never been interrupted) remain (Nigeria, India, Pakistan and Afghanistan)3.
Humans, commonly among those with inapparent infections particularly children.
Mode of transmission
- Person to person spread via the faecal-oral route is the most common mode of transmission.
- Pharyngeal spread also known.
- Poliovirus is highly infectious, with sero conversion rates among susceptible household contact of children nearly 100%.
Commonly 7-14 days (particularly for paralytic cases), with a range of 3 -35 days.
Period of Communicability
- Transmission possible as long as virus is excreted.
- Individuals infected with the poliovirus are most infectious from 7-10 days before and after the onset of symptoms.
- Poliovirus may be shed in the stool for a period of 3-6 weeks.
- Long term carriage does not occur
Prevention and control
- Heymann D L, editor, Control of Communicable Disease Manual. 18th ed. American Public Health Association; 2004.
- Hawker J, Begg N, Blair I, Reintjes R, Weinberg J. Communicable Disease Control Handbook, Blackwell, 2005.
- CDC, Global Polio Eradication, Polio News, issue 26, spring 2006, available at: http://www.polioeradication.org/content/polionews/PolioNews26.pdf
WHO/UNICEF/CDC/Rotary International - Global Polio Eradication Initiative www.polioeradication.org
© CM Kirwan 2006