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Poliomyelitis

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Disease

Poliomyelitis is a disease of the central nervous system caused by three closely related enteroviruses, poliovirus types 1, 2 and 3. The virus is spread predominantly by the faecal–oral route, although rare outbreaks caused by contaminated food or water have occurred. After the virus enters the mouth, the primary site of infection is the intestine, although the virus can also be found in the pharynx. Poliomyelitis is also known as “infantile paralysis” because it most frequently caused paralysis in infants and young children in the pre-vaccine era in industrialized countries. In developing countries, 60–70% of cases currently occur in children under 3 years of age and 90% in children under 5 years of age. The resulting paralysis is permanent, although some recovery of function is possible. There is no cure.

Occurrence

Significant progress has been made towards global eradication of poliomyelitis. More than 125 countries were endemic for polio in 1988; by 2006, only four countries – Afghanistan, India, Nigeria and Pakistan (see map), where wild poliovirus transmission has never been interrupted – remained endemic. A number of previously polio-free countries have been affected by wild-virus importation that has resulted in subsequent outbreaks, e.g. Namibia – a popular country for tourists – in the summer of 2006. Until all countries have stopped wild poliovirus transmission, all areas remain at high risk of importations and even of the re-establishment of endemic transmission.

Risk for travellers

The consequences of polio infection are life-threatening or crippling. Infection and paralysis may occur in non-immune individuals and are by no means confined to infants. Infected travellers are potent vectors for transmission and possible reintroduction of the virus into polio-free zones, now that worldwide eradication is near. Until the disease has been certified as eradicated globally, the risks of acquiring polio (for travellers to infected areas), and of re-infection of polio-free areas (by travellers from infected areas), remain. Travellers to and from endemic countries should be fully protected by vaccination.

Vaccine

All travellers to and from polio-infected areas should be up to date with vaccination against poliomyelitis according to national immunization policy. There are two types of vaccine: inactivated (IPV), which is given by injection, and oral (OPV). OPV is composed of the three types of live attenuated polioviruses. Because of the low cost and ease of administration of the vaccine and its superiority in conferring intestinal immunity, OPV has been the vaccine of choice for controlling epidemic poliomyelitis in many countries. On very rare occasions (2–4 cases per million births per year) OPV causes vaccine-associated paralytic poliomyelitis (VAPP). The risk of VAPP is higher with the first dose of OPV than with subsequent doses. VAPP is more common in individuals who are immunocompromised, for whom IPV is the vaccine of choice.

Most industrialized countries use IPV, either as the sole vaccine against poliomyelitis or in schedules combined with OPV. Although IPV suppresses pharyngeal excretion of wild poliovirus, this vaccine has only limited effects in reducing intestinal excretion of poliovirus. For unvaccinated older children and adults, the second dose is given 1–2 months, and the third 6–12 months, after the first dose. A booster dose is recommended after 4–6 years. IPV is also the vaccine of choice for travellers with no history of OPV use, as well as for immunocompromised individuals and their contacts and family members.

For those who have received three or more doses of OPV in the past, it is advisable to offer another dose of polio vaccine as a once-only dose to those travelling to endemic areas of the world. Any unimmunized individuals intending to travel to such areas require a complete course of vaccine. Countries differ in recommending IPV or OPV in these circumstances: the advantage of IPV is that any risk of VAPP is avoided, but this vaccine is more expensive and may not prevent faecal excretion of the virus.

In order to limit further international spread of wild poliovirus to polio-free areas, travellers from an infected country or area should have a full course of vaccination against polio before leaving their country of residence, with a minimum one dose of OPV before departure. Some polio-free countries may also require travellers from endemic countries to be immunized against polio in order to obtain an entry visa.

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