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| POLIOMYELITIS |
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 faeco–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. 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; as of 2007, there are only four endemic countries where wild poliovirus transmission has never been interrupted: Afghanistan, India, Nigeria and Pakistan. Wild poliovirus importations from the four endemic countries into previously polio-free countries continue to occur, with some resulting in new outbreaks. As of mid-2007, imported wild poliovirus was circulating in six previously polio-free countries: Angola, Chad, Democratic Republic of the Congo, Myanmar and Niger. Until wild poliovirus transmission has been stopped globally, all polio-free countries and areas remain at risk of importation and of renewed outbreaks.
Risks associated with international travel
The consequences of polio infection are crippling and sometimes life-threatening. Infection and paralysis may occur in non-immune individuals of any age. Infected travellers are potential 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 reinfection of polio-free areas (by travellers from infected areas), remain. Travellers to and from endemic and reinfected countries should be fully protected by vaccination.
Vaccine
There are two types of poliomyelitis vaccine, inactivated poliomyelitis vaccine and live (oral) poliomyelitis vaccine. Oral polio vaccine is being used as a part of global immunization programme especially in developing countries; whereas injection of polio vaccine containing inactivated strains of human poliovirus types 1, 2 and 3 is being used in developed countries.
A course of primary immunization consists of 3 doses starting at 2 months of age with intervals of 1 month between doses.
Two booster doses of a preparation containing inactivated poliomyelitis vaccine are recommended, the first before school entry and the second before leaving school. Further booster doses are only necessary for adults at special risk, such as travellers to endemic areas, or laboratory staff likely to be exposed to the viruses, or healthcare workers in possible contact with cases; booster doses should be given to such individuals every 10 years.
The live (oral) vaccine poses a very rare risk of vaccine-associated paralytic polio because the attenuated strain of the virus can revert to a virulent I form. For this reason the live (oral) vaccine must not be used for immunosuppressed individuals or their household contacts. The use of inactivated poliomyelitis vaccine removes the risk of vaccine-associated paralytic polio altogether.
Now a days preparations containing macfoaided poliomyelitis vaccine are being used in place of oral polio vaccine or to complete an immunization course with one polio vaccine. Some persons advocate the use of both oral as well injectable vaccine in developing countries.
Travellers: Unimmunised travellers to areas with a high incidence of poliomyelitis should receive a full course of a preparation containing inactivated poliomyelitis vaccine. Those who have not been vaccinated in the last 10 years should receive a booster dose of adsorbed diphtheria [low dose], tetanus and inactivated poliomyelitis vaccine.
Vaccine
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 now 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.
A course of primary immunization consists of 3 doses, starting at 2 months of age with interval of 1 month between doses. In India no booster dose is required after the age of 5 years where as in developed or polio free countries booster doses are required first before school entry and second before leaving school. Unvaccinated older children and adults also receive 3 doses, second dose after 1-2 months and third dose after 6-12 months after the 1st dose. IPV is the vaccine of choice to protect travellers with no history of OPV use, as well as for immuno compromised individuals, their contacts and family members.
Travellers to polio-infected countries and areas who have received three or more doses of OPV in the past should be offered another dose of polio vaccine before departure. 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 a polio-infected country or area should have a full course of vaccination against polio preferably with OPV, before leaving their country of residence, with a minimum one dose of OPV before departure. Some polio-free countries may also require travellers from polio-infected countries to be immunized against polio in order to obtain an entry visa. |
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