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, 65–75% of cases currently occur in children under 3 years of age and 95% in children under 5 years of age. The resulting paralysis is permanent, although some recovery of function is possible. There is no cure.
Occurrence
Signifi cant progress has been made towards global eradication of poliomyelitis. More than 125 countries were endemic for polio in 1988; as of 2008, there are only four countries where indigenous wild poliovirus transmission has never been interrupted: Afghanistan, India, Nigeria and Pakistan (see Map). 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-2008, imported wild poliovirus was circulating in seven previously polio-free countries: Angola, Chad, Democratic Republic of the Congo, Ethiopia, Ghana, Niger and the Sudan. 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 certifi ed 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. Updates on countries with ongoing transmission of indigenous and imported wild poliovirus and countries with recent transmission of imported wild poliovirus can be found at http://www. polioeradication.org/casecount.asp.
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 endemic and 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 fi rst 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. Following the fi rst dose, previously unvaccinated older children and adults receive the second dose 1–2 months later, with the third dose after 6–12 months. An additional dose is recommended after 4–6 years. For adults who have received a primary series of vaccination in childhood, a one-time only additional dose is recommended in some countries. IPV is also the vaccine of choice to protect travellers to polio infected areas who have no history of OPV use, as well as for immunocompromised individuals and their contacts and family members.
Travellers to polio-infected countries and areas who have previously received three or more doses of OPV or IPV should be offered another dose of polio vaccine before departure. Non- immunized individuals intending to travel to polio-infected countries and areas require a complete course of vaccine. Countries recommend OPV or IPV in these circumstances, depending on national policy. OPV has the advantage of boosting intestinal mucosal immunity to poliovirus and reducing the risk of intestinal infection while travellers are in polio-infected areas as well as subsequent virus excretion by returning travellers. For frequent travellers to polio-infected areas for brief periods of time, a one-time-only additional dose of polio vaccine after the primary series should be suffi cient to prevent disease. Travellers who are resident in a polio-infected country or area should have a full course of vaccination against polio, preferably with OPV, before leaving their country of residence to reduce the risk of international spread of wild poliovirus to polio-free areas. To optimize intestinal mucosal immunity against poliovirus, and minimize the risk of inadvertent intestinal carriage, such travellers should receive an additional dose of OPV between 1 and 12 months prior to each international travel. In cases of urgent travel, a minimum of one dose of OPV should be given before departure.
Some polio-free countries (e.g. Saudi Arabia) may require travellers from polioinfected countries to be immunized against polio in order to obtain an entry visa and/or to receive an additional dose on arrival in the country.