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| RABIES |
Disease
Rabies is an acute viral disease of the central nervous system (CNS) that is transmitted to humans by infected animals. After a prodromal phase, rabies manifests most often as encephalitis – or less frequently as a paralytic form of the disease – and then progresses to coma and death. The disease is transmitted by infective saliva that gains entry into the body by an animal bite or sometime an open wound. The incubation period may range from 10 days to many years but is usually 3-7 weeks depending upon the distance of the wound from the CNS.
Occurrence
Rabies is generally transmitted to humans by the bite of a rabid (infected) animal. Dogs have been the prime reservoirs and vector for rabies and they still remain the major source of transmission in humans in Asia and Africa. Other than dogs bats, raccoons, skunks and foxes have been source of endemic virus infection. Except bat and dogs, all the other mentioned reservoirs are geographically restricted.
Risk for travellers
The risk to travellers in rabies-endemic areas is proportional to their contact with potentially rabid mammals. For instance, it is estimated that 13% of visitors to one country in South-East Asia come into contact with local animals. Dogs, both owned and ownerless, are very common, with an estimated 1:10 ratio of dogs to humans in most developing countries.
An average of 100 suspected rabid dogs bite per 100 000 inhabitants in endemic countries with dog rabies. According to a recent survey conducted in India, 1.6% of the total population received a dog bite during a 12-month period. Veterinarians and people working with dogs are at the greatest risk. Most travellers in tourist resorts are at very low risk. There is a greater risk for children, however, who may have more contact with animals and may not report suspect incidents. It is prudent to avoid walking in areas where dogs roam. Following suspect contact, especially bites or scratches, medical advice should be sought at once at a competent medical centre, ideally in the rabies treatment centre of a major city hospital. First-aid measures should be started immediately.
Travellers should avoid contact with free-roaming animals, especially dogs and cats, and with wild and captive animals. For travellers who participate in caving/spelunking, casual exposure to cave air is not a concern, but cavers should be warned not to handle bats.
Vaccine
Vaccination against rabies is used in two distinct situations:
- to protect those who are at risk of exposure to rabies, i.e. pre-exposure vaccination;
- to prevent clinical rabies occurrence after exposure has occurred, usually following the bite of an animal suspected of having rabies, i.e. post-exposure prophylaxis.
The vaccines used for pre-exposure and post-exposure vaccination are the same, but the immunization schedule differs according to the type of application. Rabies immunoglobulin is used only for post-exposure prophylaxis. Modern vaccines of cell-culture or embryonated egg origin are safer and more effective than the older vaccines, which were produced in brain tissue. These modern rabies vaccines are now available in major urban centres of most countries of the developing world. Rabies immunoglobulin, on the other hand, is in short supply worldwide and may not be available even in major urban centres in many dog rabies-infected countries.
Pre-exposure vaccination
Pre-exposure vaccination should be offered to people at high risk of exposure to rabies, such as laboratory staff working with rabies virus, veterinarians, animal handlers and wildlife officers, and other individuals living in or travelling to areas where rabies is endemic. Travellers with extensive outdoor exposure in rural areas – such as might occur while running, bicycling, hiking, camping, backpacking, etc. – may be at risk, even if the duration of travel is short. Pre-exposure vaccination is advisable for children living in or visiting rabies-endemic areas, where they provide an easy target for rabid animals. Pre-exposure vaccination is also recommended for persons travelling to isolated areas or to areas where immediate access to appropriate medical care is limited or to countries where biologicals are in short supply and locally available rabies vaccines might be unsafe and/or ineffective.
Pre-exposure vaccination consists of three full intramuscular doses of cell-culture or embryonated egg origin rabies vaccine given on days 0, 7 and 21 or 28 (a few days’ variation in the timing is not important). For adults, the vaccine should always be administered in the deltoid area of the arm; for young children (under 2 years of age), the anterolateral area of the thigh is recommended. Rabies vaccine should never be administered in the gluteal area: administration in this manner will result in lower neutralizing antibody titres.
To reduce the cost of cell-derived vaccines for pre-exposure rabies vaccination, intradermal vaccination in 0.1-ml volumes on days 0, 7 and either 21 or 28 may be considered. This method of administration is an acceptable alternative to the standard intramuscular administration, but it is technically more demanding and requires appropriate staff training and qualified medical supervision. As an open vial should not be kept for more than 6 hours, wastage can be avoided by vaccinating several people during that period. Concurrent use of chloroquine can reduce the antibody response to intradermal application of cell-culture rabies vaccines. People who are currently receiving malaria prophylaxis or who are unable to complete the entire three dose pre exposure series before starting malarial prophylaxis should therefore receive pre-exposure vaccination by the intramuscular route.
Rabies vaccines will induce long-lasting memory cells, giving rise to an accelerated immune response when a booster dose of vaccine is administered. Periodic booster injections are therefore not recommended for general travellers. Periodic booster injections are recommended only for people whose occupations put them at continuous or frequent risk of rabies exposure, e.g. rabies researchers, staff in diagnostic laboratories where rabies virus is present. For persons at continuous or frequent risk of rabies exposure who have previously received pre-exposure rabies vaccination, a booster vaccination consists of one dose of a cell-culture or embryonated egg rabies vaccine. In this case, a routine booster vaccination is administered if the serological titre of the person at risk falls below 0.5 IU/ml, the antibody level considered to be adequate. |
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