Rabies

also called hydrophobia , or lyssa acute, usually fatal, viral infectious disease of the central nervous system. The disease is usually spread among domestic dogs and wild carnivorous animals; all warm-blooded animals are susceptible to rabies infection. The virus, a rhabdovirus, is often present in the salivary glands of rabid animals and is excreted in the saliva; thus, the bite of the infected animal introduces the virus into a fresh wound. Under favourable conditions, the virus propagates along nerve tissue from the wound to the brain and becomes established in the central nervous system. The disease develops most often between four and six weeks after infection, but the incubation period may vary from 10 days to eight months.

Rabies often begins with excitation of the central nervous system expressed as irritability and viciousness. A rabid animal is most dangerous during the early stages of the disease because it appears to be healthy and may seem friendly but will bite at the slightest provocation. Wild animals that appear to be tame and that approach people or human habitations in the daytime should be suspected of having rabies.

Infected dogs usually show a short excitation phase that is characterized by restlessness, nervousness, irritability, and viciousness and is followed by depression and paralysis. Sudden death without recognizable signs of illness is common. Dogs that develop the predominantly excited type of rabies invariably die of the infection, usually within three to five days after the onset of symptoms. Those that develop the paralytic type of rabies without any evidence of excitation or viciousness may recover on rare occasions. Paralysis of the “voice” muscles in rabid dogs may produce a characteristic change in the sound of the bark.

Rabies in humans is similar to that in animals. Symptoms include depression, headache, nausea, seizures, anorexia, muscle stiffness, and increased production of saliva. Abnormal sensations, such as itching, around the site of exposure are a common early symptom. Repeated episodes of painful contraction of the muscles of the throat may occur upon attempting to swallow or may be elicited by the sight of water. This reaction to water is called hydrophobia (“fear of water”). Rabies in humans is almost always fatal. Death ordinarily occurs within three to five days after the onset of symptoms due to cardiac or respiratory failure. Sometimes rabies is characterized by paralysis without any evidence of excitation of the nervous system. In such cases the course of the disease may be prolonged to a week or more.

If administered soon after infection, serum or vaccine can be effective in combating the disease. This is a type of passive immunization whereby animals are immunized with attenuated rabies virus, and antibodies from these animals are injected into infected persons to give them temporary immunity to rabies. The treatment is effective if given within 24 hours after exposure but has little, if any, value if given three or more days after infection by rabies. Immediate treatment of animal-bite wounds by cleansing with soap and water is extremely important because much, if not all, of the virus can be thus removed.

Vaccines prepared from rabies virus can be used to protect people who are likely to be in contact with infected animals. The safest and most effective vaccines are human diploidcell vaccine (HDCV), purified chick embryo cell culture (PCEC), and rabies vaccine adsorbed (RVA). When a person not protected by previous immunization is bitten by a rabid animal, treatment is a dose of serum followed by a series of vaccinations. With the older vaccines, at least 16 injections were required, whereas with HDCV, PCEC, or RVA, 5 are usually sufficient.