Rabies

Definition

Rabies is an acute viral disease of the central nervous system that affects humans and other mammals. It is almost exclusively transmitted through saliva from the bite of an infected animal. Another name for the disease is hydrophobia, which literally means "fear of water," a symptom shared by half of all people infected with rabies. Other symptoms include fever, depression, confusion, painful muscle spasms, sensitivity to touch, loud noise, and light, extreme thirst, painful swallowing, excessive salivation, and loss of muscle tone. If rabies is not prevented by immunization, it is essentially always fatal.

Description

Worldwide, approximately 15,000 cases of human rabies continue to occur annually. Remarkably, although more than one million persons in the United States are bitten each year by animals, on average, only one or two persons die from the disease each year. Nevertheless, with the continued encroachment of humans on animal habitats, both for housing and recreational purposes, rabies remains a public health concern.

Both domestic and wild animals may transmit rabies. With the widespread vaccination of domesticated animals in the United States, dogs in particular, the number of cases of rabies has significantly declined. In 1955 domesticated animals, especially dogs, constituted 47% of the reported rabies cases. By 1994, fewer than 2% of positive tests occurred in dogs. In fact, in the 1990s, cats outnumbered dogs as transmitters of the disease. As of 1997, most cases of rabies are in wild animals, particularly bats, raccoons, skunks, foxes, wolves, and coyotes.

Anyone who has been bitten by an animal, regardless of age or sex, can contract rabies. However, people whose occupations involve routine exposure to a domestic animal that has not been immunized or to wildlife are at a greater risk for getting the disease. As a result, cave explorers, farm and ranch workers, animal trainers and caretakers, forest rangers, animal exterminators, some laboratory workers, and veterinarians are at a higher risk.

Rabies is caused by a rod-or bullet-shaped virus in the family Rhabdoviridae. The virus is usually transmitted via an animal bite, however, cases have also been reported in which the virus penetrated the body through infected saliva, moist tissues such as the eyes or lips, a scratch on the skin, or the transplantation of infected tissues. Inhalation of the virus in the air, as might occur in a highly populated bat cave, is also thought to occur.

From the bite or other area of penetration, the virus multiplies as it spreads along nerves that travel away from the spinal cord and brain (efferent nerves) and into the salivary glands. The rabies virus may lie dormant in the body for several weeks or months, but rarely much longer, before symptoms appear. Initially, the area around the bite may burn and be painful. Early symptoms may also include a sore throat, low-grade fever, headache, loss of appetite, nausea and vomiting, and diarrhea. Painful spasms develop in the muscles that control breathing and swallowing. The individual may begin to drool thick saliva and may have dilated or irregular pupils, increased tears and perspiration, and low blood pressure.

Later, as the disease progresses, the patient becomes agitated and combative and may exhibit increased mental confusion. The affected person usually becomes sensitive to touch, loud noises, and bright lights. The victim also becomes extremely thirsty, but is unable to drink because swallowing is painful. Some patients begin to dread water because of the painful spasms that occur. Other severe symptoms during the later stage of the disease include excessive salivation, dehydration, and loss of muscle tone. Death usually occurs three to 20 days after symptoms have developed. Unfortunately, recovery is very rare.

Because of the extremely serious nature of a rabies infection, the need for rabies immunizations will be carefully considered for anyone who has been bitten by an animal, based on a personal history and results of diagnostic tests.

If necessary, treatment includes the following:

• The wound is washed thoroughly with medicinal soap and water. Deep puncture wounds should be flushed with a catheter and soapy water. Unless absolutely necessary, a wound should not be sutured.
• Tetanus toxoid and antibiotics will usually be administered.
• Rabies vaccination may or not be given, based on the available information. If the individual was bitten by a domestic animal and the animal was captured, the animal will be placed under observation in quarantine for ten days. If the animal does not develop rabies within four to seven days, then no immunizations are required. If the animal is suspected of being rabid, it is killed, and the brain is examined for evidence of rabies infection. In cases involving bites from domestic animals where the animal is not available for examination, the decision for vaccination is made based on the prevalence of rabies within the region where the bite occurred. If the bite was from a wild animal and the animal was captured, it is generally killed because the incubation period of rabies is unknown in most wild animals.
• If necessary, the patient is vaccinated immediately, generally through the administration of human rabies immune globulin (HRIG) for passive immunization, followed by human diploid cell vaccine (HDCV) or rabies vaccine adsorbed (RVA) for active immunization. Passive immunization is designed to provide the individual with antibodies from an already immunized individual, while active immunization involves stimulating the individual's own immune system to produce antibodies against the rabies virus. Both rabies vaccines are equally effective and carry a lower risk of side effects than some earlier treatments. Unfortunately, however, in underdeveloped countries, these newer vaccines are usually not available. Antibodies are administered to the patient in a process called passive immunization. To do this, the HRIG vaccine is administered once, at the beginning of treatment. Half of the dose is given around the bite area, and the rest is given in the muscle. Inactivated viral material (antigenic) is then given to stimulate the patient's own immune system to produce antibodies against rabies. For active immunization, either the HDCV or RVA vaccine is given in a series of five injections. Immunizations are typically given on days one, three, seven, 14, and 28.

In those rare instances in which rabies has progressed beyond the point where immunization would be effective, the patient will be given medication to prevent seizures, relieve some of the anxiety, and relieve painful muscle spasms. Pain relievers will also be given. In the later stages, aggressive supportive care will be provided to maintain breathing and heart function. Survival is rare but can occur.