Rabies

Rabies

Definition

Rabies is a viral illness that can affect any mammal but is most common in carnivores (flesh-eaters). It is sometimes referred to as a zoonosis, or disease of animals that can be communicated to humans. Rabies is usually transmitted in the saliva through a bite wound. The virus attacks the central nervous system, and is fatal once symptoms begin, with very rare exceptions.

Description

Rabies, also known as hydrophobia, belongs to the rhabdovirus family. Fewer than 10% of animal cases reported in the United States in 1998 were in domestic animals. Raccoons accounted for the largest number of cases in wild animals. Cases of rabies in humans are very infrequent in the United States, averaging one or two a year (down from over 100 cases annually in 1900), but the worldwide incidence is estimated to be between 30,000 and 50,000 cases each year. These figures are based on data collected by the World Health Organization (WHO) in 1997 and updated in 2002. Rabies is most common in developing countries in Africa, Latin America, and Asia, particularly India. Dog bites are the major origin of infection for humans in developing countries, but other important host animals may include the wolf, mongoose, and bat. Most deaths from rabies in the United States result from bat bites; the most recent victim was a man in Iowa who died in September 2002.

People whose work frequently brings them in contact with animals are considered to be at higher risk than the general population. This would include those in the fields of veterinary medicine, animal control, wildlife work, and laboratory work involving live rabies virus. People in these occupations and residents of or travelers to areas where rabies is a widespread problem should consider being immunized.

In late 2002, rabies re-emerged as an important public health issue. Dr. Charles E. Rupprecht, director of the World Health Organization (WHO) Collaborating Center for Rabies Reference and Research, has listed several factors responsible for the increase in the number of rabies cases worldwide:

• Rapid evolution of the rabies virus. Bats in the United States have developed a particularly infectious form of the virus.
• Increased diversity of animal hosts for the disease.
• Changes in the environment that are bringing people and domestic pets into closer contact with infected wildlife.
• Increased movement of people and animals across international borders. In one recent case, a man who had contracted rabies in the Philippines was not diagnosed until he began to feel ill in the United Kingdom.
• Lack of advocacy about rabies.

Causes & symptoms

The most common way to contract rabies is from the bite of an infected animal. Although bats are the most frequent source of human infection in the United States, dogs are the primary vector of rabies in most parts of the world. The disease may also be transmitted by tissues and body fluids other than saliva. Rare cases have occurred as a result of infection through corneal transplantation.

Rabies travels from the site of the bite along the peripheral nerves to the brain. The average incubation period in humans is 30–50 days, although it varies from 10 days to over a year. The initial symptoms are flu-like and nonspecific. They may include fever, headache , muscle pain, sore throat, fatigue, nausea , and vomiting . Altered sensation and muscle twitching in the area of the bite are signs that are more suspicious of rabies. When the virus reaches the brain, signs related to encephalitis (local or general inflammation of brain tissue) appear. This typically involves agitation, progressing to confusion,

combativeness, seizures, and localized areas of paralysis. There may also be hypersensitivity to light, sound, and touch. The patient may be coherent at times, but less so as the disease progresses. Many viruses causing encephalitis may produce similar signs. The next stage is dysfunction of the brainstem. The well-known phenomenon of foaming at the mouth is caused by excessive saliva production combined with difficulty swallowing. Many patients will refuse liquids at this point due to the painful muscle contractions caused by swallowing. This is how rabies came to be known as hydrophobia, which means "fear of water." Coma ensues soon after brainstem involvement, and death occurs when the respiratory center is affected. The course of the disease is four to 20 days after symptoms appear, unless life support is used.

Diagnosis

Early in its course, and without a known history of an animal bite, rabies can be difficult to diagnose. Symptoms of the early encephalitic (brain tissue inflammation) phase are similar to those of most viral types of encephalitis. When signs of brainstem dysfunction appear shortly after this time, rabies becomes a more likely possibility. Several tests are available for rabies diagnosis, but none are extremely reliable in the living patient. Part of the challenge is that rabies is so limited to nerve tissue until the late phases of the disease. The examination of brain tissue reveals a characteristic known as a Negri body, which is diagnostic. Direct fluorescent antibody (dFA) staining of saliva, skin biopsy, and corneal impressions may also yield a diagnosis.

Treatment

Local wound cleansing is important. Anyone who has experienced an animal bite should wash it thoroughly with soap and water. Rabies is a fatal illness, so a bite that breaks the skin warrants a call to a health care provider for evaluation of whether post-exposure prophylaxis (PEP) is necessary. Alternative treatments are recommended as a complementary therapy to conventional treatment in the case of rabies. Observation of the animal for signs of rabies is recommended whenever possible.

Allopathic treatment

If a person is bitten by a domestic animal and the owner is known, vaccination status should be checked. People bitten by healthy, immunized animals are unlikely to need post-exposure prophylaxis (PEP). The animal can be confined for 10 days. If it is healthy at the end of that time, it is presumed not to have been capable of transmitting rabies at the time of the bite, so PEP for the person bitten is not necessary.

Wild animals that have bitten can be captured, destroyed, and tested for rabies. Postexposure vaccine and specific immune globulin can be given if deemed necessary. In the United States, if the person who was bitten has not had prophylactic immunization and has a high-risk bite, generally five rabies vaccinations and one injection of human rabies immune globulin are given. There have been no cases in this country of people contracting rabies after receiving correctly administered PEP.

Bites from mice, rats, or squirrels rarely require rabies prevention because these rodents are typically killed by any encounter with a larger, rabid animal, and would therefore be less likely to be carriers. Bites from raccoons, bats, or unvaccinated dogs or cats are more suspect. Anyone bitten by a bat in the United States should receive PEP unless the bat is captured and proven not to be rabid.

If a pet is bitten by an animal suspected to have rabies, its owner should contact a veterinarian immediately and notify the local animal control authorities. Domestic pets with current vaccinations should be revaccinated immediately; unvaccinated dogs, cats, or ferrets are usually euthanized (put to sleep). Further information about domestic pets and rabies is available on the American Veterinary Medical Association (AVMA) web site.

Expected results

Survival of rabies after the appearance of symptoms is exceedingly rare.

Prevention

The following precautions should be observed in environments where humans and animals may likely come into contact. Domesticated animals, including household pets, should be vaccinated against rabies. Booster shots, given according to the manufacturer's recommendations, are required to maintain immunity. Wild animals should not be touched or petted, no matter how friendly an animal may appear. It is also important not to touch an animal that appears ill or passive, or whose behavior seems odd, such as failing to show the normal fear of humans. These are all possible signs of rabies. Many animals, such as raccoons and skunks, are nocturnal and their activity during the day should be regarded as suspicious. People should not interfere with fights between animals. Because rabies is transmitted through saliva, a person should wear rubber gloves when handling a pet that has had an encounter with a wild animal. Windows and doors should be screened. Some victims of rabies have been attacked by infected animals, particularly bats, that entered through unprotected openings. Finally, garbage or pet food should not be left outside because it may attract wild or stray animals.

Members of the high-risk occupations mentioned above should consider prophylactic immunization. Those who receive this pre-exposure vaccine still require PEP in the event of a potentially infective episode, but they have several advantages. One is that they require fewer post-exposure vaccines. A second advantage is that the timing of the PEP may be less critical for people who are in remote areas, or don't have ready access to vaccine for other reasons. Last, some people may be exposed without being aware of it, and the prophylactic vaccine might protect them.

Resources

BOOKS

"Central Nervous System Viral Diseases: Rabies (Hydrophobia)." Section 13, Chapter 162 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.

Corey, Lawrence. "Rabies Virus and Other Rhabdoviruses." Harrison's Principles of Internal Medicine, edited by Anthony S. Fauci et al., 14th ed.; New York: McGraw-Hill, 1998.

PERIODICALS

Fooks, A. R., N. Johnson, S. M. Brookes, et al. "Risk Factors Associated with Travel to Rabies Endemic Countries." Journal of Applied Microbiology 94 (2003) (Supplement): 31S–36S.

"Human Rabies—Iowa, 2002." Morbidity and Mortality Weekly Report 52 (January 24, 2003): 47–48.

Messenger, S. L., J. S. Smith, L. A. Orciari, et al. "Emerging Pattern of Rabies Deaths and Increased Viral Infectivity." Emerging Infectious Diseases 9 (February 2003): 151–154.

Smith, J., L. McElhinney, G. Parsons, et al. "Case Report: Rapid Ante-Mortem Diagnosis of a Human Case of Rabies Imported Into the UK from the Philippines." Journal of Medical Virology 69 (January 2003): 150–155.

Stringer, C. "Post-Exposure Rabies Vaccination." Nursing Standard 17 (February 5-11, 2003): 41–42.

Weiss, R. A. "Cross-Species Infections." Current Topics in Microbiology and Immunology 278 (2003): 47–71.

ORGANIZATIONS

American Veterinary Medical Association (AVMA). 1931 North Meacham Road, Suite 100, Schaumburg, IL 60173-4360. <http://www.avma.org>.

Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311. <http://www.cdc.gov>.

Institut Pasteur. 25-28, rue du Dr. Roux, 75015 Paris, France. +33 (0) 1 45 68 80 00. <http://www.pasteur.fr/haut_ext.html>.

OTHER

CDC. "Epidemiology of Rabies." http://www.cdc.gov/ncidod/dvrd/rabies/Epidemiology/Epidemiology.htm.

National Association of State Public Health Veterinarians, Inc. "Compendium of Animal Rabies Prevention and Control, 2003." Morbidity and Mortality Weekly Report Recommendations and Reports 52 (March 21, 2003) (RR-5): 1–6.

Judith Turner

Rebecca J. Frey, PhD