Courtesy of: www.ivis.org

 

Rabies

Rabies is a fatal (rare exceptions) encephalitis of all warm-blooded mammals caused by a lyssavirus and manifested mainly in either a furious or dumb (paralytic) form. The infection usually originates in a bite wound and ascends a nerve trunk to the cord and brain. The incubation period is variable and, on occasions, has been longer than six months.

Distribution

The disease occurs worldwide except for Australia (a rabies variant virus recovered from bats) , New Zealand, the British Isles, Hawaii, the Scandinavian countries, Cyprus and Japan. All domestic animals are susceptible. It is frequently endemic in wild animals including the skunk, fox, raccoon, wolf, bobcat and coyote. There are periodic epidemics among wild animals. Asymptomatic salivary gland infections occur in vampire bats resulting in prolonged viremia. Insectivorous bats may also be infected. In recent years in the US there have been more feline cases than canine, probably because cats are less frequently vaccinated.

Etiology

A labile, single-stranded RNA virus in the Rhabdoviridae family. The following variants of the classic rabies virus have been identified employing molecular techniques: Lagos bat virus (Africa), Mokola virus (Africa), Duvenhage virus (Africa), European bat virus 1 and 2 and Australian bat virus. It is concluded that there is only one immunotype of the virus. Because the FA reagent is prepared from a pool of monoclonals of different variants. It is thought that the standard FA test will identify the variants. The variants cause fatal rabies-like infections on rare occasions.

Mode of Infection and Transmission

The virus is shed in the saliva from infected salivary glands. The disease almost always results from the bites or scratches of infected or rabid animals. Several cases in humans have resulted from aerosol exposure. There have been two known recoveries in humans. Although susceptible to common disinfectants and ultraviolet light, the virus retains its viability in tissues for several weeks at room or refrigerator temperatures.

Susceptibility

The various animal species vary in their susceptibility from very high (wolves, coyotes and jackals) to low (American opossum). Domestic animals including the dog and cat are moderately susceptible, as are humans, racoons, bats and skunks.

Clinical Signs

The incubation period is usually 2 - 8 weeks but can be several months. The course is 3 - 10 days.
The disease is seen in the forms described below; however, these forms are not always clearly demarcated.

  • Prodromal form. Animals show apprehension, anxiety and changes in temperament and behavior. Severe pruritis may develop at the site of exposure. This stage lasts from 1 - 3 days. Paralysis ensues rapidly and death occurs within 10 days after the first signs are seen.
  • Furious form. Aimless wandering; bumps into objects; excitement; irritability; bites or attempts to bite animals, people and inanimate objects (mad-dog syndrome); depraved appetite; voice altered; muscle paralysis, salivation, convulsions, ataxia, paralysis and death.
  • Paralytic form. This form is most common. The animal is lethargic and hides; doesn't usually bite; muscular tremors; perceived difficulty in swallowing; terminal paralysis.
  • Inapparent form. This form has been observed in dogs, cats, skunks and bats. These animals may seroconvert, survive and serve as a source of the virus for extended periods. Bats may be asymptomatic or have protracted clinical signs with transmission of the virus for months.
Diagnosis
  • The different forms of the disease make a clinical diagnosis difficult. History of exposure to potentially infected wild animals should be considered. Canine distemper, pseudorabies, canine hepatitis, feline infectious peritonitis, listeriosis, cryptococcosis, toxoplasmosis, other infections of the CNS and poisonings due to lead, strychnine and various pesticides which produce neurologic signs should be considered.
  • Rabies can only be definitively diagnosed by laboratory means.
  • After human exposure to a dog or cat suspected of having rabies, the dog or cat may be killed and submitted to the laboratory immediately, or be confined for 10 days. If suspicious signs develop, the animal should be killed and submitted to the laboratory for tests.
  • Specimens: The entire carcass or head, or the live animal. The fluorescent antibody (FA) procedure is widely used and is the preferred method. It is used on animals that have died or been killed and is recommended for the immediate examination of wild animals that cannot be readily held for observation. Smears of the hippocampus major are usually employed, but they can also be made from the salivary gland. Mice are inoculated with selected negative specimens. Correlation of 99.9 % between FA and mouse inoculation results is reported. Other procedures that are employed for identification of rabies virus are immunoperoxidase staining and the polymerase chain reaction.
  • As a back-up measure mice are sometimes inoculated intracerebrally with a suspension of the suspect brain.
Treatment

None.

Control

A variety of live attenuated and inactivated vaccines are available.

  • Avian flury strain: A modified live virus (MLV) vaccine. The high egg passage can be used in dogs, cats and cattle. The low egg passage can only be used in dogs.
  • Cell culture MLV vaccines: Causes fewer allergic reactions than the avian strain. They are given intramuscularly. There have been postvaccinal reactions in cats (see discussion of Vaccinosarcoma).
  • Cell culture produced, inactivated vaccines have largely supplanted the MLV vaccines because of safety considerations. Only inactivated vaccines are used in the US.
Vaccination Schedule

Maternal antibodies from vaccinated females will protect most neonates until three months of age. There are several types of vaccines but the first injection should be given at three months (dogs and cats) and the second injection at one year of age. Animals should be revaccinated every 2 - 3 years.

Human Exposure

Decisions frequently have to be made by veterinarians regarding human exposure. It is important to remember that the virus can be in the saliva of infected animals from 1 to 13 days before clinical signs occur. The location and severity of the bite are quite important. Persons bitten in the head region by a stray animal should begin treatment immediately if the animal cannot be found. When the animal is available and has not been vaccinated, it should be sacrificed and the brain examined by the FA test. Attempts, still largely in the experimental stage, are being made to immunize certain wild animals in endemic areas with vaccine-laced baits in order to prevent rabies and thus halt its spread.

Public Health Signicance

All individuals at high risk of exposure, e.g., veterinary practitioners, staff of veterinary clinics, diagnostic laboratory workers, animal control personnel, park rangers, etc..., should be regularly vaccinated. Vaccination may be advisable for those travelling to countries where rabies is endemic. In the event that a vaccinated individual is exposed they are given the same post exposure regimen that an unvaccinated person would receive. It is most important that all bite or scratch wounds be thoroughly cleaned and washed with plenty of water and soap or detergent. The risk of rabies transmission to humans needs to be carefully evaluated. To be considered are the prevalence of the disease in the area, the nature and extent of the exposure and the behavioral status of the suspected animal. Dogs, cats, wild carnivores and bats are the most frequent sources of human infections. If the animal responsible for the possible exposure, regardless of its behavior, can be sacrificed it can be subjected to immediate diagnostic testing. As was mentioned above, suspected animals should be confined for ten days. If suspicious signs develop, the animal should be killed and submitted to the laboratory for tests. Regimens employing inactivated human or animal cell culture vaccines are use for prevention in high-risk personnel and those exposed. Rabies immune globulin, or antirabies serum (equine) are widely used for prophylaxis in exposed humans.

 

Bill & Wendy Johnson
Mount Airy, NC 27030
(336) 786-8391

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