Introduction:
Rabies is an acute viral encephalomyelitis that principally affects carnivores and insectivorous bats, although it can affect any mammal. It is almost invariably fatal once clinical signs appear. Although rabies occurs throughout the world, a few countries are free of the disease due to successful eradication programs or to their island status and enforcement of rigorous quarantine regulations.
Etiology and Epidemiology: Rabies is a rhabdovirus that characteristically is confined to one species in a given geographic area, although extension to other species is common. Identification of different variants, or ecotypes, by laboratory procedures such as monoclonal antibody analysis and polymerase chain reaction (PCR) has greatly enhanced understanding of rabies epidemiology. Generally, each ecotype is responsible for rabies transmitted between members of the same species in a given geographic area. From an epidemiologic perspective, it is preferable to use the name of the species acting as the reservoir as an adjective: rabies maintained by dog-to-dog transmission is canine rabies, whereas rabies in a dog as a result of extension from a different reservoir, eg, skunk (or fox), should be referred to as skunk (or fox, etc) rabies in a dog.
No cat-to-cat transmission of rabies has been recorded, and no feline ecotype is known. However, cats are very susceptible to all ecotypes, and extension is common. Virus is present in the saliva of rabid cats, and there have been reports of people developing rabies after being bitten by a rabid cat. Reported cases in domestic cats have outnumbered those in dogs in the USA every year since 1987.
Transmission and Pathogenesis: Transmission is almost always by introduction of virus-laden saliva into the tissues, usually by the bite of a rabid animal. However, virus from saliva or tissue fluids may be introduced into fresh wounds or through intact mucous membrane (eg, ingestion). Virus may be present in the saliva and transmitted by an infected animal several days before onset of clinical signs (usually 3-5 days in domestic dogs and cats and up to 8 days in striped skunks). Rabies virus has not been isolated from skunk musk (spray).
The incubation period is both prolonged and variable; typically, the virus remains at the inoculation site for a considerable time. The unusual length of the incubation period is why postexposure treatment, including in man the practice of locally infiltrating hyperimmune serum, is possible. Most cases in dogs occur within 21-80 days after exposure, but the incubation period may be shorter or considerably longer.
After replication within muscle cells near the site of inoculation, the virus travels via the peripheral nerves to the spinal cord and ascends to the brain. After reaching the brain, the virus usually travels efferently via peripheral nerves to the salivary glands. Therefore, it is assumed that if an animal was capable of transmitting rabies via its saliva, virus will be detectable in the brain.
Clinical Findings: Clinical signs of rabies are rarely definitive. Rabid animals of all species exhibit typical signs of CNS disturbance, with minor variations among species. The most reliable signs, regardless of species, are behavioral changes and unexplained paralysis. Behavioral changes may include anorexia, signs of apprehension or nervousness, irritability, and hyper excitability (including priapism). The animal may seek solitude. Ataxia, altered phonation, and changes in temperament are apparent. Uncharacteristic aggressiveness may develop-a normally docile animal may suddenly become vicious. Commonly, rabid wild animals lose their fear of man, and species that are normally nocturnal may be seen wandering about during the daytime.
The clinical course is divided into three phases-prodromal, excitative, and paralytic. However, this division is of limited practical value because of the variability of signs and the irregular lengths of the phases. During the prodromal period, which lasts 1-3 days, animals show only vague CNS signs, which intensify rapidly. The disease progresses rapidly after the onset of paralysis, and death is virtually certain within 10 days after the initial onset of signs. Some animals die rapidly without marked clinical signs.
The term "furious rabies" refers to animals in which aggression (the excitative phase) is pronounced. "Dumb or paralytic rabies" refers to animals in which the behavioral changes are minimal or absent, and the disease is manifest principally by paralysis.
Furious Form: This is the classical "mad-dog syndrome," although it occurs in all species. There is rarely any evidence of paralysis during this stage. The animal becomes irrational and, with the slightest provocation, may viciously and aggressively use its teeth, claws, horns, or hooves. The posture and expression is one of alertness and anxiety, with pupils dilated. Noise invites attack. Such animals lose all caution and fear of natural enemies. Carnivores with this form of rabies frequently roam extensively, attacking other animals, including people, and any moving object. They commonly swallow foreign objects, e.g., feces, straw, sticks, and stones. Rabid dogs chew the wire and frame of their cages, breaking their teeth, and will follow a hand moved in front of the cage, attempting to bite. Young pups apparently seek human companionship and are overly playful, but bite even when petted, usually becoming vicious in a few hours. Rabid skunks appear to seek out and attack litters of puppies or kittens. Rabid domestic cats and bobcats attack suddenly, biting and scratching viciously. As the disease progresses, muscular incoordination and seizures are common. Death is the result of progressive paralysis.
Paralytic Form: This is first manifest by paralysis of the throat and masseter muscles, often with profuse salivation and inability to swallow. Dropping of the lower jaw is common in dogs. Owners frequently examine the mouth of dogs and livestock searching for a foreign body or administer medication with their bare hands, thereby exposing themselves to rabies. These animals are not vicious and rarely attempt to bite. The paralysis progresses rapidly to all parts of the body, and coma and death follow in a few hours.
Control: Comprehensive guidelines for control in dogs have been prepared by the World Health Organization and include the following: 1) notification of suspected cases, and destruction of dogs with clinical signs and dogs bitten by a suspected rabid animal; 2) reduction of contact rates between susceptible dogs by leash laws, dog movement control, and quarantine; 3) mass immunization of dogs by campaigns and by continuing vaccination of young dogs; 4) stray dog control and destruction of unvaccinated dogs with low levels of dependency on, or restriction by, man; and 5) dog registration.
The Compendium of Animal Rabies Control, compiled and updated annually by the National Association of State Public Health Veterinarians (NASPHV), summarizes the most current recommendations for the USA and lists all USDA-licensed rabies vaccines. Many effective vaccines, both modified live virus and inactivated types, are available for use in dogs throughout the world; in the USA, all currently marketed vaccines (for any species) are inactivated. Recommended vaccination frequency is 1 or 3 yr. Several vaccines are also available for use in cats, and a few for use in ferrets, horses, cattle, and sheep. Because of the increasing importance of rabies in cats, vaccination of cats is extremely important. No vaccine is approved for use in wildlife kept as pets, and protective immunity from the commercially available vaccines has not been demonstrated in these species.
Management of Suspected Rabies Cases-Exposure of Pets:
Where terrestrial wildlife or bat rabies is known to occur, any animal bitten or scratched by a wild, carnivorous mammal (or a bat) not available for testing should be regarded as having been exposed to rabies. The NASPHV recommends that any unvaccinated dog or cat exposed to rabies be destroyed immediately. If the owner is unwilling to do this, the animal should be placed in strict isolation for 6 mo and vaccinated against rabies 1 mo before release. Some rabies authorities recommend vaccination at the beginning of the isolation period. If an exposed animal is currently vaccinated, it should be revaccinated immediately and closely observed for 45 days
