An Overview of Rabies
Rabies, a serious disease caused by a rhabdovirus, is one of the most feared infections of all time. References to the illness go back thousands of years and, despite today’s modern vaccination programs, the virus is still a threat to a significant number of human and mammal lives every year. This disease is made particularly dangerous by the fact that all warm-blooded mammals are susceptible. Because the virus is classified as a major zoonosis (disease which may be transmitted from animals to humans under natural conditions), its spread has a considerable impact on public as well as veterinary health. Vaccinations have helped to diminish the number of cases annually, but rabies still exists even in developed countries and runs rampant through Third World countries in Asia. The only nations that remain untouched by the virus are Australia, New Zealand, the British Isles, Hawaii, Scandinavia, Cyprus, and Japan.
Appearance of disease symptoms after initial infection may take anywhere from a few weeks to several months, but once they have appeared there is no cure for the disease. Vaccination of your pets, including indoor cats, is extremely important.
Why and how might my dog become infected?
Rabies remains a threat due to its high prevalence among unvaccinated wild animals. Species that are particularly likely to carry the virus are known as “reservoir species.” These reservoir species vary widely depending upon geographic location, but in the United States they include raccoons, skunks, bats, foxes, and coyotes. Highly susceptible animals in other parts of the world include wolves, mongooses, and jackals.
The most common way in which domesticated animals become infected with the rabies virus is via a bite. Transmission has also occurred via ingestion of infected tissue or by aerosol exposure, but bites remain by far the most prevalent mode of infection. The amount of time that passes between initial infection and onset of clinical symptoms is highly variable, and what follows is a general overview of the average duration of each stage of infection.
Once in the body, viral particles undergo replication in muscular tissue at or near the point of entry. The virus remains in the muscle for a few days, then travels to local nerves and begins its journey to the brain. It is here, in the peripheral nerves, that the virus incubates. How long it takes to reach the central nervous system (CNS) and brain is somewhat dependent upon the site of infection; the closer it is to the brain, the shorter the incubation period will be. Even with this guideline however, incubation times are highly variable and impossible to predict.
Ultimately of course the virus completes its journey to the spinal cord and brain. Two days after its initial arrival in the CNS, viral particles are present in all body secretions and the victim is fully contagious. Clinical symptoms appear at or soon after this point.
How will my vet reach a diagnosis of rabies?
There is no definitive set of clinical symptoms that leads to a diagnosis of rabies. Certain symptoms are however associated almost exclusively with the disease. The progression of rabies infection has traditionally been divided into three stages – prodromal, furious, and paralytic (dumb). It should be noted that though these stages provide a rough outline of disease progression, there is a remarkably high level of variation associated with rabies, and every case is unique. Death occurs within 10 days after onset of clinical signs.
Animals experiencing the prodromal stage, which may persist for 2 to 5 days, often exhibit vague, nonspecific symptoms. Signs include apprehension, restlessness, loss of appetite, temperament changes and sometimes vomiting. This initial phase is followed by either the paralytic or furious form of the disease.
Approximately 25 to 30 percent of infected animals will progress from the prodromal stage to the furious form, which is much more common in cats than in dogs and generally lasts 2 to 4 days. This phase is characterized by an increased level of restlessness, wandering, viciousness, howling, panting, drooling, and occasionally convulsions. The classic “mad dog” image of rabies is perpetrated by the furious form. Affected animals will often attempt to attack objects that may or may not be real.
The paralytic form of rabies, which also lasts between 2 and 4 days, is most common in dogs. Those animals not progressing to the furious form will develop the paralytic form instead. Symptoms include ascending paralysis, beginning near the bite site and gradually progressing up the body, paralysis of the lower jaw, and facial paralysis. Biting is uncommon with this form, but excessive drooling does occur. Victims have difficulty eating and drinking. In dogs, a noticeable change in how the bark sounds occurs as the larynx becomes paralyzed. Symptoms progress to coma and death from respiratory paralysis.
Laboratory tests are the only certain way to diagnose rabies in animals, and must be carried out postmortem. In the case of a suspicious death, the head or entire body of the deceased animal is either donated to the local animal control office or given to a veterinarian who can send the specimen to the appropriate authority.
What are the treatment options for dogs with rabies?
Once clinical symptoms have appeared, there is no treatment. Rabies is nearly 100 percent fatal at this point. Any animal that has been exposed to rabies and is not properly immunized should be euthanized. If a vaccinated animal is exposed to the virus he should be re-immunized and kept under close observation for at least 3 months. Of course, if you ever suspect that your pet has been exposed to rabies, contact your veterinarian immediately.
How do I vaccinate my pet against rabies?
Vaccines are complicated substances and often raise many questions. For a brief introduction to vaccines and how they work, please goto An Introduction to the World of Vaccines.
Rabies vaccines should be considered a requirement for any domestic dog or cat. Vaccination is governed by law in most states. Rabies vaccines are generally administered between 16 and 26 weeks of age and require a booster shot that is given one year later. Beyond this point, recommendations and laws vary widely from state to state. A specific vaccination schedule should be discussed with your veterinarian.
How else can I help prevent the disease?
Contact with all wild animals, not just suspicious looking ones, should be avoided. Beyond that logical step, vaccination is the key!
Additional resources on the web:
All pet owners should be informed as to the seriousness of rabies both in the context of their pet’s health and of their own health. Further information regarding this virus may be found by perusing the following links.
The Centers for Disease Control provide links to a range of information regarding every aspect of the virus, from natural history to epidemiology to diagnosis and prevention.
This site powered by the National Institutes of Health and MEDLINE contains links to a vast array of articles on rabies, including general overviews, information on prevention and screening, and statistical data.
The World Health Organization site contains links to a variety of internationally oriented sites on rabies in humans and animals. The Pan-American Health Organization, a division of the WHO, provides weekly outbreak updates and statistics on rabies in the Americas.
For information on specific vaccines, visit the manufacturers' websites:
Pfizer Animal Health (the Defensor ® 1 and Defensor ® 3 Vaccines)
Intervet (the Prorab ®-1 Vaccine).
Schering-Plough Animal Health (the Rabdomun ® 1 Vaccine)