Rabies is a viral disease of warm blooded animals (you won’t see Rabies in birds and reptiles) that occurs world wide, with significant human health significance due to its fatal nature. Various outbreaks have occurred in the United States in the last decade. Adequate vaccination of dogs and cats is the primary line of defense in preventing outbreaks.
The Centers for Disease Control and Prevention, along with the veterinary community, produce reports detailing the incidence of rabies in specific areas of the country. Thousands of animal rabies cases are reported every year, with the actual number of cases being much higher. The majority of rabies cases in animals in the U.S. occur along the East coast, with pockets of rabies in various other states.
Approximately 500 cases of human rabies are reported yearly, with the actual number again being probably much higher. Discrepancies in diagnosis and reporting make actual numbers hard to come by. Several countries are free of rabies, and institute extreme quarantine measures to prevent spread.
We have never seen a case of rabies in our hospital, a testimony to the effectiveness of the rabies vaccine.
Rabies is caused by an RNA virus belonging to the order Mononegavirales. All warm-blooded animals, including humans, are susceptible. Raccoons, skunks, bats and foxes, account for 90% of the cases in the U.S., with raccoons making up the majority, followed by skunks, bats, and foxes. Forty years ago it was the domestic animals that accounted for the majority of the cases. More cats get rabies than dogs.
When a wild animal is bitten by a rabid animal the virus enters the bloodstream, eventually spreading to the spinal cord and brain. It remains there for up to 3 months, during which time the affected animal has no symptoms of rabies. When the virus passes to the salivary glands the animal shows symptoms, and will usually die within 7 days. It is during this time that it can infect another wild animal, a human, or a dog or cat.
The main animals that infect humans are dogs, cats, cattle and horses, because they are exposed to these animals much more than wildlife. In addition to bite wounds, the virus can rarely be transmitted through the mucous membranes, as an aerosol, and through cornea transplants.
Clinical signs of rabies are quite variable, with a change in behavior being one of them more consistent findings. This behavior change can be as subtle as apprehension, or as extreme as biting in a normally friendly dog. Dogs might chew at the site they were bitten when they became infected, and can even maim themselves.
As the disease progresses dogs may show increased irritability, viciousness, excitability, and eating unusual objects (pica) like wood. These dogs may hide in dark or quiet places, and will bite when provoked. Central nervous system signs like seizures will exhibit, and there may be paralysis prior to death.
A phase of the disease causes paralysis of the muscles in the throat. This leads to excessive drooling and choking sounds due to an inability to swallow, and is the sign most people think of when describing rabies. It is also common for people to think their dog has something stuck in its throat, and cause themselves to be exposed to virus laden saliva when attempting to removed the suspected foreign body.
The disease is suspected in dogs that show neurological signs consistent with rabies, and may or may not have been bitten by another animal. Since these signs are so variable rabies needs to be considered in any dog showing behavioral changes. Blood samples are not helpful in the diagnosis. The only way to confirm a diagnosis of rabies in animals is to have the brain examined. A test called Fluorescent Antibody (FA) is performed on the brain cells of a dead animal.
In humans tests for the virus are performed on saliva, serum, spinal fluid and skin biopsies. In some cases the test checks for antibodies to the rabies virus, in other cases it looks for the virus itself.
Animals that have rabies are not treated because they can shed the virus in their saliva for extended periods.
The vaccination of dogs by a licensed veterinarian is the most effective means to control rabies. Every state has specific laws regarding vaccines. They are usually given to dogs at 4-6 months of age, repeated one year later, then every 3 years. In order to get a dog license, a certificate of vaccination by a veterinarian licensed in that state must be presented. Rabies vaccine is given to animals only under the supervision of a veterinarian licensed in that state. Cats are also given rabies vaccines.
In humans a pre-exposure vaccine is given to high risk groups, usually veterinarians, animal handlers, and laboratory workers. By giving this vaccine prior to any exposure to rabies, a person that eventually gets exposed to the rabies virus will need less post-exposure treatment, and will partially protect people that were exposed to rabies without realizing it. approximately 18,000 people per year received this pre-exposure vaccine, while 40,000 people per year receive the vaccine after they have been exposed.
Public Health Significance
Rabies has extreme human health significance due to the fatal nature of this disease. Symptoms include fever, headache, anxiety, confusion, hypersalivation, paralysis, and ultimately even death. In the early 1900’s more than 100 people died annually from rabies in the U.S. That number is down to 1-2 per year, because of vaccination of domestic animals and post exposure treatment. Most people in the U.S. die from rabies because they were not aware they were exposed to the virus, and never sought treatment. Post exposure treatment in humans has to be instituted before any symptoms appear for it to be effective in preventing death.
Most humans are infected by a dog bite, therefore aggressive wound cleansing can be of help. Tens of thousand of people are given rabies shots after being bitten, this therapy has proven to be highly effective. High risk groups (people that work extensively with sick animals like veterinary hospital personnel) can receive vaccines prior to exposure. animals that have bitten people must be quarantined for 5-10 days, depending on local laws, to observe for any signs of disease. Animals that have bitten someone are not euthanized unless they have successfully passed their quarantine period or their brain is scheduled for an examination.
The Centers for Disease Control and Prevention has an extensive section on rabies if you would like more information.