Back Top Bookmark this page!

THE PET HEALTH LIBRARY
By Wendy C. Brooks, DVM, DipABVP
Educational Director, VeterinaryPartner.com

Print this article  Email this article
Rabies in Animals

Photo courtesy of U.S. Fish and Wildlife Service

Descriptions of rabies go back thousands of years as rabies has classically been one of the most feared infections of all time. It is caused by a bullet-shaped rhabdovirus that is relatively unstable in the environment; establishing infection requires direct contact with infected mucous membranes. In most cases, disease is transmitted by a bite wound. Only mammals are susceptible to infection, wildlife being the primary animal group where infection occurs. When wildlife come into contact with humans or domestic animals, rabies becomes a public health problem. Despite vaccination being readily available, every year the U.S. reports hundreds of dog and cat deaths from rabies, not to mention several human deaths. Worldwide some 55,000 human deaths from rabies occur and rabies remains an important and nearly untreatable illness even now in the 21st century.

Photo courtesy of U.S. Fish and Wildlife Service

Rabies is nearly untreatable once symptoms begin despite all the resources of modern medicine; it is important to take its threat seriously. It is because of rabies that most municipalities have dog licensing requirements to ensure that the community's dogs (and in some areas, cats as well) are vaccinated.

The most common wildlife species to spread rabies to domestic animals and humans in the Northern Hemisphere are the skunk, bat, raccoon, fox, and coyote. It should be noted in particular that wildlife, bats especially, are able to gain access to indoor areas and potentially infect pets and people.

Many people do not realize how fast death occurs from rabies.

While it may take a long time for the virus to incubate, once even mild symptoms begin, death occurs within 10 days.

Course Of The Disease

Photo courtesy of U.S. Fish and Wildlife Service

Virus in the infected animal's saliva enters the victim's tissues during the bite. The virus attaches to the local muscle cells for a couple of days before penetrating to local nerves and beginning its slow ascent to the brain. Once within nervous tissue, the virus is not accessible to the immune system and may safely proceed, though the journey is slow and takes up to one year (average time between bite and detectable virus in the brain is 20 to 30 days). Virus ultimately reaches the brain and in two to three days more is evident in all body secretions, including saliva. At this point, the disease becomes transmissible and symptoms begin.

It can take up to a year from the time of the initial bite before symptoms begin to show. In dogs, typical incubation periods are 21 to 80 days. Once symptoms show, treatment is nearly impossible.

Photo courtesy of U.S. Fish and Wildlife Service

Prodromal Stage (The First 1.5 Days After Symptoms Have Started)
A change in personality is noted. Friendly animals become shy, etc. The larynx begins to spasm and a voice change may be noted (especially true in rabid cattle). Most infected animals will actively lick or scratch the site of the original bite.

Excitative Stage (Next 2-3 Days)
Classically, this would be the "mad dog" stage. The animal has no fear and suffers from hallucinations. If confined, the animal often attacks the bars of the cage.

Paralytic Or Dumb Stage (Next 2 Days)
Weakness/paralysis sets in. The larynx is paralyzed, resulting in an inability to swallow, thus drooling and "foaming at the mouth" result. The animal dies when the intercostal muscles that control breathing are paralyzed. It is from animals in this stage where most human exposure occurs. There is no treatment for animals or humans once clinical signs appear.

Photo courtesy of U.S. Fish and Wildlife Service

Once the virus has been released to body secretions, it is again accessible to the immune system; however, the patient dies before an adequate immune response is mounted.

The classical symptoms of rabies described above may not be obviously recognizable, making diagnosis difficult if not impossible in a living animal. Long quarantines are often needed to determine if infection has occurred.

When human exposure to the animal in question is involved, what happens depends on an assortment of criteria. If the animal in question is dead, its brain can be tested for rabies. There is no test for rabies in a living animal but since we know that death follows quickly after the virus becomes contagious, a living animal can be confined for 10 days. If the animal is still alive 10 days after biting a person, then the bite could not have transmitted rabies.

Prevention

Bullet-shaped rabies virus particles in brain tissue (Photo courtesy of CDC)

Happily, rabies prevention is accomplished with vaccination and limiting exposure to wildlife. The standard killed-virus vaccines are available for both dogs and cats and, after the initial dose which is good for one year, subsequent doses are generally good for three years.  Because tumors have developed in cats who received killed virus vaccine, a recombinant product is now available that uses a portion of rabies viral DNA cloned into a harmless canarypox virus. This arrangement allows for the pet to realize the benefits of live virus vaccination without any risk of exposure to living rabies virus. Rabies vaccination protocols are typically controlled by municipal regulations. Most communities legally require vaccination of all dogs. The American Association of Feline Practitioners recommends rabies vaccination for all cats living in rabies endemic areas.

For pets not current on rabies vaccination that have been exposed to biting wildlife, the Texas Post-Exposure Rabies Prophylaxis Protocol has been particularly helpful. In this situation, the pet should be vaccinated for rabies as soon as possible after the wildlife bite with booster vaccines given 3 weeks post-bite and 8 weeks post-bite. The pet should be strictly isolated for 90 days. This protocol has been extremely successful in preventing rabies symptoms and contagion when normal rabies vaccination had lapsed. 

The Law Regarding Animal Bites (Against Humans)

This dog is suspected of being rabid as it had been exhibiting signs of restlessness and overall uncharacteristic aggressive behavior, which are two symptoms of rabies. Other symptoms of rabies in pets may include impaired walking, eating and drinking. Photo courtesy of CDC.

In my area (Los Angeles), if the biting animal has been legally vaccinated against rabies, only routine first aid may be necessary; bacterial infection of the wound may still be possible. If the animal has not been currently vaccinated, it must be confined for 10 days at the owner's expense for observation and then vaccinated at the end of that period.

The purpose of the ten-day period is to determine if rabies virus could have been in the animal's saliva at the time of the bite. An animal infected with rabies and shedding virus will certainly be dead within ten days.

If the biting animal is known to have been exposed to wildlife, the situation is different. A vaccinated animal must be re-vaccinated within 48 hours and confined for observation for 30 days. Unvaccinated animals must either be confined for 6 months or be euthanized and tested for rabies. All bites that break the skin are reportable to the health department. All dogs must be vaccinated against rabies. Wildlife/pet incidents leading to bites on the pet are of no concern to public health officials as long as no humans have been bitten.

Laws regarding biting dogs and rabies vaccination are highly regional. Check with your veterinarian or local animal regulation department to find out what you need to know.

If You Are Exposed

A fresh bite wound should be washed out with water quickly as this may wash out viral particles. The time it takes for the virus to reach the brain depends on the amount of virus there is in addition to how close the wound is to the head.

If the animal is dead, the head of the biting animal is submitted to the health department for fluorescent antibody testing for the virus. This process only takes a matter of hours so that any bite victims can know right away if they will require rabies treatment. If the biting animal is living, its vaccination status should be confirmed as soon as possible and it will need to be confined. The bite wound should be reported to the health department as soon as possible.

Hyperimmune (antibody rich) serum is flushed into the wound in hope of inactivating the virus before it penetrates to the nerves. The patient receives a vaccination on a regular schedule for about a month. In this way, when the virus comes out in secretions, a strong immune response is waiting to put down the infection.

For complete details, the CDC has information on post-exposure rabies.

Anyone pursuing a career with animals should consider vaccination against rabies.

Veterinarians, for example, have a rabies exposure risk more than 300 times that of the general population.

Quarantines When Traveling

Great Britain, Hawaii, and several other island areas have successfully eradicated rabies from their territory. These places are EXTREMELY cautious about allowing potential carriers of rabies in. Because of the long incubation period of rabies, a very long quarantine is needed; however, this must be balanced by the expense associated with quarantine and owners' reluctance to be separated from their pets. Most places that have eradicated rabies have protocols for avoiding or minimizing quarantine. Typically, a microchip is implanted in the pet for identification purposes, a rabies antibody titer (a measurement of vaccine-induced protection) must be performed at an approved laboratory, and rabies vaccine documentation is necessary.

For listings of what each state requires for entry, go to the USDA

For travel to another country check with that country's consulate, but guidelines are also available at USDA.

Other Links

The CDC's rabies home page has, in addition to basic information, a children's education area that is particularly helpful for families who go camping.

The annual World Rabies Day calls attention to this problem to raise awareness and provides information about rabies in both humans and animals.

 

Back Top Bookmark this page!