Veterinary Q & A: Rabies

How is Rabies spread?
Most often, the rabies virus is spread via bite wounds. The virus lives in the saliva of infected animals, and is passed into the tissues of the victim after getting bitten. The virus can also be spread by getting the virus-laden saliva in an open wound, splashed in an eye, or other mucous membrane, such as the mouth. A rare method of transmission is via aerosol -- breathing in infected droplets. This can happen in a cave housing infected bats -- the wing beats can aerosolize the saliva secretions of the bats¹. Important to note: bat bites are very small, and may be completely unnoticed by humans. Any exposure or contact with a bat should be reported to your physician for discussion.

What happens after being bitten by a rabid animal?
The time between bite wound and showing signs is variable and often prolonged. After the bite, the bullet-shaped virus particle travels up the nearby nerves to the spinal cord, then the brain. A bite on the head or neck will usually produce signs quicker than a bite on the hind leg. Time of bite to clinical signs varies greatly - from few days to a few months. There are 3 clinical phases of the clinical course of rabies:

1. Prodromal period - first 1 to 3 days after the rabies virus reaches the brain.
   Vague neurologic signs that progress rapidly - Some animals may appear more tame, some will drool more. Death usually follows within 10 days due to paralysis.
2. Excitative stage - next 2 to 3 days.
   This is the "furious rabies" stage - tame animals suddenly become vicious, attacking humans and other animals as they roam and wander. Some animals will chew and eat odd objects (rocks, sticks, etc.). Paralysis is setting in, and losing the ability to swallow will cause frothing at the mouth.
3. Paralytic stage - follows excitative stage, or is the main clinical presentation for some animals.
   The throat and chewing muscles are paralyzed, and the animal is unable to swallow, causing excessive drool. The lower jaw often drops. This is a dangerous time for human exposure with domestic animals such as cows and horses - "choke"; (foreign body in the throat) can be a misdiagnosis, causing humans to be exposed as they investigate. Same with dogs who appear to be choking (drooling and dropped jaw). This is also the time when wildlife appear tame to humans and nocturnal animals appear in daylight. The paralysis progresses from the neck and jaw to all areas of the body, the animal falls into a coma, and death is within a few hours.

How is rabies diagnosed?
It is difficult to diagnose rabies based on clinical signs, since they can mimic other diseases, behavioral aggression, or toxin exposure. The preferred way to definitively diagnose rabies is to examine brain tissue. Here are some images of a rabies positive tissue samples from various animals. To do this, the deceased animal's head must be sent to the lab for examination of the brain tissue itself. There are also tests available to diagnose rabies in living humans (using tissues and samples other than the brain).

For those who are vaccinated against rabies (at-risk human workers and pets), there are serologic (blood) tests that measure antibodies to rabies produced in humans and animals vaccinated against rabies. This tests immunity and level of protection after vaccination, it is not used to diagnose clinical cases of rabies.

What is the treatment for rabies?
No treatment exists for animals and humans showing clinical signs of rabies. However, because it takes a long time for the virus to reach the spinal cord and brain (several days, weeks, or months), there is time to get immunoglobin shots and vaccinations against rabies to offer protection. Treat any bite wounds or other exposure as a possible exposure to rabies until proven otherwise.

How can I protect myself and my pets?
Vaccination of domestic animals is a must to protect against this fatal disease. Some exotic species are excepted, check with your veterinarian about your specific pets. High risk workers, such as veterinary staff, animal control officers and the like should also be vaccinated, and have their titers checked as recommended by their physician.

Regarding wildlife in the wild, do NOT approach wild animals! Especially those who are wandering in broad daylight and who appear "tame". For wildlife that enter the house, call animal control if possible for assistance. If that is not an option, contain the animal until authorities can be called. Do not touch the animal, even if deceased. Again, treat any bite wounds or other exposure as a possible exposure to rabies until proven otherwise.

¹The Merck Veterinary Manual, 8th ed., pp. 966-970.

Text: Copyright © Janet Tobiassen Crosby. All rights reserved.

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