Forms of Rabies

by Food and Drug Administration (FDA)

(Page 2 of 3)

There are two forms of rabies. "Furious" rabies largely affects the brain and causes an infected animal to be aggressive or excitable (the very picture of the foaming-at-the-mouth, "mad dog" image people have of rabies). "Paralytic" or "dumb" rabies, mainly affects the spinal cord, causing the animal to be weak-limbed, lethargic, and unable to raise its head or make sounds because neck and throat muscles are paralyzed. In the beginning phase of paralytic rabies, an animal may also appear to be choking.

A wild animal acting unusually tame can also be an early warning sign for rabies. Both types of rabies are the result of the virus multiplying as it moves to the central nervous system and, finally, to all body organs and tissues.

Because rabies may be contagious before any clinical signs appear, a healthy-looking animal can transmit the disease. Rabies' incubation period, the time between infection and the appearance of symptoms, can range from a few weeks to a year or longer in humans, although 30 to 50 days is average. Animals usually develop symptoms between 20 and 60 days after exposure. The incubation period depends on bite location and the dose of virus received.

In humans, a string of worsening symptoms appears: itching or burning at the bite site, fever, headache, and appetite loss. The rabies sufferer grows restless, may experience hydrophobia (a fear of water, because of difficulty swallowing), have convulsions, or hallucinate. Signs of nervous system damage, such as paralysis or disorientation, follow; the only treatment is sedatives and painkillers. The person may slip into a coma, or die suddenly of respiratory or cardiac arrest.

Control in Animals

Experts agree that preventing rabies in people depends on controlling rabies in animals. Animal rabies vaccines are available for domesticated animals like dogs, cats, sheep, cattle, horses, and even ferrets. But controlling rabies in wildlife populations is a more challenging problem.

The sharp rise in animal rabies, particularly among raccoons, the most common animal carrier, led to a 1995 conditional license approval for an oral rabies vaccine for raccoons. The conditional license was issued by the Animal and Plant Health Inspection Service (APHIS), an arm of the U.S. Department of Agriculture, which regulates animal vaccines. The conditional license allows the vaccine to be dropped to wildlife habitats in vaccine-laced bait. Such conditional licenses are issued to meet emergency situations or "special circumstances," limited markets, or local situations, in this case defined by APHIS as a raccoon rabies "epizootic," or outbreak, stretching from Maine to Florida.

The oral raccoon vaccine is unique because it was created using genetic engineering.

"Scientists took a gene from the rabies virus and inserted it into a vaccinia virus using recombinant DNA technology, or technology where the genetic material in a cell is manipulated," explains Robert B. Miller, D.V.M., chief of virology in Veterinary Biologics with APHIS. "When the modified vaccinia virus, a member of the pox virus family, infects a cell, it produces a protein normally produced by the rabies virus. The animal's body recognizes the protein as foreign, and the animal develops an active immunity."

Preliminary studies showed that if the vaccine went directly to the stomach, it would be killed by stomach acids, so the vaccine is placed inside a tube of fish meal and is absorbed by the animal's tonsils, prompting the immune response. Although aimed at raccoons, Miller says the recombinant virus was tested for safety in over 50 different animal species before the conditional license was issued.

Because of how the vaccine is distributed, Miller says few humans are expected to be exposed. APHIS advised that local public health authorities in the areas where the vaccine is to be distributed be notified before bait distribution, and said authorizing state or federal agencies should be notified of any reports of human contact with vaccine-laden baits.

Nevertheless, Miller is concerned about the vaccine's use.

"One of my concerns is that we license its use but we don't control how it's used. I think a lot of art will be required in the use of this vaccine, because not only does it have to be effective, but [bait] placement requires special knowledge," he explains. "Baits are attractive to animals other than raccoons, so you need the proper timing of application and the appropriate number of baits so you get the most benefit."

CDC's Rupprecht says he is "encouraged" so far by the vaccine's use.

About the Author

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