Thursday, November 27, 2014

Definition

Rabies is a disease (caused by the rabies virus) primarily of animals, including both wild and domestic animals and human beings.

Although people usually associate rabies with dogs, among domesticated animals in the U.S., rabies today is more likely to be found in cats.

Description

Cats, dogs and cattle account for nearly 90 percent of rabies cases in domestic animals, with horses, mules, sheep, goats and ferrets making up the remaining cases.

Among wild animals, the disease is most often reported in skunks and raccoons.

Other wild species in this country in which rabies is commonly found include bats, foxes, and rodents.

The rabies virus, present in the saliva of an infected animal, is usually spread by a bite or scratch that punctures the victim's skin.

The virus has a strong affinity for cells of the nervous system. It enters nerve cells at the site of the wound, travels to the brain, and then follows other nerve pathways to muscles and organs that are especially affected by rabies.

There are at least two other ways in which humans have been known to have contracted rabies, both extremely rare. Two people were exposed by breathing the air in caves inhabited by rabid bats. And six people contracted rabies following implants of corneas from donors who had undiagnosed rabies.

The virus concentrates in the salivary glands, which explains why it is usually spread by bites. It also invades and damages the muscles involved in drinking and swallowing.

Most human victims, and apparently lower animals as well, suffer excruciating pain on swallowing liquids. Though they suffer from thirst, animal and human rabies victims can be terrified by the sight of water, hence another name for the disease, hydrophobia.

Symptoms

Symptoms usually develop between 20 and 60 days after exposure. Rabid animals may become aggressive, combative, and highly sensitive to touch and other kinds of stimulation. And they can be vicious. This is the "furious" form of rabies, the kind traditionally associated with mad dogs.

There is also a "dumb" form of the disease in which the animal is lethargic, weak in one or more limbs, and unable to raise its head or make sounds because its throat and neck muscles are paralyzed. In both kinds of animal rabies, death occurs a few days after symptoms appear, usually from respiratory failure.

In humans, the course is similar. After a symptom-free incubation period that ranges from 10 days to a year or longer (the average is 30 to 50 days), the patient complains of malaise, loss of appetite, fatigue, headache, and fever. Over half of all patients have pain (sometimes itching) or numbness at the site of exposure. They may complain of insomnia or depression.

Two to 10 days later, signs of nervous system damage appear, hyperactivity and hypersensitivity, disorientation, hallucinations, seizures, and paralysis. Death may be sudden, due to cardiac or respiratory arrest, or follow a period of coma that can last for months with the aid of life-support measures.

Questions

How is the rabies virus transmitted?

What are the symptoms in animals and humans?

How effective is the rabies vaccine?

Why is the vaccine given after exposure to the virus?

How long is the incubation period?

What can be done to prevent exposure to rabies?

Prevention

The advent of scientific medicine makes rabies control possible, not by cure but by prevention.

Unlike other immunizations, the rabies vaccine is administered after exposure to the virus. This unusual technique is successful because the rabies virus takes a comparatively long time to induce disease, a minimum of 10 days, and in rare cases, up to a year.

The length of the incubation period apparently depends on both the location of the wound - the farther from the brain, the longer the incubation - and the dose of virus received.

No matter where the wound, authorities emphasize that the first and most valuable preventive measure is thorough cleaning of the site with soap and water, and immediate medical attention.

If rabies vaccine treatment is called for, it should be started as soon as possible after exposure. Counting the first day of vaccine treatment as day 0, injections are administered on days 0, 3, 7, 14, and 28.

In addition to vaccine, patients who have not previously been vaccinated for rabies also receive an injection of rabies immune globulin (RIG) on the day they get the first vaccine (day 0).

RIG is prepared from the blood of persons who have been immunized against rabies and contains antibodies to the rabies virus. This "passive" immunity helps protect patients during the period before the rabies vaccine causes their own immune system to counter the virus (active immunity).