Rabies

Rabies Virus Infection

What Is Rabies?

Rabies is a disease caused by a virus that affects the brain and spinal cord function of mammals. Most cases in the United States occur in wild animals, including raccoons, skunks, bats, foxes and coyotes. It is also a well-known disease of domesticated animals, such as cats, cattle and dogs.

The rabies virus is a member of the Lyssavirus genus. It has a nearly global distribution and can be found on every continent except Antarctica, though it has been eliminated from parts of Western Europe. The World Health Organization (WHO) estimates that worldwide the disease causes 55,000 deaths each year, with most occurring in Africa and Asia.

Rabies occurs throughout most of the United States. The only exception is Hawaii, which has not had any infection occurring in the state. According to the U.S. Centers for Disease Control and Prevention (CDC), rabies was reported in nearly 7,500 animals in 2001. Of those cases, 93 percent occurred in wild animals. In the same year, there were no cases in humans. Since 1980, only one to two human deaths due to rabies have occurred each year in the United States according to the CDC.

Infection with rabies typically follows a bite or scratch by an animal with the disease. While once largely associated with dogs and other domesticated animals, pet vaccination programs in the United States and other industrial nations have significantly reduced the number of these cases that occur each year. Most recent cases of human disease result from exposure to bats. Between 1980 and 1997, more than half of the cases in the United States resulted from bats, according to the CDC. Outside the United States, dogs remain a prominent transmitter of the disease. The CDC estimates that rabid dogs cause 90 percent of human exposure and more than 99 percent of worldwide deaths due to the disease. Rabies may also become airborne, but transmission through this route has only been recorded in a laboratory setting.

After the rabies virus enters the body, it moves through the nervous system toward the brain and spinal cord (central nervous system or CNS). Once in the brain, the disease causes the most severe symptoms. Because of this movement pattern through the body, the incubation period is longer in a patient wounded on the foot than in one wounded on the head or neck. After the virus reaches the CNS, it begins to reproduce. These new viruses move into the salivary glands, where they can be transmitted.

Risk factors and causes of rabies

Rabies is a zoonosis that is usually transmitted to humans in the saliva of infected animals. The most common route of infection is a bite or scratch. The virus can also be transmitted if saliva comes into contact with the mucous membranes (e.g., eyes, nose and mouth) of an uninfected animal or person. Other means of transmission includes organ and corneal transplants, though control measures have been improved to reduce the risk of this mode of disease spread. Person-to-person transmission from bites and scratches is also possible but rare.

Other potential routes of rabies transmission exist. Transmission of the virus through respiratory exposure or skin contact with urine, feces or blood of infected bats is possible. The virus can become airborne, though transmission through this route is rare and has only been documented in a laboratory setting. Scientists also believe that rabies may potentially be spread through unpasteurized milk from an infected cow, but there are no reported cases of this occurring.

The rabies virus is unable to survive outside a host for long. It is highly susceptible to drying and ultraviolet light. This means that the virus is unlikely to be passed through normal contact between people or with animals.

Certain risk factors make a person more likely to be infected with rabies. These include:

  • Working with animals
  • Working with live rabies vaccine in a laboratory setting
  • Traveling to areas without domesticated animal vaccination programs

Signs and symptoms of rabies

After the signs and symptoms of rabies begin to appear, the disease is nearly always fatal. Infection with the virus affects the body’s central nervous system. It often results in inflammation of the brain (encephalitis) and spinal cord (myelitis).

Symptoms of rabies usually appear within one to three months of exposure but the incubation period may be just a few days or longer than a year. The location and severity of the infecting bite or wound affect how quickly symptoms begin. A severe bite to the head or neck will develop faster than one to the extremities.

Symptoms of rabies are often nonspecific in the beginning. They are similar to those of the flu and include fever, headache and a feeling of discomfort. These symptoms typically last for a few days before progressing to the more severe phase of the disease.

Symptoms of this phase are likely to include:

  • Insomnia
  • Anxiety
  • Confusion
  • Partial paralysis
  • Excitation
  • Hallucinations
  • Agitation
  • Excessive salivation
  • Difficulty swallowing
  • Fear of water (hydrophobia)

Some practitioners may classify the disease based on dominant symptoms. It may be considered “furious rabies” if hyperactivity is present or “dumb rabies” if the patient is paralyzed. Other symptoms and characteristics of the disease are common for both forms, so the use of this classification is largely uncommon.

The symptoms of rabies may last from two to 10 days before the patient becomes comatose. After symptoms begin, rabies is nearly always fatal due to the failure of either the respiratory or circulatory systems.

Diagnosis methods for rabies

Most frequently, a rabies diagnosis is based on the patient receiving a bite, scratch or other injury and the circumstances of the injury. The physician is likely to request further information to determine the possible risk of rabies exposure. Questions the physician may ask include:

  • Where did the incident take place?
  • What kind of animal was involved?
  • Was the exposure provoked (e.g., attempted handling, feeding) or unprovoked?
  • If the animal is domesticated, has it been vaccinated against rabies?
  • Can the animal involved be safely captured and observed or tested for rabies?
  • If the animal involved was killed, has its body been saved to be tested for rabies?

The information provided by the patient is important for the physician to determine whether the animal may have been rabid and the course of future treatment. An unprovoked bite by a skunk, raccoon, fox or other wild animal is likely to be presumed to have spread the virus unless the animal can be captured and tests negative for the disease. The physician may also contact local wildlife officials for more information on the prevalence of rabies in the animal population and the species’ potential for carrying the virus.

Bat bites are the current major route of rabies transmission in the United States. Bite marks are difficult to detect and the actual bite may occur while the patient is asleep. Whenever a bat is discovered in a bedroom, it is recommended that it be presumed positive for rabies until it can be safely captured and tested for the disease. It is also recommended that anyone who could possibly have been bitten by a bat while sleeping, or had direct contact with a bat, consult a physician.

While rabies may be suspected based on a known or suspected animal bite, multiple tests are required to confirm the diagnosis. Individually, the tests do not provide the sensitivity required to confirm whether a patient has been infected. However, when combined the tests are believed to be much more accurate.

Tissue samples used include:

  • Saliva. A sample from the patient’s mouth may be used to isolate and culture the virus or to be tested for genetic material of rabies.
  • Blood serum and spinal fluid. Blood tests and a spinal tap may be used to obtain samples to examine for antibodies produced by the body against the rabies virus.
  • Skin biopsy at nape of neck. Once in the body, rabies antigens move towards the brain and spinal cord. Before symptoms begin, they may be found in the nerves from the base of hair follicles.

These tests take time and the physician may choose to proceed with treatment while waiting for the results. If the animal suspected to be carrying rabies can be captured, it can be tested, with results typically available within hours. These tests examine brain tissue for antibodies and can only be performed on a dead animal. If the animal is a pet or other domesticated animal, it may be observed for up to 10 days for signs of rabies. If no indication of disease is seen during this period, it is likely not rabid.

The U.S. Centers for Disease Control and Prevention (CDC) developed the following recommendations for postexposure prevention or prophylactic treatment of rabies:


Animal Type Evaluation and Disposition of Animal Post-exposure Recommendations
Dogs, cats and ferrets Healthy animal that is available for 10 days observation Persons should not begin prophylaxis unless animal develops clinical signs of rabies.
Rabid or suspected rabid animal Immediately vaccinate the exposed person
Unknown Consult public health officials
Livestock, small rodents, lagomorphs (rabbits and hares), large rodents (woodchucks and beavers) and other mammals Consider each case individually Consult public health officials. Bites from squirrels, hamsters, guinea pigs, gerbils, chipmunks, rats, mice, other small rodents, rabbits and hares almost never require postexposure prophylaxis.

Skunks, raccoons, foxes and most other carnivores; bats

Regarded as rabid unless animal proved negative by laboratory tests

Consider immediate vaccination of exposed person. If the person had direct contact with a bat, vaccination may be recommended.

Treatment options for rabies

Rabies is almost always fatal without proper and timely treatment. The first step after receiving a scratch or bite is to wash the wound with soap and water. If available, the use of an over-the-counter antiseptic solution containing povidone-iodine is recommended to kill viruses and bacteria that could infect the wound. After the wound is thoroughly cleaned, it is recommended that the patient seek immediate medical attention.

If the physician suspects possible rabies infection, postexposure prophylactics are likely to be initiated. According to estimates by the U.S. Centers for Disease Control and Prevention, about 40,000 rabies postexposure treatments are given each year. For most people, treatment requires one dose of antibody (immune globulin) and five doses of vaccine over a four-week period.

Patients who have been previously vaccinated against rabies have a slightly different postexposure treatment schedule. They do not require the dose of immune globulin and patients receive two doses of vaccine. One injection of vaccine is given immediately and the second is given three days later.

If postexposure prophylactic treatment is not initiated before symptoms appear, it is not likely to be effective. Supportive treatment is recommended to make patients more comfortable as the rabies symptoms progress. This may require sedation and medications to reduce pain. Possible complications of rabies may occur and typically involve the respiratory, cardiovascular and nervous systems.

Most cases of rabies can be treated with prompt attention and most deaths occur because the patient did not seek treatment. If rabies does not prove to be fatal despite delayed treatment, the patient still is likely to have reduced neurological function.

Rabies is considered a reportable disease in both people and animals. All cases must be reported to local and state health officials who will pass the information to national organizations, and possibly the World Health Organization (WHO). These officials may require further information and may contact the patient or his or her family.

Prevention methods for rabies

A vaccine is available to help prevent rabies infection. The vaccine consists of rabies viruses grown in a laboratory and then inactivated. Within the body, the inactive virus causes the immune system to produce antibodies that become active in case of actual infection. To properly develop immunity, the vaccine is given in three doses. The schedule is as follows:

  • One dose on day 0
  • One dose on day 7
  • One dose between days 14 and 21

There are potential side effects of the rabies vaccine. These typically are minor and include pain, reddening of skin, and swelling or itching at the injection site. Other side effects are uncommon, but may include headache, nausea, abdominal and muscle pain and dizziness.

For most people, the risk of being infected with rabies is low and the vaccine is not recommended. In people at high risk of exposure to rabies, the vaccine may reduce the chance of being infected. The vaccine is recommended for certain people including:

  • Veterinarians
  • Animal handlers
  • Laboratory workers who handle live rabies virus
  • Anyone who may have regular contact with potentially rabid domesticated or wild animals

After vaccination, the body maintains a level of immunity for approximately two years. Booster shots may be taken to maintain immunity for those people at greatest risk of infection.

Despite the vaccine, if rabies exposure is suspected, further treatment is still required. Following exposure, those who have been vaccinated require fewer additional doses of vaccine and do not require a dose of immune globulin. Previously vaccinated patients also tend to have fewer side effects than those receiving the vaccine for the first time.

For people who are not in a high-risk category, vaccinating pets is highly recommended. Many areas require dogs, cats and ferrets to receive a regular rabies vaccination. This reduces the pet’s risks of being infected by another animal and then passing it on to others. Vaccination programs have been highly effective in the United States and other industrialized nations. Rabies has been eliminated or never spread to certain areas, such as Hawaii and Great Britain. Such areas may require proof of vaccination of incoming pets or have a quarantine period for the animals. In the United States, most cases of human infection result from exposure to wild animals.

Questions for your doctor

Patients may wish to ask their doctor the following questions related to rabies:

  • What animals are potential carriers of rabies?
  • What can I do to prevent rabies in my family?
  • What does rabies vaccination involve?
  • Who should get vaccinated against rabies?
  • Does the rabies vaccine completely protect a person from infection?
  • What are the treatment options for rabies?
  • Is it possible to tell if an animal has rabies?
  • If I am bitten by an animal that might be rabid, what should I do?
  • Other than animal bites, are there other ways rabies is transmitted?
  • Will being infected with rabies once prevent its reoccurrence?