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Animal Bite Infections

Definition, Description, Causes and symptoms, Diagnosis, Treatment, Prognosis, Prevention

The most common problem following an animal bite is simple infection. The saliva of dogs and cats is known to contain a wide variety of bacteria. According to one recent study, bacteria or other pathogens show up in about 85 percent of bites. When an animal bites, it can then transmit pathogens into the wound. These microorganisms may grow within the wound and cause an infection. The consequences of infection range from mild discomfort to life-threatening complications.

Description

Two to 4.5 million animal bites occur each year in the United States and about 1% of bites require hospitalization. Animal bites result in 334,000 emergency room visits per year, which represents approximately 1% of all emergency hospital visits, at an annual cost of $100 million dollars in health care expenses and lost income.

Children are the most frequent victims of dog bites, with 5-9 year-old boys having the highest incidence. Men are more often bitten by dogs than are women (3:1), whereas women are more often bitten by cats (3:1).

Dog bites make up 80-85% of all reported incidents. Cats account for about 10% of reported bites, and other animals (including rodents, rabbits, horses, raccoons, bats, skunks, and monkeys) make up the remaining 5-10%. Cat bites become infected more frequently than dog bites. A dog's mouth is rich in bacteria, but only 15-20% of dog bites become infected. In contrast, approximately 30-50% of cat bites become infected.

Many factors contribute to the infection rates, such as, for example, the type of wound inflicted, the location of the wound, pre-existing health conditions in the bitten person, the extent of delay before treatment, patient compliance and the presence of a foreign body in the wound. Dogs usually inflict crush injuries because they have rounded teeth and strong jaws; thus, the bite of an adult dog can exert up to 200 pounds per square inch of pressure. This pressure usually results in a crushing injury, causing damage to deep structures such as bones, blood vessels, tendons, muscles, and nerves. Dog canine teeth are also sharp and strong and also inflict lacerations. Cats, with their needle-like fangs, typically cause puncture wounds. Puncture wounds appear innocuous on the surface, but the underlying injury goes deep. Cat teeth essentially inject bacteria into the bite, and the deep, narrow wound is difficult to clean. Persons with impaired immunocompetence—for example, individuals with HIV infection—are especially vulnerable to infection.

The bacterial species most commonly found in bite wounds include Pasteurella multocida, Staphylococcus aureus, Pseudomonas sp, and Streptococcus sp. P. multocida, the root cause of pasteurellosis, is especially prominent in cat bite infections. Other infectious diseases from animal bites include cat-scratch disease, tetanus and rabies.

Causes and symptoms

The most common sign of infection from an animal bite is inflammation. The skin around the wound is red and feels warm, and the wound may exude pus. Nearby lymph glands may be swollen. Complications can arise if the infection is not treated and spreads into deeper structures or into the bloodstream. If the bite is deep or occurs on the hand or at a joint, complications are more likely.

Live, disease-causing bacteria within the bloodstream and tissues cause complications far from the wound site. Such complications include meningitis, brain abscesses, pneumonia and lung abscesses, and heart infections, among others. These complications can be fatal. Deep bites or bites near joints can damage joints and bones, causing inflammation of the bone and bone marrow or septic arthritis.

Cat-scratch disease is caused by Bartonella henselae, a bacterium that is carried in cat saliva; infection may be transmitted by a bite or scratch. Approximately 22,000 cases are reported each year in the United States; worldwide, nine out of every 100,000 individuals become infected. More than 80% of reported cases occur in persons under the age of 21. The disease is not normally severe in individuals with healthy immune systems. Symptoms may become serious, however, in immunocompromised individuals, such as those with acquired immune deficiency syndrome (AIDS) or those undergoing chemotherapy. Common symptoms include an inflamed sore in the area of the bite or scratch, swollen lymph nodes, fever, fatigue, and rash.

Rabies is caused by a virus that is transmitted through the bite of an animal that is already infected. More than 90% of animal rabies cases occur in wild animals such as skunks, bats, and raccoons, with domestic animals such as dogs and cats accounting for less than 10%. The World Health Organization (WHO) estimates that between 35,000 and 50,000 individuals worldwide die each year as a result of rabies. The highest incidence of rabies occurs in Asia where, in 1997, over 33,000 deaths were noted, most occurring in India. Rabies is nowadays rare in the U.S. due to good animal control practice. The delay of onset is usually weeks to months after being bitten. Early symptoms of rabies include fever, headache, and flu-like symptoms. These progress to anxiety, hallucinations, muscle spasms, partial paralysis, fear of water (hydrophobia), and other neurological symptoms as the virus spreads to the central nervous system. Medical treatment must be sought soon after exposure because death invariably follows once the infection becomes established.

Diagnosis

A medical examination involves taking the history of the injury and assessing the wound type and damage. Tetanus immunization and general health status are checked. An x ray may be ordered to assess bone damage and to check for foreign objects in the wound. Wound cultures are done for infected bites if the victim is at high risk for complications or if the infection does not respond to treatment. Evaluation of possible exposure to rabies is also important. A biting animal suspected of having rabies is usually apprehended, tested, and observed for a period of time for evidence of pre-existing infection.

Treatment

Treatment depends on the wound type, its site, and risk factors for infection. All wounds are cleaned and disinfected

This snake breeder shows the scar from his surgery after he was bitten by a venomous West African Gabon viper. His arm was cut open in order to relieve swelling from the snake bite in his middle finger. (Photograph by Joe Crocetta, AP/Wide World Photo. Reproduced by permission.) This snake breeder shows the scar from his surgery after he was bitten by a venomous West African Gabon viper. His arm was cut open in order to relieve swelling from the snake bite in his middle finger. (Photograph by Joe Crocetta, AP/Wide World Photo. Reproduced by permission.)

as thoroughly as possible. Bites to the head and face usually receive sutures, as do severe lacerations elsewhere. Puncture wounds are left open. If abscess formation occurs, the physician may perform an incision so as to drain the abscess.

If infection occurs, antibiotics are prescribed. Antibiotics may also be used for infection prevention. Since a single bite wound may contain many different types of bacteria, no single antibiotic is always effective. Commonly prescribed antibiotics are penicillin or a combination of amoxicillin and clavulanate potassium.

Because rabies is caused by a virus, antibiotics are not effective. In addition, as of 2001, there is no known cure for the disease once symptoms become apparent. It is therefore recommended that individuals with a high risk of contracting the disease (veterinarians, animal handlers, some laboratory workers) receive preexposure vaccination. Individuals bitten by an unknown or potentially rapid animal should receive postexposure vaccination, also called postexposure prophylaxis (PEP). The PEP regimen consists of one vaccine given at the initial visit as well as one dose of human immune globulin. Additional vaccines given on days 3, 7, 14, and 28.

Prognosis

Once a bacterial infection is halted, the bite victim usually recovers fully. There is no known cure for rabies once symptoms become evident and death is almost certain. WHO reports that 114 rabies deaths occurred in the Americas in 1997, with only four deaths occurring that year in the United States, thus emphasizing the importance of good animal control practice and postexposure prophylaxis.

Prevention

Preventing bites obviously prevents subsequent infections. Children under 12 years of age are at a higher risk for bites due to their small size and their inexperience with animals; therefore, they should be supervised with animals and taught to act appropriately around them. An animal that is unusually aggressive or behaving strangely (e.g. a raccoon that is active during the daytime) should be avoided and reported to the local animal control authorities; it may be infected with the rabies virus. All pets should be vaccinated against rabies and wild animals should not be taken in as pets. People should also avoid trying to break up fights between animals and should as a rule approach unknown cats and dogs very cautiously, especially on their territory. Finally, animals should not be trained to fight.

Resources

BOOKS

Kizer, Kenneth W. "Animal Bites." In Infectious Diseases. 2nd ed. Ed. Sherwood L. Gorbach, John G. Bartlett, and Neil R. Blacklow. Philadelphia: W. B. Saunders, 1998.

PERIODICALS

Dumyati, Ghinwa K., Nayef El-Daher, and C. Richard Magnussen. "Animal and Human Bite Wounds: Immunization, Prophylaxis, and Treatment." Consultant 37 (June 1997): 1501.

Lewis, Katherine T., and Melissa Stiles. "Management of Cat and Dog Bites." American Family Physician 52 (August 1995): 479.

Presutti, R. John. "Bite Wounds: Early Treatment and Prophylaxis Against Infectious Complications." Postgraduate Medicine 101 (April 1997): 243-54.

OTHER

"Farm Animal Zoonotic and Reportable Diseases." University of Arizona, University Animal Care. 2000. <http://www.ahsc.arizona.edu/uac/notes/classes/farmzoodisease00/farmzoo2000.html>.

"Rabies Situation and Trends." World Health Organization. 2001. <http://www.who.int/emc/diseases/zoo/rabies.html>.

Julia Barrett

KEY TERMS


Canines—The two sharp teeth located next to the front incisor teeth in mammals that are used to grip and tear.

Culture—A laboratory procedure in which a sample from a wound, the blood or other body fluid is taken from an infected person. The sample is placed in conditions under which bacteria can grow. If bacteria grow, identification tests are done to determine the bacteria species causing the infection.

Immunocompetence—An individual's ability to fight off infection.

Microorganisms—Microscopic organisms, such as bacteria, viruses, algae and fungi.

Pasteurellosis—A bacterial infection caused by Pasteurella multocida. Pasteurellosis is characterized by inflammation around the wound site and may be accompanied by bacteria in the bloodstream and infection in tissues and organs.

Pathogen—Any disease producing microorganism.

Postexposure prophylaxis (PEP)—Any treatment given after exposure to a disease to try to prevent the disease from occurring. In the case of rabies, PEP involves a series of vaccines given to an individual who has been bitten by an unknown animal or one that is potentially infected with the rabies virus.

Additional topics

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