Animal bite infections

Definition

The most common problem following an animal bite is simple infection. The saliva of dogs, cats, ferrets, and rabbits 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; about 1% of these 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, including 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 such deep structures as bones, blood vessels, tendons, muscles, and nerves. The canine teeth in a dog's mouth are also sharp and strong, often inflicting lacerations. Cats, with their needle-like incisors and carnassial teeth, 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 from cat bites.

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.

Doctors are increasingly aware of the importance of checking animal bite wounds for anaerobic organisms, which are microbes that can live and multiply in the absence of air or oxygen. A study published in 2003 reported that about two-thirds of animal bite wounds contain anaerobes. These organisms can produce such complications as septic arthritis, tenosynovitis, meningitis, and infections of the lymphatic system.

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. It is classified as a zoonosis, which is a term that refers to any disease of animals that can be transmitted to humans. More than 90% of animal rabies cases occur in such wild animals as skunks, bats, and raccoons, with such domestic animals as dogs and cats accounting for fewer than 10% of cases. 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 United States, as a result of good animal control practices. Onset is delayed, usually weeks to months after the person has been 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.

Most deaths from rabies in the United States result from bat bites; the most recent victim was a man in Iowa who died in September 2002.

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 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. Aztreonam has been reported to be effective in treating infections caused by P. multocida.

Because rabies is caused by a virus, antibiotics are not effective. In addition, as of 2003, 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 dose of vaccine given at the initial visit as well as one dose of human immune globulin. Additional doses of vaccine are 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. Wild animals should not be taken in as pets, and garbage or pet food that might attract wild animals should not be left outside the home or camp site. 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.

Domestic pets should be vaccinated against rabies; people should consult a veterinarian for advice about the frequency of booster vaccinations for the area in which they live. In addition, people who are traveling to countries where rabies is endemic should consider vaccination before leaving the United States.

Key Terms

Anaerobic

Referring to an organism that can live in the absence of air or oxygen. About two-thirds of animal bites are found to contain anaerobic disease-producing organisms.

Canines

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

Carnassials

The last upper premolar teeth in the mouths of cats and other carnivores, adapted to shear or puncture food. Carnassial teeth often cause puncture wounds when a cat bites a human.

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 . 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.

Tenosynovitis

Inflammation of the sheath of tissue that surrounds a tendon. Tenosynovitis is a common complication of animal bites containing anaerobic bacteria.

Zoonosis (plural, zoonoses)

Any disease of animals that can be transmitted to humans. Rabies is an example of a zoonosis.

For Your Information

Resources

Books

• "Central Nervous System Viral Diseases: Rabies (Hydrophobia)." Section 13, Chapter 162 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.
• "Infections of Bones and Joints." Section 5, Chapter 54 in The Merck Manual of Diagnosis and Therapy, edited by Mark H. Beers, MD, and Robert Berkow, MD. Whitehouse Station, NJ: Merck Research Laboratories, 1999.
• 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.

• Brook, I. "Microbiology and Management of Human and Animal Bite Wound Infections." Primary Care 30 (March 2003): 25-39.
• Dumyati, Ghinwa K., Nayef El-Daher, and C. Richard Magnussen. "Animal and Human Bite Wounds: Immunization, Prophylaxis, and Treatment." Consultant 37 (June 1997): 1501.
• Fooks, A. R., N. Johnson, S. M. Brookes, et al. "Risk Factors Associated with Travel to Rabies Endemic Countries." Journal of Applied Microbiology 94 (2003) (Supplement): 31S-36S.
• Garcia Triana, M., M. A. Fernandez Echevarria, R. L. Alvaro, et al. "Pasteurella multocida Tenosynovitis of the Hand: Sonographic Findings." Journal of Clinical Ultrasound 31 (March-April 2003): 159-162.
• "Human Rabies-Iowa, 2002." Morbidity and Mortality Weekly Report 52 (January 24, 2003): 47-48.
• Le Moal, G., C. Landron, G. Grollier, et al. "Meningitis Due to Capnocytophaga canimorsus After Receipt of a Dog Bite: Case Report and Review of the Literature." Clinical Infectious Diseases 36 (February 1, 2003): 42-46.
• Messenger, S. L., J. S. Smith, L. A. Orciari, et al. "Emerging Pattern of Rabies Deaths and Increased Viral Infectivity." Emerging Infectious Diseases 9 (February 2003): 151-154.
• Presutti, R. John. "Bite Wounds: Early Treatment and Prophylaxis Against Infectious Complications." Postgraduate Medicine 101 (April 1997): 243-54.
• Weiss, R. A. "Cross-Species Infections." Current Topics in Microbiology and Immunology 278 (2003): 47-71.
• Winner, J. S., C. A. Gentry, L. J. Machado, and P. Cornea. "Aztreonam Treatment of Pasteurella multocida Cellulitis and Bacteremia." Annals of Pharmacotherapy 37 (March 2003): 392-394.

• American Veterinary Medical Association (AVMA). 1931 North Meacham Road, Suite 100, Schaumburg, IL 60173-4360. http://www.avma.org.
• Centers for Disease Control and Prevention. 1600 Clifton Rd., NE, Atlanta, GA 30333. (800) 311-3435, (404) 639-3311. http://www.cdc.gov

• "Farm Animal Zoonotic and Reportable Diseases." University of Arizona, University Animal Care. 2000.
• National Association of State Public Health Veterinarians, Inc. "Compendium of Animal Rabies Prevention and Control, 2003." Morbidity and Mortality Weekly Report Recommendations and Reports 52 (March 21, 2003) (RR-5): 1-6.
• "Rabies Situation and Trends." Paris: World Health Organization. 2001. http://www.who.int/emc/diseases/zoo/rabies.html

Source: Gale Encyclopedia of Medicine, Published December, 2002 by the Gale Group

The Essay Authors are Julia Barrett, Rebecca J. Frey, PhD.

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