The three most common animal bites — dog, cat, and human — account for an estimated one million to two million emergency department visits a year, and the literature often has conflicting recommendations about treatment (antibiotic choices and closure). Although the microbiology of mouth flora is always mixed, each animal has unique organisms and some shared with the others.
Dog bites account for the vast majority of animal bites to humans. There are some 55 million dogs in the U.S., and the breeds most likely to bite a human include German shepherds, Rottweilers, Doberman pinschers, pit bulls, and malamutes.1 Children are more common victims than adults, are more likely to be killed, and although are more commonly bitten in an extremity, are more likely to have severe craniofacial injuries. Most feel that is because the child's head is at the same height as the dog's. One important point concerning dog bites to infant's heads is the possibility of cranial puncture; the potentially high pressure of a dog's jaw can easily penetrate the soft skull of an infant. Adults are more likely to be bitten on an extremity, either upper or lower.
The mechanism of the dog bite is a combination of tearing, puncture, and crush (dogs' jaws can produce 150 to 450 psi). These characteristics, unique compared with cat or human bites, have implications in management. Although the literature varies, the approximate complication rate from dog bites is 10 percent.
The microbiology of dog bites, like all bite wounds, is polymicrobial. Several organisms need to be considered in management and disposition. Pasteurella canis is the most common organism cultured from infected dog bites (approximately 50%), followed by Streptococcus and Staphylococcus.2Pasteurella is a gram-negative coccobacillus, and an opportunistic pathogen in humans.
The microbiology of dog bites, like all bite wounds, is polymicrobial
The signature characteristic of infection with this organism is its rapid onset (usually with 24 hours of bite). Capnocytophaga canimorsus (also known as CDC group DF-2) is a particularly virulent organism associated with dog bites, especially to immunosuppressed or asplenic individuals. DF-2 infections have been known to cause sepsis, gangrene, and DIC within two weeks of inoculation. Leptospirosis, a gram-negative spirochete that lives in amphibians, reptiles, and mammals, is another possible infection transmitted by dog bites. Infection with leptospirosis is characterized by a viral-like illness, headache, myalgias, vomiting, diarrhea, chills, and cough. Patients also develop aseptic meningitis, hepatic and renal failure (Weil's disease), iritis, and other ocular manifestations. The bacteria can be cultured from the blood, urine, or CSF; treatment is either PO doxycycline or IV penicillin.3
The rabies virus is a common concern with dog and cat bites although domestic animals only account for nine percent of human rabies in the U.S.4 Dogs and cats only carry five percent of animal rabies in the U.S.5 The rabies virus lives in the saliva of an infected animal, and once the skin is violated (by bite or saliva on broken skin), the virus infects the central nervous system. Untreated, it is nearly 100 percent fatal.
Bats, raccoons, coyotes, skunks, and foxes are the most common rabies carriers in the U.S. For dogs and cat bites, the Center for Disease Control and Prevention recommends basing the decision to begin PEP on several factors:
- ▪ Geography/epidemiology. The U.S.-Mexico border has the highest incidence of canine rabies in the U.S.
- ▪ Whether the animal has been immunized. Immunization is very effective in preventing infection.
- ▪ Whether the bite was unprovoked. Feeding or handling the animal is considered provoking the animal.
- ▪ Whether the animal can be observed for 10 days. If not, beginning PEP is reasonable if the bite was unprovoked.
Persons who routinely handle animals (veterinarians, animal handlers) may have received the rabies vaccine, which is protective for up to two years. A high-risk exposure, even within the two years, requires a modified protocol, which consists of just two doses of the vaccine. For previously unvaccinated individuals who require rabies PEP, the protocol is on day 0 to give a dose of the rabies immunoglobulin (RIG) at 20 IU/kg to provide the patient an immediate immune response. Part of or the entire dose should be infiltrated into the wound if possible, with any remaining IM distant from the wound.
In the U.S. there are three commercially available vaccines, and each 1 ml dose should be given IM in the deltoid on days 0, 3, 7, 14, and 28. The vaccine is considered safe in pregnancy. Two final points concerning these bites: Tetanus status should always be addressed, and washing the wound with a virucidal agent (i.e., povidone-iodine) has been shown in animal studies to decrease the incidence of transmission.6
Next month: cat and human bites.
1. Avener JR, Baker MD. Dog bites in urban children. Pediatrics
2. Talan DA, Citron DM, Abrahamian FM, et al. Bacteriological analysis of infected dog and cat bites. N Engl J Med
3. Mandel WJ, ed. Douglas and Bennett's Principle and Practice of Infectious Disease
. Churchill Livingstone. 2000, Vol. 1, 5th
4. Krebs JW, Rupprecht CE, Childs JE. Rabies surveillance in the United States during 1999. J Am Vet Med Assoc
5. Moran GJ. Dogs, cats, raccoons and bats: Where is the real risk of rabies? Ann Emerg Med
6. Human rabies prevention: United States, 1999. Recommendations of the Advisory Committee on Immunization Practices (ACIP). MMWR