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Antibiotic Treatment Essential for Human and Animal Bites

Isaacs, Lawrence MD

ID Rounds: Part 2 in a Series

Dr. Isaacs is a clinical assistant professor of emergency medicine at the Temple University School of Medicine, an emergency physician at Lourdes Hospital of Burlington County in Willingboro, NJ, and the medical director of the Willingboro Ambulance Service, Endeavor.

Cat bites differ from dog bites in many ways. The cat's tooth is thin and sharp in contrast to the thicker dog's tooth. This results in cat bites generally being small, relatively deep wounds rather than the tearing, thicker punctures or lacerations of dog bites. The microbiology also is less complex, with Pasteurella multocida accounting for approximately 70 percent of infected bite wounds and Streptococcus and Staphylococcus species accounting for the most of the remaining 30 percent. It is important to remember that most animal bite wounds are polymicrobial, and a study by Talan et al1 revealed a median of five isolates from infected dog and cat bites.

Full-thickness cat bites are considered high-risk wounds, and should receive antibiotic therapy. Cat bites to the hand are especially dangerous because inoculation of the tendon is possible. Because they are puncture wounds, cat bites are impossible to irrigate, which probably plays a role in the higher incidence of infection. Overall, approximately 30 percent of all cat bites and scratches lead to some complications. As with P. canis infections, P. multocida is characterized by early onset of signs and symptoms, usually within 24 hours. The indications for rabies PEP is the same as with dog bites.

Cat-scratch disease is an illness that most commonly occurs after exposure to cat's saliva via a lick, scratch, or bite. Interestingly, it also has been reported after exposure to dog's saliva, although cats account for the vast majority of the disease. It is caused by Bartonella henselae, a gram-negative organism, and most patients are under age 21.

After a three- to 10-day incubation period, a tender papule appears at the site of inoculation. About two weeks later, very large regional lymphadenopathy, fever, and headache develop. These signs and symptoms will spontaneously resolve after several weeks to months. The treatment is controversial, with some stating that no antibiotic treatment is needed, and some stating that treatment with erythromycin, ciprofloxacin, or azithromycin will hasten recovery.

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Human Bites

Human bites, which are the third most common animal bite in the U.S., are most commonly polymicrobial. The hand is involved the majority of the time. There are two types of wounds, occlusional bites and closed-fist injuries. Occlusional bites are when the teeth are sunk into the skin, causing crush, lacerations, contusions, and abrasions. Closed-fist injuries usually occur in fistfights when the patient strikes his opponent in the mouth. The teeth lacerate the skin, usually over the third or fourth metacarpophalangeal joint, inject saliva into the joint, and possibly lacerate the extensor tendon. The wound is often about 1 cm long, and within hours becomes swollen and draining.

These are special wounds that require irrigation, antibiotics, and either close follow-up or admission for IV antibiotics and/or exploration and repair. S. aureus and streptococcal species are the two most common organisms cultured from infected human bites, but anaerobes including Proteus, Serratia, and Eikenella corrodens also cause infection. E. corrodens is a slow-growing gram-negative rod synergistic with S. aureus and Streptococcus, and is isolated from 10 percent to 30 percent of infected human bites. It is more common in closed-fist injuries.2

The two most controversial aspects on this topic are antibiotic use and wound closure. Remember that wound infections developing within 24 hours are highly suspicious for Pasteurella species, and wounds becoming infected after 24 hours are more likely polymicrobial. Simply put, amoxicillin-clavulanic acid is the reasonable empiric choice in human, dog, or cat bites. In patients who cannot afford or tolerate this drug, the choices require a little more thought. Patients with cat bites, especially those where P. multocida is suspected (early onset of infection), should receive penicillin, an extended-spectrum quinolone, TMP/SMX, or a tetracycline.

For empiric treatment when one does not or cannot suspect P. multocida and amoxicillin-clavulanic acid is not an option, consider doxycycline alone, a newer macrolide (clarithromycin or azithromycin), or a newer quinolone. The duration of empiric treatment should be five days while therapy for already infected wounds should be seven to 10 days. Cat bites that need to be treated are any full-thickness punctures, hand or lower extremity wounds, and those in patients over 50. In general, wound irrigation and closure is impossible so this rarely becomes an issue.

Dog bites are less likely to get infected and are more amenable to closure. Indications for empiric antibiotic treatment are the same for cat bites, with the addition of any wound that is debrided or closed. Choices, besides amoxicillin-clavulanic acid, include clindamycin and a quinolone or TMP/SMX. Because dog bites are the least likely to become infected of the three discussed here, the guidelines are to close primarily any facial wounds and wounds greater than 1.5 cm to scalp, trunk, or proximal extremities after irrigation and debridement, if necessary. Obviously infected wounds and hand and foot lacerations should be left open, and it is reasonable to delay primary closure for 48 to 72 hours for uninfected wounds.3

All patients with full thickness human bites should receive empiric antibiotic treatment. Again, amoxicillin-clavulanic acid is the drug of choice. Other options include azithromycin, clarithromycin, clindamycin, and the extended spectrum quinolones. As a rule, no human bites should be closed primarily. Delayed primary closure is an option in human bites. The practice of loose closure using sutures or steri-strips is the equivalent of full closure, so any wound not amenable to suture should not be loosely closed.

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1. Talan DA, Citron DM, Abrahamian FM, et al. Bacteriological analysis of infected dog and cat bites. N Engl J Med 1999;340:85.
2. Basadre JO, Parry SW. Indications for surgical debridement in 125 human bites to the hand. Arch Surg 1999;126:65 (abstract).
3. Capellan O, Hollander J. Management of lacerations in the emergency department. Emerg Med Clin NA 2003;21:205.
© 2004 Lippincott Williams & Wilkins, Inc.