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InFocus

Culture that Infected Cat Bite (and Give Antibiotics)

Roberts, James R. MD

doi: 10.1097/01.EEM.0000650972.61390.cb
    cat bite. Infectious flexor tenosynovitis developed only three days after a cat bite to the finger. Empiric V antibiotics, such as ampicillin-sulbactam (Unasyn) or piperacillin-tazobactam (Zosyn), monotherapy with a beta-lactam and a beta-lactamase inhibitor, are indicated. Also appropriate are ceftriaxone plus metronidazole or ciprofloxacin/levofloxacin plus metronidazole. Most importantly, this infection also requires urgent surgery. This finger demonstrates all four Kanavel signs of infectious tenosynovitis: fusiform swelling, the finger held in flexion, tenderness over the flexor tendon, and pain with passive extension. Keeping this condition in mind is paramount because patients do not always display all four signs.
    Figure
    Figure

    About 35 percent of households in the United States have pet cats. They are generally docile creatures, but can be unpredictable and occasionally bite or scratch for no good reason. Numerous diseases can be transmitted from cats to humans, primarily by a bite or scratch. Their long teeth, though small, can cause puncture wounds, while dog bites usually caused lacerations.

    Puncture wounds cannot be irrigated, accounting for a relatively high infection rate following a cat bite. Cat scratches are usually superficial and benign, but can also cause infection. Infections following cat puncture wounds are relatively common, and 50 to 80 percent of cat bites become infected. Patients occasionally experience septic sequelae such as meningitis, endocarditis, and osteomyelitis.

    The microbiology of cat bite infections is relatively complex, and it has been clarified that the organisms were causing deep wound infections and abscesses. Such infections are usually polymicrobial. Pasteurella, Streptococcus, Staphylococcus, and anaerobic organisms are isolated from most infections. The bacteria recovered from an infected cat bite usually reflect the oral flora of the cat, but some infections originate from the victim's own skin or the physical environment.

    Bacteriologic Analysis of Infected Dog and Cat Bites

    Talan D, et al.

    New Engl J Med.

    1999;340(2):85

    http://bit.ly/2NOTUBk

    This is one of only a few prospective studies evaluating the microbiology of infected cat bites that present to the ED. The authors noted that most infections from cat bites are benign, but there are occasional complicated wound infections, abscesses, meningitis, endocarditis, septic arthritis, and septic shock. The report includes data from 18 university-affiliated EDs, evaluating patients who met one of three major criteria for a bite wound infection.

    Wounds were considered infected if the patient had a fever, abscess formation, or lymphangitis and demonstrated erythema, tenderness around the bite, wound discharge, or leukocytosis. Wounds were cultured by a research microbiology laboratory specializing in isolating microorganisms from bite wounds. This specialized laboratory isolated organisms not previously identified by hospital laboratories as human pathogens. Fifty-five wound cultures were also sent to the local microbiology laboratories, but significantly fewer organisms grew in these cultures than in cultures sent to the reference laboratory (median: 1; range: 0 to 5; P<0.001).

    The study evaluated cat bites seen over an 18-month period, and patients had to have a cat bite that was large enough that a miniswab could be inserted into the wound to obtain a culture. The study group enrolled 57 patients with infected cat bites. The mean age was 39, and most patients were relatively healthy. About one-third of patients had an associated medical condition, including diabetes, steroid use, alcoholism, or chronic renal disease. Eighty-five percent of the cat bites were puncture wounds, and only three percent were lacerations. Nineteen wounds involved tendons, and one involved the metacarpophalangeal joint. The majority of the wounds were on the hands and arms.

    Most patients were administered some form of local wound care, such as washing with soap and water, peroxide, or topical alcohol. About 10 percent of patients had a fever, and 45 percent had an elevated peripheral white blood cell count. The median time between a bite and the collection of specimens was only 25 hours, attesting to the rapid onset of infection.

    The median number of isolates per culture was five organisms. Aerobes and anaerobes were isolated from 56 percent of the wounds, aerobes alone from 36 percent, and anaerobes alone from one percent. Seven percent of cultures had no growth. About 16 percent of cases presented with an abscess, and 48 percent had purulence or lymphangitis. The most common organism isolated in about 75 percent of infected cat bites was a Pasteurella species, usually P. multocida. Streptococcus and Staphylococcus were common aerobic organisms, but a number of anaerobes were also identified. About a third of patients were hospitalized and initially treated with various IV antibiotics. The most common IV antibiotic was a combination of a beta-lactam antibiotic and a beta-lactamase inhibitor, typically ampicillin and sulbactam (Unasyn). A large number of cephalosporins and penicillins were administered, often in combination. The majority of patients received antibiotics for 10 days.

    Although Pasteurella species were the most common pathogens identified in cat bites, a number of unusual anaerobes were also isolated. Those with Pasteurella infections had a rapid onset of infection following a bite, a previously reported phenomenon.

    These authors suggest the empiric therapy for infected cat bites should be directed against Pasteurella, Streptococcus, Staphylococcus, and anaerobes. Pasteurella was usually susceptible to a wide variety of antibiotics, some as simple as penicillin and others as complicated as third-generation cephalosporins. Many were susceptible to doxycycline and trimethoprim-sulfamethoxazole and fluoroquinolones. Of note, first-generation cephalosporins alone were often unsuccessful in treating Pasteurella infections. These authors concluded that empiric treatment with penicillin, ampicillin, or first-generation cephalosporins alone, common choices, was less than optimal. Suggested was a combination of antibiotics, usually a beta-lactam antibiotic and a variety of other antibiotics.

    Figure
    Figure:
    Typical puncture wounds from a cat bite. Note the swelling and redness, signs of an early infection. Most infected cat bites harbor an average of five organisms when a specialty lab performs the culture (aerobic and anaerobic organisms), but don't expect the same results from your basic hospital lab. A rapidly developing infection within 24 hours of the bite is typical for Pasteurella multocida infection. Antibiotics commonly used for soft tissue infection, such as cephalexin, dicloxacillin, clindamycin, and erythromycin, are not effective for cat bites infected with this organism. Amoxicillin-clavulanate (Augmentin) is a reasonable empiric single choice for prophylaxis and treating infected cat bites in outpatients.

    Comment: This is probably the largest prospective study on treating obviously infected cat bites. As we know from other studies, cat bites that become infected within 24 hours are usually caused by Pasturella multocida, but there are a number of Pasturella species. Only hospitalized patients were considered; it is not known how many patients were treated and released from the ED. The studied patients had more severe infections, but most infected cat bites can be treated as outpatients.

    Interestingly, most cat bite infections were polymicrobial, and this study revealed an average of five organisms isolated from an infected bite. Don't expect your hospital lab to give you this information. Many of the anaerobes were unusual and not commonly reported in infections, and they included many other organisms not routinely identified by clinical microbiology laboratories.

    It is unknown whether all of the organisms have to be treated to cure the infection. Antibiotics typically used for routine infections of skin and soft tissue, such as antistaphylococcal penicillins, first-generation cephalosporins, clindamycin, and erythromycin, are less effective against Pasteurella in vitro. Treatment with a first-generation cephalosporin alone in this study was unsuccessful in three cases in which Pasteurella was cultured. Suffice it to say that when patients are admitted with an infected cat bite, antibiotics other than those used for other types of soft-tissue and wound infections are required.

    It is generally agreed that amoxicillin-clavulanic acid (Augmentin) is a reasonable empiric single choice for treating cat bites in outpatients. P. multocida should be suspected for those who have had an early onset of infection. Amoxicillin-clavulanate would be effective, but so would alternatives such as penicillin and an extended spectrum quinolone, TMP/SMX, or tetracycline. It is my experience, however, that only a single antibiotic is usually prescribed, often only penicillin, and that seems to be effective. The literature suggests adding metronidazole to single outpatient antibiotics except amoxicillin-clavulanate, but I don't think this is often done in clinical practice. Note that antibiotics commonly prescribed for soft-tissue infections, cephalexin (Keflex), clindamycin, erythromycin, and dicloxacillin, are poor choices for treating or for prophylaxis of cat bites.

    Whether to use prophylactic antibiotics in a patient who presents with a noninfected cat bite is somewhat controversial, but it is usually recommended. Most clinicians would administer a prophylactic antibiotic to any patient with a cat bite who presents with a noninfected puncture wound, and a five-day course of amoxicillin-clavulanic acid is a good choice.

    Table
    Table:
    The Kanavel Signs of Infected Flexor Tenosynovitis*
    Table
    Table:
    Prophylactic and Empiric Outpatient Therapy for Cat Bites*

    Antibiotics are usually prescribed for bites that are not yet clinically infected, seemingly a common and suggested practice. A Cochrane review concluded that no evidence supports that prophylactic antibiotics are effective for cat bites. (Cochrane Database Syst Rev. 2001;[2]:CD001738.) This review, however, hedges on hand bites, the type of wounds that frequently become infected. Essentially, the Cochrane review and the general medical literature seem to support using prophylactic antibiotics after cat bites in hand wounds. The rationale for prophylactic antibiotics can be supported because the infection incidence after a cat bite is almost 50 percent.

    It seems reasonable to culture an infected cat bite puncture, but a microswab must be used. Trying to irrigate a puncture wound is counterproductive and not recommended. Cat puncture wounds that are not infected are not cultured, but almost all are given antibiotics in the hope they will prevent an infection. Hospitalization is often suggested for patients with an obvious severe infection, particularly immunocompromised ones, with an evaluation for septic arthritis, osteomyelitis, tenosynovitis, or rarely, endocarditis. It should be pointed out that a piece of the cat's tooth might be imbedded in a wound, although finding it would be quite a challenge even if suspected.

    Many cat bites occur on the hand, so be aware of the potential for infectious tenosynovitis, a surgical urgency. Most clinicians are familiar with the Kanavel signs, which strongly suggest the presence of infectious tenosynovitis and require hospital admission and rapid surgical consultation from the ED. See the great article on tenosynovitis in EMN's blog, The Case Files. (2018 Aug 21;40[8B]; http://bit.ly/2WPqmaT.)

    The medical literature is essentially devoid of studies or recommendations on what to do with simple cat scratches. Scratches alone can transmit cat scratch disease, but local infection does not seem to be a problem. Most clinicians advise local care and perhaps an antibiotic ointment for noninfected cat scratches. It seems that there are no recommendations to treat uninfected cat scratches with prophylactic antibiotics to prevent cat scratch disease. This is confusing to me because the disease is caused by bacteria introduced by the scratches.

    Finally, the literature suggests that indications for rabies prophylaxis are the same for cat bites as those for dog bites. Interestingly, there are more cases of rabies in cats than dogs in the United States. Rabies is rarely considered in my experience nor is prophylaxis provided for a pet cat bite. One should consider rabies prophylaxis if the patient has been bitten by a wild cat or when the bite was produced under aggressive circumstances by a sick cat.

    Empiric Intravenous Antibiotics for Serious Cat Bite Infections

    Preferred

    • Ampicillin-sulbactam (Unasyn)
    • Piperacillin-tazobactam (Zosyn)

    Alternatives

    • Ceftriaxone plus metronidazole
    • Ciprofloxacin/levofloxacin plus metronidazole
    • Meropenem
    • Ertapenem

    Not Recommended

    • Vancomycin
    • Erythromycin
    • Clindamycin
    • Cephalexin
    • Dicloxacillin

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    Correction

    The November InFocus article, “Racial and Insurance Disparities Shape ED Dental Care,” incorrectly described delivering anesthesia to the teeth with a mental nerve block or infraorbital nerve block. Neither of these blocks will provide anesthesia to the teeth. The infraorbital nerve block anesthetizes the upper lip, lateral nose, lower eyelid, and medial cheek. The mental nerve block provides anesthesia to the skin and mucosal surface of the lower lip and chin. An inferior alveolar nerve block will provide anesthesia to the pulp of the mandibular teeth, lower lip, chin, and anterior tongue. Supraperiosteal infiltration—placing local anesthetic at the apex of the root tip of individual teeth—provides anesthesia to each tooth, specifically the maxillary incisors, canines, and premolars. EMN regrets the error.

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    Dr. Robertsis a professor of emergency medicine and toxicology at the Drexel University College of Medicine in Philadelphia. Read the Procedural Pause, a blog by Dr. Roberts and his daughter, Martha Roberts, ACNP, PNP, athttp://bit.ly/EMN-ProceduralPause, and read his past columns athttp://bit.ly/EMN-InFocus.

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