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Feature Article

Human and animal bites: managing and preventing infection

STEPHEN MUHI, JUSTIN DENHOLM
OPEN ACCESS

Human bites

Human bite injuries comprise clenched-fist injuries, sustained when a closed fist strikes the teeth of another person, and occlusive bites, resulting from direct closure of teeth on tissue.5 Clenched-fist injuries are more common than occlusive bites, particularly in men,6 with most human bites occurring on the hands.7 Human bites result in a greater infection and complication rate than animal bites. Cultures of human bites are typically polymicrobial.7 Mixed aerobic and anaerobic organisms are common, with the most common isolates including Streptococcus spp. and Eikenella corrodens, which occurs in up to one-third of isolates.7 E. corrodens is a fastidious Gram-negative organism that has been associated with septic arthritis and culture-negative infective endocarditis (as part of the HACEK group [Haemophilus spp., Aggregatibacter spp., Cardiobacterium hominis, E. corrodens and Kingella spp.]).8

Some authors suggest that all patients with human bites should be commenced on antibiotic prophylaxis, given the high risk of infection.6 The choice of antibiotic therapy should cover E. corrodens, which is resistant to first-generation cephalosporins (such as cefalexin), flucloxacillin and clindamycin, antibiotics that are often used for skin and soft tissue infections. Table 1 provides the Australian Therapeutic Guidelines’ antibiotic recommendations for empirical therapy of bite infections,9 and Table 2 lists additional antibiotic recommendations according to specific animal and pathogens.5-8,10-45

Blood-borne viruses, particularly hepatitis B virus (HBV), may also be transmitted by human bites, although the risk is low and evidence is limited to case reports.6 Baseline serology, including human immunodeficiency virus (HIV) serology; HBV surface Ab (HBsAb), core Ab (HBcAb) and surface Ag (HBsAg); and hepatitis C virus (HCV) serology, should be performed in human bite recipients. HIV post-exposure prophylaxis is generally not recommended, but may be considered in very high-risk situations (i.e. in patients with bites from known HIV-positive sources where complicated wounds occur).46 If the recipient has no immunity to hepatitis B, and chronic HBV infection has been excluded, vaccination should be initiated. If the recipient is unvaccinated, and the source is known to be HBsAg positive, or if the HBV status of the source cannot be ascertained, the exposed person should receive a dose of hepatitis B immunoglobulin (HBIG) within 72 hours of a percutaneous exposure.47 HCV serological testing is performed three months after exposure.46 When relevant and possible, testing of both parties for blood-borne viruses is also recommended, as these can also be transmitted from the patient to the biter (reverse exposure).48

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Dog bites

Most mammalian bites (about 80%) are dog bites,2 usually occurring from a dog that is known to the bite recipient.49 ­Children are especially vulnerable to dog bites, and have increased risk of head and neck injury due to their smaller size, and possibly also due to increased provocation.2 Most dog bites occur in males (56%), who also suffer most animal-bite fatalities.2

Mixed anaerobic and aerobic cultures are often present in dog bites.10 Almost ­one-fifth of dog bites become infected, ­especially when the hand is involved.10 The most ­common pathogens in dog and cat bites are Pasteurella spp., isolated in most cat bites and in half of all dog bites.10 Pasteurella canis is most often isolated from dog bites, whereas Pasteurella multocida and Pasteurella septica are more often ­isolated from cat bites.10 This is relevant for treatment, as Pasteurella spp. are generally resistant to many antibiotics used to treat cellulitis, including anti­staphylococcal ­penicillins (such as flucloxacillin), first-­generation cephalosporins and clinda­my­cin.10 However, P. multocida is usually ­susceptible to penicillin, amoxi­cillin, quinolones, doxycy­cline and trimethoprim-sulfamethoxazole.11

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Although Capnocytophaga canimorsus is rarely associated with dog bite wounds,10 it can invade the host and cause a fulminant infection, including disseminated intravascular coagulation and septic shock, usually affecting immunocompromised and asplenic patients.6 C. canimorsus has been described as an emerging infection with a case fatality rate of 26% in one recent study.11 Penicillin is the drug of choice, and third-generation cephalosporins and beta- lactamase inhibitor combinations are other options with good activity.

Cat bites

Cat bites are the second most common type of animal bite, responsible for more than 7% of mammalian bites in Victoria, most of which occur in females.2 Although less prevalent, cat bites become infected in up to 80% of cases, with P. multocida isolated in three-quarters of infections.12 One ­proposed mechanism for the very high infection rate is that the sharp teeth of the cat may result in deep tissue puncture and subsequent inoculation of bacteria.12 Patients with P. multocida infection may, therefore, present with deep infection, including tenosynovitis and septic arthritis,12 which may initially be underestimated. In contrast, Pasteurella spp. cellulitis is often characterised by rapid onset of spreading cellulitis (Figure 1), usually within 24 hours of the bite injury.13

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Bartonella henselae is a fastidious, Gram-negative bacillus, and the aetiological agent of cat-scratch disease (CSD).14 Although classically associated with cat (and particularly kitten50) scratches, it has been described following cat bites.51 CSD is characterised by the development of a vesicle at the inoculation site, followed by a self-limited regional lymphadeno­pathy about two weeks later that can persist for weeks to several months. Low-grade fever, malaise and myalgia are often reported by affected patients.14 Most cases of CSD are self-limited and do not require antibiotic therapy,14 although a short course of ­azithromycin may be used for severe or unresolving lymphadenitis. Other manifestations include culture-­negative endocarditis, neuroretinitis, pyrexia of unknown origin and bacillary angiomatosis in ­immunocompromised individuals, particularly those with AIDS.52 Specialist referral is required for patients with these complex conditions.

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Dr Muhi is an Infectious Diseases Registrar at the Victorian Infectious Diseases Service, Royal Melbourne Hospital. Associate Professor Denholm is Medical Director of the Victorian Tuberculosis Program; Senior Staff Specialist at the Victorian Infectious Diseases Service, Royal Melbourne Hospital; and Principal Research Fellow in the Department of Microbiology and Immunology, University of Melbourne, Melbourne, Vic.