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Feature Article

Human and animal bites: managing and preventing infection

STEPHEN MUHI, JUSTIN DENHOLM
OPEN ACCESS

As its name suggests, the Tasmanian devil is a more aggressive marsupial which has resulted cases of bite infection, including P. multocida.61 The devastating devil facial tumour disease (DFTD), a unique infectious cancer in which tumour cells themselves transmit between biting hosts, has thankfully not been reported in humans.62

Reptile bites

Lizards are becoming increasingly common as pets, and are known to transmit Salmonella species.33 Generally, lizard oral flora is polymicrobial and reflective of the gut and skin flora of their recent meals. Although most lizards do not bite, case reports have described Serratia marcescens cellulitis following iguana bite,34 and Pseudomonas aeruginosa septic arthritis following a monitor lizard bite.35 Until recently, Komodo dragons were thought to harbour a deadly cocktail of oral flora, resulting in the gradual death of its prey following a bite. This has been challenged with data suggesting Komodo flora is comparable to that of other carnivorous species, with injury resulting from venom produced in previously unidentified venom glands in the lower jaw.36

Due to their territorial nature, crocodile attacks are often unprovoked, and survivors are likely to suffer deep and complex infections following the powerful bite of a crocodile.37 If the patient survives the injury (noting a 27% mortality rate in Australian attacks37), polymicrobial infection may result, including Clostridium spp, Aeromonas hydrophila, P. aeruginosa and Salmonella spp. involvement.37 In Australia crocodile bite wounds are also at risk of becoming infected with the environmental bacterium Burkholderia pseudomallei, which causes the potentially fatal infection melioidosis.37 Some Australian authors have therefore suggested that patients with crocodile wound infections be treated broadly with antibiotics, including ceftazidime for B. pseudomallei (and most Aeromonas spp.), penicillins for Clostridium spp. and metronidazole for anaerobes.38 

Snake bites in Australia rarely lead to severe localised tissue necrosis, and Australian snake venom is believed to be ­antibacterial in nature.31 Routine use of antibiotic prophylaxis is not recommended, although referral of patients for antivenom should always be considered.63

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Fresh and marine animal bites

Girt by sea, Australians may sustain wounds in marine environments, exposing them to a unique range of organisms not often encountered in everyday practice. These include Vibrio spp., A. hydrophila, Erysipelothrix rhusiopathiae and Mycobacterium marinum.39 Bites transmitting these organisms may result in a range of presentations, from simple cellulitis to necrotising fasciitis and septic shock. Although bites are an uncommon cause of marine animal injury in Australia,40 the significant morbidity and high mortality of subsequent infection warrants further consideration.

Vibrio spp. are classically located in warm estuarine waters, with a worldwide distribution. Vibrio vulnificus is a highly virulent pathogen associated with rapid and severe necrotising skin and soft tissue infection following traumatic injury, often requiring aggressive surgical debridement.39 Cases have been reported following shark and stingray bites.41,42 Blood cultures are positive in 30% of wound infections,43 and should be taken before antibiotic ­therapy is started. On the basis of animal studies demonstrating efficacy, the US Centers for Disease Control and Prevention recommends combination treatment with a third-generation cephalosporin plus doxy­cycline as initial empirical antibiotic for V. vulnificus infection,39 although other authors suggest doxycycline monotherapy for empirical Vibrio spp. therapy.44

A. hydrophila and other Aeromonas spp. are ubiquitous in aquatic environments, particularly fresh and brackish water, and are universally resistant to penicillins, amoxicillin/clavulanic acid and first­generation cephalosporins.39 Carbapenems, fluoroquinolones and aminoglycosides usually demonstrate activity against these organisms. Empirical therapy for bite infection following aquatic exposure includes a carbapenem or fluoroquinolone to cover these organisms and other co-infecting Gram-negative bacteria.39

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Infection with E. rhusiopathiae, a Gram- positive bacillus, is mainly seen following occupational cutaneous exposure to fish and shellfish,64 and is known as ‘shrimp picker’s disease’, ‘crab poisoning’ or ‘fish poisoning’. The bacterium is also ubiquitous in birds, reptiles and mammals, including pigs (which are thought to be a major reservoir), cats and dogs, which have also transmitted it following bites.10 E. rhusiopathiae also causes erysipeloid, a disease characterised by well-circumscribed, violaceous plaques, most often on the hands. Unlike erysipelas, erysipeloid occurs slowly over the course of one week and usually resolves within four weeks, even without treatment.65 Rarely, bacteraemia and endocarditis may also occur.39 Diagnosis is confirmed by isolating the organisms in culture, and it remains susceptible to penicillins, cephalosporins, clindamycin and ciprofloxacin. Of note, E. rhusiopathiae is intrinsically resistant to vancomycin, which is often used to empirically treat Gram-positive bacteraemia.39

‘Fish tank granuloma’ is caused by M. marinum, a slow-growing acid-fast bacteria, characterised by nodular, erythematous lesions most often on the hands and arms (Figure 2). It occurs in fish handlers and aquarium workers with a history of lacerations or punctures. Bites are one mechanism of inoculation, with case reports including dolphin bite and exotic fish bite.66,67 Diagnosis includes mycobacterial culture of biopsy samples or of the wound discharge, and/or PCR identification.45 Treatment with a prolonged course of combination clarithromycin and ethambutol is recommended,45 and usually continued for two months after symptoms have resolved.39

Although uncommon, seal bites are known to cause ‘seal finger’ due to Mycoplasma spp., including Mycoplasma phocacerebrale, which has been isolated from the bite of a seal-trainer’s finger.68 Infected bites present as painful, erythematous lesions, which respond rapidly to doxycycline monotherapy.68

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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.