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Feature Article

Human and animal bites: managing and preventing infection

STEPHEN MUHI, JUSTIN DENHOLM
OPEN ACCESS

In addition to vaccination, HRIG is recommended for all non-immune patients with category III exposures, as long as no more than seven days has elapsed since administration of the first dose of vaccine (or the vaccine may be inactivated by immunoglobulin).47 HRIG should be infiltrated in and around the wound, and any remaining HRIG should be administered in an area proximal to the wound.47

Bites from ungulates

Ungulates (hoofed animals) include horses, cows, sheep and pigs. Bites from these animals are often polymicrobial, including mixed anaerobes, Gram-positive bacteria including Staphylococcus and Streptococcus spp. and Gram-negative bacteria including Pasteurella and Actinobacillus species.13

Actinobacillus spp. are Gram-negative bacilli that are part of the normal flora of many ungulates (including horses, sheep, cattle and pigs).13 Numerous cases of bites infected by Actinobacillus spp. have been reported, and they may result in a purulent, malodorous abscess at the site of injury.21 Actinobacillus spp. are closely related to Pasteurella spp., so additional molecular testing to identify the organism should be requested in the appropriate setting.22 Actinobacillus spp. are broadly sensitive to many classes of antibiotics used for bite-­associated skin and soft ­tissue infection.13

Cattle, sheep and goat bites

Parapoxviruses include the orf virus, transmitted from sheep and goats (‘scabby mouth’), and bovine papular stomatitis virus, transmitted from cattle.23 These viruses may produce a lesion at the site of inoculation (usually on the fingers or hands), developing from a papule to a vesicle and eventually a red target-shaped ulcer over the course of one to two months.23 The lesion often resolves by six weeks, although infection in the immunocompromised may require antiviral therapy and requires specialist referral.

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Horse bites

Similar to other ungulates, horse bite infection is often polymicrobial and associated with several infections, particularly Actinobacillus spp. infection. Horse bite infection was shown to be the third leading cause of animal bite injury in one Victorian study.2

Streptococcus equi is a commensal of the upper respiratory tract of horses; infection in horses results in massive lymphadenopathy and airway obstruction known as ‘strangles’.21 Human disease due to this organism has rarely been reported but has resulted in serious disease. Transmission of S. equi subspecies zooepidemicus after horse bite has been reported, causing meningitis and brain abscesses with long-term neurological sequelae.24

Rhodococcus equi is a Gram-positive bacteria carried in the gut of many domesticated animals and distributed worldwide.25 It is weakly acid-fast and related to Nocardia spp. R. equi infection is typically associated with horses and particularly foals. Transmission is incompletely understood, but may occur via inhalation or inoculation into a wound or mucous membrane.25 Various symptoms may occur, although many have pulmonary involvement.22 The incidence of this infection has increased markedly, particularly as an opportunistic pathogen in the immunocompromised host.25

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Hendra virus is transmitted by physical contact with oral and nasal secretions of an ill or dead horse. Only seven cases have occurred since the first detection of this virus in 1994, and four people have died.26 Although transmission via horse bite has not been reported, personal protective equipment should be used when handling an unwell horse and prevention with equine vaccination is vital.27 Given the high case fatality rate, patients with suspected ­Hendra virus infection should be referred to hospital for specialist assessment. Diagnosis relies on polymerase chain reaction (PCR) identification and serology, which is available at reference laboratories.26 No specific treatment or human vaccine is available.

Pig bites

Of all ungulates, pigs are the most likely to bite and are often associated with polymicrobial infection including Pasteurella and Actinobacillus spp.28 Streptococcus suis is an emerging zoonotic infection, responsible for septicaemia and meningitis in pigs and humans with significant occupational exposure.29 More than 1600 cases in humans have been reported, predominantly in Asia, with transmission occurring cutaneously,29 including following pig bite.30

Bites from possums and other native species

Wombats, koalas and kangaroos seldom bite, although the rising urban exposure to possums increases the risk of bites.31 Possum bites have been associated with
a number of infections, including tula­raemia.59 Tularaemia is caused by the Gram-negative intracellular bacterium Francisella tularensis, which is widespread in the northern hemisphere and rarely causes infection in Australia. Tularaemia may be transmitted from a wide variety of animals, causing several classical syndromes depending on the route of exposure.59 The most common form (about 80% of cases) is ulceroglandular tularaemia, with ulcerated skin lesions and painful lymphadenopathy.59 Only a handful of cases have ever been reported in Australia, most recently in 2011, when two women were bitten by a ringtail possum in Tasmania and developed the ulceroglandular syndrome.59 This organism’s high virulence and infectivity (as few as 10 organisms required for infection) has led to its reputation as a potential bioterrorism agent.60

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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.