Human and Animal Bites

Epidemiology^[1]

• Substantially more dog bites occur than cat bites.
• It has been estimated that dog bites account for 60-90% of bites, cat bites for 5-20% and human bites for 4-23%.
• One study reported that of an estimated 740 people per 100,000 annually bitten by dogs, only a small proportion seek medical attention. 2.6 in 100,000 require hospital admission. Half of all children had been bitten by dogs at some time, boys more frequently than girls. A telephone surgery of 1,184 families concluded that the annual incidence of bites in children aged under 15 years was 22/1,000.^[2]
• German shepherds, pit bull terriers, chows and Rottweilers are the most likely to bite but all dogs should be considered potentially dangerous.^[2]
• It is difficult to determine prevalence as many people do not report bites, but it is estimated that 175-740 per 100,000 people per year are bitten by dogs in Europe and the USA.
• The majority who present with animal bites are children, most commonly bitten by a household pet. Children under 5 years old were significantly more likely than older children to provoke animals before being bitten and are most at risk of serious injury.^[3]
• Most human bites occur during fights, although they may occur during sports or sexual activity. Clenched fist injuries, causing small wounds over the metacarpophalangeal joints, are particularly prone to infection.
• A UK study on management of human bites at an emergency medicine department found that, in a four-year period, 421 human bites were identified out of 3,136 case notes, approximately one every three days. Most patients were males aged between 16-25.^[4]

Assessment^[1]

All bites

• Document how and when the bite occurred.
• Document location and appearance of bite, and assess for underlying damage to tendons, arteries, nerves, joints or bones.
• Assess whether the person is at increased risk of infection, due to, for example:
• Diabetes.
• Immunosuppression.
• Site or nature of bite.
• Assess whether the wound is infected. (Look for redness, swelling, discharge, pain, cellulitis, lymphadenopathy and fever.) Take swabs if it appears to be infected.
• Establish history of tetanus vaccination.
• Ask about allergies to medication.
• Consider X-ray in:
• Clenched fist injuries.
• Cases where there is a crush injury, a possibility of fracture or foreign body in the wound.
• Penetrating scalp injuries.

Human bites

• Consider risk of blood-borne viruses (hepatitis B or C, HIV):
• Ask about the status of the person who bit them: known positive for hepatitis or HIV, known drug user.
• Establish the status of the person who has been bitten, in terms of known infection or vaccination against blood-borne viruses.
• Consider the circumstances of the bite:
• Alcohol.
• Domestic violence.
• Non-accidental injury or child neglect.

Dog and cat bites

Consider risk of rabies:

• Ask if bitten outside the UK (no risk if bitten in the UK by a cat or dog).
• Ask if the bite was from a domestic or a wild animal.
• Establish rabies vaccination status if the bite occurred outside the UK.

Differential diagnosis

• Consider non-accidental injury if there is an adult human bite inflicted on a child.
• For animal bites inflicted on children, consider poor parenting and supervision.
• Without a clear history, you may have to infer whether a bite has been received from a human, an animal or an insect.
• Non-traumatic infections can give an appearance resembling a bite.

Management^[1]

Initial management

• Irrigate the wound thoroughly with warm running tap water or normal saline to remove dirt and bacteria.
• Consider the need for tetanus immunisation and human tetanus immunoglobulin. Give a tetanus booster if primary vaccination course is not complete, or boosters are not up to date. Consider human tetanus immunoglobulin if it is a high-risk wound - eg, if it is contaminated with soil or manure.
• Bite wounds are rarely suitable for closure unless very fresh and with minimal risk of infection. See 'Wound closure', below.
• Advise over-the-counter analgesia where required.
• Consider the need for prophylactic antibiotics - see 'Antibiotic prophylaxis', below.
• If the patient has sustained a dog bite, however minor, in a country in which rabies is known to occur, in England contact the Health Protection Agency (HPA), which is now part of Public Health England (PHE).^[5]They will assess and follow the HPA guidelines for management.^[6]Vaccines are not supplied through this route for Scotland or Northern Ireland. Those in Scotland should discuss urgently with the local infectious disease consultant, and in Northern Ireland the Regional Virology Service or the Public Health Agency duty room. Details are available in the Green Book.^[7]
• To assess the need for prophylaxis, staff will enquire about the following:
  • Previous vaccination status.
  • Country where bitten.
  • Site and date of bite.
  • Provoked or unprovoked bite.
  • Domestic or feral dog.
  • Current health of animal - if known.
• Consider referral to A&E or plastic surgery for:
  • Bites involving arteries, nerves, muscles, tendons, or bones.
  • Penetrating bites to the hands or feet.
  • Facial wounds (excluding very minor wounds).
  • Bites where there is the possibility of a foreign body in the wound.
  • Devitalised wounds where extensive debridement is required.
  • Bites where the severity of the injury is difficult to assess.
  • People with infected wounds who are systemically unwell.

Wound closure

• Primary closure with sutures is not generally recommended for non-facial bite wounds, especially deep punctures, bites to the hand and clinically infected wounds. Delayed closure is usually more appropriate and should also be considered for wounds more than six hours old.
• Facial wounds and larger lacerations may require sutures (or Steri-strips®) to prevent scarring and improve cosmetic outcome. There is uncertainty about the risks of this but, in most cases, it is safe providing the person has presented early and the wound has been adequately cleaned.
• Delayed primary closure (after 3-5 days) is advisable for bites to the hand, bites with extensive crush injury, wounds needing a considerable amount of debridement and wounds more than six hours old.
• Cover with a sterile, non-adhesive dressing to protect the wound.

Antibiotic prophylaxis - when to prescribe

Prescribe prophylactic antibiotics for:^[1]

• All cat bites.
• All human bites under 72 hours old.
• All dog bites to the face, hand or foot, genitals or those that require surgical debridement.
• All bites which affect underlying structures such as tendons, ligaments, joints or bones.
• Wounds that have undergone primary closure.
• People at increased risk of infection due to:
• Diabetes.
• Cirrhosis.
• Immunosuppression.
• Asplenia.
• Prosthetic valves or prosthetic joints.

There is some difference between guidelines about the need to treat human and dog bites. As fewer than 20% of dog bites become infected, many will not need antibiotics, especially if superficial and easily cleaned. Studies are inadequate to date and the last Cochrane review was in 2001 and found very weak evidence for the benefit of prophylactic antibiotics.^[8]Similarly there is no strong evidence for the benefit of prophylactic antibiotics in human bites, and there is a case to be made for only treating those at higher risk.^[9]Studies suggest that 'low-risk' human bites (ie those which do not penetrate the epidermal layer) probably do not need antibiotic prophylaxis as long as they do not involve the hands, feet, joints, or cartilaginous structures.

Antibiotic prophylaxis - which antibiotic?^{[1, 10]}

Co-amoxiclav is recommended as first-line for all domestic animal bites. Alternatives for those allergic to penicillin include doxycycline plus metronidazole, oxytetracycline plus metronidazole, or clindamycin plus ciprofloxacin. Erythromycin alone is not effective.

Treatment should be for five days.

Treatment of established infection

• Most infections resulting from bites are polymicrobial, often including anaerobes.
• Pasteurella multocida is the most common infection, causing the most morbidity. It is resistant to flucloxacillin and erythromycin, and present in virtually all animal mouths.^[10]It is ten times more likely to cause infection after a cat bite than a dog bite.
• Infected dog bites often contain multiple species of bacteria, including:
  • Staphylococcus spp.
  • Streptococcus spp.
  • Eikenella spp.
  • Pasteurella spp.
  • Proteus spp.
  • Klebsiella spp.
  • Haemophilus spp.
  • Enterobacter spp.
  • Capnocytophaga canimorsus (formerly known as DF-2).
  • Bacteroides spp.
  • Corynebacterium auriscanis.
• Infected cat bites are usually due to P. multocida. Staphylococcus spp., Streptococcus spp., and anaerobes are also important pathogens.
• Infected human bites usually contain Streptococcus spp., Staphylococcus aureus, Eikenella corrodens, and anaerobic bacteria.
• Co-amoxiclav is recommended as first-line treatment for mild-to-moderate infections following a dog, cat, or human bite.
• If the person is allergic to penicillin, first-line treatment is with doxycycline plus metronidazole.^[11]
• Oral treatment is usually given unless the infection is severe.

Hepatitis B, hepatitis C and HIV

• Blood-borne viruses are potentially transmissible by a human bite if the skin is broken.
• The risk from a bite is thought to be about twenty times less than for needlestick injury.^[1]
• There are some cases reported where hepatitis C has been transmitted through bites. Although there is no prophylaxis available, serological testing and follow-up should be arranged in accordance with local guidelines.
• The risk of transmission from human bites is extremely low, as blood-borne viruses are not transmitted in saliva, unless there is blood present.^[12]
• If there is any suggestion of risk or genuine uncertainty then local guidelines should be followed for post-exposure prophylaxis with hepatitis B immunoglobulin and hepatitis B vaccine, and/or the use of antiretroviral drugs following exposure to HIV infection.
• If exposure prophylaxis for hepatitis B with immunoglobulin is considered necessary, it should be started preferably within 12 hours and not later than one week after exposure.^[12]
• If post-exposure prophylaxis for HIV is considered necessary it should be started as soon as possible, ie within an hour and not if presentation is after 72 hours.^[12]

Complications^[1]

• In children, dog bites frequently involve the face, potentially resulting in severe lacerations and scarring.
• Wound infection: occurs in 3-18% of dog bites, 20-80% of cat bites and 9-50% of human bites.^{[1, 13]}Risk of infection is particularly high in: puncture wounds, hand injuries, full-thickness wounds, wounds requiring surgical debridement and wounds involving joints, tendons, ligaments, or fractures.
• Less frequent complications include tetanus, rabies, septicaemia, septic arthritis, tenosynovitis, tendonitis, fractures, osteomyelitis, peritonitis, endocarditis, endophthalmitis, meningitis and disfiguring wounds from severe mauling.
• Necrotising fasciitis has been reported in a person with diabetes after an animal bite and can occur after a minor injury in immunocompromised patients.^[14]
• Psychological:
  • Increased anxiety.
  • Children may have nightmares and flashbacks.
  • Children who have suffered severe and multiple dog bites are at risk of developing post-traumatic stress disorder.
  • Permanent scarring and disfigurement may lead to depression and decreased self-esteem.
• Cat-scratch disease is caused by Bartonella henselae and can follow a bite or scratch from a cat or dog.
• C. canimorsus (part of the normal canine oral flora) has been associated with severe infections in immunocompromised patients, which may result in meningitis, endocarditis, kidney failure and septicaemia.^[15]

Prevention^[16]

Dog owners can be prosecuted by the police under the Dangerous Dogs Act 1991, which makes ownership of certain breeds (such as pit bull terriers) illegal. An amendment in 1997 removed the mandatory destruction order provisions of the 1991 Act by giving the courts discretion on sentencing and re-opened the Index of Exempted Dogs for those prohibited dogs which the courts consider would not pose a risk to the public. A civil claim can also be made against the owners for damages under the Animal Act 1991.