Animal bite

Introduction

The majority of animal bites received by humans are dog bites (85–90%), followed by cat bites (5–10%) and rodent bites (2–3%).

Human bites constitute 2–3% of animal bites received by humans.

Dog bite

• Dog bites occur more commonly in children aged under 5 years than in older children. In adults, they are more common in males than in females.
• People are most often bitten by their own dogs or dogs known to them.
• Unsterilised dogs, male dogs, and dogs aged 3 months or older are most commonly involved in reported dog bite cases.
• Dog bites are usually unprovoked.

Most dog bites in young children occur on the head and neck. In older children and adults, they are more commonly situated on the arms and legs. This is likely due to the level of these body parts in relation to the mouths of large dogs.

Dog bites commonly result in crush injuries, but may also cause scratches, deep cuts, puncture wounds, and tearing of the skin and underlying flesh.

Cat bite

• Cat bites occur more commonly in children and female adults than in male adults.
• Most cat bites are provoked.
• Cat bites are more likely than dog bites to become infected.

Most cat bites occur on the arms and the hands. Cat bites typically result in either scratches or deep puncture wounds.

Rodent bite

• The majority of rodent bites are caused by rats
• Rat bites occur most commonly on the hands or face of children aged 5 years or younger
• Rat bites usually occur in the bedroom at night, and are more common in warmer months

Complications of animal bite

Complications of animal bites include:

• Tissue damage from crush injury
• Psychological distress
• Wound infection

Animal bite wound infection

Wound infection can lead to:

• Subcutaneous abscess
• Osteomyelitis (infection of a bone)
• Septic arthritis (infection of a joint)
• Tendonitis
• Bacteraemia (bloodborne infection).

Up to 20% of dog bite wounds become infected. Most dog bite wound infections are polymicrobial. The most common pathogens are:

• Pasteurella spp. (50%)
• Staphylococcus aureus (20%)
• Streptococcus pyogenes
• Capnocytophaga canimorsus
• Various anaerobes.

Cat bite wounds are more likely to become infected than dog bite wounds, due to the tendency of cat bites to result in deep puncture wounds. Bites located on the hands have a greater risk of leading to further complications such as osteomyelitis or septic arthritis. The most common pathogens are:

• Pasteurella multocida (up to 75%).
• Staphylococcus aureus (20%)
• Streptococcus pyogenes
• Various anaerobes.

Bartonella henselae, the causative agent of cat scratch disease, can also be transmitted via a bite from an infected cat.

Approximately 20% of rodent bite wounds become infected. The most common pathogens are:

• Streptobacillus moniliformis
• Spirillum minus
• Salmonella spp.

Bites from rodents infected with Streptobacillus moniliformis or Spirillum minus can result in rat bite fever, a rare systemic illness that can be serious or fatal if untreated. Patients with rat bite fever typically present 10 days to 4 weeks after a rodent bite with fever, rash, and septic arthritis.

Although New Zealand is free of rabies, the risk of an animal bite being infected with rabies must be considered if the bite occurred outside of New Zealand.

Initial management of animal bite

An animal bite wound should be carefully examined and treated as soon as possible after it has occurred to reduce the risk of complications.

• Apply direct pressure to the wound to stop bleeding
• Clean the wound thoroughly with soap and running water or normal saline
• Remove foreign particles (dirt, teeth, visible debris)
• Elevate the wound during the first 48–72 hours

When to see a doctor

If you have been bitten by an animal, consult a doctor:

• If there are any signs of infection, such as:
  • Fever
  • Redness or swelling around the wound
  • Purulent discharge from the wound (yellow or brown pus)
• If you have a deep bite on your hand or foot
• If you are not up to date with your tetanus vaccinations
• If you are at increased risk of infection (for example, if you are diabetic, on immunosuppressive medications, or have an artificial heart valve)

Medical and surgical management of an animal bite wound

Assessment of the wound

• Clinically infected wounds should be irrigated, debrided, incised and drained as required.
• Cultures should be taken from clinically infected wounds. They are not appropriate in clean wounds.
• Most animal wounds should be left open due to the risk of infection. Primary closure may be considered for low-risk wounds on the head and neck (with antibiotic prophylaxis), as enhanced blood supply in this region reduces the risk of infection.
• Cover the wound with a sterile, non-adhesive dressing.
• All bites that are infected should be treated with antibiotics (see below)

Antibiotics for animal wounds

Antibiotic prophylaxis should be considered for high risk wounds, such as:

• All cat bites (unless superficial scratches)
• All dog bites to the face, hand, foot or genitals
• Wounds requiring surgical intervention (debridement, drainage, reconstruction)
• Bites involving bones, joints, tendons or nerves
• Wounds that have undergone primary closure
• Bites in immunocompromised hosts, such as people with diabetes, cirrhosis, immunosuppression, asplenia, or prosthetic valves or joints

The most common bacteria cultured from animal wounds, Pasteurella, should be considered resistant to flucloxacillin, first generation cephalosporins, erythromycin, and clindamycin.

Amoxicillin clavulanate is the first-line antibiotic for animal wounds. Other options are:

• Penicillin plus metronidazole
• Metronidazole plus doxycycline
• Metronidazole plus trimethoprim + sulphamethoxazole

Rat bite fever can be treated with penicillin or doxycycline.

Indications for hospital referral include:

• Signs of systemic infection, such as fever and/or swollen lymph glands
• Severe or rapidly spreading cellulitis
• Wounds requiring surgical intervention
• When considering x-ray imaging, eg in case of retained animal teeth in a wound.

A tetanus booster (tetanus toxoid) should be offered to patients if it has been more than 5 years since their last dose and they have completed their primary vaccination course. If they have not previously been fully immunised against tetanus, they should be offered tetanus toxoid plus tetanus immunoglobulin.

Rabies post-exposure prophylaxis should be considered for all bites that occurred overseas in a rabies endemic area.