Introduction

Bite wounds are a common presentation in veterinary practice, and the consequences can range from relatively trivial to potentially life-threatening. One study found an overall mortality rate of 7% for all bite wounds. The initial assessment of a wound is critical to returning the animal back to full health.

Pathophysiology

The effect of a bite wound should be considered in two ways.

The Local Effect of the Bite

 The bite is most likely to cause a puncture wound to the affected area, although sometimes tearing of surrounding tissues will be seen.

 Crush injury to the surrounding tissue can also be significant: there is initially bruising to the area caused by damage to vessels and vasospasm, followed by necrosis in some cases if local blood supply is more seriously affected.

 Tissues may be torn away from their underlying attachments, so creating dead space. Haematoma or seroma fluid will then often form, an ideal environment for bacterial multiplication.

 Bacteria will be inoculated through the bite wounds and under the skin, and so all bite wounds are initially heavily contaminated. These bacteria may originate from the victim's own skin, or more likely from the aggressor's oral cavity.

 Hair and debris taken under the skin act as a further focus for infection.

 Bacterial multiplication under the skin will then almost inevitably happen, with a range of both aerobic and anaerobic bacteria:

 Aerobic bacteria in wounds reported in a recent study were most commonly Staphylococcus intermedius, Enterococcus and Escherichia coli.

 The most commonly found anaerobic bacteria were Bacillus, Clostridium and Corynebacterium.

 Interestingly this study found that no single antibiotic was effective for all bacteria found, and so ideally tissue samples should be taken for aerobic and anaerobic culture and sensitivity testing.

 If left untreated, abscess formation will result, a very common presentation in feline practice.

 Other local effects may be seen, such as oedema around the wound, or damage to underlying muscle or connective tissue.

Systemic or Life-Threatening Injury Caused by the Bite

It is very easy when assessing a patient with bite wounds to miss more serious underlying disease. The majority of bite wounds are uncomplicated and do not have more serious injuries, but the possibility should not be ignored.

Firstly, serious complications could be as a result of septic or hypovolaemic shock. If acutely presented, the animal may have haemorrhaged significantly or may be in hypovolaemic shock as a result of the trauma and tissue damage. If the wound is older and there is an active infection present, the animal may be in septic shock. The animal should be initially assessed for haemorrhage and/or shock and stabilised appropriately (see below).

Secondly, the bite wound may have penetrated a body cavity, which would have potentially life-threatening implications:

 Bite wounds to the chest or other areas of the respiratory tract (such as the trachea within the neck) may cause acute respiratory clinical signs (dyspnoea due to pneumothorax, for example) or clinical signs days to weeks later due to infection (pyothorax). Rib fractures are sometimes seen with bite wounds over the thorax.

 Bite wounds over the abdomen may cause a septic abdomen, although the effects of this will not usually be seen for a couple of days after the injury. They may occasionally cause damage to intraabdominal organs, leading to haemorrhage from the liver, or rupture of the bladder or gastrointestinal tract.

 Bites may also damage other structures, such as the ears, eyes, external genitalia and limbs.

Initial Approach to the Acutely Bitten Animal

Assess Systemic Health

 This should be a priority before concentrating on management of the wound.

 A basic TPR (temperature, pulse and respiration) is helpful, as well as assessing how bright the patient looks, and observing its breathing pattern.

 Evidence of shortness of breath or excessive effort should be treated with urgency--radiography of the chest or neck will be required in most cases. Some animals may just be breathing fast because they are in pain.

 If the animal appears to be in hypovolaemic shock, then blood may be taken to assess renal perfusion, packed cell volume and plasma proteins.

Patient Stabilisation

 The order that treatment is started depends on the injuries in any one individual, but if the animal is in hypovolaemic shock then an intravenous catheter should be placed and fluid therapy commenced. Further treatments, such as with colloids or blood transfusion, are not often required. Anaesthesia or more involved treatments for the wound should not begin until the hypovolaemia is corrected.

 The bite wounds should ideally be covered, initially using sterile swabs soaked in sterile isotonic solution (such as saline or Hartmann's) .

 The patient will be in discomfort following the injury, and so analgesics should be administered if appropriate. An opioid such as methadone or buprenorphine at standard doses may be useful, along with a non-steroidal anti-inflammatory drug (NSAID).

 Antibiotics can be administered--a broad-spectrum bactericidal drug that is effective against anaerobes (such as amoxicillin) is used initially. Animals with very serious injuries may benefit from administration of antibiotics intravenously. Antibiotics may need to be subsequently changed if samples are taken for culture and results show that the initial antibiotic is not effective.

 If the animal is not going to be anaesthetised or sedated for further treatment, then food and drink can be offered.

Treatment of the Wound

This obviously depends on the extent of the injuries but for welfare reasons is best performed under heavy sedation or general anaesthetic. If there is penetration of the bite wound into the chest or abdomen, surgical exploration is required once the animal is stable.

The following applies if there is no evidence of body cavity penetration:

 Hair should be removed from a large area around the wounds and so should be carefully clipped away. Clipped hair should be prevented from entering the wounds by packing them with either moistened sterile swabs or sterile K-Y jelly.

 Hair or other debris should be carefully removed with sterile surgical instruments.

 Any obviously necrotic tissue should be debrided. Some tissue from deep in the wound can be taken and submitted for bacterial culture and sensitivity. Surface tissues should not be sampled as an unrepresentative sample may be taken.

 Simple puncture wounds that are more than a few hours old will sometimes close over with fibrin. In most cases it is better to open up these puncture wounds again (using a pair of curved artery forceps, for instance) to allow ongoing drainage.

 The wounds should be flushed with copious volumes of sterile isotonic solution such as saline or Hartmann's. Solutions containing glucose should be avoided as they will provide a nutrient source for bacteria. Flushing should ideally be repeated twice daily until healthy granulation tissue forms in the wound, as this indicates infection has been cleared from the wound.

 Ongoing debridement with dressings such as wet-to-dry should be used where necessary.

 In general, bite wounds should not be sutured, to allow continued drainage. Second intention healing will eventually occur when any infection has been resolved. If there are flaps of skin present due to a tear injury, then delayed primary closure should be performed. This is where debriding dressings are used along with antibiotics for several days to a couple of weeks to eliminate infection, at which point the wound can be closed.

 If there are any pockets of skin communicating with the bites that are not draining well, a Penrose drain should be placed to allow egress of potentially infected fluid and debris.

 If there are very large pockets of skin that are not draining, an active suction drain may be more effective than a passive drain.

 Broad-spectrum bactericidal antibiotics should be continued until signs of infection are under control. The most useful sign to look for is the presence of granulation tissue in the wound: once this is present, antibiotics can usually be stopped.

 Ongoing analgesia in the form of further NSAIDs is often necessary, and ongoing opioid medication is required in some cases.