Presenting problem: Bite Wounds

Key Points

  • Triage the bite wound patient for life threatening internal injuries such as coelomic penetration.
  • Stabilize the patient before initiating wound management. Provide hemostasis, supplemental oxygen, and supplemental heat as needed.
  • The superficial appearance of a bite wound can be misleading. Deeper structures often suffer from hematoma formation, necrosis, and bacterial invasion.
  • Bite wound infections are often polymicrobial, involving a broad mixture of aerobic and anaerobic bacteria.
  • The danger of infection caused by Pasteurella spp. is high in victims of cat bite wounds.
  • Reduce the risk of bite wounds in reptiles by feeding pre-killed prey and removing uneaten prey items within 15-30 minutes.
  • A broad spectrum beta-lactamase inhibiting antibiotic like amoxicillin-clavulanate is a commonly used to treat bite wounds in patients with simple guts such as ferrets and parrots.
  • The risk of dysbiosis makes beta-lactams a dangerous choice for hindgut fermenters, like rabbits and many rodents, therefore potentiated sulfa drugs or fluroquinolones are popular choices in these species.
  • Good bandaging practice protects the wound from contamination and supports the wound while it heals.

Introduction

Bite wounds are a common and significant problem in clinical practice (Fig 1). The incidence of bite wounds increases with a history of exposure to the outdoors or to other animals. The owner may even report a fight or interaction that results in a bite wound.

Bite wounds are a common problem in clinical practice. Image provided by Dr. Isabelle Langlois.

Figure 1. Bite wounds are a common problem in clinical practice. Image provided by Dr. Isabelle Langlois. Click image to enlarge.

Bite wounds are complex problems. Dog teeth can exert a pressure of 150 to 450 pounds per square inch (psi). This pressure is strong enough to perforate sheet metal and exerts shearing, tearing, and crushing forces (Brook 2003, Campbell 2012).

Box 1. Biting force measured in various species (Campbell 2012, Dog Facts 2008, Lindner 1995)
Species Biting force (pounds per square inch or psi)Biting force (Newtons)
Dogs320 on average (range: 150-450)256 on average (13-1394)
Hyacinth macaw375+n/a
Snapping turtle1000n/a

These crush injuries lead to devitalized tissue rather than mere laceration. In fact the outward appearance of a bite wound is often the mere “tip of the iceberg”. Despite minimal superficial damage, there is substantial damage to deeper tissues including hematoma formation, necrosis, and bacterial invasion.

Common changes in the deep structures affected by a puncture wound:

  • Necrosis
  • Bacterial infection
  • Hematoma
  • Large dead space

Bite wound infections are generally polymicrobial, meaning a variety of aerobic and anaerobic microorganisms can be isolated (Abrahamian 2011) (Box 2). These pathogens primarily arise from the animal’s oral flora (Fischer 1994), which can also be influenced by the microbiome of their ingested prey and other foods. Bacteria can also be introduced into the wound from the victim’s own skin or from the physical environment at the time of injury (Abrahamian 2011, Brook 2003). The most common pathogens associated with dog and cat bite wounds include Streptococcus spp., Staphylococcus spp., Pasteurella multocida, and anaerobic bacteria (Yaqub 2004).

Box 2. Bacteria commonly isolated from cat bite wounds
(Abrahamian 2011, Madsen 2011)
Aerobic organismsPasteurella
Streptococcus
Staphylococcus
Neisseria
Moraxella
Corynebacterium
Enterococcus
Bacillus
Anaerobic organismsFusobacterium
Porphyromonas
Bacteroides
Prevotella
Proionibacterium

A systemic inflammatory response often develops in animals with multiple severe bite wounds (Holt 2011). Severe tissue trauma activates not only an inflammatory response, but also immunologic, coagulation, and fibrinolytic cascades (Holt 2011). In small animals, the patient is deemed to have systemic inflammatory response syndrome or SIRS when they suffer from two or more of the following conditions:

  • Tachypnea
  • Tachycardia
  • Hyperthermia
  • Leukocytosis or leukopenia

A confirmed diagnosis of sepsis exists when a SIRS patient develops a documented infection. Sepsis is common in animals with bite wounds, and is obviously a very serious condition. Septicemia caused by certain isolates of Pasteurella multocida causes pathology in major organ systems while endotoxin release affects the vascular system (Riggs 2004).

 

Bite wounds from predators versus prey species

Bite wounds can arise from predator species, like cats, dogs, and ferrets, prey species, like rats and mice, or individuals can be bitten by animals from their own taxonomic group (e.g. big bird on small bird injury).

 

Predator bite wounds

Pasteurella spp. is a gram-negative organism categorized morphologically as a coccobacillus. Pasteurella is a part of the normal flora of the nasopharynx and oropharynx in many animal hosts, but it also serves as an opportunistic pathogen (Adler 2011). Pasteurella multocida is commonly isolated from predator bite injuries, especially cat bites (Collins 2012). Cat bite wounds can initially appear superficial and uncomplicated, but they often become severely infected with cellulitis, necrotizing myositis and dermatitis, as well as septicemia (Collins 2012, Quesenberry 1997).

 

Prey bite wounds

Bite wounds caused by prey species can be prevented by feeding pre-killed prey. Rodents, like rats and mice, can bite in self-defense creating wounds on the tongue, lips, eyes, or head. More commonly the live rodent is left unattended in the cage with a reptile or amphibian that is not eating. The rodent then gnaws on the reptile (or amphibian), creating wounds along the sides and back that can range from superficial and minor to massive tissue destruction (Fig 2).

Rat bites in a boa constrictor

Figure 2. Rodent prey left unattended in a reptile enclosure often gnaw on the animal’s sides and back. Image from Alta Rancho Pet & Bird Hospital.

Pathogens isolated from human bite wounds caused by rats have included Staphylococcus epidermis, Bacillus subtilis, and alpha-hemolytic Streptococcus (Ordog 1985). Rat bite-associated human infections have been caused by Corynebacterium kutscheri and leptospirosis (Holmes 2007, Gollop 1993, Luzzi 1987). There is also a case report of acute septicemic P. multocida infection in a grey parrot bitten by a pet rat (Lumeij 1986).

Invertebrate prey left uneaten can also “attack”. Crickets can feed on lizards creating small, multifocal skin lesions; roaches can also create very similar lesions.

For this reason, live prey should never be left in an enclosure for longer than 15 to 30 minutes. If the reptile or amphibian has not seized its prey in this time frame, it will probably not eat that day (Barten 2006).

 

Reptile bite wounds

Although bite wounds caused by reptiles can involve a variety of microorganisms, Gram-negative coverage appears particularly important. Septic arthritis caused by Pseudomonas aeruginosa developed in a person bitten by a monitor lizard (Tehrani 2008) while an 8-year old boy bitten on his finger by a green iguana (Iguana iguana) developed purulent cellulitis that grew Serratia marcescens and Stapylococcus auerus (Hsieh 1999). There are also two case reports of S. marcescens bullous cellulitis in adults following iguana bites (Grim 2010).

 

Bite wounds in birds

Bite wounds in birds caused by mammals are often inflicted over the back or tail base. Wing or tail feathers can also be ripped from the follicles, tearing the skin or leaving skin severely bruised (Quesenberry 1997).

When larger birds bite smaller birds, the resulting wound often involves crushing injuries of the beak or head (Quesenberry 1997). There are no organisms classically isolated from bird bite wounds, and human case reports involving infection secondary to bird bite wounds are rare (Abrahamian 2011). For instance, there is a case report of a brain abscess caused by Streptococcus bovis, Clostridium tertium, and Aspergillus niger in an infant pecked on the head by a rooster (Berkowitz 1987). There are also human case reports of anaerobes isolated from cellulitis secondary to owl and swan bites (Eberly 1997, Davis 1992).

 

Urgent Care

  1.   Stabilize immediate life-threatening problems
    • Avoid tunnel vision. Trauma sufficient to cause a bite wound often results in other injuries so begin an aggressive period of stabilization before initiating wound management (see case management below).
    • Triage the patient for life threatening internal trauma such as penetration of the coelom, shock, anemia, and/or sepsis.
    • Screen the patient for serious concurrent conditions such as severe malnutrition, hypoglycemia, hypothermia, or dehydration.
    • Control hemorrhage:  In most instances, applying direct pressure followed by a temporary pressure bandage can control active hemorrhage. Placing a blood pressure cuff proximal to the bleeding vessel can rapidly control bleeding on a distal extremity. Inflate the cuff to a pressure greater than systemic blood pressure until hemorrhage has slowed or stopped (Holt 2011). In dogs and cats, these cuffs are left on up to 1 hour (Garzotto 2009). Surgical intervention is required if these measures cannot control bleeding.In most instances, applying direct pressure followed by a
      temporary pressure bandage can control active hemorrhage.
  2. Address oxygen and circulatory needs: Provide fluids, supplemental oxygen, and supplemental heat as needed. Fluid therapy is also indicated for the febrile or anorectic patient (Macintire 2006).
  3. After stabilizing any immediate life-threatening problems, perform a more thorough physical examination (Holt 2011). This complete examination should include an orthopedic and neurologic exam, however keep in mind that routine function tests used for domestic animals may not be useful in wild species or stressed exotic animals, and results can be unpredictable or decreased.
  4. Also note any evidence of bruising in the tissue, which indicates wound duration of at least 2 to 3 days. Bruises in birds are green tinged due to the accumulation of biliverdin pigment in the tissues subsequent to hemoglobin breakdown.
  5. It is also important to identify all bite wounds. Remember that predator bite wounds commonly result from the penetration of both upper and lower teeth. The location of the wound can also be significant (Box 3). For instance, an abdominal injury can cause intestinal puncture.
    Box 3. Bite wound location (Holt 2011)
    Location Potential result
    Cranial vaultComa, death
    Neck Cervical spinal cord injury and subsequent tetraparesis, paralysis, or hypoventilation
    Body wallPneumothorax in mammals
    Peritonitis
    Bite wounds penetrating the coelom, abdominal cavity, or thorax require special consideration and necessitate thorough exploration of the affected body cavity (see case management below).
  6. Provide wound first aid: Keep the wound moist and covered until definitive therapy can be initiated.
    • Apply saline soaked gauze sponges and/or sterile, water-soluble lubricant to the wounds.
    • Utilize a temporary, protective, soft, padded bandage to support the wound.

    These measures prevent further wound contamination and trauma while minimizing the risk of nosocomial infection.

Case management

Before initiating definitive treatment, provide the owner with an estimate for not only short-term treatment but long-term management as well.

 

Diagnostics

Perform a complete physical examination if this has not already been performed (see urgent care tips above). Collect a minimum database, complete blood count, biochemistry panel, and urinalysis, to provide baseline or reference values. Additional tests performed will vary but can include survey whole body or chest/abdominal radiographs, abdominocentesis, thoracocentesis, endoscopy if the cervical region is involved, culture/sensitivity, and gram stain cytology. Cultures of uninfected wounds do not predict which bacteria will later cause infection. If the wound is obviously infected, culture deep tissue and/or purulent material (Campbell 2012). Pending culture results, cytology can help guide appropriate antimicrobial therapy in obviously infected wounds.

 

Therapeutics

Wound management should only be initiated once the patient is stable. Although the steps involved apply to all animals, the approach to your individual patient may need to vary dramatically with your species of interest. Minimize handling of delicate patients and reduce stress by assembling all materials required beforehand. Be sure to address pre-emptive analgesia, and before getting started photograph all bite wounds that may require legal documentation.

  1. Chemically restrain your patient using sedation, local anesthesia, and/or general anesthesia. Take this opportunity to carefully examine injured tissue in more depth and assess the integrity of nerves, bones, and muscles.
  2. Clean the wound using aseptic technique (Ford 2012, Garzotto 2009).

-Don examination gloves to protect yourself and your patient

-Apply sterile water-soluble lubricant to  protect the exposed wound from contamination.

-Widely clip or pluck wound margins.

-Clean skin surrounding the wound, moving from the inner margin outward to minimize wound contamination (Ford 2012).

  • Remove gross debris from the wound via lavage or gentle scrubbing.

Grossly contaminated wounds can be rinsed first with warm tap water (Ford 2012).Hydrogen peroxide can also be used for this first initial irrigation of the dirty wound. Peroxide is a very ineffective antiseptic, but a 3% concentration does not appear to have detrimental effects in vivo on wound healing. Never deliver hydrogen peroxide to wounds under pressure. The foaming action can force debris between tissue planes, enlarging the wound, and causing gas to collect in tissues (Devey 2012).

Perform sterile lavage to remove particulate debris and reduce bacterial contamination using sterile saline or lactated Ringer’s solution.

The effectiveness of lavage is proportional to the volume of solution used—Holt 2011

To manually irrigate the wound, a liter fluid bag can be placed in an inflated pressure cuff. Alternatively a 35-ml syringe can be attached to a 3-way stopcock and fluid bag. Interestingly, needle gauze size does not appear to be relevant (Campbell 2012).
Use of antibiotics or antiseptics in lavage solution is controversial (Lagutchik 2012), however infected wounds and chronic, non-healing wounds appear to benefit most from the use of antiseptics (Devey 2012). Recommended dilutions include 0.1-1% povidone-iodine or 0.05% chlorhexidine (Campbell 2012, Lozier 1992).
When preparing a dilute antiseptic solution, visual clues have been found to be inaccurate. Follow an exact recipe (Box 4) or post a color photograph in the treatment area that depicts the exact color of the correct dilution (Campbell 2012).

Box 4. Recipe for a 0.05% chlorhexidine solution
IngredientVolume
2% chlorhexidine25-ml
Sterile water or 0.9% saline1-Liter
    1. Explore all bite wounds as far as they will go.Explore wounds even when they appear superficial. Probe the lesion for dead space or pockets using mosquito forceps (Garzotto 2009).When a puncture wound extends into a body cavity, the surgeon initially explores and debrides the wound(s) itself. The affected body cavity is then entered by a separate, ventral midline incision.
    2. Surgically debride necrotic tissue.Remove all soft tissue that is obviously not viable using sharp dissection, keeping in mind that bruised tissue normally appears green in birds.Questionable tissue should remain: “When in skin, leave it in”. The rule of thumb for deeper tissues: “When in doubt, take it out” unless the tissue is essential to life (Campbell 2012). Also strive to preserve as much bone, tendon, nerve, and blood vessel as possible.
    3. Bite wounds are rarely closely completely.Unless all necrotic or questionable tissue is removed, leave the wound open for reevaluation and debridement on a daily basis.
      • All necrotic tissue has been removed
      • Dead space has been eliminated
      • Closure is tension free

      Healing by secondary intention can take months, or even more than a year, in reptiles. Defects from bite wounds can appear unchanged between periods of shed, but then appear partially epithelialized after a shed (Barten 2006). Use the minimal amount of suture possible when closing a bite wound. As the amount of suture increases, the risk of vascular compromise and infection also rises. It is also important establish drainage and bandage all closed wounds.

    4. Bandage all but the most minor open wounds.The type of the bandage selected will vary with the wound. Non-adherent bandages promote epithelialization while wet-to-dry bandages are indicated for wounds with foreign or necrotic debris or devitalized tissue. Honey or sugar bandages are also a good choice for contaminated wounds. Perform bandage changes daily or every other day. Visit Wound Management in Exotic Animals for additional information.Snakes are particularly challenging to bandage. The long list of materials that can be tried (from condoms to elastic wrap) is a testament to the difficulties encountered. An alternative to bandaging the snake is to house the animal in a waterproof container like a small aquarium or plastic picnic chest with adequate ventilation for the first few days of treatment. Line the enclosure with clean towels soaked in dilute chlorhexidine or povidone-iodine. Towels and solution are changed daily (Barten 2006).
    5. Antibiotic use is controversial.Topical antibiotics are indicated for contaminated bite wounds. Good choices for wounds infected with Pseudomonas spp. and other Gram-negative bacteria are 1% silver sulfadiazine cream or mupirocin ointment (Bactoderm, SmithKline Beecham) (Barten 2006).Some authors favor systemic antimicrobial use in all bite wound cases while other sources recommend antibiotics only for wounds at high risk of infection (Holt 2011, Cummings 1994).
      Risk factors determining the likelihood of wound infection include (Brook 2003):
  • 24 hour delay in seeking treatment
  • Puncture wounds
  • Immunocompromised patients
  • Critically ill patients
  • Patients with deep wounds or cat bite wounds

Since bacteria are directly introduced into the wound, infection can develop quite rapidly. For instance, without prompt, aggressive antimicrobial treatment, patients with cat bite wounds are susceptible to bacterial sepsis and death within 24 to 72 hours (Wade 2002, Lumeij 1986).

Administer antibiotics for a minimum of 7 days to high-risk patients. If the bite involves bones or joints, administer antibiotics for 3 weeks minimum (Campbell 2012, Brook 2003).
When selecting a systemic antibiotic keep the possible pathogen involved in mind:

  • Pasteurella spp. is often resistant to many of the antibiotics typically used to empirically treat skin infections. For infected cat or dog bites, amoxicillin-clavulanate is the treatment of choice in species with a simple gastrointestinal tract such as the ferret and parrot. Because dysbiosis is a danger in hindgut fermenters, like the rabbit, chinchilla, or guinea pig, select sulfa-trimethoprim or a fluoroquinolone like enrofloxacin.
  • Serratia is often susceptible to fluroquinolones, carbapenems, and aminoglycosides (Abrahamian 2011).
  • Multi-drug therapy may be necessary to adequately cover anaerobic bacteria.

Follow-up care

Follow-up care of the bite wound patient should focus on minimizing stress, managing pain, and providing recumbent patient care such as proper padding and prevention of soiling. Adequate nutrition is also required to ensure proper wound closure and support of the immune system (Liss 2012).

Nursing care for the bite wound patient

  • Minimize stress
  • Pain management
  • Recumbent patient care
  • Bandage care
  • Nutritional support
  • Antimicrobial therapy
  • Fluid therapy

Standard bandage care focuses on keeping the bandage clean and dry. The limb must also be monitored for swelling and the bandage monitored for slippage. Elect judicious use of the Elizabethan collar to protect the bandage or wound (Liss 2012).

Particulate bedding can stick to wounds. Exotic animals housed on particulate bedding or shavings, should be placed in a bare cage or in one line with newspaper, butcher paper, paper towels, or cloth towels.

Potential complications

  1. Refractory or recurrent infections can be caused by resistant or unusual pathogens, immunosuppression, foreign bodies within the wound–or in most cases—inadequate debridement or drainage (Macintire 2006).
  2. The introduction of microbes beneath the skin by a bite wound can lead to an abscess in most mammals or a granuloma in rabbits, reptiles, and birds.In addition to firm swellings, the owner may report non-specific signs of illness such as lethargy and anorexia. Physical examination can reveal fever and non-specific signs of illness.Lance and ventrally drain abscesses. Granulomas require surgical debridement. Also ensure appropriate antimicrobial coverage. Anaerobic bacteria play a prominent role in bite wound infections, and can be isolated from approximately 75% of dog and cat bite wound infections, particularly those with abscess formation (Brook 2003).
  3. The scars that frequently develop after a wound has healed, can lead to difficult sheds or dysecdysis in reptiles. The area around healing wounds can shed incompletely, and may require soaking and manual removal of old skin (Barten 2006).
  4. Bite wounds on raptor feet or legs are serious, high-risk bite wounds. The presence of avascular tendon and sheath spaces makes these wounds an important cause of tendon injury in birds of prey (Riggs 2004, Brook 2003).

Prognosis

Prognosis varies but should always be considered guarded for cat bites, even when the wound is superficial (Quesenberry 1997).

Conclusion

Although the nature of bite wounds can vary in exotic animals, the classic bite wound is a puncture wound. Punctures are penetrating wounds with minimal superficial damage yet substantial damage to deeper tissues. Careful patient assessment is critical since life-threatening injuries such as penetration of the coelom can occur. Early estimation of infection risk and, if indicated, surgical treatment, are the cornerstones of successful management of bite wounds.
Although medical literature frequently describes antibiotic use in the face of bite wounds as controversial, contamination and subsequent infection is common in cat bite victims. Many exotic animal and wildlife patients are also immunosuppressed due to severe stress and/or substandard husbandry practices, which increases the risk of infection.

References