Animal Bite Infections
Animal bite infections develop in humans when an animal's teeth break the skin and introduce saliva containing disease organisms below the skin surface. The saliva of dogs, cats, ferrets, and rabbits is known to contain a wide variety of bacteria. According to one study, bacteria or other pathogens show up in about 85 percent of animal bites. These microorganisms may grow within the wound and cause an infection. The consequences of infection from these bites range from mild discomfort to life-threatening complications.
Animal bites may occur in a variety of circumstances, ranging from unprovoked attacks in the wild by rabid or naturally aggressive animals to injuries inflicted by household pets who do not feel well, are frightened, are interrupted during their meal, or are annoyed by a child's teasing or overly rough play. The bite may be a simple warning to back off (as in most household cats), an assertion of dominance and control (as in many dogs), or an intention to seriously injure or kill (as in a few breeds of dogs and some wild animals). Animal bites can range from small injuries that barely break the skin to severe wounds that can cause a person to lose the use of a hand, eye, or foot or even bleed to death.
The number of animal bites that occur in the United States each year is difficult to estimate because many of these injuries are treated successfully at home. Still, U.S. figures range from 1 million to 4.5 million animal bites each year. About 1 percent of these bites requires hospital inpatient treatment. Cat and dog bites result in 334,000 emergency room visits per year, which represents approximately 1 percent of all emergency hospital visits, at an annual cost of $100 million dollars in health-care expenses and lost income. Children are the most frequent victims of dog bites, with five to nine year-old boys having the highest incidence. The average age of a dog bite victim is 13, whereas the average age of a cat bite victim is 19 or 20. Men are more often bitten by dogs than are women (3:1), whereas women are more often bitten by cats (3:1).
Children are more likely than adults to suffer dog bites on the face and neck, partly because they are shorter than adults. Cat bites in children as well as adults are far more likely to injure the hands or lower arms rather than other parts of the body.
Dog bites make up 80 to 85 percent of all reported animal bites in the United States and Canada. Cats account for about 10 percent of reported bites, and other animals (including rats, hamsters, ferrets, rabbits, horses, sheep, raccoons, bats, skunks, and monkeys) make up the remaining 5 to 10 percent. Cat bites, however, become infected more frequently than dog bites. A dog's mouth is rich in bacteria, but only 15 to 20 percent of dog bites become infected. In contrast, approximately
Figures on bite injuries from animals other than cats and dogs are difficult to obtain, although bites from pet hamsters and ferrets have been reported more frequently since the late 1990s. Rat bites are becoming more common, particularly in large cities where the rat population has been increasing in the early 2000s. Bites from such wild animals as mountain lions and bears are also reported more frequently as humans explore or move into their natural habitats.
Many factors contribute to the risk of infection from an animal bite, including the type of wound inflicted, the location of the wound, pre-existing health conditions in the bitten person, the extent of delay before treatment, compliance with treatment, and the presence of a foreign body in the wound. Dogs usually inflict crush injuries because they have rounded teeth and strong jaws; thus, the bite of an adult dog can exert up to 200 pounds per square inch of pressure. This pressure usually results in a crushing injury, causing damage to such deep structures as bones, blood vessels, tendons, muscles, and nerves. The canine teeth are sharp and strong, often inflicting lacerations. Cats, with their needle-like incisors and carnassial teeth, typically cause puncture wounds. Puncture wounds appear innocuous on the surface, but the underlying injury goes deep. The teeth of a cat essentially inject bacteria deep within the bite, and the deep, narrow wound is difficult to clean. Persons with impaired immune systems—for example, individuals with HIV infection—are especially vulnerable to infection from cat bites.
The bacterial species most commonly found in animal bite wounds include Pasteurella multocida, Staphylococcus aureus, Pseudomonas sp., and Streptococcus sp., P. multocida, the root cause of pasteurellosis, is especially prominent in cat bite infections. Other infectious diseases from animal bites include cat-scratch disease, tetanus, and rabies.
Doctors are increasingly aware of the importance of checking animal bite wounds for anaerobic organisms, which are microbes that can live and multiply in the absence of air or oxygen. A study published in 2003 reported that about two-thirds of animal bite wounds contain anaerobes. These organisms can produce such complications as septic arthritis, tenosynovitis, meningitis, and infections of the lymphatic system.
With regard to the most common types of domestic pets, it is useful to note that biting and other aggressive behavior has different causes in dogs and cats. To some extent these differences are rooted in divergent evolutionary pathways, but they have also been influenced by human interference through selective breeding. Dogs were first domesticated by humans as early as 10,000 B.C. for hunting and as guard or attack dogs. Many species travel in packs or groups in the wild, and many human fatalities resulting from dog bites involve a large group of dogs attacking one or two persons. In addition, dogs typically relate to humans according to a hierarchical model of dominance and submission, and many of the techniques of dog training are intended to teach the dog to respect human authority. Certain breeds of dogs are much more likely to attack humans than others; those most often involved in fatal attacks are pit bulls, Rottweilers, German shepherds, huskies, and mastiffs. According to the Centers for Disease Control (CDC), there are between 15 and 20 fatal dog attacks on humans in the United States each year. There are several assessment or evaluation scales that veterinarians or animal trainers can use to score individual dogs and screen them for dominant or aggressive behavior.
Unlike dogs, cats were not domesticated until about 3000 B.C., and were important to ancient civilizations as rodent catchers and household companions rather than as protectors or hunters of wild game. Biologists classify cats as solitary predators rather than as pack or herd animals; as a result, cats do not relate to humans as authority figures in the same way that dogs do, and they do not form groups that attack humans when threatened or provoked. In addition, domestic cats have been selectively bred for appearance rather than for fierceness or aggression. Most cat bites are the result of fear on the cat's part (as when being placed in a carrier for a trip to the vet) or a phenomenon known as petting-induced aggression. Petting-induced aggression is a behavior in which a cat that has been apparently enjoying contact with a human suddenly turns on the human and bites. This behavior appears to be more common in cats that had no contact with humans during their first seven weeks of life. In other cats, this type of aggression appears to be related to a hypersensitive nervous system; petting or cuddling that was pleasurable to the cat for a few seconds or minutes becomes irritating, and the cat bites as a way of indicating that it has had enough. In older cats, petting-induced aggression is often a sign that the cat feels pain from touching or pressure on arthritic joints in its neck or back.
General signs of infection
The most common sign of infection from an animal bite is inflammation, which usually develops within
Live disease-causing bacteria within the bloodstream and tissues may cause complications far from the wound site. Such complications include meningitis, brain abscesses, pneumonia and lung abscesses, and heart infections, among others. These complications can be fatal. Deep bites or bites near joints can damage joints and bones, causing inflammation of the bone and bone marrow or septic arthritis.
Cat-scratch disease is caused by Bartonella henselae, a bacterium that is carried in cat saliva; infection may be transmitted by a bite or scratch. Approximately 22,000 cases are reported each year in the United States; worldwide, nine out of every 100,000 individuals become infected. More than 80 percent of reported cases occur in persons under the age of 21. The disease is not normally severe in individuals with healthy immune systems. Symptoms may become serious, however, in immunocompromised individuals, such as those with acquired immune deficiency syndrome (AIDS) or those undergoing chemotherapy. Common symptoms include an inflamed sore in the area of the bite or scratch, swollen lymph nodes, fever, fatigue, and rash.
Rabies is caused by a virus that is transmitted through the bite of an animal that is already infected. It is classified as a zoonosis, which is a term that refers to any disease of animals that can be transmitted to humans. More than 90 percent of animal rabies cases occur in such wild animals as skunks, bats, and raccoons, with such domestic animals as dogs and cats accounting for fewer than 10 percent of cases. The World Health Organization (WHO) estimates that between 35,000 and 50,000 individuals worldwide die each year as a result of rabies. The highest incidence of rabies occurs in Asia where, in 1997, over 33,000 deaths were noted, most occurring in India. Rabies is in the early 2000s rare in the United States, as a result of good animal control practices. Onset is delayed, usually weeks to months after the person has been bitten. Early symptoms of rabies include fever, headache, and flu-like symptoms. These conditions progress to anxiety, hallucinations, muscle spasms, partial paralysis, fear of water (hydrophobia), and other neurological symptoms as the virus spreads to the central nervous system. Medical treatment must be sought soon after exposure because death invariably follows once the infection becomes established.
Most deaths from rabies in the United States in the late twentieth and early twenty-first centuries have resulted from bat rather than dog bites; one victim was a man in Iowa who died in September 2002.
When to call the doctor
Minor animal bites and scratches (those that just break the surface of the skin) can be treated satisfactorily at home. The American Academy of Family Practice (AAFP) recommends the following steps:
- Wash the bite with soap and water, and rinse well.
- Apply pressure with a clean towel or cloth to stop the bleeding.
- Cover the wound with a sterile dressing or bandage.
- Hold the injured area above the level of the heart to lower the risk of tissue swelling and infection.
- Apply antibiotic ointment to the bite twice a day until it heals.
- Report the incident to the local animal control authority or police.
Parents should, however, take their child to a doctor or the emergency room in any of the following circumstances:
- The bite was inflicted by a cat.
- The child was bitten by a dog on the hand, foot, head, or neck.
- The wound is deep or gaping.
- The child has an immunodeficiency disease, cancer, diabetes, a kidney or liver disorder, or any other condition that affects the body's ability to fight off infection.
- The wound already shows signs of infection.
- The bleeding does not stop after 15 minutes of pressure.
- The child has a broken bone, nerve damage, injury to the skull, or other serious injury in addition to the bite.
- The child has not had a tetanus shot or booster within the past five years.
Most animal bites that cannot be treated at home are examined by a doctor in the emergency room of a
Evaluation of possible exposure to rabies is also important. A biting animal suspected of having rabies is usually caught and restrained, tested, and observed for a period of time for evidence of pre-existing infection.
Treatment depends on the wound type, its site, and risk factors for infection. All wounds from animal bites are cleaned and disinfected as thoroughly as possible. The doctor begins by injecting a local anesthetic in order to examine the wound thoroughly without causing additional pain to the child. The next step is to remove dead tissue, foreign matter, and blood clots, all of which can become sources of infection. This removal is called debridement. After debriding the wound, the doctor will cut away the edges of the tissue, as clean edges heal faster and are less likely to form scar tissue. The doctor then irrigates, or flushes, the wound with saline solution forced through a syringe under pressure. Irrigation is highly effective in lowering the risk of infection from animal bites; in one study of 45 children with a total of 145 dog bite wounds on the face, pressure irrigation combined with trimming the edges of the wounds resulted in an infection rate of 0.4 percent.
Bites to the head and face usually receive sutures, as do severe lacerations elsewhere. Puncture wounds are left open. If an abscess forms, the physician may perform an incision in order to drain the abscess.
If infection does occur, the doctor will prescribe antibiotic medications. Antibiotics may also be used for infection prevention. Since a single bite wound may contain many different types of bacteria, no single antibiotic is always effective. Commonly prescribed antibiotics are penicillin or a combination of amoxicillin and clavulanate potassium (Augmentin). Aztreonam (Azactam) has been reported to be effective in treating infections caused by P. multocida. In most cases, antibiotics taken by mouth are sufficient; however, some deep bites may require treatment with intravenous antibiotics.
Because rabies is caused by a virus, antibiotics are not effective against it. In addition, there is no known cure for the disease as of 2004 once symptoms become apparent. People with a high risk of contracting the disease should receive preexposure vaccination. Individuals bitten by an unknown or potentially rabid animal should receive postexposure vaccination, also called postexposure prophylaxis (PEP). The PEP regimen consists of one dose of vaccine given at the initial visit as well as one dose of human immune globulin. Additional doses of vaccine are given on days three, seven, 14, and 28.
Once a bacterial infection is halted, the bite victim usually recovers fully. There is no known cure for rabies once symptoms become evident and death is almost certain. WHO reports that 114 rabies deaths occurred in the Americas in 1997, with only four deaths occurring that year in the United States, thus emphasizing the importance of good animal control practice and postexposure prophylaxis.
The prognosis for restoring the function or appearance of a hand or other body part following a severe bite depends on the location of the bite, the promptness of treatment, and the availability of specialized surgical repair.
Preventing bites obviously prevents subsequent infections. With regard to domestic pets, parents should inform themselves about the aggression level and other characteristics of a particular breed before bringing a purebred pet dog into the family and consider having a specific dog evaluated by a veterinarian or animal behaviorist before adopting it. In addition, parents should make sure that the dog has been neutered or spayed, since intact dogs of either sex are more likely to bite than those that have been altered. Cat bites can often be prevented by learning about a cat's body language and recognizing the signs of petting-induced aggression. These include dilating pupils, a low growl, stiffening of the body, twitching of the tail, and flattening the ears backward against the head.
Children under 12 years of age are at a higher risk for bites due to their small size and their inexperience with animals; therefore, they should be supervised with animals and taught to act appropriately around them. In particular, children should be taught not to tease a dog by pulling its fur or tail; to leave a dog alone while it is eating; and to avoid running or screaming in the presence of a dog, as the animal is more likely to chase a moving object. Direct eye contact with a threatening dog should be avoided, as the dog may interpret that as aggression. It is best to stand still if at all possible, with feet together and arms against the chest; most dogs will lose interest in an object that is not moving and will eventually go away.
A wild animal that is unusually aggressive or behaving strangely (e.g. a raccoon or bat that is active during the daytime or is physically uncoordinated) should be avoided and reported to the local animal control authorities; it may be infected with the rabies virus. Wild animals should not be taken in as pets, and garbage or pet food that might attract wild animals should not be left outside the home or campsite. People should also avoid trying to break up fights between animals and should as a rule approach unknown cats and dogs very cautiously, especially on their territory. Finally, animals should not be trained to fight.
Domestic pets should be vaccinated against rabies; people should consult a veterinarian for advice about the frequency of booster vaccinations for the area in which they live. In addition, families planning to travel to countries where rabies is endemic should consider vaccination before leaving the United States. The AAFP frequently posts updated travel advisories for rabies immunizations.
Anaerobic—An organism that grows and thrives in an oxygen-free environment.
Canines—The two sharp teeth located next to the front incisor teeth in mammals that are used to grip and tear. Also called cuspids.
Carnassials—The last upper premolar teeth in the mouths of cats and other carnivores, adapted to shear or puncture food. Carnassial teeth often cause puncture wounds when a cat bites a human.
Culture—A test in which a sample of body fluid is placed on materials specially formulated to grow microorganisms. A culture is used to learn what type of bacterium is causing infection.
Irrigation—Cleansing a wound with large amounts of water and/or an antiseptic solution. Also refers to the technique of removing wax (cerumen) from the ear canal by flushing it with water.
Microorganism—An organism that is too small to be seen with the naked eye, such as a bacterium, virus, or fungus.
Pasteurellosis—A bacterial wound infection caused by Pasteurella multocida. Pasteurellosis is characterized by inflammation around the wound site and may be accompanied by bacteria in the bloodstream and infection in tissues and organs.
Pathogen—Any disease-producing microorganism.
Postexposure prophylaxis—Any treatment given after exposure to a disease to try to prevent the disease from occurring. In the case of rabies, postexposure prophylaxis involves a series of vaccines given to an individual who has been bitten by an unknown animal or one that is potentially infected with the rabies virus.
Tenosynovitis—Inflammation of a tendon and its enveloping sheath, usually resulting from overuse injury.
Zoonosis—Any disease of animals that can be transmitted to humans. Rabies is an example of a zoonosis.
Given prompt treatment, most animal bites are not cause for major concern; as has been mentioned, minor bites can be treated at home without a visit to the doctor.
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Julia Barrett Rebecca Frey, PhD