Animal Bite Infections Health Article

Definition

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.

Description

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.

Demographics

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 30 to 50 percent of cat bites become infected because a cat's teeth can penetrate more deeply than a dog's and carry bacteria deeper into a wound.

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.

Causes

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.