Animal Bite Infections

Human skin is a protective layer and it is the first line of defense against invading microorganisms. During an animal bite, that protective barrier is broken. In addition, various microorganisms that colonize the animal’s mouth enter into the human body via an animal bite.

In United States alone, millions of animal bites occur every year. Most animal bites are inflicted by dogs and cats. Annual incidence of dog and cat bites are around 300 per 100,000 populations in United States (Harrison 17th). However, other animal bites also occur in significant proportions.

Dog bites

Rabies is the most feared infection a dog bite can cause. However, it is rare in United States (in 2006 only 6940 animal cases were identified). Due to the lethality of rabies, any dog bite should be analyzed thoroughly to exclude the possibility of getting rabies and a prophylaxis vaccination should be given if necessary. Almost all mammalian bites can cause rabies, with the exception of rat bites.

The most common infection that occurs after a dog bite is cellulitis of the surrounding skin area, characterized by redness, swelling and pain. If not treated quickly, it can result in localized tissue destruction. Septic arthritis and osteomyelitis can develop if the canine tooth penetrates bones or joints. Another common complication associated with dog bites is tetanus. Therefore, tetanus toxoid should be given if the person has not been immunized.

Rarely, dog bites can cause widespread bacterial infections such as septicemia, meningitis, brain abscess or endocarditis. These infections are much more common in immunocompromised people. 

Another rare infection is C. canimorsus infection, which results in septicemia, bleeding and kidney failure. It occurs especially in immunocompromised individuals, people with liver failure and in people who underwent splenectomy.

Cat bites and scratches

Cat bites are more likely to cause infections, because the sharp incisors of the cat can penetrate deep into the tissues.  Cat bites can cause Pasteurella multocida infection (which sometimes can also be caused by dog bites). This organism is a part of the normal bacterial flora of the cat’s mouth. It tends to advance rapidly, usually within hours. It causes a discharge of pus and severe inflammation. 

Another infection is cat scratch disease, caused by Bartonella henselae. Cat scratch disease is characterized by fever, malaise, formation of a vesicular nodule in the area scratched by the cat and painful lymph node enlargement. It will usually resolves within three months. In addition, Tumlaremia is also reported with cat bites and scratches.

Other animal bites

Apart from venomous effects, snake bites can result in severe local infections and cellulitis due to bacteria that colonizes the snake’s mouth. Bites from Old World monkeys (Macaca) could result in severe central nervous system infection due to transmission of Herpesvirus simiae or B virus. Bites from seals and walruses may result in seal finger, which is caused by Mycoplasma sp, which colonize the seal’s mouth.

In addition, bites from small rodents such as rats can cause rat bite fever. It is characterized by fever, muscle pain and joint pains. However, it can be treated with antibiotics.


The most common animal bites that can cause infections are dogs, cats, snakes, monkeys and rats. All the mammalian bites except rats can cause rabies. In addition, most of the other infections are due to the bacterial flora that are found in an animal’s mouth and infective agents in the animal’s body.


Harrisons Principles of Internal Medicine 17th edition