Pet vectors: MRSA infections transmitted between humans, animals

Key Points

Tampa, Fla. - People and their pets are often said to take on the morphology and mannerisms of each other over time. Regardless of whether that is true, it's clear that they do come to share diseases, and, increasingly, that means methicillin-resistant Staphylococcus aureus (MRSA).

"There is no question that there has been an increase in dermatological problems in both pets and humans attributed to community-associated strain of USA300 MRSA over the last five to 10 years," says Richard Oehler, M.D., an infectious disease specialist at the University of South Florida College of Medicine.

"There is a foundation of evidence that there is a direct relationship between MRSA in humans and in pets; recent literature has solidified that," he says.

Two things inspired Dr. Oehler's review of household pet zoonoses and their treatment, which appeared in the July issue of Lancet Infectious Diseases. One was the decision to get a dog for his children - he says he grew up around cats but lacked experience with dogs - and the other was an unusual infection at his hospital.

"A patient on dialysis had a life-threatening bloodstream infection. It turned out to be Pasteurella ssp." The organism is a common, benign part of the oral flora of dogs and particularly cats. Given that lead, a careful questioning of the patient "revealed that the pet had licked the dialysis catheter," he says.

That exposure led to development of the infection.

Dr. Oehler emphasizes the need to keep wounds and implanted medical devices clean, covered and away from pets.

"A lot of times germs are harmless until you put them in some place outside of their natural environment. That is the case with the normal bacteria that occur in the mouths of our pets," he says.

Vulnerable groups

Certain populations are more vulnerable to these pathogens - the very young with immature immune systems, the elderly with declining immune function, and patients in whom skin barrier function is reduced. Also, many types of therapy wreak havoc with immune protection.

Hands are the primary mode of human contact with pets, Dr. Oehler notes. That makes them particularly at risk for bites. Hands also are the most likely vector of transmission in both directions, from contact with an open wound on the animal or a colonization site on the human.

"Hands are perhaps the most complicated structure in the body, with many ligaments, nerves, blood vessels, multiple compartments, and planes of tissue that can be penetrated by puncture wounds from dogs or cats. They can lead very easily to deep space infections," Dr. Oehler warns.

Cats pose more risk

Cats pose the greater risk of this type of infection because their teeth are very sharp and needle-like and "can puncture the joints of the hand, potentially leading to debilitating septic arthritis."

With dog bites, there is greater risk of traumatic destruction of tissue, "due to (dogs') larger teeth and more vigorous mechanical force," Dr. Oehler says.

The pet does not have to go out of the household to come in contact with infectious bacteria, he says.

"With MRSA, many times the pet is acquiring it from people in the household. We bring it into the household, the pets acquire it from us, and someone else in the household - a child, someone with a chronic medical condition, somebody just back from the hospital - can acquire that infection from their pet unknowingly," he says.

"Good infection control in the household - hand washing - is the most important thing you can do," Dr. Oehler tells his patients.

"No. 2, if the patient develops any kind of a pustular wound that is greater than 2 centimeters, that may require medical attention from a physician.

"Any rapidly expanding skin condition that looks like an infection should receive prompt medical attention," he says. "The presence of fever associated with a skin infection is also a warning sign."

Dr. Oehler says any animal bit that is "more than just the casual nip" should receive medical attention.

"With a cat bite, you can never be sure how deeply the puncture wound has been," he says. "In some cases, you can prevent a more complicated infection with good medical attention."

Unprovoked dog bites always merit medical attention, he says, because of the risk of rabies, particularly if the vaccination record of the animal is unknown.

Dr. Oehler advises that any use of antibiotics should include coverage against MRSA, because of the growing prevalence of the pathogen.

"We should remember that when a pet bites a person, there also is the risk of transmission of the patient's own staph germs from the skin into deeper tissue," bypassing the skin barrier, he says.

"The MRSA doesn't have to come from the pet; it can come from us but be introduced into the wrong place, where it can cause a potential infection," Dr. Oehler says.

Disclosures: Dr. Oehler reports no relevant financial interests.