Cat Scratch Fever: A Link Between Pet and Human Health

Part of the veterinary oath states that a veterinarian does "solemnly swear to use my scientific knowledge and skills for the benefit of society through the … promotion of public health," which is why diseases that can be transmitted from animals to people (called zoonoses or zoonotic diseases) get special attention from us. When we can effectively treat or prevent a zoonotic disease in a pet, we are protecting both animal and human health.

A study appearing in the journal Emerging Infectious Diseases points to a new link between a type of bacteria carried by cats and poor human health. The bacteria are Bartonella henselae, which most commonly cause bartonellosis in cats and cat scratch disease in people. Add to that list the possibility that Bartonella bacteria may also play a role in the development of some human cases of rheumatoid arthritis (RA).

The researchers tested blood samples taken from 296 human patients exhibiting symptoms consistent with RA, and 62 percent had antibodies to Bartonella bacteria, indicating they had been exposed at some point in the past. Bacterial DNA was found in 41 percent of patient samples, which is consistent with an active infection. B. henselae was one of the most prevalent but not the only species of Bartonella identified in these patients. B. vinsonii subsp. berkhoffii, which causes bartonellosis in dogs, was also found.

B. henselae bacteria are usually transmitted from infected to non-infected cats through fleas (The vector for B. vinsonii subsp. berkhoffii appears to be ticks). People often come in contact with the B. henselae when they are scratched or bitten by a cat that has the bacteria around their nails or in their mouths. Immunosuppressed people are at a higher than average risk of diseases associated with B. henselae infection. Cats may carry Bartonella henselae bacteria in and on their bodies without becoming sick themselves. If they are affected, typical symptoms include fever, enlarged lymph nodes, and/or chronic inflammation of the digestive tract.

Diagnosing a cat with bartonellosis is not always easy. Blood tests capable of identifying cats that have been exposed to the bacteria do not determine whether or not an animal’s current illness is related to Bartonella or if they can transmit disease to people. Other tests reveal the presence of an active infection, but they tend to misdiagnose cats with low-level infections as being free from disease. It is generally not recommended that healthy cats be tested for bartonellosis because of the results can be so difficult to interpret. Treating bartonellosis in cats is also difficult. The antibiotic azithromycin is the drug of choice, but it often fails to completely clear the infection. Cats with symptoms that are thought to be linked to Bartonella bacteria often improve with antibiotics but commonly relapse after treatment is stopped.

The link between Bartonella infections in cats (and potentially dogs) and rheumatoid arthritis in people is still tenuous, but certainly points to a need for more research into these harmful bacteria.