The Human Disease

Infection with Bartonella henselae in an immunocompetent person (i.e., a normal, healthy person) person leads to cat scratch disease. The inoculation site, a scratch from a claw containing bits of flea dirt, develops a small red bump called a papule. About 2 to 3 weeks following contact with the infected cat, the lymph node in the area will swell and become painful, and a fever will develop. These signs generally resolve on their own, and the condition is minor, though lymph node enlargement can persist for several months.

If the patient does not have a competent immune system (in other words, they are very old, very young, debilitated from another condition, etc.), one of several far more serious syndromes can result. The infection goes deeper into the body and causes spleen enlargement, and potentially encephalitis, heart valve infection, and other conditions. These syndromes are rare, but they are potentially quite serious.

How Likely Is It for a Cat To Be Infected?

Since fleas carry the bacteria, cats with insufficient flea control are at the highest risk. This means cats living in climates that are warm and humid (conditions where fleas thrive best) are most likely to be infected. If conditions are right, up to 40% of cats in an area may be infected. If a person is diagnosed with cat scratch disease, there is a 90% chance that the cats they own will be found infected as well.

This sounds somewhat concerning for the cat-owners in a flea area but it is important to realize that an infected cat cannot transmit the infection without a claw full of flea dirt. If the fleas are removed from the infected cat, there will be no flea dirt in the coat and no risk of disease transmission. Experimental studies have not been able to demonstrate cat-to-cat transmission in cats that mutually groom each other, fight with each other, mate with each other, or share food or litter with each other, nor has transmission been demonstrated between infected mother cats and their kittens. Short of an actual blood transfusion from an infected cat, fleas or ticks are needed to spread the infection.

Do Infected Cats Get Sick?

This is a highly controversial question. It was only relatively recently discovered (1992) that cats themselves were more than simple carriers of Bartonella henselae and that they could actually become actively infected themselves.

Several illnesses seem to have been associated with Bartonella infection (fever, deep eye inflammation, lymph node enlargement, muscle pain, reproductive failure, and bacterial heart valve deposits called endocarditis), but these seem to be isolated cases for the most part. The Bartonella organisms are highly adapted to live in the feline body without causing disease, and it is only in rare situations that cats actually experience issues from this infection.

It has been suggested that Bartonella infection may be at the root of numerous chronic inflammatory conditions of the cat. With such high numbers of infected cats occurring regionally (up to 40%), it is going to be difficult to prove one way or the other whether there is a real association or just coincidence.

Can Dogs Get Infected?

The short answer is yes, though the species they get is Bartonella vinsonii rather than Bartonella henslae. Fleas may carry the infection as they do for cats, plus it appears that ticks may also be carriers. Since there are numerous infectious agents spread by ticks and it is not unusual for a dog to have multiple tick-borne infections, it is difficult to determine which infection is causing which signs.

Is My Cat Infected?

Testing is not recommended for the average cat, even if the cat is sick. Testing would be a good idea for any cat that may become a blood donor, for the rare sick cat where Bartonella-related disease is actually being considered if a human in the home has been diagnosed with a Bartonella-related disease, or if there is an immune-compromised person in the home, in which case testing is actually important.

There are five tests available to detect Bartonella henselae: ELISA, IFA, PCR, Culture, and Western Blot. All the tests have pros and cons, and no method seems to shine above the others.

The ELISA, IFA, and Western Blot tests are tests for antibody detection, the idea being that if antibodies against Bartonella are there, then Bartonella must be there as well. For most diseases where antibody levels are used to establish a diagnosis, a minimum titer or antibody amount is considered necessary to say “Yes, this patient is infected.”  A problem with this is that we know that up to 11 percent of cats with Bartonella organisms happily circulating in their bloodstreams will not make antibodies and will thus test negative. At least this means that when the test is negative, there is an 89 percent or greater chance that the cat is truly negative. For screening, this kind of antibody testing is frequently paired with PCR testing.

PCR is a very sensitive DNA test for Bartonella, but because the organism only intermittently circulates, false negatives can result if not very much Bartonella DNA is circulating. Results from PCR testing can rapidly distinguish different species of Bartonella. 

The most reliable test is the blood culture; however, several consecutive cultures are needed because the organism tends to circulate only intermittently. A positive culture is proof of infection, though a negative culture may simply not have been taken at the time when the organism is circulating. 

In humans, a delayed hypersensitivity skin test is used as part of the diagnostic criteria for cat scratch disease but this test has not been useful in cats. In this test, similar to the tuberculosis test most of us are familiar with, a scratch on the skin is made, and a reaction to the introduced antigens may occur either right away or in approximately 48 hours (delayed hypersensitivity reaction). Cats are poor delayed hypersensitivity responders. 

All in all, it is hard to rule out a cat as a carrier of a Bartonella organism. If the results of testing are suggestive of infection, treatment can be considered if the cat is truly believed to have an illness from the infection or if there is realistic concern about exposure to an immune-compromised person.

Treatment for Cats

An assortment of antibiotics have been used against Bartonella henselae in cats: clavulanic acid-amoxicillin combination, azithromycin, doxycycline, and quinolone class medications (enrofloxacin, marbofloxacin, etc). Efficacy has been mixed, and Bartonella henselae rapidly becomes resistant to therapy. Treatment is currently recommended for cats showing symptoms of disease or for cats living with immune-compromised individuals.  Treatment lasts a minimum of three weeks.

Preventing Human Infection of Bartonella: Guidelines from the Centers for Disease Control and Prevention

• Cats belonging to immune-compromised people should be kept indoors only and should not have contact with cats that go outdoors.
• Claws should be kept trimmed.
• Cat scratches should be promptly washed. (This goes for everyone, not just immune-compromised people.)
• Flea control should be strict. 

Preventing Disease

No data support chemoprophylaxis (i.e., preventive drug treatment) for Bartonella-associated disease. In simpler terms, if a suspicious cat scratch has occurred, there is no point in using medication to prevent infection.