For years there has been a growing body of research linking cat ownership and depression, especially in women. A number of studies have looked at his from different angles, trying to determine the actual cause of this, and a new study may have come across an unexpected cause: Bartonella bacteria.

Previous Research

Data analysis of patient records for 1.3 million patients previously discovered a statistical association between cat bites and human depression. That study’s authors suggested that the women could be infected with Toxoplasmosa gondii, which made them depressed. The researchers did not suggest that the cat bites were necessarily the cause of the Toxoplasmosa infection, but it was being noticed as a common characteristic of the depressed women who all had cats.

Previous research had already linked depression and suicide attempts to the Toxoplasmosis infection. One theory is this happens that when humans are infected with Toxoplasmosa, the immune system kicks in to try to ward off the attacker by producing molecules called cytokines that tell your immune cells to get to work. High levels of cytokines have been linked to both depression and suicide attempts.

New Research on Animal-Inflicted Injuries and Depression

A pair of researchers in the Czech Republic wanted to take a closer look at the correlation between animal-inflicted injuries and depression. Study subjects filled out a questionnaire where they identified how closely they lived with dogs or cats during their whole lives and whether they had been bitten or scratched by a dog or cat.

The research subjects also took the Beck Depression Inventory, a standard diagnostic tool to measure the severity of depression, and they also identified whether they had ever been diagnosed as infected with Toxoplasmosa gondii.

Detailed analysis of the survey results found that neither cat bites nor Toxoplasmosa were the primary correlation to depression in women. Instead, depression occurred more often in women who had been scratched by cats.

This led the researchers question whether something other than Toxoplasmosa might be triggering depression in women. The study’s authors propose that the Bartonella bacteria, which is responsible for cat-scratch disease, might actually be the cause.

Cat-scratch disease, also known as cat-scratch fever, is carried by fleas and ticks and transmitted to humans primarily by scratches from cats, usually when the cats are kittens. Cats get the Bartonella bacteria that causes cat-scratch disease by being bitten by infected fleas, and it is found in up to 40% of cats.

Human medicine usually focuses on the physical symptoms caused by Bartonella, which can include a bump or blister at the wound site where the bacteria entered the body, low grade fever, enlarged lymph nodes, and fatigue. Newer research about Bartonella in humans shows that it can become a chronic condition that returns after it seems to be eradicated by antibiotics, and it can also cause psychiatric issues, including treatment-resistant depression.

Just as the previous correlation of depression in cat owners to Toxoplasmosa infection was not fully explored, the statistical link between cat scratches, potential Bartonella infection, and depression is still in its early stages, and the study authors suggest that a wider study be performed.

Hopefully science will soon help us learn more about whether this is really the root cause of the observed link between living with cats and depression. In the meantime, it is a good idea to reinforce during kittenhood that human hands and feet aren’t toys to bite and scratch, trim your cat’s nails, and to try to quickly get rid of flea infestations. Those are good practices regardless of the outcome of future research.

---

Research and further reading:

Parasites & Vectors, Cat scratches, not bites, are associated with unipolar depression – cross-sectional study
Lancet, Feasibility of neonatal screening for toxoplasma infection in the absence of prenatal treatment
The Journal of Nervous and Mental Disease, Toxoplasma gondii Seropositivity and Suicide Rates in Women
PLOS One, Describing the Relationship between Cat Bites and Human Depression Using Data from an Electronic Health Record
Centers for Disease Control and Prevention, Bartonella Infection – for Veterinarians
Journal of Clinical Microbiology, Bartonella sp. Bacteremia in Patients with Neurological and Neurocognitive Dysfunction
Medscape General Medicine, Do Bartonella Infections Cause Agitation, Panic Disorder, and Treatment-Resistant Depression?

Photo credits:

depositphotos, photo-deti
depositphotos, OlegTroino
flickr creative commons, Tambako the Jaguar