A question that has been simmering for years in the veterinarian community is now attracting the interest of physicians as well: Do the bacteria that cause cat scratch disease—a typically mild illness with flulike symptoms—also cause chronic fatigue syndrome? Decades of case reports hint at associations between fatigue, chronic headaches, numbness, pain and cognitive impairment and infection with Bartonella. Yet researchers still do not have clear answers.

Recent research found fragments of Bartonella species' DNA in 41 percent of 296 patients examined by a rheumatologist. Many of them had visited multiple specialists without finding relief from their symptoms. The findings, published in May 2012 in Emerging Infectious Diseases, drew criticism in two letters to the editor, published last November, which expressed concerns with patient inclusion criteria and a low threshold cited as evidence of infection. “We must be cautious before attributing illnesses X, Y and Z to Bartonella infection without solid evidence,” says Christina Nelson, a medical epidemiologist at the U.S. Centers for Disease Control and Prevention, who adds that the study results were difficult to interpret.

Complicating matters is the pathogen's elusive biology: it evades detection within hosts by changing proteins on its surface and by hiding inside blood vessels. In addition, the organism can shift strategies depending on whether it is in a mammalian host, such as a cat or dog, or an insect vector, such as a flea or tick. “We are not even at the tip of the iceberg” when it comes to understanding Bartonella, says Jane Koehler, a professor of medicine at the University of California, San Francisco.