Study: Texas Stray Cats Carrying Fleas Linked To Two Serious Human Diseases

Fleas on South Texas Cats Test Positive for Bacteria Behind Typhus and Cat Scratch Disease

Fleas were found all over the stray cats examined in a new study from the Rio Grande Valley, and many of those fleas carried bacteria linked to two diseases that regularly land people in the hospital: murine typhus and cat scratch disease. Researchers from Texas A&M University’s School of Veterinary Medicine combed fleas off 167 stray cats collected by animal control across south Texas, then tested them for disease-causing bacteria. The findings, published in Parasites & Vectors, suggest free-roaming cats in this stretch of the state sometimes carry bacteria capable of making people seriously ill.

Murine typhus, also known as flea-borne typhus, can cause fever, headache, chills, rash, and stomach or kidney trouble severe enough to require hospitalization. Cat scratch disease, caused by a different bacterium usually picked up through a bite or scratch, can trigger fever, swollen lymph nodes, and rare nervous system complications.

Neither illness is typically fatal in healthy people, but both have been climbing for years. Texas saw murine typhus cases jump from 348 in the 1990s to more than 3,100 between 2010 and 2018, and Hidalgo County, where this study took place, has repeatedly logged the highest case count of any county in the state, with 710 cases between 2017 and 2023 alone.

Cat Fleas Tested Positive for Dangerous Bacteria

That backdrop sent researchers into the Rio Grande Valley itself, a subtropical stretch along the Texas-Mexico border where warm, humid weather gives fleas ideal conditions to thrive. They examined 167 mostly stray cats gathered by animal control from 14 towns and rural areas around the valley, all of which had passed through a large Hidalgo County shelter that takes in roughly 9,000 cats a year. The team combed fleas from each cat, drew blood, and pulled tissue from some animals, then ran DNA tests to see which bacteria were lurking in both fleas and hosts.

Most of the cats, 83 percent, carried fleas, adding up to 721 fleas total. Male cats were flea magnets more often than females, with about nine in ten infested versus roughly three in four. Cats carried about four fleas each on average, though one unlucky cat had 41, and nearly all the fleas collected were a single species, the common cat flea.

Testing showed the bacterium behind murine typhus in about 4% of flea samples. A lesser-known relative, whose health effects on people remain unclear, appeared far more often, in about 20% of samples. Cats with heavier flea loads were more likely to have this typhus-related bacteria show up, a link not seen with the other bacteria studied.

Cat Fleas and the Rise of Cat Scratch Disease

Numbers climbed higher for the bacteria behind cat scratch disease. Bartonella henselae, the species most often responsible for the illness in humans, showed up in about 42% of flea samples. A second, related species turned up in a few samples too. B. henselae itself circulated in the blood of about 22% of the cats, and thirty cats had it in both blood and fleas at once, a sign the infection was actively passing between parasite and host.

Cat scratch disease sends an estimated 4.5 to 9.3 out of every 100,000 Americans to the doctor each year, hitting children ages 5 to 9 hardest, with the highest rates in the South. Most healthy people recover without lasting harm, but weakened immune systems can turn the infection life-threatening, and even healthy patients have occasionally developed nervous system complications, including eye problems.

Perhaps the most unsettling discovery: fleas carrying both bacteria at once showed up far more often than chance would predict. Twenty-five flea samples tested positive for both groups. Even more strikingly, five of the six samples carrying the murine typhus bacterium also carried Bartonella, four specifically B. henselae. Two cats carried fleas positive for both bacteria at once. Researchers floated the idea that these cats could serve as sentinels, early warning signs of broader disease risk nearby, though they stopped short of applying that idea to every cat studied.

Flea Control May Be the Best Defense for Households

Cats that never come indoors likely pose less direct risk than the ones curling up on the couch or mingling with other pets, researchers noted. That risk grows once a flea-carrying cat crosses a home’s threshold, potentially introducing what other scientists call “zoonoses in the bedroom,” diseases that jump from animals to people in close living quarters.

Keeping fleas under control on pet cats could help keep these bacteria out of people’s homes, the study’s authors suggest. Public health experts generally agree routine flea prevention is a smart move, though this study did not test specific treatments. Finding bacterial DNA in fleas and blood is not proof those fleas infected a person, and this sample leaned heavily toward shelter-bound strays rather than pampered house pets.

Still, the takeaway is hard to wave off. Public health surveillance for these diseases has focused mostly on human patients, but the cats and fleas living nearby may be quietly circulating the same bacteria for years. An 18-year review found 213 children hospitalized in south Texas with confirmed murine typhus. In a region where typhus cases have climbed for decades, the flea on a stray cat’s back may carry more risk than most realize.

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Disclaimer: This article summarizes findings from a peer-reviewed scientific study and is intended for general informational purposes only. It is not a substitute for professional veterinary or medical advice. Anyone with concerns about flea exposure, pet health, or symptoms consistent with the diseases described here should consult a veterinarian or physician.

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