Higher Temperatures Make Some Ticks Pick Humans Over Dogs

The study’s results suggest certain tick-borne diseases could become even more prevalent as climate change heats up the planet

American dog tick
An American dog tick (Dermacentor variabilis) on a human finger. Jerry Kirkhart via Flickr under CC 2.0

Disease-carrying ticks that normally prefer dogs may start to prefer the blood of humans as climate change cranks up the heat around the world, according to new research presented at the annual meeting of the American Society of Tropical Medicine and Hygiene (ASTMH).

Experiments revealed that brown dog ticks, which can transmit the life-threatening Rocky Mountain spotted fever (RMSF) and normally prefer to feed on dogs, become more likely to switch up their usual dining habits and go for people as the mercury rises, reports Patrick Barkham for the Guardian.

“Our work indicates that when the weather gets hot, we should be much more vigilant for infections of Rocky Mountain Spotted Fever in humans,” says Laura Backus, a Ph.D. student in veterinary medicine at the University of California, Davis who led the study, in a statement.

RMSF can be treated with antibiotics, but if left untreated, kills 30 percent of those infected. Early detection is crucial to treating the disease, as it can quickly escalate if an infected person isn’t treated within five days of their first symptoms. Those symptoms include fever, rash, severe headache, swelling around the eyes and backs of the hands, as well as nausea or vomiting, writes Kathleen Doheny for WebMD Health News. According to the Centers for Disease Control and Prevention, cases of RMSF are on the rise. In 2017, there were more than 6,000 cases, a 12-fold increase compared to the 495 cases reported in 2000.

Spurred by prior research that suggested brown dog ticks were more aggressive towards humans in warmer weather, Backus and her colleagues devised an unenviable experiment to test the finding. They constructed two large wooden boxes and connected them with a clear plastic tube. Once the researchers had placed a hapless graduate student in one box and a dog in the other, they released 20 ticks into the plastic tube, per the statement.

Higher Temperatures Make Some Ticks Pick Humans Over Dogs
One of the dogs inside the wooden boxes used in the experiments. Don Preisler, UC Davis

For 20 minutes, the researchers observed which way the dueling scents of the two different potential meals led the insects to crawl. Though mesh barriers prevented the ticks from reaching their quarry, it was clear from the ticks’ scuttling that the hotter it got, the more likely they were to head towards the person. Compared to 74 degrees, 100-degree temperatures made the ticks two and a half times more likely to prefer the human over the dog, reports Ashley P. Taylor for Live Science.

“The findings from the use of this simple but effective laboratory experiment to gauge how rising temperatures might lead to more human infections with a very dangerous tick-borne pathogen adds to the growing evidence of the increasing connection between climate change and its impact on health,” says Joel Breman, the president of ASTMH, in the statement.

“Climate change is moving so quickly that it is critical to keep pace with the many ways it may alter and intensify the risk of a wide range of infectious diseases so we are better prepared to diagnose, treat and prevent them.”