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Warming Planet Ups Risk of Deadly Tick-Borne Fever

photo of tick close up

Nov. 16, 2020 -- Climate change, already linked to more frequent wildfires, longer droughts, and more tropical storms, may also increase the risk of getting the potentially deadly tick-borne disease known as Rocky Mountain spotted fever, new research suggests.

When temperatures rise, the brown dog tick, which carries the bacteria causing the disease, is more than twice as likely to shift its feeding preference from dogs to humans, say researchers from the University of California, Davis. They will present the research today at the American Society of Tropical Medicine and Hygiene annual meeting.

"That risk [of contracting the disease] may increase as climate change causes us to have more frequent hot weather environments," says researcher Laura Backus, a UC Davis graduate student.

Rocky Mountain spotted fever, spread by various types of ticks in the U.S., has a fatality rate of 30% and can kill quickly if it’s not treated within a 5-day window after symptoms appear, the CDC says. Among the symptoms are fever, rash, severe headache, swelling around the eyes and back of the hands, and stomach issues such as vomiting or nausea.

A blood test can help to make the diagnosis. It’s usually treated with the antibiotic doxycycline for 5-7 days.

Cases of Rocky Mountain spotted fever and related diseases, known collectively as spotted fever rickettsiosis, have increased greatly over the last 20 years. In 2000, 495 cases were reported in the U.S.; by 2017, the total was more than 6,000. Cases in 2018 declined somewhat, the CDC says.

Human vs. Dog Experiment

To observe the effect of temperature on a tick's preference to feed on dogs or people, the researchers built two large wooden boxes, about 3 feet high and 2 feet wide, connected by a clear plastic tube. A person sat in one box and a dog in the other as ticks were released into the tube.

For 20 minutes, the researchers observed whether the ticks headed to the dogs or the people, once when the temperature was 74 degrees and then when it was 100 degrees.

Researchers tested the ticks ahead of time to be sure they weren't infected. They placed mesh at either end of the tube, so the ticks could not make contact with dogs or people.

The researchers studied two types of brown dog ticks -- known as temperate and tropical -- both capable of carrying the disease. The tropical lineage ticks greatly shifted their preference from dogs to people; the temperate did too, but less so, Backus says. The researchers can't say why.

Perspective

The research suggests that ''warmer climates are going to have a greater risk of Rocky Mountain spotted fever transmission by this vector," says Kathleen Walker, PhD, an associate specialist and associate professor of entomology at the University of Arizona, Tucson. She reviewed the findings but wasn’t involved in the study.

This tick lives in and around houses, she says. "People find these ticks in their beds." The best prevention is to treat the dogs -- with a tick collar, oral tick medicine prescribed by a veterinarian, or a topical tick preparation.

"The way these come into contact [with people] is through dogs," Walker says. "If you protect the dog, you protect yourself."

Walker also suggests taking all tick bites seriously. "Get it off ASAP," she says, using a forceps to pull it out. Keep an eye on the area. If you get a fever or rash, get medical attention right away, she says. Be sure to tell medical providers you have been bitten.

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Sources

TropMed2020, annual meeting of the American Society of Tropical Medicine & Hygiene, Nov. 16, 2020.

CDC: "Rocky Mountain Spotted Fever (RMSF)."

Laura Backus, PhD student, University of California, Davis.

Kathleen Walker, PhD, associate specialist and associate professor of entomology, University of Arizona, Tucson.

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