In Mexicali, an uncontrolled epidemic of Rocky Mountain spotted fever has affected more than 1,000 people since 2008.
Researchers examined dogs, ticks, and surveyed households in 200 neighborhoods. Half of the neighborhoods in the study had diagnosed human cases of the disease. The team discovered that even though citywide only one in 1,000 ticks were infected, there were neighborhoods at very high risk where almost one in 10 ticks were infected.
“If you live in one of these high-risk neighborhoods and you get five brown dog tick bites, that means you have a pretty good chance of being exposed to Rocky Mountain spotted fever,” says lead author Janet Foley, with the department of medicine and epidemiology at the University of California, Davis, School of Veterinary Medicine.
The brown dog tick, which feeds on dogs and people, spreads Rocky Mountain spotted fever. The insect thrives in hot, arid climates. Previous studies have shown that poverty, numerous stray dogs, and brown dog ticks increased the risk of getting Rocky Mountain spotted fever. In Mexicali, risks were higher along the edges of poorer neighborhoods or outside of the city in rural areas.
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Half of the 284 dogs the researchers examined were infested with ticks. Some dogs carried thousands of ticks.
“Almost three-quarters of the dogs we tested had been infected with the agent of Rocky Mountain spotted fever at some point in their life,” says Foley. “That’s astronomical.”
People with Rocky Mountain spotted fever typically develop symptoms one to two weeks after an infected tick bites them. They can develop fever, nausea, headache, and muscle pain. As the bacteria infect blood vessel linings, blood begins to pool under the skin, resulting in a rash that can look like red splotches or spots. The longer people wait before seeing a doctor and starting treatment with antibiotics, the greater the chance of death.
The study, which appears in the American Journal of Tropical Medicine and Hygiene, also gauged people’s knowledge about Rocky Mountain spotted fever. It found 80 percent of residents had heard of the disease, but fewer than half used pesticides to prevent bites.
Foley says a Rocky Mountain spotted fever epidemic on the scale of that in Mexicali is not as likely in the United States as long as dog ticks are well managed. But as temperatures warm with climate change, there are concerns that the particular human-feeding brown dog tick strain will continue to move north, resulting in more human cases. Some studies have suggested the hotter it gets, the more active and aggressive the ticks become.
The binational research team included academic researchers, health workers, epidemiologists, veterinarians, agency officials, medical doctors and students, who aided in the need to communicate in Spanish and English, address canine and human disease, understand fundamental epidemiological patterns, and protect public health. The US Centers for Disease Control and Prevention, the UC Davis School of Veterinary Medicine, and the Autonomous University of Baja California funded the work.