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Common Tick Spreads Rocky Mountain Spotted Fever Across Arizona, Mexico

Published: Wednesday, September 5, 2018 - 5:23pm
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A female adult brown dog tick (Rhipicephalus sanguineus).
Sizes of the larva, nymph, and adult stages of the brown dog tick compared with a human finger.

Since 2003, Rocky Mountain Spotted Fever (RMSF) has struck eight Arizona counties where it formerly was not found, according to the Centers for Disease Control and Prevention.

From 2003 to 2017, officials reported nearly 380 cases of the bacterial disease in Arizona, mainly among Native Americans, 10 percent of whom died.

Since the 1940s, the disease has spread across Mexico, where its early flu-like symptoms — fever, headache, vomiting, muscle pain and diarrhea — can be misdiagnosed as the more common dengue virus.

Diagnosis is further complicated by the fact that symptoms appear 3-12 days after a bite that the patient might not remember receiving. One study found that roughly 40 percent of RMSF patients in the U.S. did not notice or recall being bitten.

Delays in treatment of RMSF can result in amputations, and more than 20 percent of untreated cases are fatal. But the disease responds well to early treatment with the antibiotic doxycycline.

RMSF gets its name from the rash of small, flat, pink spots that sometimes go with it. The rash, which does not itch, might not occur for five days after the onset of fever, and in some patients will never develop.

Recent outbreaks have also occurred in Sonora and Baja California. The 2009-10 Baja California outbreak took place in Mexicali, where officials reported 967 cases with 132 deaths. In Sonora, from 2003 to 2017, 1,394 cases were reported, accounting for 247 deaths. The disease was particularly prevalent among indigenous migrant agricultural workers in low-income, rural communities.

Kathleen Walker, a vector ecologist at University of Arizona, says the disease often strikes people who lack resources.

"People who live far from medical care, people who don't have health insurance, people who don't have money to protect their dog. Those are the people that we should really be concerned for."

The key to combating RMSF is to control ticks on dogs, particularly free-roaming ones, and to clear yards of potential safe harbors like toys, old tires and other detritus.

But Walker emphasized focusing on the dogs, and on people who think they might have the disease advocating for their own health.

"Everybody should protect their dog, and then be aware that, if you see signs that look like it could possibly be Rocky Mountain Spotted Fever, you should get medical care immediately," she said.

Some Arizona American Indian communities have had success with communal tick control plans, equipping dogs with tick collars and treating homes and lawns. However, the expense of such programs can block their implementation.

Elsewhere, RMSF is spread by other ticks, but in Arizona and Mexico it spreads via the brown dog tick (Rhipicephalus sanguineus), which is found throughout the world. Prior to around 20 years ago, it was not a known vector for Rickettsia rickettsia, the bacterium that causes RMSF in Arizona.

Researchers are still investigating the origin of that change.