A wide-ranging tick previously considered to be little more than a nuisance to people is responsible for at least 11 cases of Rocky Mountain spotted fever in eastern Arizona, researchers report. The lethal bacterial disease had been virtually unknown in that area.
The cases represent the first documented U.S. outbreak of the disease directly attributable to the brown dog tick, Rhipicephalus sanguineus. Health officials had presumed that the fever spreads exclusively through American dog ticks and Rocky Mountain wood ticks—species that typically feed on wild rodents and other small mammals, including dogs. These ticks also prefer moist climates. In contrast, brown dog ticks feed almost exclusively on dogs and are more common in dry climates than the other ticks are.
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In Arizona, feral dogs that lived in and around two towns carried the disease, says epidemiologist Linda J. Demma of the Centers for Disease Control and Prevention in Atlanta. Although residents didn’t allow the dogs into their homes, they fed the animals and children played with them.
Demma went to Arizona in 2002 to investigate an unusual report of Rocky Mountain spotted fever. She wasn’t surprised to learn that people who showed symptoms of the disease had been exposed to tick-covered dogs. “We found more than 100 ticks on some of the dogs,” she says.
But Demma was stunned when she found only brown dog ticks on the dogs and near the patients’ houses. However, laboratory analysis soon confirmed that some of the ticks carried Rickettsia rickettsii, the bacterium that causes Rocky Mountain spotted fever. Demma later learned that lab tests in 1933 had shown that such ticks could indeed be intentionally infected with R. rickettsii.
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Demma’s team identified 16 people, 12 of them children, with symptoms of Rocky Mountain spotted fever: headache, abdominal pain, fever, sore throat, and a telltale rash. Various analyses of blood and skin samples from these people revealed that 11 of the group had clear-cut cases of Rocky Mountain spotted fever and the other five had probable cases.
Two of the 16 patients, children ages 14 months and 5 years, died. The others received the antibiotic doxycycline and recovered, the scientists report in the Aug. 11 New England Journal of Medicine.
“This is a really excellent study,” says pathologist David H. Walker of the University of Texas Medical Branch in Galveston. He notes that it raises the question of how the bacterium gets into brown dog ticks in the first place. One possibility is that they feed on dogs or other mammals infected with R. rickettsii by one of the other tick species. “Somehow, there’s a bridge in nature to get [the bacterium] into the brown dog ticks,” says Walker.
Although Rocky Mountain spotted fever was discovered in Idaho in 1896, today it occurs primarily in the area stretching from Oklahoma east to the Carolinas, Demma says.