Rocky Mountain spotted fever epidemic at US border, study says

What began in Mexicali in 2008 has become an epidemiologic emergency there and has affected 4,000 people as of 2018, the report states

Brown dog tick (Rhipicephalus sanguineus) Photo courtesy Companion Vector-Borne Diseases

A study published by the School of Veterinary Medicine at Universidad Autonoma de Baja California and the University of California, Davis says ticks are causing a deadly Rocky Mountain spotted fever (RMSF) epidemic in Mexico, and health officials are concerned it could spread to the U.S.

Dr. Luis Tinoco-Gracia, a research professor in the School of Veterinary Medicine at Universidad Autonoma de Baja California and director of the Laboratory of Veterinary Public Health Sciences, in Mexicali, Mexico, and colleagues from the University of California, Davis published the report in the September 2018 issue of Emerging Infectious Diseases, the Centers for Disease Control and Prevention’s public health journal.

The RMSF epidemic began in 2008 in Mexicali, adjacent to the U.S. border in Baja California. In 2014, a fatal human case was reported in Imperial County, Calif. In 2015, the Mexican Ministry of Health declared an epidemiologic emergency, which as of 2018 has affected approximately 4,000 people.

Since that time, four people who have been carrying the disease and crossed the border to the U.S. have died. Overall, since 2000, the incidence of RMSF in the U.S. has increased markedly, the study authors said.

“Overall, since 2000, in the United States, the incidence of RMSF has reportedly increased fourfold; this dramatic increase may be caused in part by increased transmission via the brown dog tick, but also by changes in reporting and inclusion of false-positive test results in case diagnoses.”

RMSF, caused by Rickettsia rickettsii, is responsible for more human deaths than any other tickborne disease in North America, according to the authors, citing a total of 80 fatal cases reported from Sonora, Mexico, during 1999 to 2007.

According to the authors, the brown dog tick (Rhipicephalus sanguineus) has more recently emerged as a vector of R. rickettsia, which increases the threat to humans because of R. sanguineus’ ability to adapt to indoor living.

The RMSF epidemic in Mexicali has not been contained and may be spreading to other parts of Baja California and into the U.S., according to the authors.

“More data are needed before we can understand why this epidemic emerged, where the specific areas of high risk for exposure to infected ticks are located, and whether the particular R. rickettsia strain or relationship with this R. sanguineus tick strain is likely to be particularly invasive or virulent,” the authors said.

“This large epidemic in a major city will require a far greater and more creative public health response,” the authors continued. “Studying this epidemic offers an opportunity to understand the origin and dynamics of this epidemic and can inform response to emerging tickborne diseases in general.”

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