Brown Dog Tick Causes RMSF Epidemic in Mexico, Could Spread Stateside

A recent molecular study has confirmed the causative agent involved in a large epidemic of Rocky Mountain spotted fever in humans and dogs in Mexicali, in Baja California, Mexico.

A recent molecular study has confirmed the causative agent involved in a large epidemic of Rocky Mountain spotted fever (RMSF) in humans and dogs in Mexicali, in Baja California, Mexico. Because Mexicali is a town adjacent to the border with the United States, investigators now express concern that the epidemic may be spreading stateside.

Dr. Luis Tinoco-Gracia, from the Universidad Autónoma de Baja California, Mexicali, Mexico, and colleagues from the University of California, Davis, USA, published the report in the September 2018 issue of Emerging Infectious Diseases, the monthly peer-reviewed public health journal of the US Centers for Disease Control and Prevention (CDC).

RMSF is a tick-borne disease that is caused by the bacterium Rickettsia rickettsii. Infection with this bacterial agent results in severe, widespread vasculitis that can result in organ failure and death. RMSF causes more human deaths than any other tick-borne disease in North America, the authors say.

In the United States, the American dog tick (Dermacentor variabilis) and Dermacentor andersoni (the Rocky Mountain wood tick) are the 2 major vectors of R rickettsii. D variabilis is widely distributed east of the Rocky Mountain, whereas D andersoni is prominent in the western United States.

However, according to the authors, the brown dog tick (Rhipicephalus sanguineus) has also more recently emerged as a vector of R rickettsii. This raises the threat to humans, the authors note, because although the tick predominantly targets dogs as hosts, dogs frequently bring them into home environments, and thus, into direct contact with humans. Unlike Dermacentor ticks, which are best adapted to living on wild animals, the brown dog tick easily adapts to living indoors.

In 2008, an epidemic of RMSF began in the border town of Mexicali, in Baja California, Mexico. As of 2018, this epidemic has now affected approximately 4,000 individuals. One human death in the United States, in Imperial County, California, was also probably linked to this epidemic, Dr. Tinoco-Gracia and colleagues say. Recent epidemics have also arisen in Arizona, the USA, and in the Mexican state Sonora, they add.

“Overall, since 2000, in the United States, the incidence of RMSF has reportedly increased »4-fold; 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.”

While investigating the RMSF in Mexicali, the group from the Universidad Autónoma de Baja California tested dogs in the area and found that 81% of them were infected with R rickettsii. Using polymerase chain reaction (PCR) testing, they also confirmed the presence of active R rickettsii infection in kidney tissue from one individual. Additionally, they found that 30 of 120 patients from Mexicali who had experienced clinical signs consistent with RMSF, tested PCR-positive for the rickettsial gltA gene.

The group from the University of California also used molecular techniques to further characterize strains of R rickettsii and R sanguineus ticks from the Mexicali outbreak.

In particular, using PCR, they tested blood samples from 16 patients affected by the epidemic in Mexico. Ten samples tested positive for the R rickettsii citrate synthase gene, and 5 were positive for 2 other rickettsial genes, ompA and the 17kDa antigen gene.

“We provide definitive molecular confirmation of the identity of the disease agent causing the Mexicali epidemic,” the authors write.

However, they stress that the RMSF epidemic in Mexicali has not yet been contained, and could even spread into the United States.

“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 rickettsii strain or relationship with this R sanguineus tick strain is likely to be particularly invasive or virulent,” the authors conclude.

Dr. Parry graduated from the University of Liverpool, England in 1997 and is a board-certified veterinary pathologist. After 13 years working in academia, she founded Midwest Veterinary Pathology, LLC where she now works as a private consultant. She is passionate about veterinary education and serves on the Indiana Veterinary Medical Association’s Continuing Education Committee. She regularly writes continuing education articles for veterinary organizations and journals and has also served on the American College of Veterinary Pathologists’ Examination Committee and Education Committee.