An uncontrolled outbreak of Rocky Mountain spotted fever (RMSF) has reached epidemic proportions in one northern Mexico town and started spreading to the United States last year, according to a 2018 study from the Centers for Disease Control and Prevention (CDC).

Historically, most cases of RMSF reported in the US have been transmitted by the bite of a Dermacentor variabilis (also known as the American dog tick or wood tick) that is infected with Rickettsia bacteria. But recent epidemics in Sonora, Mexico, and Arizona have been associated with the brown dog tick (Rhipicephaluls sanguineus), a tick whose preferred host is a dog.

The current outbreak, which began in 2008 in the Mexican border town of Mexicali, has affected more than 1,000 people and an unknown number of dogs. Several hundred people have died of the disease in Mexico and at least one died in the US after crossing the border from Mexico.

Before the advent of antibiotics, RMSF had a mortality rate of up to 30%. Even today, it remains the most common fatal tickborne disease in the United States, with a mortality rate of up to 5%.

Now, a new study details the efforts of researchers to identify the risk factors for RMSF in Mexicali.

The researchers examined dogs and 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 1 in 1,000 ticks were infected with Rickettsia, there were neighborhoods at very high risk, where almost 1 in 10 ticks were infected.

Half of the 284 dogs the researchers examined were infested with ticks—and some of the dogs carried thousands.

NEWStatreached out to Janet Foley, DVM, PhD, MS, professor of epidemiology at the University of California, Davis, School of Veterinary Medicine and lead author of both studies, to find out more about their findings.

NEWStat: As of last summer, the CDC said they didn’t know the epidemic emerged. Are researchers any closer to figuring out why it happened?

Janet Foley: The ticks have been there, so our current hypothesis is that the Rickettsia is spreading among the ticks. However, not controlling dogs and ticks is a serious problem. So it’s a combination of tick numbers going up, stray and mobile dogs, and probably introduction and spread of the bacteria among the ticks.

NEWStat: Are health officials making any headway against the epidemic?

JF: [It’s] early days. A few “bad” neighborhoods have been cleaned up but the infection spreads to new areas, so we constantly have to monitor and manage; and overall, [the number of] cases hasn’t come down enough. So we still need to find new tools, collect data, and try to prevent and manage cases.

NEWStat: What are the possibilities the epidemic will spread to the US?

JF: The tropical lineage of the tick associated with the northern Mexico epidemic is now present in the southern US. We have seen a fatal human case in California but there was [a history of travel from Mexico]. Quite a few dogs on the border, in Southern California and Arizona, have been exposed. So it’s a possibility, although high rates of spay/neuter, control of dog movement, and proactive public health might provide protection.

NEWStat: What can US veterinarians do to help mitigate any possible impact on their clients and their pets?

JF: Honor international regulations for dog movement. If dogs come in from Mexico or anywhere else, ensure they are healthy or treated on import! Ensure they are free of ectoparasites. Continue international collaborative research and intervention programs. Keep up education campaigns.

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