An outbreak of Rocky Mountain Spotted Fever (RMSF) has reached epidemic proportions in one Northern Mexico town, and it’s starting to spread to the United States, according to a new study from the Centers for Disease Control and Prevention (CDC).

The outbreak, which began in 2008 in the Mexican border town of Mexicali, has affected 4,000 people and an unknown number of dogs as of 2018. Several hundred people have died of the disease in Mexico, and at least four have died in the US after crossing the border from Mexico.

RMSF, caused by the bacteria Rickettsia rickettsii, is responsible for more human deaths in North America than any other tickborne disease, killing as many as 10% of those infected. Between 1999 and 2007, 80 fatal cases were reported in Sonora, Mexico, alone. The genus Rickettsia is composed of bacteria that behave like viruses, reproducing only inside living cells. The bacteria live parasitically in ticks and are transmitted to vertebrate hosts by bite.

Of particular concern to scientists: The recent epidemic appears to be spreading through the bite of a new carrier.

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

More than 80% of the dogs in one Mexicali neighborhood were found to be infected with brown dog ticks.

The risk to humans is heightened by the brown dog ticks’ habit of living in areas adjacent to towns and cities, and the fact they often spend their off-host time indoors.

Symptoms of RMSF in humans include fever, headache, and muscle aches, accompanied by a crusty skin rash at the bite site. Although not usually fatal, RMSF can kill as many as 10% of those infected.

Symptoms in dogs include depression, lethargy, arrhythmia, and discolored spots along the skin, often bruised or purplish in color. The fatality rate in dogs infected with RMSF is unknown—one study puts the survival rate at 100%, while another put the fatality rate at 60%. The discrepancy is thought to be a combination of delayed diagnosis and more severe manifestations of the disease.

Although both species can develop the disease, it can only be transmitted through a bite by an infected tick, so dogs can’t infect humans directly and vice versa.

As to why the outbreak happened, the CDC doesn’t know.

“More data are needed before we can understand why this epidemic emerged,” wrote the authors of the CDC study. “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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