A new kind of tick is causing an epidemic of Rocky Mountain spotted fever in Mexico, and it’s threatening to spread to the U.S., researchers said Wednesday.

Rocky Mountain spotted fever is already dangerous, and the new carrier is more likely to bite people than the ticks that usually spread it, the team of U.S. and Mexican researchers said.

As ticks in general become more common as the climate warms, they’re a bigger threat, they added.

“Rocky Mountain spotted fever, caused by the bacteria Rickettsia rickettsii, is responsible for more human deaths than any other tick-borne disease in North America,” the team wrote in the journal Emerging Infectious Diseases.

Rocky Mountain spotted fever was reported in 4,269 people in the U.S. in 2016, according to the Centers for Disease Control and Prevention. It can kill up to 10 percent of victims, depending on the outbreak.

It’s usually spread by the American dog tick and the closely related Rocky Mountain wood tick. But in recent years the bacterial infection has also been spread by the brown dog tick — a completely different species.

The researchers were investigating an epidemic of the infection that broke out in the border town of Mexicali starting in 2008. It’s already sickened at least 4,000 people, according to Mexican government estimates. Several hundred have died, and at least four people have died in the U.S. after crossing the border, according to this report and others.

“That’s a very big epidemic of a fatal disease,” said Dr. Janet Foley, an expert in the spread of animal-borne disease at the University of California Davis School of Veterinary Medicine. “There are likely thousands of cases.”

The infection is not always easy to diagnose in human blood. If people get a rash and other symptoms of Rocky Mountain spotted fever, the advice is to treat quickly with the antibiotic doxycycline. Other symptoms are similar to those caused by many infections and include fever, nausea and headache.

Working with a team at the Universidad Autónoma de Baja California in Mexicali, Foley and colleagues tested the blood of 16 patients to see if they could find a characteristic signature of the infection.

“I was absolutely startled,” Foley said in an interview. The people who had been sickened in Mexicali had a heavy load of the infectious agent in their blood — something that had not been seen in past outbreaks.

Ticks that commonly bite humans. Centers for Disease Control and Prevention

The epidemic is worrisome because the brown dog tick is more likely to bite people and it adapts easily to living in a house, as opposed to living on wild animals, the researchers said.

“The Rocky Mountain spotted fever epidemic in Mexicali has not been contained and may be spreading to other parts of Baja California and into the United States,” the team wrote.

And now it’s possible that for some reason, the infection the brown dog tick transmits is more virulent, Foley said.

“We need to study this tick more to understand why it makes people so sick,” she said. “This Mexican strain seems more willing to feed on people.”

It’s a big problem in a poor, crowded city like Mexicali, where many people and many dogs live. Foley said she visited neighborhoods there where dogs were infected with hundreds or even thousands of the ticks.

“It’s pretty bad,” she said. She described one dog that was homeless but being fed by the community.

“This dog had ticks everywhere. Every millimeter of ear tissue was covered with ticks. They were down her back,” Foley said.

The Universidad Autónoma de Baja California veterinary team tested dogs in the town and found that more than 80 percent were infected.

“One community I worked in tried to get rid of their dogs,” Foley said. “That was so sad. We shouldn’t have to go so far.”

The key is to stop the ticks from biting dogs, but that can be expensive, as flea spray and flea collars are costly.

It’s less of a threat in more affluent communities, where dogs are vaccinated, groomed and licensed, but people can travel with infected dogs.

“It can be a threat as people go back and forth across borders, especially with dogs,” Foley said. “The tick is present in the U.S. and is moving northward.”

More than 72 million crossings were made by people from Mexico into California in 2015 alone, a CDC team reported.

The CDC says tick-borne diseases are on the rise. “Overall, since 2000, in the United States, the incidence of Rocky Mountain spotted fever has reportedly increased fourfold,” Foley’s team wrote.

The brown dog tick may be in part responsible but it could be that testing methods have made a difference as well, they said.