EJIDO PADRE KINO, Mexico — The boy came home from school weakened by fever, his ears burning-hot. Over the next few days, the 7-year-old got sicker — vomiting and complaining of abdominal pain, his mother recalled. Then, the telltale red spots appeared on his hands. But none of the doctors in this rural community along Mexico’s Pacific coast recognized the warning sign for one of the most lethal infectious diseases in the Americas — Rocky Mountain spotted fever. A week later, the boy was dead.
The following year, in 2020, the disease killed a 5-year-old boy in a nearby house. Then last October, a few blocks away, another 7-year-old succumbed to the same scourge.
The disease, spread through the bite of an infected tick that lives primarily on dogs, is rare, but its incidence is rising. It has reemerged at epidemic levels in northern Mexico, where more than 2,000 cases, resulting in hundreds of deaths, have been reported in the past five years. Young children have been hit the hardest. In the Mexican state of Baja California, where Ejido Padre Kino is located, there were 92 cases in 2022, more than double the previous year, according to state data.
The outbreak prompted a team of Mexican and U.S. scientists to descend upon this small town more than four hours south of San Diego to pluck ticks off dogs, scour the crevices around homes for larvae, and warn residents to keep their dogs from roaming the dusty streets.
“It’s very, very hard, because it’s a totally 100 percent preventable disease,” said Oscar Efrén Zazueta, epidemiologist for Baja California and part of the research team. “Kids are the ones who are in contact with dogs, and they die so very, very quickly … in a matter of days.”
Alarm has risen in recent years as warming temperatures intensify tick activity and disease risk. Cases of malaria, dengue fever, West Nile virus and Lyme disease — infections transmitted by ticks and mosquitoes — have skyrocketed. Scientists worry that Rocky Mountain spotted fever, first identified in western Montana at the beginning of the 20th century, could spread to more regions.
“What’s the tipping point? We don’t know for sure,” said Laura Backus, a postdoctoral researcher at the Laboratory of Infectious Disease Ecology at the University of California at Davis and another member of the research team. “Weather directly affects how fast ticks reproduce. When it’s hot and dry, they get more desperate.”
The brown dog tick, one of the species that transmits Rocky Mountain spotted fever, becomes more aggressive toward humans in seeking blood meals in hotter, drier climates, such as that in northern Mexico and the southwestern United States, said Backus, who led a 2021 study that found that the ticks are twice as likely to choose humans over dogs when temperatures rise.
With the yearly number of days topping 100 degrees expected to increase across most of the continental United States in the next decade, the study warned of an “increasing concern for heat-driven emergence of tick-borne disease.”
Climate change extends the length of time ticks actively feed on humans and animals, enabling ticks to develop and reproduce faster.
“They begin biting people earlier in the year and stay out longer,” said Ben Beard, deputy director of the division of vector-borne diseases at the Centers for Disease Control and Prevention. “More people are being exposed potentially to the bites of infected ticks and, as a result, more cases of tick-borne diseases.”
“Only a few degrees’ difference in annual average temperature can have a huge impact,” he said.
The spread of Rocky Mountain spotted fever is hard to predict, Backus said, because it often simmers undetected, erupts, then disappears again. Originally called “black measles” — a rash in the disease’s late stages often turns the skin black — it is one of the deadliest tick-borne diseases in the Americas.
The bacterium that causes it — Rickettsia rickettsii, commonly spread by tick species that feed on wildlife in predominantly wooded areas — is present in almost every U.S. state. But scientists were surprised to discover the brown dog tick — which lives around and inside homes where dogs are present — as a new vector for the disease in the United States in 2003.
Since then, nearly 500 cases and 28 deaths have been reported on tribal lands in Arizona, CDC officials said. In California, 88 cases were reported between 2013 and 2022, more than triple the 26 cases reported the previous decade, according to state data.
Public health authorities are especially alarmed by the deadly outbreaks in low-income communities such as Ejido Padre Kino that have large numbers of free-roaming dogs. Migrant laborers from poorer parts of Mexico, many from Indigenous communities, have flocked here in recent years to work in the raspberry and strawberry fields. The dog population has also boomed after local governments stopped collecting strays — and spaying and neutering them — during the coronavirus pandemic. The increased interaction between dogs and humans is the underlying factor fueling the rise of Rocky Mountain spotted fever here, researchers said.
Ticks carry the bacteria and spread the disease when they bite dogs or humans. Previously uninfected ticks that bite infected dogs can acquire and transfer the pathogen. The disease can also cause serious illness — even death — in dogs. Dogs who do not receive regular veterinary care, as in poorer communities in Mexico or on tribal lands in Arizona, are more likely to perpetuate the disease.
Untreated, the disease kills 4 out of every 10 people infected in Baja California, Zazueta said. Children younger than 10 are at highest risk because research shows they have more contact with dogs, and the early symptoms of Rocky Mountain spotted fever — fever, pain, malaise — resemble those of so many common childhood illnesses that the disease is often not diagnosed in time to be treated. Deaths can be prevented with the right antibiotic, if it’s administered within a few days of an infection taking hold.
For Emmanuel Juárez Flores, the 7-year-old boy who returned home from school with a fever, it was already too late. The local hospital sent the boy to the bustling port city of Ensenada, two hours away, in hopes that a larger hospital might have intravenous doses of doxycycline, the only treatment for late stages of the disease.
During the ambulance ride, “my son was crying and screaming because he was already in a lot of pain,” his mother, Olivia Flores Legardia, recalled as she brushed tears from her cheek in a recent interview. “He was biting me… because he was acting as if he didn’t know me.”
The nurse explained to her that the bacteria, which infect the lining of blood vessels, had already caused massive internal bleeding and damaged her son’s brains and lungs. Medicine would not work.
Flores blames herself. In the months before her son’s 2019 death, she had temporarily relocated the family to Ensenada for better health care for her high-risk pregnancy with Emmanuel’s younger brother. Neighbors were supposed to care for the family’s two dogs, but when they returned, the animals were skinny — and full of ticks.
“I tried to fumigate,” she said quietly.
Scouring for ticks
Multiple deadly outbreaks of Rocky Mountain spotted fever ravaged parts of Mexico in the 1940s and ’50s before the disease went dormant, then reemerged in Baja California in 2008 and other northern states.
Disease detectives tracking the epidemic’s spread are looking for clues about its ecology here in the San Quintín Valley to understand how the epidemic moved south in recent years from cities along the U.S. border to smaller towns like this one.
“¡Buenos días! ¡Venimos de la Secretaría de Salud!” shouted Zazueta, announcing his official presence from the state health department as he stood outside a home in June. Wearing a bright-yellow vest emblazoned with “brigada epidemiológica” (“epidemiology brigade”), he peered over a brown mesh fence. His greeting set off barks from a small white dog chained in the front yard before homeowner Ignacia Cruz Cruz emerged.
Zazueta explained that the team of researchers was going house-to-house to survey families about their dogs and inspect for ticks because of recent cases of the disease, known here simply as rickettsia. Cruz said the family owns two dogs — Tobi, the white one, and Coca, who roams the streets — and no one had been bitten by ticks in the past year.
Researchers checked the white dog’s ears and toes for ticks; it was clean. Another team member, crouching as she walked slowly along the house’s cement foundation, searched intently for signs of tick larvae, nymphs or adults. Female ticks lay eggs in any crack or crevice around baseboards, window frames and door frames. An adult brown dog tick is reddish brown, about the size of a sesame seed. After feeding on blood, it turns gray and can stretch to the size of an olive.
After Zazueta explained that free-roaming dogs have a greater chance of being infested, Cruz headed into the street, grabbed Coca’s collar and pulled the reluctant dog inside the yard for inspection. Nine ticks.
The ticks live in the dirt, along tiny cracks in cinder-block walls, under rocks and plastic buckets, and in the wood of doghouses. Public health officials spray the neighborhoods with pesticides, but not often enough, families say. Officials acknowledge that the pesticide is not as effective as tick collars in lowering incidence of disease, but the cost — up to $60 for long-acting ones — is prohibitive for many families who struggle to access even running water.
Cruz said she has been bitten by ticks before and is aware of rickettsia’s dangers. But she doesn’t worry about dying of the disease. God’s will, she said, shrugging. Like other homeowners here, she said she needs the dogs for protection from burglars. If Coca wants to roam, Cruz said, she will let that continue.
At another house, the resident told researchers that the previous occupants had lost a child to rickettsia, but the family had moved, along with their dog. Yet researchers discovered the abandoned doghouse and other parts of the home crawling with ticks.
With gloved hands, Andrés López Pérez, a professor of disease ecology at the Institute of Ecology A.C., a Mexican government-run public research center in Veracruz, pulled a chunk of rotting wood from the side of the house and examined it closely. “This one is a male. Here is the female,” he said, as he plucked the adult ticks out with tweezers and dropped them swiftly in a test tube for later analysis. “Down here, you can see the nymphs,” he said, hovering his tweezers over the gray specks. “And here is the larvae. So we have the whole family.”
That week, researchers would visit nearly 300 households in Ejido Padre Kino and a neighboring town, drawing blood from more than 500 dogs and collecting thousands of ticks, to learn what conditions might pose the greatest risk for human infection.
DNA testing will allow the researchers to see what portion of ticks are infected with the bacteria, and what portion of dogs have antibodies in their blood or already have infections.
López Pérez said he hopes to have some preliminary analysis completed by the end of the year. “If we are able to find a pattern, we will have more ideas on how to control the disease,” he said. “We already know that one of most important things is to keep the dogs in the house.”
Brown dog tick discovered in Arizona
Health officials in the United States have had some success limiting transmission from brown dogs ticks.
The 2003 death of a 14-month-old boy on a tribal reservation in southeastern Arizona caught health officials off guard. The case was the first time the brown dog tick was found to transmit Rocky Mountain spotted fever in the United States. The other tick species that transmit the disease are typically isolated in wooded areas, where they are less likely to come into contact with humans.
Sixteen people — nearly all of them children — fell ill with the disease on that reservation and an adjacent one between 2002 and 2004, according to CDC and Indian Health Service investigators.
“It was a big surprise because this was not a part of the country where the tick that spreads it occurred,” said Jennifer McQuiston, one of the researchers, who is now deputy director of the CDC’s division of high-consequence pathogens. She said investigators found “thousands and thousands of brown dog ticks” in the local environment — in the cracks of stucco walls, in crawl spaces under houses, and in discarded upholstered furniture outdoors, where children played and free-roaming dogs rested.
By the mid-2000s, the disease had spread to six reservations; annual incidence on the three most heavily affected tribal lands combined was more than 130 times the national average, according to a 2014 study by tribal, federal and state health officials.
After intensive efforts by tribal communities and health officials to raise awareness and promote the use of long-acting tick collars and pesticides, the average incidence on two of the most affected reservations dropped to 50 times the national rate between 2015 and 2019, CDC officials said. No deaths have been reported since 2019.
But the disease remains a threat. And public health and tribal leaders remain vigilant in a community where many families own three to five dogs.
During a tick awareness campaign on the San Carlos Apache reservation in July, some dogs had as many as 30 ticks, said Julie Cassadore, who runs Geronimo Animal Rescue Team, a nonprofit that provides veterinary services to the 13,000 people who live on the reservation. The group placed more than 50 tick collars on dogs, including strays.
Now, people know the importance of getting early medical treatment if they develop symptoms of infection, said Cassadore, who was raised on the reservation. “Whereas before, people didn’t know what was going on,” she said. “They thought it was just flu and they’ll get over it, and they ended up dying.”
The doctors who treated Daniela Villanueva León’s 5-year-old son, Axel, in 2020 didn’t know why his fever wouldn’t go down. In desperation one night, she submerged the boy in a tub filled with ice.
After he began vomiting and had a seizure, she rushed him to the main regional hospital, where he stayed for two days. “A doctor said it was probably rickettsia, but they weren’t sure,” Villanueva recalled. Doctors sent them to a hospital in Ensenada, where their neighbor’s son Emmanuel had died the previous year.
The hospital confirmed the diagnosis, but Axel’s organs were failing. It was too late for further treatment.
Axel died on his third night in the Ensenada hospital, without his mother by his bedside because of hospital protocol during the coronavirus pandemic. “I was in the hallway when they told me,” she said.
The family never owned dogs. But Axel liked to play soccer in the streets, where many dogs roam.
On Sundays, Villanueva would walk the 20 minutes from her home to visit Axel’s grave with her parents and her other children, Haisha, 11, and Aron, 7. Plastic flowers and a small candle sit in front of the headstone. But she hasn’t visited recently. She is pregnant, due in November, and tires easily during the walk uphill beneath the blazing sun.
Villanueva worries about protecting her other children. Even though the family has paid private companies to spray their dirt yard with pesticides, she wonders if that will be enough. Dogs trot behind children coming home from school. They gather around boys playing in the street. And her home is surrounded by neighbors who have dogs.
“The ticks,” Villanueva said, “you see them everywhere.”
Two days after the researchers left Villanueva’s home, they received jolting news from the infectious-disease director at the local hospital.
Another child lost to rickettsia. The first in the San Quintín Valley to die this year. She had just turned 8.
Gabriela Martínez in Mexico City contributed to this report.
A previous version of this article incorrectly said that the 2003 death of a 14-month-old boy from Rocky Mountain spotted fever occurred on the San Carlos Apache tribal reservation. The boy died on a different reservation in southeastern Arizona. The article has been corrected.