It’s rare, but in some cases a tick bite can cause a person to develop an allergy to red meat.
Most cases, scientists believe, are linked to the lone star tick, a species typically found in the Southeastern U.S. and the lower Midwest.
Two case reports published this week, however, suggest that two other species — found elsewhere in the country — may also trigger the allergy.
When ticks bite, they release a steady stream of saliva that contains chemicals that keep their victim’s blood flowing as they feed. Sometimes, the saliva contains a sugar molecule called alpha-gal. Alpha-gal syndrome, a reaction to the molecule that occurs in some people, causes a red meat allergy.
“Alpha-gal syndrome is relatively rare, but those who have it can have a full-on anaphylactic shock,” said Douglas Norris, a professor of molecular microbiology and immunology at the Johns Hopkins Bloomberg School of Public Health.
The Center for Disease Control and Prevention estimates nearly half a million people in the U.S. have alpha-gal syndrome.
“We do believe the lone star tick is still responsible for most of the cases of alpha-gal syndrome in the U.S.,” said Dr. Johanna Salzer, a veterinary medical officer and epidemiologist with the CDC’s Division of Vector-Borne Diseases. Salzer worked on both of the new case reports, which were published Wednesday in the agency’s journal, Emerging Infectious Diseases.
Those reports found cases of alpha-gal syndrome in Washington and Maine in two women who hadn’t traveled to areas where the lone star tick typically lives.
Both women reported being bitten by ticks, which suggests that other species — including Ixodes scapularis, also called black-legged or deer ticks — may also be associated with alpha-gal syndrome, though much more rarely. A related tick, Ixodes pacificus, or the western black-legged tick, was also linked to the Washington case.
The CDC’s map of suspected cases — which shows high concentrations in Alabama, Arkansas, Georgia, Mississippi, Missouri, Kansas, Kentucky, Oklahoma and Tennessee as well as Maryland, North Carolina and Virginia — “would look very different if Ixodes scapularis was widely causing it,” Salzer said.
However, documented cases being associated with other ticks in the U.S. is “certainly a big deal,” Matthew Aliota, an associate professor in the department of veterinary and biomedical sciences at the University of Minnesota.
Unlike Lyme disease or the Heartland virus, alpha-gal syndrome is not caused by a bacterium or virus. The syndrome is instead an immune reaction to alpha-gal, a sugar found in almost all nonprimate animals, including cows, horses and deer and which can be present in tick saliva.
For some people, being exposed to alpha-gal through a tick bite triggers an immunoglobulin E, or IgE, antibody response in the body. People who mount this response develop an allergy to meat that can take weeks to months to appear. Alpha-gal syndrome was discovered in 2009, and researchers are still working to answer a lot of questions about it — including why some people react to alpha-gal while many do not.
For most people who do develop alpha-gal syndrome, the allergic reaction occurs hours after they eat red meat.
“It’s unique because it’s a delayed reaction, roughly 2-8 hours, so you can have a hard time knowing the trigger,” Salzer said, noting that with other IgE allergies, such as peanuts or shellfish, the reaction is almost immediate.
Different people can also have different reactions, which can further complicate diagnosis.
In one case, a 61-year-old woman in Washington state went into anaphylactic shock 29 days after she remembers being bitten by a tick on her shoulder: After eating beef tacos, she broke out in hives and her tongue and throat swelled. It happened again after she ate pork.
In the other case, a 45-year-old woman in Maine had stomach pain and general malaise about 2 ½ hours after she ate roasted rabbit. About nine days earlier, she had removed a deer tick from her arm. After that first incident, she’d get sick a few hours after she ate any type of red meat. Beef elicited a bout of vomiting and diarrhea that eventually landed her in her doctor’s office.
Both women tested positive for alpha-gal antibodies, and both recalled their bites being itchy and inflamed.
“Not everyone will have this local reaction, but it is pretty common,” Salzer said, referring to the itchy, inflamed bites. “But there is a subset of people with alpha-gal syndrome who do not recall being bitten by ticks,” she said.
Increasing risk for tick-borne illness
In 2023, the Maine CDC documented 23 cases of confirmed alpha-gal syndrome, but it’s unclear how many may have encountered the ticks while traveling. (Lone star ticks are uncommon in Maine and Washington, but they have been found there.)
It’s also possible that these other tick species in the U.S. have always been associated with alpha-gal, Norris said, but doctors haven’t detected it nearly as often as with lone star ticks. Indeed, ticks in Africa, Australia, Central America and Europe and Central America are also known to trigger alpha-gal syndrome.
One thing is certain: Ticks, and the diseases they are associated with, are thriving.
“Tick populations in general, and particularly with lone star ticks, are exploding all over the United States,” Aliota said. “Lone star ticks are moving into bigger geographic areas, and that range is continuing to expand with climate change.”
For this reason, it’s important to take precautions to avoid tick bites. This includes using bug repellant and wearing long-sleeved shirts, pants and socks when outdoors during tick season, which typically begins in April. If you’ve been in wooded areas or tall grass, shower off and do a “tick check” when you get home.
“The most important thing is preventing the tick bite altogether,” Salzer said.
