Where Alpha-Gal Syndrome Is Found: Suspected Cases in the U.S., 2017–2022

Alpha-gal syndrome (AGS) is an emerging allergic condition linked to tick bites: a potentially life-threatening immunoglobulin E (IgE) reaction to galactose-alpha-1,3-galactose (alpha-gal), a sugar found in the meat of most non-primate mammals and in products made from them — milk, other dairy products, and some medicines. Symptoms and severity vary, and there is no treatment or cure. More than 34,000 suspected cases were identified in the United States in 2010–2018, but little was known about where they occur.

How the study worked

• Data: de-identified results of alpha-gal–specific IgE tests from Eurofins Viracor, the laboratory that did nearly all such testing in the United States before 2022 — age, sex, test date, result, and state and zip code of residence, but no clinical or travel information.
• Suspected AGS: at least one positive result (0.1 kU/L or more).
• Maps: suspected cases per 1 million population per year, by county — low (under 11), medium (11–87) or high (over 87).

Results, 2017–2022

• 357,119 tests from 295,400 people; 90,018 (30.5%) tested positive.
• Positives rose from 13,371 in 2017 to 18,885 in 2021 (averaging 15,003 a year). About 30,000–70,000 people were tested each year, peaking at 66,106 in 2021, before other laboratories began testing.
• 63.8% of those tested were women, but 42% of men tested positive, against 24% of women. People who tested positive were older (average 48 vs 41 years); among people 70 and older, 44.6% were positive.

Suspected alpha-gal syndrome cases per 1 million population per year, by county, 2017–2022. Figure from CDC's report.

Where:

• Most cases were in counties of the South, Midwest and mid-Atlantic — a nearly continuous region across parts of Oklahoma, Kansas, Arkansas, Missouri, Mississippi, Tennessee, Kentucky, Illinois, Indiana, North Carolina, Virginia, Maryland and Delaware.
• The most cases: Suffolk County, New York (3,746 — 4% of all suspected cases) and Bedford County, Virginia (1,511). The highest rates: Charlotte County, Virginia (12,273 per million per year) and Muhlenberg County, Kentucky (6,107).
• Cases clustered where the lone star tick (Amblyomma americanum) is established or reported — much like ehrlichiosis, which the same tick spreads.
• Separate clusters in Minnesota and Wisconsin (238 suspected cases), outside the lone star tick's known range, need more study. Alpha-gal has been found in the saliva of other ticks, and other species are linked to AGS elsewhere in the world.

What it means

• Health care providers often don't know AGS: in one survey, 42% had never heard of it and 35% weren't confident diagnosing or managing it. Testing by knowledgeable providers is 98% sensitive and 92% specific, and unpublished state data suggest about 90% of people who test positive have symptoms consistent with AGS.
• Whether the rise reflects more awareness, more disease, or both is unclear.
• Testing isn't equal: people tested are more often non-Hispanic White, with higher incomes and education, and diagnosis can take years of visits.
• Counting 110,229 suspected cases in 2010–2022, and allowing for how many are true AGS and how many people ever get tested, the authors estimate that 96,000–450,000 people in the United States may have been affected since 2010.
• As the lone star tick's range expands, cases are expected to rise. Needed: tick bite prevention education, provider training, and national surveillance; AGS belongs in tickborne disease outreach, since its effects can last a lifetime.

Limits

• Other laboratories also tested — academic and allergy labs before 2022, and commercial labs from August 2021 — so these results undercount, and help explain the dip in 2022.
• Home addresses may not be where the tick bite happened.
• The test's 92% specificity means some false positives.
• Fewer than 1% of records were excluded for invalid locations.

Sources

Based on Thompson JM, Carpenter A, Kersh GJ, Wachs T, Commins SP, Salzer JS, "Geographic Distribution of Suspected Alpha-gal Syndrome Cases — United States, January 2017–December 2022," MMWR Morbidity and Mortality Weekly Report, volume 72, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's discussion says new suspected cases "increased by approximately 15,000 each year"; its results show about 15,000 positives a year (13,371–18,885), which is the figure given here.