Alpha-gal syndrome

What it is

A bite from a lone star tick can trigger a meat allergy in some people.⁹ CDC calls this alpha-gal syndrome (AGS), a serious, potentially life-threatening allergy and tickborne disease, also known as red meat allergy or tick bite meat allergy.¹

Alpha-gal is a sugar molecule found naturally in most mammals and in the saliva of some ticks.⁷ When a tick bites, it can transfer alpha-gal from its saliva into a person’s blood.¹ Humans do not naturally produce alpha-gal, so the immune system sometimes mistakenly identifies it as a threat, and when a person with AGS later eats red meat or is exposed to other products with alpha-gal, the allergic response can range from mild to very severe.⁷

AGS is unusual among food allergies: most food allergies are directed against a protein, but alpha-gal is a carbohydrate.⁸

Most cases have been reported in adults, but people of all ages can develop AGS.¹ In July 2023, CDC reported that more than 110,000 suspected cases were identified in the US from 2010 to 2022, and estimated that as many as 450,000 people might have been affected.⁵

Symptoms

CDC says symptoms usually appear 2 to 6 hours after eating red meat or dairy products.² AAAAI says symptoms from eating red meat such as pork, lamb or beef may be delayed, occurring 3 to 8 hours after eating.⁸ ACAAI says symptoms commonly occur 3 to 8 hours after eating red meat, especially beef.⁹

CDC’s list of reactions includes hives or an itchy rash; nausea or vomiting; severe stomach pain; heartburn or indigestion; diarrhea; cough, shortness of breath or difficulty breathing; a drop in blood pressure; swelling of the lips, throat, tongue or eyelids; dizziness or faintness; and a combination of symptoms called anaphylaxis.² AAAAI’s list of possible signs and symptoms also includes wheezing.⁸

Reactions can range from mild to severe, and some may be life-threatening.² People with AGS might not react to every product containing alpha-gal, and some people have different symptoms or reactions each time they are exposed, even to the same product.²

ACAAI contrasts the delay with traditional food allergy reactions, which are usually immediate, occurring within minutes to one hour after exposure.⁹ With AGS, symptoms can be so delayed that a person may only wake up with them in the middle of the night after an evening meal.⁸ AAFA says the usually delayed onset makes it hard to connect symptoms with red meat eaten many hours before.¹⁰ Researchers have linked tick bites to many cases of anaphylaxis that would previously have been classified as idiopathic, or of unknown cause.⁸

Some redness at the bite is normal: most people bitten by insects have pain, redness, itching and minor swelling around the bite.¹⁰ Beef allergy in children is not always AGS, either: some children with cow’s milk allergy may also develop a beef allergy, but this is different from AGS.⁹

What triggers it

In the US, AGS is primarily associated with the bite of a lone star tick, and less commonly with the bite of a blacklegged tick or a western blacklegged tick.¹ Not every person bitten by these ticks will develop AGS.¹ It can take weeks to months after a tick bite for a person to have symptoms.² AAAAI notes that people bitten by the tick are at risk of becoming sensitized, and that allergic reactions may then occur to red meat, to later tick bites, and even to drugs that contain alpha-gal.⁸

Most reported cases of AGS in the US occur in South, East and Central states.¹ A CDC study of 2017 to 2022 alpha-gal antibody test results found the highest prevalence of suspected cases across a nearly contiguous region of the southern, midwestern and mid-Atlantic US, and also identified suspected cases in areas outside the lone star tick’s range.⁶ The lone star tick lives primarily in Texas, Oklahoma, Arkansas, Tennessee, North Carolina and Virginia, with increasing populations in the Northeast, including New Jersey, New York, Connecticut and Maine.⁹ AAFA says these ticks are mainly active from April to late August.¹⁰ AAAAI says you are more likely to be at risk if you often go outdoors in wooded areas for activities such as hiking, fishing or hunting.⁸

A person with AGS is more likely to have an allergic reaction after exposure to meat from mammals, such as beef, pork, lamb, venison or rabbit.³ Alpha-gal may also be found in mammal organ meat, such as liver, kidneys, intestines or sweetbreads.³ Some people with AGS may also be sensitive to alpha-gal in gelatin made from beef or pork, in products made or cooked with mammal fat such as lard, tallow or suet, and in meat broth, bouillon, stock and gravy.³ Milk products may contain alpha-gal as well, but many patients with AGS can tolerate them.³ Some drugs and vaccines may contain alpha-gal in additives, stabilizers or coatings, though not all patients with AGS react to these ingredients.³ Animal-derived medical products, such as heart valves from pigs or cows, may contain it too.³

When to see an allergist

Seek immediate emergency care if you are having a severe allergic reaction or anaphylaxis.² Anaphylaxis is a serious allergic reaction that can progress rapidly.¹⁰ AGS reactions may be delayed, so symptoms can begin hours after a meal, even in the middle of the night after an evening meal.⁸

If you think you may have AGS, CDC says it is important to see your healthcare provider or an allergist.⁵ CDC says AGS is diagnosed through a detailed patient history, a physical examination and a blood test that looks for specific IgE antibodies to alpha-gal, and allergy skin testing may also be used.⁴ A positive alpha-gal blood test does not mean a person has AGS: positive results may occur in people who do not have it, particularly in regions with lone star ticks.⁴ CDC advises providers to consider the test along with symptoms, whether allergic reactions are delayed, and any history of tick bites or outdoor exposure.⁴ ACAAI says screening everyone bitten by a tick is not how AGS should be diagnosed, and that false positives are common.⁹ Any information you can give your allergist, such as a food diary that shows when you ate a certain food and how you felt after eating it, can help.⁹

AGS should be managed under the care of an allergist or other healthcare provider, so talk to an allergist about what applies to you.¹