When I say, fears of “a bug bite that turns you into a vegetarian” I'm of course talking about the ‘meat allergy’ alpha-gal syndrome (AGS), caused, somewhat surprisingly, by ticks. We know that ticks can carry and transmit an array of diseases, but few probably knew that they could be the cause of a food allergy. Thousands each year are diagnosed with alpha-gal syndrome, an allergy that can be life threatening and which is on the rise around the world. Australia has the highest number of alpha-gal cases globally, and the allergy is also prevalent in the US, but climate change may lead to an increase in cases outside of these hotspots.
What causes mammalian meat allergy?
Food allergies are most commonly mediated by proteins; however, mammalian meat allergy, or alpha-gal syndrome, is an allergy caused by a carbohydrate. Alpha-gal (galactose-alpha-1,3-galactose) is an oligosaccharide that is present in the saliva of ticks and is transferred to our blood when we are bitten. The main culprits are the lone star tick (Amblyomma americanum) in the US, the Australian paralysis tick (Ixodes holocyclus) in Australia, and the castor bean tick (Ixodes ricinus) in Europe. When alpha-gal was first identified, it was found to be present in some New World monkey and lemur species, but absent in Old World monkeys and humans. The significance of alpha-gal in causing allergies was understood when, in the early 2000s, patients were experiencing allergic reactions to the mammal-derived cancer drug Cetuximab. It wasn’t until a few years later, however, that research groups in the US and Australia began to identify the link between tick bites and the red meat allergy.
Alpha-gal is now known to be naturally produced in the bodies of most mammals, except humans, and its presence in mammalian meat is the reason for the allergic reaction. The carbohydrate can also be found in meat derived products like gelatine, heparin, and sometimes dairy products. The Alpha-gal allergy stems from an immune reaction in our bodies; when bitten by a tick with alpha-gal in its saliva, the body produces immunoglobulin E (IgE) antibodies that target the antigen. When those that have previously been exposed to alpha-gal through a tick bite are re-exposed to it upon eating red meat, the immune response is triggered, IgE antibodies are produced, and an allergic reaction ensues.
The difficulties in spotting and treating alpha-gal syndrome
The symptomology and severity of alpha-gal syndrome vary between people, but the characteristic feature is a delayed allergic reaction, occurring 2-6 or even up to 8 hours after consuming red meat. Because of this delay, people may not suspect that the food they have eaten could be the cause of their symptoms. Symptoms are known to include urticaria (hives), angioedema (swelling), and gastrointestinal issues like vomiting, nausea, stomach pains and diarrhoea. Most alarmingly, the allergy can cause anaphylaxis which can be life threatening. Due to the variable symptoms and delayed reaction, diagnosis has historically been difficult. The delay means that the oral provocation test, used to diagnose most food allergies, is not suitable, and the skin prick test is also unreliable. Diagnosis is therefore based on a person’s history of delayed hypersensitivity after eating meat, history of tick bites, alpha-gal-specific IgE detected via serological tests, and whether a patient improves upon beginning a diet that avoids mammalian meat and meat derived products.
There is no current treatment for alpha-gal syndrome. Instead most healthcare providers will suggest that patients suspected to have the allergy stop eating red meat and potentially avoid dairy products too (avoiding further tick bites is also preferable). Many wouldn’t be too inconvenienced by this dietary change; however, some have described how hard it has been. For example, a woman from Tennessee expressed how important BBQ is to her local food culture. There are an array of additional reasons dietary change can be difficult and existing barriers to meat reduction that need to be addressed. Unlike most food allergies, after some time without exposure to further alpha-gal via tick bites or red meat consumption, some people are able to start slowly reintroducing meat back into their diet.
The future of alpha-gal syndrome
Cases of alpha-gal are on the rise, a reason for it circulating in the news again this year. In the US, the range of the Lone star tick has been expanding over the past few decades, from its original home in the southeast, into the upper Midwest and Northeast (and as far north as Martha’s Vineyard). With climate change increasing the number of suitable habitats for ticks, the ranges of these AGS-causing species are likely to increase. Much is still unknown about the syndrome, for example why some people become sensitised to alpha-gal following a tick bite while others don’t, and how some who are sensitised never develop the allergy. Furthermore, there is currently no treatment or cure. To advance progress on treatment and prevention further research is needed to understand the underlying immune-related mechanisms of the syndrome. I often have a period of slight ‘tick fear’ after writing these blogs, but as a vegetarian, this time round I’m feeling quite at ease!