After a series of strange medical cases, researchers discovered that bites from the lone star tick can induce a severe and persistent allergy to red meat. In the six or so years since this was identified, it has been determined that bites from multiple other tick species can also cause it. Now, evidence indicates that bites from another type of small, parasitic arachnid known as a “chigger” may do the same.

The allergy arises after the immune system, responding to the foreign molecules introduced by the parasite, makes the unhelpful decision to hone in on a sugar called alpha-gal. Alpha-gal is present on the surface of cells in most species of mammals, save for apes and Old World monkeys, so ticks that have fed on any wild or domesticated mammals are likely to have it in their saliva or on their mouthparts. Having identified this sugar as a potential threat, white blood cells create antibodies that will quickly target and neutralize it when the body encounters it again; which, for many non-vegetarians, is a daily occurrence. 

When a human who has been sensitized to alpha-gal consumes mammalian meat, an allergic response characterized by nausea, diarrhea, hives, headache, runny nose, and – in some instances – anaphylaxis, will onset several hours later. The only way to manage the condition is to avoid all mammalian meat.

The current study from Wake Forest Baptist Medical Center and the University of Virginia, published online in The Journal of Allergy and Clinical Immunology: In Practice, features the cases of three patients who developed severe alpha-gal sensitization soon after experiencing bites from creatures they believed to be chiggers, also called harvest mites, red bugs, scrub-itch mites, and aoutas. Chiggers are found in forests and grasslands across the world and will leap from vegetation onto the bodies of passing mammals.

As tiny critters, chiggers are quite difficult to spot on one’s body, but they make up for this with their tendency to attack en masse. Most people who encounter chiggers end up with bites from many individuals, and those who report seeing the tiny invertebrates in action count clusters of dozens to hundreds swarming their flesh and/or clothing.

To gather further support for their theory, the team also surveyed 311 people with alpha-gal allergies about their past exposure to different arachnid bites. Of 301 who could recall an incident in the past decade, 17 reported a history of chigger bites but no tick bites.

However, before we jump to conclusions about a possible new vector for alpha-gal sensitization, the authors note that it is extremely difficult to distinguish the appearance and bites of chiggers from that of larval-stage "seed" ticks.

“Whether these bites are from chiggers or seed ticks, they appear to be associated with very high levels of [antibodies] to alpha-gal,” they wrote, adding that further studies should investigate whether the immune reactive part of the alpha-gal molecule is common in the gastrointestinal tracts of chiggers.

"In the meantime, we want allergists to be aware that patients may report chigger bites, and based on that fact alone should not dismiss alpha-gal sensitization as a possible diagnosis," lead author Dr Russell Scott Traister said in a statement.