Meet the new 6-legged cause of infection, allergy, and life-threatening disease

From infections to unusual allergies to life-threatening diseases, there’s a new source of mysterious illnesses on the playing field—chiggers. 

Also known as berry bugs, harvest mites, red bugs, scrub-itch mites, and aoutas, chiggers are a close relative of ticks and spiders and the most well-known denizens of the Trombiculidae family. They are almost microscopic, measuring in at just 0.4 mm, and have an orange-reddish color. Chiggers live in forests, grasslands, woodlands, berry bushes, orchards, and along lakes and streams. They can also be found on lawns, golf courses, and parks.

In their 6-legged larval stage, chiggers feed on the skin cells of animals, such as rabbits, toads, box turtles, quail, and even humans. Their bites can cause an itchy rash, usually around the ankles and the waistline. After feeding on their hosts, larvae mature into 8-legged adults, which are harmless to humans. In the United States, chigger bites are most common in the late spring, summer, and early fall.

In addition to causing bites that are itchy, painful, and capable of causing infection, chiggers have now been implicated in triggering alpha-gal allergy, according a recent report published in The Journal of Allergy and Clinical Immunology: In Practice.

Alpha-gal allergy is a reaction to a carbohydrate molecule found in mammalian meat—such as beef, pork, lamb, and venison—and can result in anaphylaxis. It was previously believed to be caused only by ticks, but researchers from Wake Forest Baptist Health, Winston-Salem, NC, and the University of Virginia, Charlottesville, VA, have now implicated chiggers as well.

“If a patient comes in telling me they ate red meat for dinner and then hours later woke up with anaphylaxis, I suspect an alpha-gal allergy,” said lead author Russell Scott Traister, MD, PhD, assistant professor of pulmonary, critical care, allergy and immunologic diseases, Wake Forest Baptist Health. “With those symptoms, doctors usually ask if the person has had a tick bite recently. But we started seeing patients with the same symptoms who said they hadn't had a tick bite, only chigger bites.”

Unlike other allergies, which usually happen immediately or within minutes of ingesting the offending allergen, an allergic reaction to alpha-gal takes 3 to 6 hours to manifest. The only remedy is to avoid eating any mammalian meat.

Dr. Traister and colleagues argued that the bites of seed ticks and chiggers can be almost indistinguishable. They presented three case studies of patients who developed alpha-gal allergies after being bitten by chiggers.

To further bolster the theory that chiggers can cause this allergy, researchers at the University of Virginia reported that, of the 311 alpha-gal IgE-sensitized subjects enrolled in ongoing studies there, 301 reported a history of either tick or chigger bites in the past 10 years; of these, 5.5% reported a history of chigger bites, but not tick bites.

Dr. Traister and colleagues call for further studies to specifically identify alpha-gal in the gastrointestinal tracts of chiggers, and thereby confirm that they can cause alpha-gal allergy.

"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," he concluded.

Scrub typhus in Chile

Chiggers have also been linked to the transmission of scrub typhus, a life-threatening zoonosis, in areas outside of their original boundaries, as detailed in a report in the New England Journal of Medicine.

The chiggers (Orientia tsutsugamushi) that spread scrub typhus are endemic to East and Southeast Asia, and Northern Australia. The geographic area they cover is a long and wide stretch known as the “tsutsugamushi triangle.”

Symptoms of scrub typhus include rash, headache, high fever, and muscle pain, and are not easily distinguishable from other diseases such as malaria and dengue fever. Effective treatment consists of oral doxycycline, and is dependent on a correct diagnosis. In fact, misdiagnosis is largely the cause of the 140,000 fatalities caused by scrub typhus annually.

But researchers are now finding that these little red creatures may not be as confined geographically as previously believed. In 2006, scrub typhus was confirmed in the Middle East and in southern Chile. Then, from January 2015 to February 2016, three patients were diagnosed with scrub typhus in Chiloé Island, off the coast of southern Chile, a discovery that has global significance, according to experts.

“Here, we report three scrub typhus cases caused by O. tsutsugamushi, in Chiloé Island in Chile; their presence suggests that this pathogen may be endemic in this part of South America, which is 12,000 km away from the tsutsugamushi triangle,” wrote the authors, led by Thomas Weitzel, MD, Clinica Alemana de Santiago, Facultad de Medicina Clinica Alemana, Universidad del Desarrollo, Concepción, Chile. “Although the distribution, reservoirs, and vectors remain unknown, this finding expands our understanding of the epidemiology of scrub typhus and suggests that there may be a much wider global distribution and higher disease burden then has previously been appreciated.”

No one is sure how this O. tsutsugamushi came to Chile. Scrub typhus is not spread like viruses; it lives inside chiggers and is transmitted to their offspring. People have to be bitten to become infected.

What is certain is that this tiny arachnid may be causing a lot more trouble with its bite than previously believed.

The study reported in the New England Journal of Medicine was supported by grants from the Comisión Nacional de Investigación Científica y Tecnológica and the Wellcome Trust.