Chigger (Harvest Mite)

Family Trombiculidae — the chiggers, also called harvest mites or red bugs — contains about 3,000 species worldwide. The remarkable biology is that ONLY the six-legged larva (not the eight-legged nymph or adult) is parasitic on vertebrates; the nymph and adult are free-living predators of soil arthropods, insect eggs, and other small prey. The larva (~0.25 mm) is bright orange-red, climbs onto a vertebrate host, and seeks out areas of thin skin (typically inside ankles, behind knees, waistband area, armpits). Once attached, the larva injects digestive saliva that liquefies the host's epidermal cells; the host's body responds by hardening the surrounding tissue into a feeding tube (the stylostome) through which the larva drinks the resulting cellular slurry for 3-5 days. The intense itching is caused by the host's immune response to mite saliva, NOT to any tissue penetration by the mite itself. The persistent viral myth that chiggers 'burrow into the skin' or 'lay eggs under the skin' is completely false: the larva is entirely on the surface during feeding and drops off when full. The bite welts last 1-2 weeks but are otherwise medically minor; some southeast Asian Trombicula species transmit Orientia tsutsugamushi, the causative agent of scrub typhus, which is a serious bacterial disease. The North American T. alfreddugesi does not transmit any pathogens.