Chiggers: The Mighty Biting Mites

By Dean Richards III, MD

The Biting Mite

Chiggers are mites; mites are arachnids, as are ticks and spiders. The imago (mature) chigger has 4 pairs of legs, antedated by the six-legged larva. A chigger's eyes are simple, unlike the compound eyes of grasshoppers and flies; mites lack antennae, unlike the butterfly and flea. Hence, mites are not insects, of class Insecta, therefore chiggers are of class Arachnida, and have no body segments, as per order Acarina.

Sample mite Families follow:^[1]

1. Acaridae (Tyroglyphidae): a large mite Family, whose members feed on all manner of organic matter-cheese, dried meats, flour and seeds, for starters.

2. Carpoglyphidae: dried-fruit-eating mites.

3. Trombidiidae: harvest mites, resembling red spider mites, but larger-scavengers, commonly found in moist leaf litter. This family includes two species of man-attacking chiggers: Trombicula alfreddugesi and T. splendens. ^[1]

Most authorities say the appellation "chigger" is a corruption of "Chigoe" and "jigger," both names given to the "biting flea" Tunga penetrans common to Central and South America, and the West Indies.

Chiggers, as with other acarids, have a four stage life cycle: egg, larva, nymph, and adult. As trombiculid mites, unlike ticks, chiggers are parasitic only in the larval stage. They seek moist areas with overgrown grass, by waterways, and areas where leafy vegetation has accumulated.

Larval chiggers, known as harvest mites and "no-see-ums," measure 0.17 mm in length. These ravenous six-legged arachnids perch on the shaded tops of leaves of grass, catching limbs of animals passing by, then, climbing aboard, they seek dark, moist, creased, proximal areas of skin, especially where limbs meet torso.

Chiggers do not draw blood-they do not have a lancet-like hypostome used by ticks to penetrate skin and blood vessels. They are red in color, but this reflects mite body pigments, not ingested blood.

Chiggers use their biting mouthparts (gnathostoma) to affix themselves to cells of the horny outer layer of skin, in pores or hair shaft invaginations. They clamp on and digest these cells with proteolytic saliva, causing the cells to rupture and creating a nutrient-rich "soup" supped by the mite.

This feeding process creates a hardening of cells surrounding the digested bite site core, driving subsequent saliva boluses deeper into the epidermis, digesting more cells in advancing waves, hardening more cells in the perimeter, forming a "tube" or stylosome over a matter of hours or 2 to 3 days.

Like a "water well" constructed in days or parts of old, the stylosome enters deepening strata of host epidermis, where pressures increase and fluid (lymph) leaks into the cylindrical cavity, raising the broth meal to the gnathostoma of the gastronomic mite.

This feeding structure, containing irritant digestive enzymes, incites inflammation and itching as it erodes skin. It is not hard to imagine the effect of digestive enzymes, reactive inflammation, and petrification of surrounding cells on local pain receptors-intense itching begins.

The miniscule mite feeds to capacity, drops of the host and rolls into leaf matter, molts, and emerges an 8-legged nymph. Eventually, nymphs metamorphose into mature imagoes 1.3 mm in diameter, visible to an acute, naked eye.

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By the time itching starts, the larval chigger has left the skin. However, the inflammation persists, inciting pain receptor firing and vasodilatation, accounting for itching, redness, and swelling at the "bite site." A "foreign body" effect or secondary infection may be problematic complications rarely encountered in chigger attacks.

In the USA, chiggers do not transmit infectious diseases, however, an acute febrile illness known as scrub typhus, or tsutsugamushi disease, spread by chigger bites in Asia, northern Australia, and the western Pacific, may carry up to a 30% mortality.^[2] Also called "chigger-borne typhus," this disease is identified as a Rickettssial disease, an agent related the Rickettsia of tick-borne Rocky Mountain Spotted Fever.

Scrub typhus is carried by Rattus rattus, the ship rat, and transmitted to feeding larval female chiggers, which mature and pass the infection transovarially to the eggs of their unlaid offspring, which later hatch into larval chiggers carrying the Rickettsia. New-hatched mites attack new hosts, spreading the disease, even to the end host--man. US forces encountered tsutsugamushi disease in the Pacific islands during WWII and in Vietnam.

Why does any Asian larval chigger infecting a human being with tsutsugamushi disease incontrovertibly receive the infection from its mother? A larval chigger feeds on a vertebrate animal only once in its life, therefore, its first animal feeding is not followed by another, so only an infected female larval chigger can pass it on-her offspring are like bombs.

Primary Prevention of mite attack: Wear protective clothing. Apply an insect repellant to feet/shoes, clothing, and hands. "Flowers of sulfur" in the shoes is helpful, but odiferous. Keep yard grass closely mowed and free of detritus; chigger intensity and swarming are greatest in summer, especially on hot, humid afternoons.

Secondary prevention: Shower, washing thoroughly with soap, e.g. Dial, or soapy soak in the tub immediately on return from yard/garden work or recreation on "chigger turf." This should remove migrating and attached chigger mites.

Tertiary prevention: Treat the rash--5 to 6 mm reddish, itching bumps or spots, with a thorough scrubbing shower with Dial soap, or a soapy soak in the bath tub, with rinse and dry, to remove any persistent mites, though most have already gained their fill and dropped off the body by this stage. Soapy washes also reduce outer skin cell build up and bacterial counts.

For bothersome itching, topical application of 1% hydrocortisone cream (Cortaid) 2 or 3 times daily to the affected areas, or Caladryl lotion, or Aveeno bath soaks, or epsom salt soaks are helpful. Keeping the areas affected well-ventilated, wearing cotton shirts or blouses, shorts, and undergarments. Use baking soda underarms if deodorant irritates the bitten tissue. Antihistamines, such as Benadryl will help reduce itching at night-caution drowsiness in the daytime or active time.

Itching should subside in a few days, unless scratching produces a secondary bacterial infection, which may respond to washing and application of Polysporin ointment or cream or triple antibiotic ointment. If the infection persists, or you see red streaks moving up your arms or legs, consult your primary care doctor immediately.

Occasionally an allergic or idiosyncratic reaction (serum sickness or "chigger id") may occur, requiring a trip to the doctor--this is very rare.

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