Thursday, 16 March 2017

Chiggers – Nasty Parasitic Mites

Chiggers create a tube called a stylostome,
through which they suck juices from host
tissues. Illustration by Bugboy52.40

The young of chigger mites prey on mammals and other animals. They occasionally bite humans, causing unbearably itchy lesions.

What Are Chigger Mites?

The group of mites collectively known as chigger mites includes a wide range of genera and species distributed over the tropical and temperate regions of the globe. In North America, the species most commonly encountered is Trombicula alfreddugesi. Like spiders and scorpions, mites are arachnids; adults have four pairs of legs. The adults of the various chiggers feed on invertebrates (worms, snails etc.) and are seldom noticed, but the larvae are parasitic on mammals, birds, reptiles, and amphibians, and cause great discomfort when they feed on humans.

How do People Get Chiggers?

Chigger mite larvae hatch from eggs deposited in the environment, usually in tall grasses, brushy areas, swamps and bogs, and other localities where plant growth is low and thick. The microscopic larvae wait in the low vegetation and climb onto any animal or human moving through the area near them. On people, the larvae tend to migrate to areas where the clothing is snug – under waistbands, under the tops of socks, or where a backpack presses against the skin, for example.

Chigger Bites

Chigger mites neither burrow under the skin nor suck blood. Instead, they attach to the skin and inject a substance that creates a pool of liquid nutrients by dissolving and liquefying the skin cells. The host’s immune system responds by trying to create a barrier between the affected cells and healthy cells. A tiny tubular hole called a stylostome forms in the skin with the larva in the center, still drawing liquefied food from the tissues below the bite as though sucking through a straw.

Chigger bites first appear as fluid-filled blisters,
but scab over later. They are notoriously itchy.
Image by Dick Culbert CC BY 2.0
Most people have no idea they’ve been invaded by microscopic mite larvae while out in the woods and fields. For the lucky few, there will be no aftereffects: not all chigger mites cause discomfort and not all individuals react to the bites. For many, however, days of suffering are just beginning. About twenty-four hours after the initial bite, a red raised lesion appears, often with a fluid blister-like center. The mite is still present; it will eventually drop off if allowed to finish its meal but many are killed at this stage when the host scratches. The intensely itchy bite progresses over the next few days, becoming larger, turning slightly bluish like a bruise and crusting over. Scratching may lead to secondary infection of the lesions.

The agony of chigger bites is usually limited to the unbearable discomfort and occasional secondary infection; however, some chiggers are known to carry diseases. Leptotrombidium sp. chiggers in Japan, Southeast Asia, and nearby countries carry an organism that causes scrub typhus, or tsutsugamushi disease, a serious infection that is occasionally fatal.

Chigger mites are often confused with another skin parasite: Tunga penetrans, the chigoe flea. The two pests are quite different however.

Further reading

Knutson, Roger M. Furtive Fauna: A Field Guide to the Creatures Who Live on Us. New York: Penguin Books, 1992.

Schmidt, Gerald D. and Larry S. Roberts. Foundations of Parasitology 6th Ed. Boston: McGraw Hill, 2000.

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