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Ectoparasites - Trombicula

Trombicula spp: Chigger mites, harvest mites

General Description: The only parasitic stage of the chigger mite life cycle is the larval phase. Trombicula larvae are reddish and 0.2 to 0.4 mm. long. Nymphs and adults feed either on plants or on invertebrates.

Life Cycle: The free-living adult lays eggs on the soil, which hatch in 10 days as larvae. After finding a host, the larva feeds for 2 to 10 days on skin fluids and tissues partially digested by salivary secretions. The mite drops to the ground, where it moults to become a nymph and then an adult. Larvae are most prevalent in the autumn, especially on grasslands and scrubby woodland. One to five generations can occur in a year, depending on species and climate.

Location: The head and neck are particularly attacked by chigger mites, but they may occur anywhere on the body.

Geographical Distribution: Significant on horses in Europe, North America, and occasionally in Australia and the Far East. Trombicula is not reported in NZ.

Significance: This parasite is widely distributed, fairly common, and a great annoyance to horses.

Effect on Host: The disease known as chigger dermatitis is characterised by terrible itching caused by digestive juices injected into the skin by larval mites while feeding. Horses may damage themselves while biting and scratching. Infested horses bite, scratch, and rub themselves.

Diagnostic Information: Finding the larvae in skin scrapings taken from the lesions.

Control: Treat infected animals with an organophosphate compound. Heavily infested pastures may be vacated and treated with miticides. Pasture rotation alone will not eradicate these parasites, as the larvae also feed on field mice and other small rodents.