Merck Manual

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Professional Version

Spider Flies/New Forest Flies of Horses


Jan Šlapeta

, MVDr, PhD, GradCertEd (Higher Ed), Sydney School of Veterinary Science, The University of Sydney

Reviewed/Revised Aug 2022 | Modified Oct 2022

The spider or new forest fly, Hippobosca equina, is widely distributed throughout Europe, Asia, and Africa. These winged keds parasitize horses and rarely cattle throughout the UK. Its bite is characterized as a nuisance and disturbance to large domestic animals. It has a predilection for feeding sites in the perineal region and between the hind legs.

The winged adult is ~10 mm long with a flattened, shriveled body. The life cycle is similar to that of Melophagus ovinus, in that the female produces one larva at a time, and at the time of maturity the larva falls away from the female fly and pupates in the soil.

Treatment is rarely indicated; however, many of the repellents used for other blood-feeding periodic parasites may be used.

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