THE MERCK VETERINARY MANUAL
Print Topic

Sections

Chapters

Mange in Horses

-
-

The last large animal mange reportable to the OIE was horse mange, but this was removed in 2006. Mange in horses was removed from the list of federally reportable diseases in the USA prior to this date, although it remains reportable to some state veterinary agencies.

Sarcoptes scabiei var equi is rare but is the most severe type of mange in horses. The first sign of infestation is intense pruritus due to hypersensitivity to mite products. Early lesions appear on the head, neck, and shoulders. Regions protected by long hair and lower parts of the extremities are usually not involved. Lesions start as small papules and vesicles that later develop into crusts. Alopecia and crusting spread, and the skin becomes lichenified, forming folds. If infestations are not treated, lesions may extend over the whole body, leading to emaciation, general weakness, and anorexia. Negative skin scrapings do not exclude the disease; biopsy may establish a diagnosis.

Hot lime sulfur spray or dip is labeled for use against sarcoptic, psoroptic, and chorioptic mites in horses. Treatment should be repeated every 12 days if needed, following the species-specific dilution on the label. Although certain spray formulations of permethrin are labeled for use against mange in horses, it is generally not considered the compound of choice. If permethrin is used, the animals should be thoroughly wet with the product and re-treated in 10−14 days. Although not labeled for treatment of mange in horses, two doses of oral ivermectin at the label dosage (200 mcg/kg) given 14 days apart (field studies), or a single treatment of oral moxidectin at the label dosage of 400 mcg/kg, have effectively treated psoroptic, chorioptic, and sarcoptic mange in horses.

Psoroptes ovis (formerly P equi) and P cuniculi (likely a variant of P ovis) both infest horses. P ovis is rare in horses. However, infestations can produce lesions on thickly haired regions of the body, such as under the forelock and mane, at the base of the tail, under the chin, between the hindlegs, on the udder, and in the axillae. P cuniculi can sometimes cause otitis externa in horses and may cause head shaking. Pruritus is characteristic. Lesions start as papules and alopecia and develop into thick, hemorrhagic crusts. Psoroptic mites are more easily recovered from skin scrapings than are sarcoptic mites. Topical and oral treatments recommended for other types of mange are effective. Hot lime sulfur is labeled for use against Psoroptes in horses (see above). Although not labeled for treatment of mange in horses, oral ivermectin at the label dosage (200 µmc/kg) given for two doses 14 days apart (field studies), or a single treatment with oral moxidectin at the label dosage (400 mcg/kg), has effectively treated psoroptic, chorioptic, and sarcoptic mange in horses.

Chorioptic mange is caused by infestation with Chorioptes bovis (formerly C equi) and is the most common form of mange in horses. Draft horses are commonly infested, although all breeds are susceptible. Lesions caused by C bovis start as a pruritic dermatitis affecting the distal limbs around the foot and fetlock. Papules are seen first, followed by alopecia, crusting, and thickening of the skin. A moist dermatitis of the fetlock develops in chronic cases. Infested horses may stamp their feet or rub one foot against the opposite leg or object. Chorioptic mange is a differential diagnosis for “greasy heel” in draft horses. The signs subside in summer but recur with the return of cold weather. The disease course is usually chronic without treatment, but the prognosis is favorable when treated. Topical and oral treatments recommended for other types of mange are effective. Hot lime sulfur is labeled for use against Chorioptes in horses (see above). Treatment is aided by clipping long hair from infested areas. Although not labeled for treatment of mange in horses, oral ivermectin at the label dosage (200 mcg/kg) given for two doses 14 days apart (field studies), or a single treatment with oral moxidectin at the label dosage (400 mcg/kg), has effectively treated psoroptic, chorioptic, and sarcoptic mange in horses.

Demodectic mange in horses is caused by infestation with Demodex equi or D caballi. Demodex mites infest hair follicles and sebaceous glands. D equi lives on the body, and D caballi on the eyelids and muzzle. Demodectic mange is rare in horses but can manifest as patchy alopecia and scaling or as nodules. Lesions appear on the face, neck, shoulders, and forelimbs. It has been reported in association with pituitary pars intermedia dysfunction (see Hirsutism Associated with Adenomas of the Pars Intermedia) and chronic corticosteroid treatment. Pruritus is absent; therefore, secondary infections due to excoriation are rare. Therapy is rarely done, although there is limited evidence that the macrocyclic lactones may be effective. Lesions have also been reported to resolve without treatment.

Trombiculid mites can parasitize the skin of horses, especially during the late summer and fall. The adult mites live on invertebrates and plants; the larvae normally feed on small rodents, but they can opportunistically feed on people and domestic animals, including horses. Lesions consist of severely pruritic papules and wheals on the face, lips, and feet. At the time of diagnosis, a topical pyrethrin or pyrethroid labeled for horses can be used to kill any remaining larvae still feeding. Symptomatic treatment with a glucocorticoid for pruritus can be added to minimize further self-trauma and associated secondary infections. Any secondary infections should be treated. Repellents may help prevent infestation.

These mites usually feed on organic material in straw and grain and can opportunistically infest the skin of horses. Papules and wheals appear on the face and neck if horses are fed from a hay rack, and on the muzzle and legs if fed from the ground. Pruritus is variable. Treatment for trombiculidiasis may be used for straw itch mite infestations.

Last full review/revision May 2015 by Mason V. Reichard, PhD; Jennifer E. Thomas, DVM

Copyright     © 2009-2015 Merck Sharp & Dohme Corp., a subsidiary of Merck & Co., Inc., Kenilworth, N.J., U.S.A.    Privacy    Terms of Use    Permissions