This is chorioptic mange – the most common mite in horses. Chorioptes equi is characterized by long legs and short pedicles.
Most commonly it is found on the lower limbs (esp. the hind legs), where it causes pruritus. Chorioptes infestation is often seen as a component of pastern dermatitis, a multi-factorial disease that is very common in draft horses, especially those breeds with heavy feathering. In non-feathered breeds, it can occur elsewhere on the body.
It is also called scratches, greasy heel, and dermatitis verrucosa. It is a surface mite and causes extreme pruritus. It is very contagious horse-to-horse (occasionally on fomites) so if one in a herd has it, the others probably do as well.
Tx: Ideal to remove the feathers (clients rarely allow this) then use some combination of topical permethrin or coumaphos, systemic or topical ivermectin/moxidectin, selenium sulphide shampoos, or topical products w/ fipronil (usually used in small animal).
Here’s a great review of chorioptic mange, with images, from the University of Liverpool.
Image courtesy of Dr. Patrick Hensel.
