Chorioptes equi

The surface mite Chorioptes occurs on horses and other equids around the world.  These hosts appear to have their own species or strain of the parasite.  The life cycle is direct and all stages occur on the horse.  In many horses Chorioptes is of little or no significance.  When clinical signs occur they most commonly affect the lower limbs, especially the feathers of heavy horses.  In horses without feathers, lesion are most common around the tail head.  For reasons that are not fully understood, some heavily infested horses do not show clinical signs.  The primary clinical signs of chorioptic mange is pruritus, which can cause a horse to stamp its feet, to bite and rub its head on affected areas.  Sometimes papules with serous exudate and scabbing will occur.  Transmission is direct horse to horse, although under ideal environmental conditions the mites might be able to survive off the host for a few weeks.  Chorioptic mange in horses is very contagious, and one affected horse in a group probably means that all horses in the group have the mites.  Chorioptes infestation can also be very difficult to treat successfully, and the mites and clinical signs can persist in the face of repeated treatments with an acaricide.

Chorioptes in horses is not known to be zoonotic.

Adult male and female mites measure up to approximately 0.5 mm in length and have eight long legs. In general, female mites are larger than the males. On all legs in the males, and on the first, second and fourth pairs in the females, there are terminal suckers on unjointed pedicels.

 

Chorioptes equi occurs on horses and other equids in many parts of the world, and is seen as a cause of clinical disease especially in heavy horses with heavily feathered legs.

The entire life cycle of C. equi occurs on the host. The mites live on the skin surface, where adult females lay eggs, which hatch to release a larva (six legs), which moults through one or more nymphal stages (eight legs) to the adult. The life cycle can be completed in a few weeks.

Life Cycle: Chorioptes equi

Transmission of C. equi is direct horse-to-horse or sometimes by objects such as grooming equipment. There is uncertainty regarding the survival of Chorioptes mites away from the host (some sources indicate up to 70 days), but it seems probable that large numbers of mites do not survive for long periods. If chorioptic mange is diagnosed in a horse, it should be assumed that all other horses in the group are also infested.

Infection with C. equi is more common than is clinical disease. Mites of the genus Chorioptes, unlike Psoroptes, do not penetrate the skin during feeding, but depend instead on debris on the skin surface. Nevertheless, the mites can cause severe pruritus. Some horses with large mite burdens do not show clinical signs. The mechanisms underlying this apparent tolerance are not known.

Clinically, the most common site for lesions of chorioptic mange is the lower limbs, especially in heavily feathered animals. The pruritus caused by the mites leads to attempts to relieve the irritation by scratching, rubbing and biting the affected areas, foot stamping and kicking. These activities may result in damage to the haircoat and skin. Papules, with exudation of serum and scabbing may occur, together with matting of the haircoat. In non-feathered horses, Chorioptes may infest the tail head and other regions of the body, with similar clinical effects. Chorioptic mange in horses is seen as a clinical problem more often in winter than at other times of the year.

The recovery of adult mites, or even fragments, from hair and coat brushings, or from skin scrapings, confirms the diagnosis.

There are no products approved for horses in Canada that are effective for chorioptic mange, but ivermectin (VARIOUS) and moxidectin (QUEST and QUEST PLUS) have been used extralabel, either orally or applied directly to the lesions. These treatments cannot be relied upon, however, to remove all the mites or resolve all the clinical signs. In some cases, shaving of the affected areas and application of a keratolytic cream may be as effective as an acaricide. All horses in a group should be treated, ideally weekly for three weeks and then monthly for three months.

Additional information on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.

Control of chorioptic mange in horses depends on rapid diagnosis and treatment, with particular attention to new arrivals in a group, and cleaning of any potential fomites, for example tack.

Chorioptes equi is not known to be zoonotic.

Pilsworth RC et al. (2005) Skin Disease Refresher: Chorioptic Mange. Equine Veterinary Education 17: 9-10.