Feather mites are among the most stubborn of parasites, causing a condition commonly called heel mange. This typically affects heavily feathered breeds such as cobs, Shires, Clydesdales and Friesians, and some of the hairier ponies.
The mite responsible is Chorioptes, which tends to cause greatest trouble in winter when horses spend more time indoors. Dwelling on the surface of the skin, these mites can live off their host in the stable environment for more than two months and are thought to survive for longer in straw bedding than shavings.
Classic horse behaviour that signals their presence includes stamping of the hooves, and biting and rubbing at the backs of the legs. Irritation is usually low down around the pastern and fetlock area, although it will occasionally spread further up towards the back of the knees. While both front and hindlegs can be affected, the condition is more common in the hindlegs.
The mites bite and irritate the skin, causing serum to leak which then dries to form scabs. Itching and trauma can result in reddening, sores and often thickening of the skin. Some animals become reluctant to allow handling of their legs, and in certain cases a secondary bacterial infection can develop with swollen limbs and lameness. The sensitivity of an individual is very variable and allergic reaction is thought to play a part.
Diagnosis is usually based on the presence of these signs. The scabs can look similar to those produced by mud fever or pastern dermatitis, but excessive irritation indicates feather mites. Skin scrapes can be taken or mites can be lifted off on a piece of sticky tape to make a definite diagnosis; because these little parasites are too small to be seen with the naked eye, analysis under a microscope is required.
In most cases, feather mites regress during the summer months. There are currently no licensed medications for treating Chorioptic mites in equines.
An injection of doramectin (commonly branded Dectomax) is often used; this is licensed to treat Chorioptes in cattle and seems to be effective. Moxidectin (branded Cydectin) is another option, but is only licensed to treat other mites in cattle and sheep.
Injectable medications will kill mites feeding from the horse’s skin, but not those living in his feathers or residing in his environment. The mite eggs are very resistant and difficult to eliminate.
Some owners may be reluctant to clip their horse’s feathers, but doing so drastically improves the chances of treatment success. Environmental decontamination is also important. Stables should be emptied of bedding and steam- cleaned, along with grooming equipment and rugs. It takes three weeks for eggs to become adult mites, so two injections are advised three weeks apart and environmental decontamination should be repeated.
An alternate approach is topical treatment, where solutions are applied directly to the skin. Clipping is again helpful to allow these treatments to reach the skin surface. Fipronyl spray (branded Effipro or Frontline and used for cats and dogs) is sometimes prescribed, but is a costly option.
Pig oil and lime sulphur is an old-fashioned and less expensive method of treatment and prevention. Studies have shown a reduction in clinical signs and absence of mites in horses treated with four applications of a 5% lime sulphur solution at weekly intervals. Any animals in contact with those affected should also be treated.
However, none of these medications are proven to entirely eradicate feather mites. Treatment targeted at both the horse and his environment is likely to be the most successful in mite management.
Ref: Horse & Hound; 22 February 2018