Feather mites are among the most stubborn of equine parasites, causing a condition commonly called heel mange (chorioptic mange). This typically affects heavily feathered breeds such as cobs, Shires, Clydesdales and Friesians, and some native ponies.
When severely affected, horses with an infestation will typically stamp their hooves, and bite or rub at the backs of the legs. With time, some cases can develop severe crusting and scaling of the skin on the lower limbs, with skin thickening and general swelling, which can become infected and quite debilitating. As a result it is important to treat and control the condition effectively in the early stages.
This common skin condition is typically more of a problem during the winter when horses spend more time stabled and the condition often improves during the summer with increased turn out.
The mite responsible is Chorioptes equi. These mites live on the surface scales of the skin and are effectively protected by the long hair on the legs of some horses. These mites can live off their host in the stable environment for some time and are thought to survive for longer in straw bedding than shavings.
The mites bite and irritate the skin, causing serum to leak that 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 sometimes severely swollen limbs causing wrinkles in the skin and occasionally lameness.
The sensitivity of an individual is very variable and allergic reaction is thought to play a part in a similar way to how flea allergies affect dogs.
Typical signs of feather mites in horses
Classic horse behaviour that signals the mites’ 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.
Diagnosis is usually based on the presence of the signs. The scabs can look similar to those produced by mud fever or pastern dermatitis, but excessive irritation indicates feather mites. The scabs may sometimes be mistaken for pastern leucocytoclastic vasculitis (PLV).
Skin scrapes can be taken, but can be hard to do properly on hairy legs, so mites can be lifted off on a piece of sticky tape to make a definite diagnosis; because these little parasites are too small (0.3-0.5mm long) to be seen with the naked eye, analysis under a microscope is required.
Treatment of feather mites in horses
Some owners are reluctant to clip their horse’s feathers, but doing so drastically improves the chances of treatment success. A mild antiseptic wash should be used to help remove any serum or scabs and reduce the skin scurf. Your vet may need to sedate the horse for this procedure, as the legs are likely to be very sore.
There are currently no licensed medications for treating Chorioptic mites in equines, but most cases are treated through a combination of topical and systemic options, combined with active decontamination of the horse’s environment.
An injection of doramectin is often used; this is licensed to treat Chorioptes in cattle and sheep and seems to be effective. Two injections are normally given 10-14 days apart, but this tends to only be effective for a short period of time and requires to be repeated frequently.
Giving oral ivermectin paste weekly for four weeks at 0.2-0.3mg/kg can help reduce the number of mites where the skin has been penetrated in chronic, long-standing cases.
If a secondary skin infection is present, antibiotics may be necessary.
While these medications will help kill mites feeding from the horse’s skin, they will not act on those living in his feathers or residing in the environment. The mite eggs, which take three weeks to become adult mites, are particularly difficult to eliminate.
Topical treatments, where solutions are applied directly to the skin include:
- Fipronyl spray (widely known under the brand name Frontline)
- Pyrethroids (eg: Deosect) applied every two weeks
- 1 – 2.5% selenium sulphide wash repeated frequently. The wash must be left on for 10mins before being thoroughly rinsed off
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.
Environmental decontamination is also important. Stables should be emptied of bedding and steam-cleaned, along with grooming equipment and rugs. Any animals in direct contact with those affected should also be treated in an ideal scenario, but it is rarely feasible to treat every horse in a yard.
Sustained treatment of all individuals beyond the three week life cycle, followed by quarantine measures and/or repeated prophylactic treatment prior to exposure to untreated horse is often curative. Unfortunately, susceptible horses seem to easily become re-infected so ongoing management is often required.
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