Leg Mange Mites

I was at a stable looking at my friend’s new mare when I was asked about the swollen leg of a Percheron gelding. A 10-year-old black Percheron was brought out from the barn with a severely swollen left hind leg. The swelling had been present for more than six months and started below the stifle down to the pastern. The leg was hard as a rock and did not “pit” (leave an impression) as seen with transient edema when horse legs “stock up.” Above the back of the fetlock, there was a thick layer of dry crusty material. The horse was very stiff legged but I observed him working in harness pulling a packer over the top of freshly seeded spring grain. He was normally housed in a standing stall and had difficulty getting his leg outstretched when he wanted to sleep in a sternal position. Since I was already at the farm, I took skin scrapings from the horse’s leg and indicated that I would call the owner and his veterinarian with the results.

Under each microscope field, numerous very busy mites were feeding on skin debris. These are probably leg mange mites of the species Chorioptes bovis. This species of mite is a non-burrowing mite which parasitizes a wide range of domesticated and wild ungulates (hooved animals). They have a predilection for draft horses and other horses with thick hair “feathers” on the distal limbs. Often, there is considerable greasy exudate at the point of infestation with a secondary bacterial dermatitis. Pruritis (severely itchy skin) with scaly to can be irritable and self-mutilate, stomp their feet and have scabby legs. Be careful. They can be testy to examine. Click here to view a video of the mites. Also, see additional photos at the bottom of this page.

There is often a lot of confusion related to the cause of skin ailments of the lower leg. On my website, the infosheet “Horses Sunburn, Photosensitivity or Contact Dermatitis in Horses” provides further details. Many of these leg ailments look similar and names are often used interchangeably. The ailments include mud fever, dermatophilus, grease heel, pastern dermatitis, or scratches. Regardless, of the appearance, one must always rule out ectoparasites and specifically chorioptes mites as the primary cause of the problem with secondary bacterial and/or fungal infections acerbating the condition.

The second question was can he be cured?

In a research paper by De Cock et al. entitled "Progressive swelling, hyperkeratosis, and fibrosis of distal limbs in Clydesdales, Shires, and Belgian draft horses" researchers describe the changes in the lower legs of affected draft horses with pastern dermatitis (grease heel, or scratches) The changes are characterized by the progressive development of thick-walled lymphatics, which were associated with chronic dermal edema, inflammation, fibrosis, the formation of numerous small blood vessels and degeneration of connective tissue elastin. Observations were that:

• • The lower legs in the end stages of the disease are also characterized by arteriosclerosis and fibrosed veins.

• The clinical signs and pathological changes in this disease of draft horses closely resemble the human condition known as elephantiasis nostras verrucosa, a disease state in which lymph edema plays a pivotal pathogenic role.

• This investigation dispelled many long-held beliefs that pastern dermatitis in draft horses was simply the result of chronic bacterial infections or parasitic infestation of the skin.

This is an interesting paper since it would explain the rock-like thickening of the leg that we are observing in this gelding. It also would confirm my suspicions that the mites may be eliminated but the lesions observed are probably irreversible. Regardless, we need to kill the mange mites, treat any secondary bacterial infection, and try to get the inflammation to settle down.

Treatment

There are no approved licensed products for the treatment of mange in horses. Licensed products are available for the control of ectoparasites in other species. Therefore, all treatment options for horses are “off-label” or “extra-label” drug use which necessitates a veterinary prescription and a valid veterinary-client-patient relationship. Treatment options include both injectable and/or topical. Whichever is chosen the product should be used at least twice at 14 day intervals.

Injectables

The most commonly used injectable products belong to the avermectin family of medications including ivermectin (Ivomec), doramectin (Dectomax), eprinomectin (Eprinex), or moxidectin (Cydectin). Oral and pour-on preparations of these products may be available but anecdotal observations would indicate they are less effective than injectable forms. Treating a large mature Percheron horse can be problematic because of the large volumes required and the need to distribute the medication in several injection sites. Injectable avermectins need to be used with some caution. In the early 1980s when ivermectin injectable was introduced for horses as a wormer, horses died of Blackleg disease (Clostridia chauvoei). It was postulated that the large volume of the injectable caused a local necrosis of muscle and the ideal environment for Clostridia to grow. My recommendations would be to use injectable penicillin concurrently. Having made that recommendation, I polled my veterinary colleagues through the listserv, Equine Clinician’s Network. None of those replying (n=4) used penicillin concurrently when administering the avermectin products by injection.

Topicals

Topical treatments include pour-on avermectins, insecticides fipronil (Frontline available in USA) and lime sulphur. Pour-on avermectins can cause hair loss at the area of application therefore some veterinarians will apply the product locally on the legs rather on the back.

It is much easier to treat the excoriated infected/infested areas if the feathers and long hair are clipped. Some horse owners will find this offensive but it will grow back and will make treating the infestation easier. It is also easier to breakdown the greasy exudates and eliminate the secondary bacterial infection if the areas are cleaned. Cleaning up the area will also discourage flies from laying their eggs in the excoriated debris. There is nothing worse than trying to treat fetlocks that are crawling and moving with thousands of maggots.

Once the legs have been clipped then your veterinarian may suggest a two to three-week treatment regime. Treatments will be dependent on medication/drug availability and your veterinarian’s success with various treatment regimes.

Regimes may include:

• • washing the legs with phoxim solution (organophosphate insecticide by Bayer available in Europe) every four to five days for 14 days to three weeks, or

  • a bacterialcidal wash/shampoo such as chlorhexidine, Malaseb or Pyoben.

Once clean the legs can be coated with:

• • a five per cent lime-sulfur dip every other day for two weeks, or

  • topical application of a corticosteroid/antibiotic combination ointment e.g., Panalog, Silvadene (silver sulfadiazine) ointment.

The shampooing is repeated either daily or every four to five days depending on severity of the skin infection. Once clipped and washed for the first time, be gentle so as to not cause bleeding. After rinsing and drying, the topical treatment of choice can be applied.

Any time there are numerous potential ways of treating a common condition like mange, it makes me suspicious that the success rate of curing the condition is less than 100 per cent. Therefore, owners must be vigilant and persistent otherwise it will plague their stable continuously.

The one aspect that is often over looked is the importance of treating all in-contact animals and the environment to prevent re-infestation. The owner of the stable will have to ensure that all equipment (brushes, harness, saddle pads etc.) are cleaned of all debris and wherever possible are disinfected. If it is possible put items through the heat cycle of the clothes drier this will kill any mites.

Leg mange has been around the horse industry for eons and unfortunately continues to plague the draft horse industry with no approved treatments to cure the problem.

References:

1. De Cock HE, Affolter VK, Wisner ER, Ferraro GL, MacLachlan NJ. Progressive swelling, hyperkeratosis, and fibrosis of distal limbs in Clydesdales, Shires, and Belgian draft horses, suggestive of primary lymphedema. Lymphat Res Biol. 2003; 1(3):191-9.

2. Paterson S, Coumbe K. An open study to evaluate topical treatment of equine chorioptic mange with shampooing and lime sulphur solution Vet Dermatol. 2009 Oct; 20(5-6):623-9.

3. Rendle DI, Cottle HJ, Love S, Hughes KJ. Comparative study of doramectin and fipronil in the treatment of equine chorioptic mange. Vet Rec. 2007 Sep 8; 161(10):335-8.

4. Ural K, Ulutas B, Kar S. Eprinomectin treatment of psoroptic mange in hunter/jumper and dressage horses: a prospective, randomized, double-blinded, placebo-controlled clinical trial. Vet Parasitol. 2008 Oct 1; 156(3-4):353-7. Epub 2008 Jun 25.

August 2012

Dr. Bob Wright, Consultant - Equine Disease Prevention and Equine Industry

6958 Wellington Rd. #16

RR#1 Belwood, Ontario Canada

N0B 1J0

519-843-1783, Fax: 519-843-3628

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