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29th December 2011

Dermatitis in Horses: Allergic and Infectious

Article taken from:- Garrards – Horse & Hound, Spring 2011

Ann Rashmir-Raven, DVM, MS, Dipl. ACVS, of Michigan State University, discussed the most common types of equine dermatitis and the best treatment options at the 2011 Western Veterinary Conference, held Feb. 20-24 in Las Vegas, Nev.

Dermatitis–simply, skin inflammation–is relatively common in horses, whether its cause is an allergic reaction, a fungal infection, or an infectious bacterium.

Insect Hypersensitivity

“Horses develop hives probably more often than any other species,” said Rashmir-Raven, who is an associate professor of Large Animal Clinical Sciences, adding that insects are the No. 1 cause of pruritus (itchiness) and are “presumed” to be the No. 1 cause of hives in horses.

Insect hypersensitivity, as the name alludes, is an allergic reaction to the saliva of biting insects such as Culicoides spp. (biting midges), black flies, horn flies, stable flies, horseflies, and mosquitoes, she explained. This is typically a seasonal disorder and tends to worsen over the years as the horse has repeated episodes.

Horses affected with insect hypersensitivity will have bumps on the skin, ranging in size from small (2-5 millimetres in diameter) lumps to large masses 20-40 centimetres in diameter. The skin surface will appear normal in mild cases, but in more severe cases the lesions can “erupt” and drain serum, causing the surrounding hair to become matted and sores to develop.

“Lesions can occasionally be so severe that they leave scars,” she described.

Additionally, Rashmir-Raven said, horses with insect hypersensitivity will be extremely itchy, some so itchy that they become unrideable.

She suggests some relatively simple techniques to help solve the insect hypersensitivity problem, and many of them involve minimizing a horse’s exposure to the insects:

  • Use long lasting fly repellent;
  • Keep horses stalled (if possible) during the peak hours of insect activity;
  • Aim a fan at the stalled horse, as many biting insects cannot fly effectively in the air currents;
  • Use an automatic fly sprayer in barns and stalls (Rashmir-Raven cautions to avoid locating these sprayers over horses’ water and food sources);
  • Protect horses with fly sheets and fly masks (preferably ones with ears) when turned out; and
  • Avoid turning horses out near “insect havens,” including standing water, manure piles, or cattle.

For the chronically itchy horse Rashmir-Raven suggests medical treatment including baths with medicated shampoos, dietary supplementation with flax based omega fatty acids (which have been shown to reduce allergic reactions to insect bites, she said), several doses of intramuscular or intravenous injection dexamethasone (a corticosteroid used to control itchiness), followed by continued oral dexamethasone administration.

Some horses may also benefit from allergy shots, but most severely affected horses will require owner vigilance to keep them comfortable.

Other Types of Hypersensitivity

Rashmir-Raven discussed a few other types of hypersensitivity seen in horses, noting that these are less common:

  • Food Hypersensitivity–She said that reactions to food are frequently caused by contact rather than ingestion (she gave an example of a horse getting feedstuffs on his body when eating or lying in his stall). The simple fix for this, she noted, is the removal of the problematic food from the stall, diet and the surrounding areas;
  • Contact Hypersensitivity–This results from an exposure to certain types of plants, bedding, topically administered agents (shampoos, sprays, medicines, etc), and tack, Rashmir-Raven explained, and the substance that causes the reaction doesn’t always have to be a recently introduced one–horses can develop reactions over time. If a substance is suspected of causing a reaction, she suggests eliminating the horse’s contact with the item in question and washing the affected area to see if the reaction clears up. She noted that some trial and error might be needed to identify the substance to which the horse is reacting; “If the causative agent can’t be identified, oral or topical (treatments) can be used to reduce inflammation,” she added.


“We tend not to think about lice too much until we have a bad outbreak,” Rashmir-Raven said of the tiny insects that spend their entire lives (from egg to adult) on the host–horses, in this case.

There are two different types of lice that affect horses: biting lice and sucking lice. Both types are transferred from one horse to another by direct contact or via tack, grooming tools, and blankets, however she noted that quarantine alone won’t stop the spread of a lice outbreak. The problem must be treated at the source.

In the event of a lice outbreak, Rashmir-Raven suggested administering the anthelmintic ivermectin two weeks apart, for two to three doses; it has been shown to kill both types of lice. She also recommends using topical treatment to eliminate any insects that weren’t eliminated by the ivermectin, and she encouraged the treatment of the horse’s environment including the stall and horse trailer with a suitable premise spray.

There are some lice treatment shampoos specifically formulated for horses, but she noted that “anything that does the job on dogs will probably work on horses.”

Drug Eruption

Essentially, drug “eruptions” are allergic reactions to the ingestion, injection, inhalation, or topical absorption of a medication, Rashmir-Raven explained. The onset of these reactions is typically within 24-48 hours of drug administration; however eruptions have been known to occur weeks–or even months–after the administration of the drug.

Rashmir-Raven explained that the drugs most commonly associated with eruptions are antibacterial agents (particularly penicillin and sulfonamides, she added), phenylbutazone or “bute”, phenothiazine-based tranquilizers, non-steroidal anti-inflammatory drugs, diuretics, and local anesthetics.

She noted that clinical signs typically associated with drug eruptions include:

  • Hives;
  • Angioedema (a vascular reaction that causes swelling due to dilation and increased permeability of the capillaries at the surface of the skin);
  • Erythema (skin redness caused by congestion of the capillaries);
  • Lesions occurring equally on both sides of the horse;
  • Intense pruritus;
  • Photosensitization (sensitivity to sun); and
  • Alopecia (hair loss) can also occur.

As with food and contact hypersensitivity, ceasing use of the medication causing the eruptions should eliminate the problem, and lesions will begin to diminish in several days; some lesions can take up to six months to disappear, however.

Tail Rubbing

Although tail rubbing might seem like little more than a sign of a horse with an itchy behind, the likely cause of this behaviour is an infestation of pinworms (Oxyuris equi). While the adult worms live in the horses’ colon and rectum females tend to lay their eggs in the horses perianal region and under the tail, Rashmir-Raven explained.

She added that this problem is typically easily fixed with a combination of oral deworming (she recommends using ivermectin, moxidectin, pyrantel pamoate, or benzimadoles based on a veterinarian’s recommendation) and a thorough cleaning of the perianal area to remove the eggs outside the body. She also suggested harrowing pastures in midsummer, as pinworm eggs can’t survive exposure to high temperatures.


A skin condition that can affect any horse is ringworm, Rashmir-Raven said, but it is most commonly found in young or debilitated horses. Like lice, this fungus can be spread horse to horse or via shared use of grooming equipment or tack.

Generally, Rashmir-Raven said, the ringworm will clear up on its own, however the process might be expedited by scrubbing the lesions with betadine shampoo, applying very dilute bleach water to the lesions (about nine parts water to one part bleach, she suggested), and using either topical or oral griseofulvin (a type of antifungal). Handlers can limit the spread of the fungus by using disposable gloves when handling horses with ringworm, she added.

Pastern Dermatitis

Scratches and mud fever are commonly used to describe pastern dermatitis, a “term used for varied etiologies that appear on the back of the pasterns and/or heel bulbs and sometimes spread up the legs,” Rashmir-Raven explained. She added that horses with severe scratches generally have a systemic infection caused by a mix bag of bacteria that accompanies the outer lesions.

Rashmir-Raven described lesions associated with scratches as generally scaly, crusty, oozy, or scabby in appearance and painful to the touch, and that affected horses might be unsound.

“Mild cases may resolve with simple steps including clipping excess hair from the pasterns to allow them to dry, gentle cleaning with chlorhexidine (a disinfectant), and applying an antibiotic ointment to the lesions,” she explained. “Severe cases will require systemic antibiotics. Severely affected horses may benefit from regional limb perfusion (in which a veterinarian places a tourniquet on the limb and injects antibiotic into a vein below the tourniquet, isolating the blood supply via the tourniquet for a brief period–usually 30 minutes or so–to the antibiotic in the lower limb long enough for the antibiotic to migrate into the surrounding tissues).”

Equine Cutaneous Pythiosis

The final and most dangerous infectious disorder Rashmir-Raven described was equine cutaneous pythiosis. She explained that this disorder was initially only found in tropical climates, but recently has been diagnosed as far north as Washington.

While technically not a fungus, pythiosis is caused by a fungus-like organism (Pythium insidiosum). Pythiosis begins as a disease of the skin and tissues immediately beneath. Wounds associated with pythiosis often resemble proud flesh and are extremely pruitic (“Many horses with pythiosis will be so itchy that they will mutilate themselves trying to relieve the itch,” Rashmir-Raven noted). She added that without proper treatment, pythiosis will progress into the tendons, joints, and bones, frequently proving fatal for horses.

“Lesions are best treated with a combination of therapies including radical surgical excision of the lesion, topical application of antifungal solutions, and immunotherapy,” Rashmir-Raven explained, adding that the prognosis is dependent on how early the pythiosis is detected and how aggressively it is treated.


Article written by:-

Ann Rashmir-Raven, DVM, MS, Dipl. ACVS, of Michigan State University.

Article taken from:- Garrards – Horse & Hound, Spring 2011