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A researcher describes ectoparasites, infections, and hypersensitivity disorders that can cause pruritus in horses.

Posted by Alexandra Beckstett, The Horse Managing Editor | Sep 28, 2021 | British Equine Veterinary Association, Hives & Skin Allergies, Horse Care, Other Skin Problems, Skin Problems, Vet and Professional

When working up a pruritus case, “history is the most important bit,” said Littlewood. “It’s probably more important than the clinical exam because you need to know the background of the case.”

Specifically, the veterinarian should determine:

“It’s also important to ask what other things they’ve been doing in addition to what’s been prescribed by the veterinarian,” Littlewood said. “Horse owners like to try lots of remedies, and they might not always be forthcoming about what they’ve used.”

The veterinarian should then perform a physical exam that includes a description of the lesions and their distribution. Littlewood recommended using the pruritus visual analogue scale, which researchers have validated for use in dogs and tweaked for use in horses.

Also perform a basic lab investigation. “Develop a relationship with your microscope,” she said. “Dermatology can be confusing, but you can get a huge amount of information from some pretty simple diagnostic tests, like hair plucks.”

Other tests include using a cardboard tongue depressor to collect surface debris from affected areas, using adhesive tape strips to capture parasites, and getting smears of lesion surfaces or exudate (secretions). The veterinarian can examine these samples under a microscope or submit them for culture.

Littlewood reviewed types of ectoparasites, some of which are quite rare:

Lice Chewing lice infestations are most common in winter and cause variable and potentially debilitating pruritus and alopecia (hair loss). You can identify chewing lice by their visible eggs and nits. Sucking lice, while less common, are quite visible and cause pruritus and anemia. Both conditions respond to treatment using pyrethroids, permethrins, and lime sulfur spray.

Chorioptes bovis var. equi This parasite causes chorioptic mange, primarily on the legs of draft and cob breeds. Cases are worse in winter and can be difficult to treat. Therapies with some evidence of efficacy include doramectin injections, fipronil spray, lime sulfur, permethrins, and selenium sulfide shampoo.

Environmental mites Examples of problematic free-living mites that occasionally affect horses include forage mites, poultry mites, and the larval stage of the harvest mite. The latter might respond to fipronil spray, while poultry mites respond to permethrins, pyrethroids, and lime sulfur. With forage mite infestations, veterinarians simply treat the symptoms and remove horses from infested hay, grain, and straw.

Bacterial and fungal skin infections in horses are unsightly and painful, said Littlewood. Malassezia dermatitis, in particular, can be a significant pruritic problem in horses, she said. It develops when dysbiosis (a microbial imbalance) causes an overgrowth of this otherwise normal commensal yeast organism in the intermammary, preputial fossa, perianal, and axilla regions. When the exudative lesions appear in a mare’s intermammary (between the udders) area, it’s often referred to as “cleavage itch.” Littlewood said you can treat it using antifungal shampoos or creams.

Littlewood then reviewed four hypersensitivity disorders that can cause pruritus in horses:

Contact dermatitis This skin inflammation usually has an irritant or allergic component and causes itching, pain, inflammation, and alopecia.

Adverse food reactions These reactions are commonly suspected but poorly understood, said Littlewood. In reality, they’re likely very rare, she said, citing only two or three published case reports that meet the proper criteria for an adverse food reaction.

Culicoides hypersensitivity This hypersensitivity to the salivary proteins of Culicoides midges (i.e., sweet itch or recurrent seasonal/summer dermatitis) is extremely common, said Littlewood. The classic signs include lesions along the dorsal or ventral midline where the midges like to feed. Currently, diagnostic options include clinical signs, intradermal testing, and serological (blood) testing. But Littlewood said recent work on recombinant salivary proteins from midges and major allergens show promise for developing better diagnostic tests and therapeutics.

Managing Culicoides hypersensitivity revolves around insect avoidance. Littlewood recommended:

Atopic dermatitis This dermatitis is a fairly common and often-seasonal sensitivity to environmental allergens such as dust mites, molds, and pollen, Littlewood explained. Veterinarians diagnose it by ruling out other causes of pruritus and running intradermal tests. Serum tests are available but show poor correlation between different laboratories and intradermal test results. Treatment includes allergen-specific immunotherapy, which researchers have shown to be beneficial in 64-85% cases. One of the challenges with atopic dermatitis, said Littlewood, is you must treat it for at least a year before you can determine whether immunotherapy is effective.

“We need to manage owner expectations,” she said. “They want it to be fixed, but it’s retraining the immune system, and we know in dogs it’s a mean of six months before it works. The advantage is it can be used in competition horses.”

As for symptomatic therapies to give affected horses relief, Littlewood recommended glucocorticoids and antihistamines. Additional options include omega-3 supplements and oclacitinib (Apoquel, which in the United States is FDA-approved for use in dogs), with further studies needed to determine their efficacy, she said.

She also emphasized the importance of avoiding the causative allergens using the following measures:

Pruritus is a broad topic in horses that encompasses a wide range of causes. It remains challenging for veterinarians to diagnose and treat, but management and prevention are key to the welfare of affected horses.

Alexandra Beckstett, Managing Editor of The Horse and a native of Houston, Texas, is a lifelong horse owner who has shown successfully on the national hunter/jumper circuit and dabbled in hunter breeding. After graduating from Duke University, she joined Blood-Horse Publications as Assistant Editor of its book division, Eclipse Press, before joining The Horse.

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