This genus of biting midges can produce a very itchy insect bite hypersensitivity in horses, says Stephen White, DVM, a professor
at the University of California-Davis College of Veterinary Medicine. The Culicoides species from North America is different from those found in other parts of the world. "Certain proteins from the saliva of
these insects will induce an allergic response in horses," White says.
Some of the midges are ventral feeders, while others feed along the dorsum. Treatment involves repelling the flies with pyrethrin
or similar substances, though there are studies of varying success of these drugs, White says. Fly masks may be helpful. "It
seems horses with hypersensitivity to Culicoides also have similar sensitivities to other allergens, such as pollens, dust and mold," he says.
Regarding some of the other fly species (e.g., black flies, deer flies, horse flies), there's debate about whether it's a
true hypersensitivity or allergy or just an irritation, since several of these fly species deliver painful bites, says White.
"If the horse is solely allergic to Culicoides, immunotherapy or allergy shots don't seem to work as well, because the allergen isn't derived only from the insect saliva
but the entire insect ground within it, making it less effective," says Rees. "Usually steroids or antihistamines are helpful.
Some use methylsulfonylmethane, an anti-inflammatory medication, or omega-3 fatty acids."
Because pemphigus foliaceus is an autoimmune disease, "presumably there are antibodies that are directed against some of the
substances that keep the epidermal cells attached to each other," says White. "While the target proteins have been pretty
well elucidated, we don't really know what it is in the horse." He says they see just one or two cases a year of pemphigus
foliaceus at UC Davis.
Pemphigus foliaceus can become severe and affect much of the body. Its acute crusting can be painful. Signs include blistering
skin, most commonly affecting areas of the head and lower extremities, although other areas may be involved.
Perform a biopsy in a horse with acute crusts that isn't responding well to other treatments. The biopsy specimen should include
a crusted area, White recommends. "Usually pemphigus foliaceus presents as a crusted disease. At first, the horse might have
a little edema on the ventrum, or their legs may stock up, or they may have a very mild anemia. Sometimes they'll start out
with urticaria, but fairly rapidly it progresses to a very crusty type of disease. It's important, prior to biopsy, to not
prep the area since the superficial crust provides the answer."
Pamela Ginn, DVM, DACVP, associate professor at the University of Florida's College of Veterinary Medicine, says she only
occasionally sees pemphigus foliaceus in her equine patients. "We see it more often in dogs and cats. We don't really know
what triggers it; sometimes it's just spontaneous. Some cases could be triggered by drug therapy, but that hasn't been documented
in the horse."
Skin disease of the lower legs and pasterns
Pastern dermatitis can be caused by myriad conditions, says White, "We tend to put a name on it, such as greasy heels or scratches, but it's not one disease. It can be caused by a number of things, such as bacterial or fungal infection, parasite infestation,
vasculitis or contact irritants. So those need to be investigated."
Draft horses with a lot of feathering on their legs seem to be particularly susceptible. Signs include inflammation and foul-smelling
crusts on the rear half of the pasterns and matted hair.