When the Body Attacks
The immune system is charged with protecting animals from external pathogenic, or disease-causing, threats such as bacteria, viruses, fungi, and parasites. But sometimes it gets a little carried away, sending its defenders out on missions to destroy things a bit more benign.
Allergies develop when the immune system responds to contact with outside “foreign” proteins, called allergens, that wouldn’t normally harm the animal. These include dust particles and components of insect saliva, pollens, and other plant and animal proteins. In horses, allergic responses most commonly arise in the skin and respiratory systems.
Basics of the Battle
Several key immune system players are involved in allergic reactions:
Antigens/allergens are any of these proteins that provoke an immune response. The immune system is designed to target the pathogens described, and it considers the generally harmless allergens a threat as antigens.
Antibodies are components the immune system produces to bind to specific antigens after initial exposure.
Mast cells are a type of immune cell.
IgE (Immunoglobulin E) is the type of antibody the immune system most commonly produces as a response to allergens. It binds to mast cells, causing them to “degranulate” and release a cascade of inflammatory mediators.
Inflammatory mediators include chemicals and enzymes such as histamine, cytokines, and leukotrienes that are meant to provoke inflammatory reactions to destroy pathogenic cells. These mediators are responsible for causing allergic signs such as skin or respiratory issues.
Allergies themselves are a hypersensitivity reaction, defined in Equine Internal Medicine, Third Edition, as “an altered state of immunoreactivity resulting in self-injury”—the body’s version of friendly fire.
In summary, under normal circumstances the immune system develops antibodies against pathogens following immunization or disease exposure. In allergic animals, however, repeated exposure to an otherwise harmless substance provokes antibody production against it. The allergen binds to the newly produced antibodies, causing mast cell degranulation and an inflammatory cascade.
In horses, allergies generally manifest in the skin as wheals, hives, crusts, pruritus (itching), and hair loss. In the respiratory tract allergy signs range from low-grade inflammatory airway disease in otherwise healthy horses to significant breathing difficulties in horses with recurrent airway obstruction (also known as RAO, or heaves).
Managing a horse’s allergies requires a two-pronged approach: minimizing the horse’s exposure to the allergen and changing the immune system’s response.
Stephen White, DVM, Dipl. ACVD, is a professor of medicine and epidemiology at the University of California, Davis, School of Veterinary Medicine, whose specialty is dermatology. Robert Judd, DVM, Dipl. ABVP, is an equine practitioner in Hewitt, Texas, with an interest in internal medicine and a consultant for the Veterinary Information Network, an online veterinary community. We’ve asked them to answer some common questions about managing equine allergies.
What are the most common triggers for equine allergies?
Both sources agree that flies and biting insects, including horseflies and mosquitoes, are to blame for most horses’ allergies.
“Probably the most common trigger worldwide is the Culicoides gnat (i.e., noseeums),” says White. “We know there are proteins in the saliva of these flies that horses become allergic to. This allergy is seasonal in many places and corresponds with warm weather.”
Environmental allergens, such as weeds, molds, and pollens, can trigger both contact and respiratory reactions. The challenge, sometimes, is determining what exactly the horse is reacting to.
“To differentiate environmental allergies from insect allergies, one has to first try to eliminate or control, as best as one can, the fly problem,” White says.
Judd says he commonly sees allergic inflammatory airway disease in young horses that have just started training and experience a change in environment from full-time turnout to stall confinement. These horses are allergic to pollen, dust in their stalls and hay, and the hay itself.
Food allergies, on the other hand, are quite rare. White says he’s only diagnosed four or five cases over the past 30 years.
“I think most of what people say is food allergy is the dust on the hay and in the grain,” Judd adds. “They’ll change hay, and the horse gets better. Was it the hay itself, or was it the dust? Often I’ll have them soak the hay, and if they’re feeding a grain that’s dusty, I’ll have them wet that down. Often with allergic airway disease that’ll make a tremendous -difference.”
With a true food allergy Judd says you generally only see cutaneous (skin) signs, such as hives or wheals. The only definitive way to determine the cause is with intradermal (skin) allergy testing.
What are some clinical signs of allergic disease in horses?
If the horse is having an allergic reaction to a topical or systemic drug, the most common reaction White says he sees is hives, but he’s also seen hair loss and crusting, among others.
“With fly allergies, whether to due to Culicoides or other species, we see lesions along the ventrum (belly), the dorsum (back), or the face. It seems to depend on the species of fly,” he says. “One of the problems is that you can have multiple species of flies in the same area; one horse in a pasture can be affected along the ventrum, one along the dorsum, and one affected in both areas.”
As far as respiratory allergic responses, says Judd, what starts as coughing and decreased performance can escalate to chronic airway changes and remodeling as horses age. “Those horses are still reactive; it’s the same syndrome,” he says. “Those horses turn into horses with heaves. With the chronic insult to airways, they get thickening of the bronchi (the passageways to each lung). With bronchospasm (where the bronchi constrict), they end up unable to compensate. They can’t dilate the bronchioles (smaller airways in the lung).”
How do you diagnose allergic conditions?
Historically, the gold standard for diagnosing contact allergies in horses has been intradermal testing, as the validity of blood (serum) tests in literature is conflicting. A blood test only measures what is going on at a certain point in time and is an immediate reaction. But White says UC Davis researchers have found that treatment (via hyposensitization, or allergy shots) for horses based on test results were equally successful regardless of whether the horses were determined to be allergic to those allergens via skin test, blood test, or both.
For respiratory allergies, veterinarians perform an endoscopy to reveal the issue. “A lot of times, the only symptom you really see in those horses is just poor performance,” Judd says. “You don’t see much of anything until you put an endoscope in them … and see large amounts of mucus in their airway. That’s the key (-indicator)—tremendous amounts of mucus in a horse that otherwise looks fine.”
The next step would be to perform a transtracheal wash, in which the veterinarian flushes sterile saline into the trachea, aspirates the fluid with tubing and a syringe, and evaluates it. A lack of significant pathogenic bacteria in the wash rules out infectious bronchitis or bronchopneumonia and points more toward an allergic cause, says Judd.
“Horses can, of course, have inflammatory airway disease but get a secondary bacterial infection,” he adds. “In those horses you treat the infection, but once you stop the antibiotics they still have problems,” indicating an allergy.
How do you treat equine allergies?
For skin allergies, White says hyposensitization—whether based on skin or serum testing—is about 70% effective. “That’s primarily looking at environmental allergens,” he says. “When you are talking about Culicoides allergies, the percentages in the literature are all over the place. Many horses don’t have allergy to just Culicoides.
“We think that horses, like other animal species, have a ‘summation of effect’ with allergies; they can have multiple allergies, and if you take care of one, the others may not be as important,” he adds.
Corticosteroids and antihistamines remain veterinarians’ treatments of choice. Extremely pruritic horses that have secondary bacterial infections might benefit from two to four weeks of antibiotics.
To reduce horses’ exposure to flies, equip them with fly sheets, stall them during evening hours with a fan, apply fly spray, and practice environmental fly control (e.g., reducing standing water areas where flies breed).
“The best thing we have used is a fly sheet, if you can keep it on the horse,” Judd says. “Fly sprays just don’t last long enough to do the trick. For all of these (skin) allergies, omega-3 fatty acids (considered natural anti-inflammatory nutrients) are effective in some cases, as are antihistamines such as hydroxyzine. Pentoxifylline (an anti-inflammatory designed to improve blood flow) has been used in some cases.”
He says his drug of choice is the corticosteroid prednisilone. He uses dexamethasone as a last resort because of its potential to induce laminitis.
For inhalant allergies, Judd recommends uses nondusty grain and wetting grain and hay as described earlier. “Depending on when the horse is allergic, especially if it’s in the winter,” says Judd, “I want to get it out of the stall and keep it out of the stall as much as possible, to decrease the amount of allergen exposure. With RAO horses I have had success using inhaled albuterol (a bronchodilator) and 30 minutes later using fluticasone (a corticosteroid). That’s also very effective in young horses with inflammatory airway disease. You don’t have all the side effects of systemic steroids when it’s given by inhaler. But this method is not inexpensive. We use human inhalers. But they do seem effective. I try to use inhalers (designed for horses) first if they don’t respond to hydroxyzine. If that doesn’t work, then I go to systemic steroids.”
Owners and veterinarians can work to manage allergies in horses by attempting to identify the allergen(s) and minimize exposure to those agents. When controlling exposure is impossible or insufficient, your veterinarian might prescribe a medication to help decrease the horse’s hypersensitivity response.
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