Just as in humans, allergic reactions in horses can be difficult to diagnose and treat and can make everyday living difficult. Unlike his human counterpart, however, a horse can’t exactly tell you all of his symptoms, so often the horseman has to do his best investigative work to discover the source of the problem.

These pesky allergies are triggered by environmental allergens such as dust, pollen and molds, or by insect bites, allergens in feeds, something put on the horse or touched by the horse, or injections. Reactions may be localized or systemic. In other words, there’s a lot out there that can cause a horse’s immune system to go haywire.

A local reaction in the skin may appear as swelling and redness (and sometimes itching) at the site of contact, or a horse can develop hives all over his body. More severe reactions involve additional body systems such as the respiratory and circulatory systems, and a severe reaction may become life-threatening unless reversed.

Sometimes the cause of a reaction is obvious—if the horse has just been given an injection or medication, a new kind of bedding, or is sensitive to mold dust and was fed dusty hay. Or, it can be as simple as a new fly spray or shampoo that caused a skin reaction. Other times, however, it can be difficult to pinpoint the triggering factor.

Some horses are sensitive to certain drugs, vaccines or pollens, just as some humans are. And like human reactions, allergies tend to be inherited. Certain family lines of horses are more prone to heaves, for instance.

Under The Skin—Skin allergies are relatively common in horses. Many hypersensitivity problems are caused by insect bites, particularly small gnats (Culicoides), and the irritation from these bites is often called sweet itch or Queensland itch.

Such an allergic reaction in the skin may sometimes cause hives or small water blisters. “These may erupt and have the potential for scarring. At a barn where I worked, a horse was brought in that had white spots from an allergic skin condition that resulted in scarring all over his body. People thought he was an Appaloosa, but he was a Quarter Horse with no Appaloosa blood in his background,” noted David Cross, DVM, PhD, of the University of Missouri.

Catch A Breath—Respiratory allergies are common, especially in horses subjected to environmental dusts and pollens. Allergic respiratory disease or heaves (also called recurrent airway obstruction) is classified into two main types. One type is caused by dusty or moldy hay or bedding; the other type affects horses on summer pasture and is caused by certain pollens in the pasture.

Laurent Viel, DVM, PhD, from the University of Guelph (Ont.) said the first type of heaves, called chronic obstructive pulmonary disease or recurrent airway obstruction, is a disease of stabled horses, those usually 7 years of age or older.

Heaves is a disease that progresses with age. Hunters and jumpers, who are regularly stabled and reach peak performance at age 12 to 14, may have a mild form at that time. But as they get older, more of them will progress to serious heaves, said Viel.

“In northern regions with a winter climate, a lot of training takes place indoors, and we tend to keep our horses in stables where it’s relatively warm. By enclosing these animals, we’re exposing them to a higher level of dust in their environment,” he said.

Even though the barn might have ventilation, there’s always more dust (from hay and bedding materials) trapped in an enclosure than the horse would be breathing outdoors.

Summer pasture heaves, by contrast, affects horses when they’re outside and is due to mold or pollens in certain pastures. Most common in Southern states, it’s also appeared in dry prairies and wetter Eastern regions.

If horses are already sensitized to molds, they’re more apt to be sensitive to other irritants. Pollen can trigger an episode of heaves in an already sensitized horse, or exacerbate an existing respiratory condition, noted Viel.

“As an example, we have problems here in Canada when horses are adjacent to fields of corn or alfalfa. If the horse already has heaves [but is doing very well outside], he may get worse when a corn crop reaches peak bloom stage in July,” said Viel.

Food For Thought—Feed allergies have been diagnosed in horses, but are rare. Skin reactions to a food allergy may be seasonal, depending on the cause.

A Real Shock—Anaphylactic shock is the most severe type of allergic reaction.

This situation can occur with any drug or vaccine that the horse has become hypersensitive to. In addition, it’s caused by blood transfusions, especially if a horse is given a second or third transfusion from the same donor horse or has been vaccinated with a blood-based vaccine.

When plasma is given to foals, there’s also potential for reaction.

Anaphylactic reactions are usually quite sudden and severe. Blood pressure drops dramatically, and the respiratory system is compromised due to internal swelling. The horse has trouble breathing due to narrowed airways. He may collapse and die unless the condition is immediately reversed with appropriate treatment.

Taking A Shot—Vaccination reactions are fairly common in horses. Many horses that develop an adverse reaction to a vaccination are reacting to the carrier in a vaccine (the fluid the antigen is placed into) rather than the antigen itself.

An allergic reaction to a vaccine may be local or systemic.

New vaccines are tested to discover if they’re effective in preventing disease and to make sure horses don’t react adversely to the carrier as well as the antigen. Most vaccines are 99 percent safe, which means there’s still a small percentage of horses that may react, explained Cross.

A horse that’s had an allergic reaction to a certain vaccine in the past may have a more severe reaction the next time. A second or third exposure to a foreign protein can produce a serious reaction.

Always discuss any vaccination reaction with your veterinarian.

A reaction may appear as hives over the body, or may quickly become a serious situation if the horse has difficulty breathing. He may go into shock at that point.

If a horse develops hives each time he receives a particular vaccine (and it’s one you don’t want to omit from your vaccination program), call this to your veterinarian’s attention. You might be able to change to another vaccine with a different carrier. “For instance, there’s more than one West Nile vaccine. If a horse had a reaction to one, you might try the other,” said Cross.

If a horse reacts to a vaccine containing several products (immunizing against several diseases at once), the next time you should try individual vaccines rather than the combination injection.