An allergy is a condition in which the body reacts adversely (locally or systemically) to a certain substance (allergen). Allergic reactions can be triggered in horses by many things–environmental allergens such as dust, pollen, and mold; insect bites; substances in feeds; things put on or touched by the horse; or injections. Reactions can be localized in the skin and appear as swelling and redness (sometimes itching) at the site of allergic contact, or show up as hives all over the body. More severe reactions might involve additional body systems such as the respiratory and circulatory systems; these conditions can become life-threatening unless reversed.
Sometimes a reaction's cause is fairly obvious–the horse has just received an injection or medication, a new type of bedding was put into his stall, or the owner started using a new fly spray that caused a skin reaction. At other times, it can be hard to pinpoint the triggering factor.
Causes and Types
Some horses are more sensitive to certain drugs, vaccines, pollens, etc., just as humans are.
Vaccination reactions are fairly common in horses. Some of these reactions are allergic and some are merely local irritation (heat and swelling at the injection site) that disappears within a day or two. New vaccines are extensively tested to make sure the vaccine is effective in preventing disease and to make sure horses don't react adversely to the antigen or carrier. Most vaccines are 99% safe, which means there is a small percentage of horses that could have a reaction, explains David Cross, DVM, PhD, clinical assistant professor at the University of Missouri. A horse with an allergic reaction to a vaccine today might have a more severe reaction the next time.
A vaccination reaction might appear as hives all over the body, or could become a serious situation in which the horse has difficulty breathing and goes into shock. If a horse gets hives when he receives a vaccine (and it's one you want to keep in your vaccination program), discuss this with your veterinarian; it might be possible to use a vaccine with a different carrier. If the horse is reacting to a vaccine that contains several products (immunizing against several diseases at once), you might try giving individual vaccines rather than the combination injection, says Cross.
Skin allergies are probably the most common type of allergic conditions in horses, and many of these hypersensitivity problems are caused by insect bites. The fly most commonly responsible is a small gnat (Culicoides), and the resultant irritation from bites is often called sweet itch.
Respiratory allergies are also 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/ moldy hay or bedding; the other type is most common in the southeastern U.S., affects horses on summer pasture, and is caused by pollens.
Food allergies have been diagnosed in horses, but they are rare.
Anaphylactic shock is the most severe type of allergic reaction. This can happen with any drug or vaccine to which the horse has become hypersensitive, or with 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 is also potential for reaction; you must watch closely and not administer it too swiftly, looking for signs of increased respiration rate, etc.," explains Cross.
Anaphylactic reactions are usually quite sudden and severe. Blood pressure drops dramatically, and the respiratory system is compromised due to internal swelling, causing the horse to have trouble breathing. He might collapse and die unless the condition is immediately reversed with medications such as epinephrine.
Skin and Blood Diagnostics
Todd C. Holbrook, DVM, Dipl. ACVIM, of Oklahoma State University, says diagnosing the cause of an allergic reaction can be difficult. Distribution of skin lesions might give clues during a clinical workup (such as where a horse has rubbed to relieve itching due to fly bites). Seasonality or response to any previous treatment help your vet figure out the cause.
A horse can develop contact dermatitis from his skin becoming sensitive to an ingredient after repeated use–such as ingredients in a certain fly spray or shampoo–or he might break out in a rash or hives after walking through nettles. "Sometimes when a horse has a hypersensitivity problem, it takes a lot of recordkeeping to figure out what might have caused it," says Holbrook. "The owner may try a process of elimination, or stop using certain feeds or products and add them back in, one by one, to see which one caused the reaction."
For a food allergy, diagnosis is typically made by exclusion. The hay is changed to a type that has not been previously fed, and all grains, additives, supplements, and treats are discontinued. Once the skin condition improves, the products are re-introduced one at a time to try and pinpoint the cause.
The veterinarian might take a skin biopsy to rule out other diseases. "Intradermal allergy testing is sometimes done, but is not foolproof, and test results need to be interpreted by the veterinarian in light of the history, clinical signs, and environment," says Holbrook.
For an intradermal skin test, the horse is sedated and a large area of skin is shaved (usually on the neck) so the results can be seen easily. Various antigens are injected into the skin on a grid pattern, with a key representing where each antigen is located. The horse is kept overnight to be monitored, and the veterinarian reads the reaction to see which caused a response.
Holbrook explains that many horses respond to various skin injection allergens. "The veterinarian has to be careful in the interpretation of test results," he says. "Some types of weeds, plants, or insects vary geographically. We are limited, as clinicians, to the commercial antigen products available. This factor is one of the limitations of skin testing.
"But skin testing can be useful in some cases to determine allergen-specific treatment protocols, or to define allergens the horse needs to avoid," he continues. "I've used it with a number of horses. It doesn't always work, but sometimes it does. The skin testing and allergy-specific treatment is often a last resort in a clinical practice."
Hilary A. Jackson, BVMS, DVM, MRCVS, Dipl. ACVD, is a veterinary dermatologist at North Carolina State University who sees a lot of seasonal skin allergies. "In the southern states, we see insect allergies and hives and may do skin tests to determine the cause," she says. "If we suspect a food allergy, however, we don't do the test; food allergens are not reliable in horses. We use the skin test for environmental allergies. Once we have the results, we may formulate an allergy vaccine for that particular horse, and put the horse on allergy shots, similar to what people get."
There are also some serum tests (blood tests) marketed as diagnostic tests for allergic conditions. Holbrook says your veterinarian might try a skin test due to conflicting information in the current literature regarding the validity of serum testing for allergic disease diagnosis in horses.
Christine Rees, DVM, Dipl. ACVD, a veterinary dermatologist at Texas A&M University (TAMU), says most veterinarians do not have the training or qualifications to perform intradermal allergy tests. "This is the reason the tests are performed most commonly by a board-certified veterinary dermatologist," she says.
"The cost to perform the skin test will vary with the region and the number of allergens tested," says Rees. "The cost of a skin test here at TAMU is around $250. This is for the test only and does not include any office call expenses or other diagnostic tests that may be required in working up an allergic horse.
"Some veterinarians perform serum allergy testing (using blood samples), but there is some debate as to whether what goes on in the skin is the same as what goes on in the blood," she says. "The other problem is that the blood test measures what is going on at a certain point in time and is an immediate reaction. With the skin testing, you evaluate the injection sites for multiple time periods. The skin test is read at 15 minutes, 30 minutes, four to six hours, and 24 hours. Some horses have an immediate reaction only, others have a delayed reaction only, and some have both immediate and delayed reactions. The skin test is thought to be more sensitive than a blood test."
Even the skin tests are looked upon with skepticism by some veterinarians. According to Philip J. Johnson, MRCVS, BVSc, MS, Dipl. ACVIM, professor and instructional leader of equine medicine and surgery at the University of Missouri, there is no credible science behind the interpretations of either intradermal or serum tests, since most horses are reactive to a multitude of different stimuli.
The first line of treatment in most allergic conditions is to eliminate the cause or avoid contact with the allergen.
The best treatment for insect sensitivities is prevention. If the horse is hypersensitive to insect bites, diligent use of a fly spray might give relief. The gnats that cause so many reactions usually attack horses at dawn and dusk. The owner might be able to put the horse in a stall with small mesh screening or use a fan in the stall to keep the air moving. This tends to discourage gnats because they are not strong fliers.
"Sometimes antihistamines are used in treatment of seasonal allergies," says Holbrook. "If we can start the horse on antihistamines early in the season, before the reactions (to insects, pollens, etc.) get so bad, this will be more effective than if given after the horse already has a problem. Once the reaction starts, we usually have to use a corticosteroid to reverse it and get it under control. I've had horses of my own with seasonal skin allergies, and you can predict when it will start."
In his experience, horses with seasonal hypersensitivities tend to get worse with age, and it helps to try to prevent problems early in the season.
Typical antihistamines used for humans are usually not very effective for horses, but an antihistamine-type drug called hydroxyzine sometimes works fairly well, according to Richard A. Mansmann, VMD, PhD, director of the equine health program at North Carolina State University.
The two main treatments for any allergy are avoidance and corticosteroids, says Mansmann. "The first goal is figuring out what's causing it and avoiding it," he says. "Usually the veterinarian is called when the horse is at its worst. That's when you need the most medication and might get the most potential side effects from it. If the skin problem is seasonal (as from fly bites) and begins in late April, the owner should start the cortisone treatment the second week in April–before the horse has a problem–and can keep the horse on a low level of dexamethasone all through the season and the problem won't get really bad. I've had horses on 4 mg of dexamethasone every other day, controlling what in earlier years had been significant skin allergies. The same holds true for seasonal respiratory disease like summer pasture heaves. Start on it before it becomes advanced. Then you can use the lower levels of dexamethasone for a longer period of time."
Sudden development of hives is quite common in horses. "I've seen the most dramatic multiple cases at horse shows," says Holbrook. "A number of horses may develop hives in a certain barn, for instance. In that circumstance (where it's more serious, cosmetically, for the owner), I tend to treat the condition more aggressively–with a small dose of steroids to reduce the swellings–so the owner can show the horse. At home, however, I might tell a client to just watch the horse and see if it worsens or goes away. Bathing the horse with a mild shampoo may help if hives were caused by contact with an allergen."
Treatment for heaves (allergic respiratory disease) involves avoiding the allergen, if possible. Horses in stalls should be kept outdoors; pasture (rather than a dusty dry lot) is the best environment for the horse. Many cannot be fed alfalfa hay because it has a higher level of mold dust than grass hay. Any hay fed should be carefully selected (clean and dust-free to begin with) and dampened or soaked in water to eliminate dust particles.
Cross recommends feeding hay on the ground and taking away any dampened hay that is not eaten right away so it does not begin to mold. "When I was in practice in Michigan, I dealt with many cases of heaves," says Cross. "People feeding round bales often had horses with problems, even if the horses were outside, because they are burying their heads in the hay bale to eat."
If the horse can't be on pasture, he might do best on a pelleted feed (no dust); there are some feeds formulated especially for horses with heaves. In severe cases, the veterinarian might use corticosteroids (systemic or inhaled) or other products (such as Ventipulmin) to reduce inflammation and open up the airways. One advantage of using inhaled medication is the rapid response, says Holbrook.
Johnson says the most important factor in heaves is "strict avoidance of the causative inhaled particles, called aeroirritants. If avoidance is impractical or ineffective, then we advocate using drugs."
Cross says that dexamethasone and bronchodilator drugs are helpful. "A mild case will generally respond to dexamethasone to ease the swelling that's constricting the airways," says Cross. "Horses that have a hard time breathing, however, may need bronchodilators such as the Ventipulmin syrup."
Treatment for heaves is only temporary if the horse is kept in a dusty environment or fed dusty hay. "I've had to euthanize horses when it's a long-term problem and they get to the point where they are struggling for breath," says Cross.
Pasture-associated respiratory problems can be hard to manage, although some horses respond when removed from the pasture and given anti-inflammatory drugs.
Treatment for anaphylactic reactions consists of immediate administration of epinephrine (adrenalin) and a steroid such as dexamethasone to try to halt inflammation and swelling and keep the horse from going into shock. Epinephrine stimulates the body in general–causing restriction of blood vessels (raising the blood pressure) and helps open the airways.
Some conditions might be mistaken for allergic reactions in horses. They include:
Sunburn–Horses with light skin or white areas might sunburn, causing redness, swelling, blistering, and peeling. Some sunscreen products for humans can create problems for horses because they contain para-aminobenzoic acid (PABA), which in horses can cause a reaction to UV light rays, resulting in dermatitis.
Photosensitization–Inflammation, redness, and painful skin can be caused by horses eating certain pasture plants that contain photodynamic agents. These are absorbed from the gut into the bloodstream and travel to the skin, where they react with the ultraviolet rays of sunlight, creating a much worse condition than simple sunburn.
Chemical changes that take place damage and kill skin cells, causing skin to slough off. In early stages, the inflammation might be mistaken for an allergic reaction, but usually only unpigmented areas of skin are affected (white blaze on the face, white socks, etc.) because dark skin has more protection against UV rays. Horses that develop liver disease might get photosensitization even if they don't eat incriminating plants because the liver is no longer filtering out normal chlorophyll products of plants; these build up in the bloodstream and are then deposited in the skin, where they react with sunlight.
Lungworms–Mansmann says some chronic respiratory problems that are non-responsive to treatment will respond to ivermectin. "It's safe, inexpensive, and may save a tremendous amount of testing," he says. "In my history-taking, I always ask about the deworming program. If the horse has not been dewormed recently with ivermectin, it may help. Lungworms can produce signs that look just like heaves. If you do a tracheal wash, you'll find some allergic cells and might assume it's heaves. Lungworms are more common in donkeys, but even without a history of donkeys on the farm, lungworms sometimes show up."
Procaine penicillin reaction–Procaine penicillin has a reputation as an injectable drug that can cause a reaction. Some horses are allergic to penicillin (just as some people are) and go into anaphylactic shock after an injection. But even more common is a reaction to the procaine carrier if it gets into the bloodstream. "This type of reaction is immediate. Horses usually show signs of excitement and may fall down and thrash," says Todd C. Holbrook, DVM, Dipl. ACVIM, of Oklahoma State University. "Usually this is a temporary situation, and by the time the owner gets on the phone and calls the vet, the horse is better." The horse is not actually allergic to penicillin, and future shots can be safely given to the horse–with no problems–as long as the procaine does not get into an artery.
Injection reactions–The most common vaccine reactions are local irritant reactions, rather than allergic reactions, says Mansmann. If you split the dose and put it in different locations for an intra-muscular injection, this may reduce the resultant swelling. Or you can give half a dose one day and half the next, he says.
While many horses have suffered from an occasional allergic reaction, most will recover on their own without further problems. However, horse owners should understand that repeated exposure to the causative agent can increase the horse's risk of having a serious reaction. Keeping the horse comfortable is important; keeping him alive is vital. Discuss any questions about your horse and allergic reactions with your veterinarian.
ALLERGY EVALUATION: Pulmonary Tests
Sarah Gardner, DVM, PhD, of the Department of Clinical Sciences at North Carolina State University, uses a non-invasive pulmonary function test to evaluate hyperreactive lungs in horses. "This equipment allows us to determine if horses have airway hyperreactivity, which can be a sign of recurrent airway obstruction (heaves) or inflammatory airway disease," she says. "The test can aid in diagnosis and also enables us to follow these horses and look at their response to therapy and environmental management to see if they are improving.
"For this testing, the horse is mildly sedated, and we attach a face mask over the nostrils," Gardner explains. "Then we perform the test, based on principles of forced oscillatory mechanics. The horse's airway reactivity is tested at rest, and if it's normal, the horse is given a histamine challenge test to determine if he has airway hyperreactivity." This gives a clue as to how that horse's lungs would react if he came in contact with an allergen in the air.
"If the test shows a horse is abnormal to begin with (heaves), then instead of doing a histamine challenge test (because the horse is already showing signs), we do a bronchodilator challenge. This helps us evaluate the reversibility of the airway obstruction and gives us an indication about how the horse might respond to that bronchodilator therapeutically."
The testing system she uses for evaluating pulmonary function is one of only two in the United States (the other is at Tufts College of Veterinary Medicine in Massachusetts). There is also one in Canada, at the University of Guelph, says Gardner. This type of test might have future potential in evaluating horses during a pre-purchase exam.
"With inflammatory airway disease, horses may only have signs of exercise intolerance and/or cough," she says. "Someone buying a high-level performance horse might want to have a pulmonary function test done on that horse."–Heather Smith Thomas
ALLERGY SHOTS FOR HORSES
Hilary A. Jackson, BVMS, DVM, MRCVS, Dipl. ACVD, a veterinary dermatologist at North Carolina State University, says a skin test often is used to pursue the possibility of allergy shots for a horse. "This is a reasonably safe form of treatment and can be continued throughout the horse's life," she says. "The treatment is usually divided into an induction phase and maintenance phase. During the induction phase, you give the horse small amounts of allergen and slowly increase it. Protocols vary, but we do this over a three-month period. Once you reach the full allergen dose, you give the shot every two weeks."
You can give shots just in summer if the horse is only affected during that time of year. In the South, however, winter is so short that horses are generally given the shots year-round, says Jackson. If you skip the shots during winter, the horse might have to go through a gradual induction phase again.
Costs vary, depending on where it's done. "Here at N.C. State, we generally charge $400 for the skin testing–to sedate the horse, do the tests, and keep the horse in overnight," she says. "For the shots, it's about $40 per month."
Allergy shots often are used with show horses to avoid treatment with steroids, which are prohibited under many jurisdictions.
"There is some debate as to how helpful shots are for horses with insect allergies, but in many cases the horse is so severely affected that we'll try it," she says. "My clinical impression is that it does help them, but there have not been any really good studies that have looked at this. The types of allergies most apt to be treated with shots are pollen allergies."
Her advice to a horse owner dealing with a horse's skin problem is to seek a specialist. "There are a number of board certified veterinary dermatologists throughout the U.S.," she says. "I usually direct horse owners to our American College of Veterinary Dermatology Web site (www.acvd.org) with a link showing where dermatologists are located. Not all of them do horse work, but they can probably recommend someone in your area who could help."–Heather Smith Thomas
ALLERGY RESEARCH: Improving Blood Tests
Bruce Hammerberg, DVM, PhD, a professor at North Carolina State University College of Veterinary Medicine, is an immunologist who is working on developing monoclonal antibodies specific for horse IgE antibodies to make blood tests more accurate. Blood testing for allergies in horses is still in its infancy. "In humans and dogs, measuring a particular type of antibody called IgE can be helpful," says Hammerberg. "IgE is a unique antibody common to all mammals and has a special role in physiology. It is associated with mast cells, which contain histamine and other inflammatory molecules. The mast cells can release these under certain circumstances. One of these circumstances is when they have acquired IgE on their surfaces. That particular antibody is specific for a particular allergen, and may also appear in the bloodstream at some level because of previous exposure.
"The IgE antibody sensitizes the mast cells to degranulate or release inflammatory mediators when they encounter an allergen–a pollen, mold, or whatever," he explains. "The allergy can start this way and be perpetuated by other inflammatory cells. IgE seems to play a very important role in that process if it develops a specificity for certain allergens. The blood may then have these antibodies, though there may be some animals that have very low levels of IgE for a particular allergen they are very sensitive to.
"This may just mean that the antibody does not accumulate in circulation, but rather in certain tissues that are reactive, like lungs, intestinal tract, or skin," Hammerberg continues. "The reaction might be associated with the important mast cells and not be in the bloodstream at high levels. If it's in the blood, this may mean you'd be sensitive to it, but not always. It's a complex system.
"Blood testing is a diagnostic aid to know what level of IgE for particular allergens is in the blood," he says. "It's a clue, but not the final story. Skin testing is the best approximation to try to pinpoint an allergy, but it's not an exact test, either, and it is very laborious. It would be nice to have a more accurate serum test, where all you had to do was take some blood and send it off to a laboratory, and have it more closely aligning with what the skin test results might be. This is what we are working on."
He is working on this with Hilary A. Jackson, BVMS, DVM, MRCVS, Dipl. ACVD, a veterinary dermatologist at North Carolina State University. "She does the skin testing and clinical evaluation and determines if the horse is clinically allergic," says Hammerberg. "Then we try to match that and see whether the assay system reflects the same thing.
"Then if you have a serological test for IgE that you have confidence in–a high level of IgE against a particular allegen in hay, for instance–you can either confirm that with a skin test or act on it to see if removing that allergen will make it better for the animal," he says.
"The test we use is an immunological assay system called an ELISA (enzyme linked immunosorbent assay) to measure antibody level," says Hammerberg. "We use a plastic surface plate with many little depressions in it. The plastic is polystyrene, which is very sticky for proteins. If we are using a protein from pollen or mold or some other substance we want to test for, we put it in solution on the plate overnight, and it adheres to the plate. We wash off the excess, then put the serum we've collected from the animal into those depressions that are now coated with the allergen. We let that incubate for a while, and if there are specific antibodies that recognize that allergen, they will bind to the protein that's stuck on the plate.
"What we do here is try to make another reagent, specific to recognize just the IgE of all the antibodies that might be seeing these proteins–to bind to the IgE antibodies that will now be binding to the protein stuck on the plate," he explains. "The reagent we have that recognizes the IgE (if it's there) will now be also bound to the plate by this chain of molecules. We've put a marker on this thing that recognizes the IgE. That marker is an enzyme which when given the proper substrate for its enzyme action will turn that substrate a color that we can detect. The intensity of the color tells us how much IgE (bound to the originally stuck protein) is on the plate. That's how we can read the test.
"The key to it is that what you use to see the horse IgE must be specific for that IgE," he says. "It cannot react with other antibodies that might also be recognizing that protein that's stuck to the plate. That's where most commercial blood test processes have problems. They may not be specific for the IgE molecule; they may cross react with other antibodies, which would confuse the test."
If this research is successful, blood testing for allergies will be a little more reliable. With some types of contact dermatitis, it can be hard to know what allergen caused it; a simple blood test to narrow it down could be a lot of help.
"We need more cases to work with," says Hammerberg. "A validated, clinically described case of allergic disease in horses (where there is serum available from that horse to work with) is not as available in the horse as in the dog. Many people have horses with allergies, but actually having a skin test to show what the horse is allergic to is not something that's done as frequently as in dogs. That's why I work with Dr. Jackson here at N.C. State, because this is her field and specialty. We need some help in finding more allergy cases that have been very rigorously worked up and are hoping more people will come forward with their horses to help."
Anyone with a horse that has been dermatologically tested can contact Hammerberg at 919/513-6226; Bruce_Hammerburg@ncsu.edu. –Heather Smith Thomas
Inflammation: Sign of Allergic Reaction
David Cross, DVM, PhD, clinical assistant professor at the University of Missouri, says one of the main signs of hypersensitivity is acute inflammation. "When an animal is exposed to an allergen, the immune system responds and there are different types of cells that play a role–B cells, mast cells, immunoglobulins like IgE, etc.," he says. "The system becomes sensitized to that particular allergen after exposure."
It's usually the second or third exposure that creates a severe reaction in the horse's body. Once the system has had a chance to react to this, it seems to have a memory and will react even more quickly and severely the next time the antigen is encountered. A second exposure sets off a cascade of events, says Cross.
"Cytokines are released by different cell types that trigger this response," he explains. "Swelling and edema can be external or internal. In a case of heaves, the swelling creates airway constriction. A contact dermatitis may create a local reaction in the skin, whereas an anaphylactic reaction to an injection may put the horse into shock due to organ system collapse.
"The inflammation may appear in different ways," Cross continues. "In a general inflammatory response, there will be a lot of inflammatory mediators released from cells in the body. There are a variety of chemicals involved, including prostaglandins, leukotrienes, histamine, and a long list of things that are normal inflammatory mediators. But in an allergic response, there are more of them released and they cause more problems. We don't always know why some horses are affected this way and others are not."–Heather Smith Thomas