Equine Allergies - Causes and Treatment

From the January 2014 issue of The Stable Magazine – www.thestablemagazine.com/january 2014

In simple terms, an allergy is when the body’s immune system over-reacts to an allergen (the substance in the environment that sets the allergy off - or what the horse is allergic to). 

Allergies in horses can be caused by many things, including medications, feeds, grooming products, certain chemicals, pollens, skin contact with plants, and insect bites. Some allergies can be so slight that they don’t really bother your horse at all - and some reactions can be so severe that the horse may have trouble breathing. Some allergic reactions can cause excessive swelling local to the initial reaction site, or all over the body - and consequently, your horse may be very uncomfortable.

An allergic reaction can be noticed by the following symptoms:

• Lumps appearing on the neck and/or body of the horse
• Swelling - localised around a site of irritation, eg) insect bite
• Swelling - around the face
• Skin abrasions, swelling or lesions caused by rubbing irritated areas
• Wheezing, excessive coughing and sneezing
• Excessive fluid from the nostrils due to irritation of the airways

HYPERSENSITIVITY

Just as in humans, allergic reactions are caused by an over-reaction of the immune system when what the body believes an ‘antigen’ is detected. An antigen (a disease carrying protein) is usually neutralised after the horse’s immune system reacts to the antigen’s presence and accordingly, mounts a defense against the perceived threat. It is normal for the body to release chemicals to assist in neutralising antigens, and these chemicals induce the symptoms that we see - which are actually designed to rid the body of the offending antigen in the first place. One of these chemicals (known for being a factor in allergic reactions!) is histamine - hence why we use anti-histamines to help to control allergic reactions. Allergic reactions are basically an over-reaction where hypersensitivity to the allergen causes the immune system to go overboard, and often, problems result from the reaction, and not from the allergen.

Skin allergies are also common - including lumps, bumps, hives, irritations or a rash - localised or otherwise. They can be caused  by a myriad of allergens, and are often seen seasonally - for example when insect activity is increased (in the case of Queensland Itch and other insect related allergies. Hives are small or large bumps that appear quite rapidly on the horse’s skin. They may be localised to a certain area, or appear all over the horse. Often they will appear in clusters as raised, flat bumps. They can dissipate as quickly as they appear, and in most cases, no further treatment is required. If your horse frequently suffers hives, investigation into the cause to determine the allergen should be undertaken - which is not always an easy task, as you’ll find out later on in this article..! Horses with hives should be monitored to ensure that the reaction is only superficial, as hives may be a pre-cursor or early warning sign to a more severe reaction.

As in humans, horses are very much individuals when it comes to allergies - some horses are more sensitive to allergens than others - and what causes a large reaction in one horse may not even bother the horse in the stable next door!

While an allergy itself may not be life threatening (although in some cases, they can be!) even a mild reaction can offset a secondary infection, as is possible with respiratory allergies, or in the case of a horse becoming so itchy and uncomfortable that he causes himself injury in an attempt to relieve the itch.

So, let’s take a look at some of the symptoms of allergic reactions in horses - how they present and the treatment that may be required...

BUMPS ON THE SKIN

If the bumps are small, firm and itchy, they are most likely insect bites, in which case you may be able to see a ‘head’ on the bite. Bumps are not a serious problem unless there has been an allergic reaction to them, or unless there are many of them. Apply antiseptic cream and monitor the bites. They should go down within a few days. If there is a severe allergic reaction to the bites, contact your vet. If the bumps are large, flat and puffy, they are most likely hives - for which the most common cause is a food allergy. The lumps may be soft or hard, and may or may not be itchy. See below for recommendation for horses suffering a feed allergy.

SWELLING IN THE HIND LEGS

This can be caused by a reaction to a diet containing too much protein, or from a severe reaction to a needle. If there are no other symptoms, check the horse’s diet. A reaction to an injection or vaccination will mean a severe reaction, (so not limited to just swelling in the legs!), and will require veterinary attention. Swelling in the hind legs can also occur when horses are immobile for a period of time - so this symptom is not always indicative of an allergic reaction.

LOCALISED SWELLING

Swellings can be caused by insect bites or reactions to injections or vaccinations. Swellings also occur when a horse has knocked himself being over-exuberant in the  paddock - so don’t jump to conclusions before checking out the severity of the swelling. For severe swellings, abnormally large (over the size of your palm) or inexplicable, obviously painful swellings, call your vet. Small swellings due to minor vaccination reactions, or bumps should subside by themselves, and are not usually cause for concern unless your horse is exhibiting other signs of ill health, such as an elevated temperature, or if the injection site is showing signs of infection. If the size of the swelling increases, or if you are at all concerned, call your vet for advice.

ITCHY SKIN

Itchy skin can be due to a number of allergic reactions, particularly from insect bites or contact with certain types of plants. Depending on whether the skin is broken, cooling the area may help to soothe the itch. If the skin is broken or the horse is constantly rubbing, placing a weather appropriate rug on to cover his skin may help to ease the itch. Be sure to check and treat sores regularly to ensure they are healing and that they are not infected.

WHEEZING, COUGHING & SNEEZING

Respiratory irritations can be due to dust in your horse’s feed, an adverse reaction to moulds or from airborne allergens such as pollen. When feeding your horse, dust can be minimised by adding a small amount of water to your horse’s feed so that the feed is slightly damp before feeding. Hay can be kept in a moist hessian bag for a couple of hours before it is fed. Chaff and pre mixed feeds must be fed straight away or the feed will go bad once it has been dampened and left. If you are dampening your horses’s meals, make sure you do it a short time before the meal is to be fed.

If the horse is suffering from an allergic reaction to airborne allergens, try to eliminate the source of the irritation. If you think it is mould or dust, and the horse is stabled - give the stable a clean up and ensure that it is as well ventilated as possible. If you think pollens are to blame, perhaps relocating the horse to another paddock, or stabling him on days where the pollen count is high might be a way to reduce his irritation.

ADVERSE REACTIONS

Allergic reactions to injections can be very, very serious. Sometimes a small swelling may occur at the site of the injection, but that is generally due to the needle not being sterilise before the injection is given - either the site has not been swabbed and contaminants have been able to enter the injection site, or the needle was not sterile.  This is not an allergic reaction. A small lump should subside by itself. If the lump looks nasty or infected, call your vet.

True allergic reactions to injections and other medications are rare, and typically occur soon after the injection or medication has been administered. Anaphylaxis can occur if the horse is truly allergic to the drug, in which case immediate veterinary treatment is required. If your horse has recently been given any medication and symptoms escalate rather than improve - or if you’re concerned, always ask your vet for advice.

CONTACT DERMATITIS

If your horse suffers an allergic reaction that is isolated to one area of the body, it may be a clue as to the allergen at fault. Horses can suffer reactions to almost any topical treatment (including grooming products, fly sprays, topical medications, etc.) Some horses may suffer reactions from an ingredient in your leather conditioner or from the dyes in your saddlecloth! Horses have been known to be allergic to wool - or more specifically, the lanolin in wool, which has caused localised irritation where woolen rugs have come into contact with the skin. In these cases, discontinue the use of the equipment that causes the irritation.

Plant allergies can also appear as isolated reactions - if the horse has come in contact with a particular weed or plant that he is allergic to, this can present as lesions on the skin.

The best prevention is to try to determine the cause of the problem. Remove weeds from your paddocks, look for natural products, or products made for horses with sensitive skin - and in the case of topical treatments, always do a test-patch and leave it for 24 hours before applying new sprays or lotions.

INSECT BITES

An allergic reaction to the bites of midges is common - also known as Queensland Itch. The allergic reaction is actually to the saliva of the insect, and causes the horse great discomfort - often until they scratch out the tops of their tales and huge areas of their coat. This can lead to secondary infections, open sores, and one miserable horse. Prevention and treatment are required in these instances. For more on Queensland Itch, check out the September issue of The Stable Magazine: http://www.thestablemagazine.com/september2013/#/12/

FEED ALLERGIES

If you think your horse is suffering from a feed allergy:

Check the horse’s diet and remove any feeds that have recently been added. Check your horse’s paddock for any new plants. Sometimes fresh, new season feed can cause a reaction, in which case introduce the new grain slowly (as you should when making any dietary changes). If the symptoms persist after removing the allergen, or if you can’t figure out what caused the reaction, call your vet.

ESTABLISHING A CAUSE

Getting to the root of the problem can be quite a tricky process. There are sensitivity tests that may be undertaken, similar to those humans undergo. The problem is, that horses may show an allergic reaction to most substances they are tested for - it’s just a matter of how sensitive they are to each allergen. To attempt to determine the cause of the allergy, you must thoroughly examine your horse’s diet and environment. Have you made any changes to your horse’s feed? Did he move from his old paddock to a new one recently? Even small things, like a changing from your normal brand of fly repellent or horse shampoo to a different one can trigger a reaction in certain horses. Try to eliminate possible causes and establish what has changed that may have brought about the reaction.

Like humans, some horses will be more sensitive than others to different allergens, so don’t always assume that because another horse didn’t react to something that it’s not the cause of your horse’s allergy. If your horse has a serious reaction - ALWAYS call your vet for swift treatment and good advice.

Thank you to The Stable Magazine for this article, which was originally published in their January 2014 issue. Check out The Stable Magazine online now for FREE. Read this article and many more at www.thestablemagazine.com