The ABC of allergies
Dr Michelle Logan looks into the seasonal problem of allergies in our equine friends IMAGES BY DUSTY PERIN
The prevalence of allergic symptoms in horses is on the rise; we look at some of the reasons behind this, and how to take care of the sufferers
Allergies are a challenge; they result in a wide range of problems and seem to be becoming extremely common. Do you know of a horse who coughs when stabled and has to have hay dampened down? Or one who comes out in strange lumps out in the paddock? Or even one who is constantly rubbing himself in the spring? Then you very likely know of a horse with an allergy problem.
What actually is an allergy?
Just like in people with hay fever or those who react strongly to insect bites, an allergic horse is one where the affected part of the body is making an inflammatory response to fight off something from outside the body which is classed as an ‘invader’ (or what we call an antigen).
The invader in this case could be pollen, insect saliva, a fungal spore, dust mite and so on. In the case of an allergic reaction, this inflammatory response has gone into overdrive; it is way over the top to what is needed to protect the body. We call this being ‘hypersensitive’.
To show a true allergic reaction, the horse has to have been exposed to the invader before, and then certain cells are programmed to respond in this exaggerated way when they get reexposed. The reaction will become stronger as the horse is exposed to the invader again. So, although it may seem as if your horse has suddenly developed an allergy, in fact it was already predisposed and just hadn’t been exposed to the trigger enough times to set off a big enough inflammatory response to show the allergic signs.
What are the different allergic reactions we see in horses?
ANAPHYLACTIC REACTION This is the most dramatic and fortunately very rare. It is the strongest allergic response and the horse can go into anaphylactic shock and can die. In people we see it more commonly with allergies to peanuts or bee stings. In horses we sometimes see it as a rare drug reaction; the airways swell and the heart can stop unless treatment can rapidly be given.
Much more commonly we see milder either respiratory or skin signs. RESPIRATORY ALLERGY SIGNS Allergic respiratory signs can look the same as other respiratory problems with coughing, runny nose and poor performance. The allergic reaction is to particles in the air that is breathed in; the particles causing the allergy can be fungal spores and dust particles, amongst others.
The allergic reaction results in the airways becoming narrower (due to the airways constricting plus extra mucous and inflammatory cells being produced in the lungs). The airways also become very reactive (hypersensitive) so any further triggers will cause more mucous, more inflammatory cells and the airways will constrict further, so less and less oxygen can be taken in. This is why we see coughing, nasal discharge and why the horse isn’t able to perform as he should – he can’t get as much oxygen into the bloodstream to reach the muscles of the legs and heart.
Recurrent airway obstruction (RAO), chronic obstructive pulmonary disease (COPD), heaves and equine asthma are all names for the allergic airway disease we see in horses. The condition usually starts in mature horses and is very common in countries where horses are stabled for a lot of the time (and so are more often exposed to dust and fungal spores found in hay), although there is a pasture-associated condition which likely involves an allergy to pollens. SKIN ALLERGY SIGNS Another place we commonly see signs of allergy is the skin. This can be due to external factors eg. pollen, insect bites and shampoos, or can be an allergic reaction to something eaten. One well known skin allergy is sweet itch; this is really a hypersensitivity to a particular type of midge (Cullicoides) which at present we don’t have in New Zealand, fortunately. We do, however, see very similar insect
hypersensitivity signs caused by other types of flying insects.
The reaction can cause intense itchiness; I have seen horses so desperate to rub themselves on fences that they have ended up injuring themselves. You usually see small lumps over the skin containing a little scab. The horse rubbing itself can then cause other signs.
Hives are another type of allergic response; these are the strange swellings that come up rapidly on the body, like a very large nettle rash. Finding the cause for this can be difficult – it may even be related to something eaten – but they usually go away fairly quickly too.
How do we diagnose an allergy?
With a suspected respiratory allergy, your veterinarian will take into account the history and the clinical signs then perform a physical exam. A lung wash, or bronchoalveolar lavage, may be needed for a definitive diagnosis.
With skin allergy, again the history, clinical signs and clinical examination are used for diagnosis and often a treatment or management trial is put in place. There are a couple of more specialised tests available; the intradermal skin test and the serum IGE test. These have not been used as much in horses as other species yet, so may still need further validation.
What can you do if your horse is prone to allergies?
RESPIRATORY SIGNS If your horse has been diagnosed with recurrent airway obstruction, inflammatory airway disease, asthma, heaves, chronic
obstructive pulmonary disease or even a respiratory virus, you can help by reducing any further triggers to the hypersensitive airways, as well as trying to remove the original source of the allergic reaction. Medication may be prescribed by your veterinarian; this can include steroids (inhaled or oral) and bronchodilators amongst others, so this should be used as well as including the following management changes: Dampen hay and cha before feeding – this is vitally important as hay contains a lot of particles that, if inhaled, can set o the reaction in an allergic horse Ideally turn out into a paddock* or use dust-free bedding and have excellent ventilation Make sure the adjoining stables have dust-free bedding too Don’t store hay near to your horse’s stable or yard Don’t sweep the yard while the horse is inside Ensure there no lingering ammonia (from urine in deep bedding) Rest your horse while lungs are hypersensitive – deep breaths of cold air will irritate the airways, prolonging recovery Feeding from the ground may help the excess mucous drain out of the nostrils *Horses who get respiratory allergy when out in the pasture (thought to be from pollen) may need to be stabled, or put in a di erent paddock away from sources of pollen. SKIN ALLERGY SIGNS If medication is needed for treatment, your veterinarian will need to prescribe this. Steroids and antihistamines may be used. If your horse has the small lumps with scabs these can become secondarily infected with bacteria and this in itself can be itchy so a medicated shampoo can be very helpful; use an iodine or chlorhexidine wash.
For prevention, if pollen is suspected as the cause, then the use of covers and trying di erent paddocks at certain times of the year or even stabling may be helpful. For insect hypersensitivity, which is probably more common, then use of good insect repellents is important. Ones containing permethrin are o en e ective. ey come in di erent forms and you may even use permethrin-impregnated ear tags attached to covers. e important thing is to keep up regular insect control.
Looking at the environment may help; for example, moving the horse away from near a stream where midges are in large numbers at certain times of the year.
For a food allergy that shows up as a skin condition – which fortunately, is quite rare in horses – it can be very di cult to work out what is the cause. Usually, we start with a very controlled diet and once the irritation is under control we may add in the food containing the suspect ingredient and see
C if the signs are up again.
Allergic issues can be a real challenge to manage. If you have a horse with allergies,
MY try to get a plan in place with your
CY veterinarian but be prepared for some trial and error along the way. ere are
K o en lots of factors involved, so treatment and management have to be quite individual although the general principles will remain the same. A horse that is prone to being allergic to one thing will o en be hypersensitive to other things, so reducing any potential irritants in the environment is only going to help.
ABOVE A nasal discharge can be a sign of a respiratory virus ABOVE RIGHT Wetting down hay before feeding is an important part of managing an allergyprone horse OPPOSITE PAGE A puffy painful eye caused by a skin allergy