Many different items commonly found in any horse’s environment can cause allergies. Stabled horses are engulfed in dust, mould spores and other allergens all day long. Even in fields where horses graze, there are many different proteins that could potentially cause an allergic reaction. Usually a horse can fight off these allergens with a healthy immune system, but not in every circumstance.
A healthy horse’s immune system makes proteins called antibodies. Antibodies are used by the body as weapons against antigens (the allergens). When the system works correctly, the antigens are eliminated. Allergic reactions, also known as hypersensitivity, occur when the immune system overreacts to an outside antigen that is introduced to a horse’s body.
Skin Allergies in Horses
Skin allergies usually appear as hives, 12 to 14 hours after the horse is exposed to the antigen. Horses may experience fever or itchy skin when they have hives. Skin allergies can be caused by food allergens or even insect bites.
Respiratory Allergies in Horses
Recurrent Airway Obstruction (RAO) is the term used for respiratory allergies in horses but is also referred to as COPD or heaves. The symptoms can include coughing, exercise intolerance and nasal discharge. Similar to human asthma, in some horses owners will notice wheezing, and there may even be a visible “heave line” in the area between the flank and the thorax. It is important to get a heaving horse treated as soon as possible because severe cases can cause weight loss.
While mould spores and lipopolysaccharides (a constituent of gram negative bacteria) are known to cause RAO in horses - the latter being higher in summer than in winter -pollen has also been identified as one of the potential allergens for susceptible horses.
Summer Pasture Associated Airway Disorder can be caused by an allergic reaction to organic dust when out at grass. While little can be done about these organisms in the air, prevention of sensitization of the horse by maintaining a low dust regime wherever possible is the best avoidance strategy for all these allergic respiratory disorders.
The warmer, stiller air conditions in summer reduce natural ventilation of the stable environment further, making it even more dusty. Horse owners should take great care to remove the dust from the horse’s breathing zone, and this means treating the fodder so that respirable particles are kept to a minimum during feeding. Studies by Moore-Colyer and Fillery, (2012) and Taylor and Moore-Colyer (2013) have shown that steaming in a specially designed hay steamer which drives steam throughout all of the hay and ensures temperatures of over 100°C are reached is the only sure way to reduce respirable particles, mould and bacteria in hay. Partial steaming in home-made steamers or soaking the hay where complete steaming at the necessary high temperatures cannot be achieved only reduced mould by 70% and actually increased bacteria in the fodder. Increasing lipopolysaccharides by stimulating bacteria growth can increase the incidence of RAO which is clearly undesirable.
When it comes to both allergies and the general health of horses it is important to minimize exposure to respirable dust and allergens where possible; forage is the largest part of the horse’s diet; it’s also a major source of respirable dust and allergens. Controlling this source by using a Haygain hay steamer can help prevent the development of hypersensitivity and assist in the management of horses presenting with allergies.
By Jacob Hepworth, Master of Science by research, student at the Royal Agricultural University, Cirencester, England