Coughing, sneezing or a cluster of skin bumps could indicate a summer allergy, as Richard Hepburn FRCVS explains
THE summer months bring increased levels of specific allergens (a molecule capable of eliciting an allergic response), namely pollens, fungi and insects.
Pollen released from grasses and trees peaks from 5am to 10am. As intact pollen is too large to enter the small airways of the lung, it is deposited on the horse’s skin and in the upper respiratory tract, mouth and eyes. Rain and moist conditions can cause pollen to shatter, liberating much smaller particles that can enter the lung.
Fungi exist in a symbiotic relationship with many grasses, and within hay are already implicated in stable-associated asthma. These fungi are also present on different parts of the growing grass plant, and the release of their spores increases with humidity – leading to peak airborne fungal spore levels in the early morning. Levels also rise during warm and windy weather.
Fungal spores can travel a great distance; in summer, your horse is not only exposed to local fungi, as he is in winter in the stable, but to those from a wider area.
Consistently warm weather creates an increase in flying insects. As they feed on your horse, they produce chemicals in their saliva which act as allergens.
Insects vary in their chosen feeding location on the horse and preferred feeding time. Culicoides midges breed near standing water, but will travel up to half a mile to feed, following a carbon dioxide trail from exhaled breath. Most active at dawn and dusk, they are poor fliers and struggle with windy conditions, congregating near hedgerows and woodland. Different midge species feed on different parts of the horse; the more species present, the more widespread the lesions.
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However, horseflies feed during the daytime on the flanks, sides of the chest and upper legs, tolerate windy conditions and breed on vegetation near water.
Understanding the allergic response
TO be allergic to something, a horse first has to become sensitised. When he is bitten by an insect or inhales pollen, a cellular response triggers the production of immunoglobulin E (IgE). This prompts the release of histamine, a chemical that increases blood flow and blood vessel permeability.
Accumulation of fluid follows, seen as skin wheals or airway oedema (swelling), leading to wheezing and the stimulation of nerve endings to cause itching, sneezing, coughing and narrowing of the airways – occurring within 15 minutes of allergen exposure.
Activation of a second inflammatory cell in the bloodstream, the basophil, leads to distant site allergic reactions. Mast cell stimulation also causes a later phase, typically four to six hours after exposure, which can last for days or weeks and is characterised by the deposition of fibrin (scar material) and remodelling of affected tissues.
Re-exposure to allergens causes the immune system to remodel, leading to a more aggressive and rapid memory response.
The most common problems
The main summer-associated allergic conditions are:
Equine pasture asthma (EPA)
Summer EPA worsens with increased turnout, especially during hot months. Signs of coughing during exercise, a high breathing rate during and after exercise, and poor performance typically begin in July, persisting until October. Many horses also show signs of breathing difficulty at rest, essentially having an asthma attack.
Mature horses are more at risk, reflecting prolonged recurrent allergen exposure. The specific cause of EPA has not been identified, but is likely to be high levels of pollens and mould spores, especially after crop harvesting.
Allergic rhinitis
Anecdotally, many owners report watery nasal discharge, sneezing and nose rubbing in their horses when the pollen count is high. Some horses also show signs of allergic conjunctivitis, with swollen eyes.
Insect bite hypersensitivity (IBH)
This is a hypersensitivity reaction to Culicoides midge species (termed sweet itch) and to other biting insects such as gnats, mosquitoes, stable and horseflies. Allergic reaction can be immediate (type one), causing urticarial wheals or hives, or delayed (type four), where itching and crusting develop a day or two after the bite. Often, horses exhibit both types of reaction.
What causes headshaking?
AN imbalance between types of T-lymphocytes (a component of the immune system) is found in allergic humans and dogs, due to genetic or environmental factors such as breed or diet, and leads to a magnified response to allergen contact. Defects in epithelial barriers (such as the nostril lining) may also contribute to human allergy, with reduced resistance found in those with chronic hay fever. Genetic variation in enzymes responsible for moderating the allergic response in the skin or respiratory tract has also been identified.
A certain cytokine (a small protein implicated in the body’s infection response) has been found in increased levels in the airways of horses with equine asthma. Similar mechanisms may mediate equine allergic disease, but it is not clear whether these changes are responsible for a horse being more likely to become allergic or have developed subsequent to him becoming allergic due to chronic low-level environmental exposure.
A recent study in the US showed that obese horses were more likely to have summer EPA. A more severe type of human asthma has emerged associated with obesity, with a poorer response to treatment. While human obesity is believed to promote a state of increased inflammation, the magnitude of its relevance to increasing the risk of allergic disease is unclear.
Management plan
DIAGNOSIS is usually based upon clinical signs, but if IBH skin lesions are unusual, a biopsy may exclude other diagnoses.
There are no definitive tests for sweet itch allergy. Blood sampling for IgE measurement or intradermal skin testing are insensitive, as it is unlikely that the test allergen will be specific to the midge species affecting an individual horse – although more detailed testing shows potential.
Intradermal skin testing can be more useful when IBH is part of a wider skin allergy problem. In EPA, airway endoscopy and sampling of airway fluid for cytology can be used to define the severity of disease and the likely duration of treatment.
Treating allergic disease depends on the severity of the clinical signs. If your horse has an allergic event, call your vet straight away. Human antihistamines are not particularly effective in horses, as their absorption from the gut is variable – meaning that a large number of tablets is needed and benefits are slow to materialise.
The mainstay of treatment is corticosteroid anti-inflammatories, given orally or by injection, on skin or inhaled into the respiratory tract. With EPA, bronchodilator drugs to relax the airways may be needed.
If your horse appears to have a summer allergic skin reaction, move him inside and hose him with cold water to help remove allergens and reduce development of skin swelling. Using colloidal oatmeal shampoo can reduce itching while you wait for the vet.
IBH-specific treatments, including allergen-specific immunotherapy and dietary manipulation using vitamin B3 and omega-3 fatty acids, have shown variable results. However, an Australian study showed that a plant extract emulsion spray significantly reduced clinical signs. Your vet can advise you about IBH management methods, such as sweet itch rugs and the fly repellent permethrin.
Management of horses with EPA is challenging, as it is difficult to remove the horse from the allergen source. An individual approach is required, based on an appropriate treatment plan, monitoring of airway health and corticosteroid treatment.
.This Vet Clinic feature is also available to read in Horse & Hound magazine, on sale date Thursday 1 July 2021
