Equine Allergies with Dr Cristina Rosales

This article is from the October 2020 Horse Deals magazine.

Cristina completed her Bachelor of Veterinary Science at the University of Melbourne in 2011, and since then has worked in both referral and ambulatory equine practice, both in Australia and the UK. She undertook an equine internship at Ballarat Veterinary Practice, followed by an equine medicine registrar position at Charles Sturt Uni before completing her medicine residency at the Equine Centre, U-Vet, Melbourne University. Cristina attained her Memberships in Equine Medicine in 2015 with the ANZCVS. She has a special interest in equine medicine cases, especially gastrointestinal disease, neonatal care, ophthalmology and endocrine diseases. She owns Scope Equine, an ambulatory internal medicine service that provides on-site diagnostics and case support for complex equine disorders, and is a medicine consultant at Barwon Equine Hospital, Geelong.

What are allergies?

The immune system’s primary role is to detect and eliminate foreign proteins or organisms (antigens). Allergies are an exaggerated immune response to certain substances (allergens) that are either inhaled, eaten, touched or injected, but are not infectious. Examples of allergens include insects, hay mould/dust, certain foods, plant pollens and some drugs. This response may vary from a mild, localised response such as uticaria (hives), to a widespread, life-threatening reaction (anaphylaxis). Common conditions caused by allergies in horses include pruritic (itchy) skin diseases, recurrent uticaria, and equine asthma.

What causes allergies?

Biting insects, pollen, certain foods (usually grain) and drugs are the most common causes of hives in horses. Exposure to stable/hay dust and airborne allergens (eg. grass/plant pollens) can trigger equine asthma in some horses. Insect bite hypersensitivity is the most common type of allergy in horses. It is caused by biting midges (Culicoides), which causes intense itching and skin disease along the dorsal midline, mane and tail base. This is also known as ‘sweet’ or ‘Queensland’ itch. Interestingly, there is a relationship between insect hypersensitivity that affects horses when then are younger and the development of equine asthma later in life.

Clinical signs of allergies

The two most common areas for allergies to develop are the skin (as hives or itchy skin) and the respiratory tract (as coughing and/or nasal discharge). Hives may appear minutes to days after exposure to an allergen and can occur anywhere on the body. Hives appear as fluid-filled, raised swellings on the skin. They may or may not be itchy. Respiratory allergies may present either as allergic rhinitis or rhinoconjunctivitis (nasal discharge and watery, red, swollen eyes) or equine asthma (characterised by coughing, poor performance, increased breathing effort plus or minus nasal discharge). Anaphylactic reactions are the most severe and potentially life-threatening type of allergy and are typically sudden-onset. There is a dramatic drop in blood pressure which can lead to collapse, as well as swelling and occlusion of the airways which causes difficulty breathing.

Challenging diagnosis

Because different causes of allergies have similar clinical presentations, it can be difficult to determine what triggered them. The body needs to be exposed more than once to develop an allergic response, and to make things more complicated clinical signs may take weeks, months or years to develop, which is why investigating allergies can be a challenge.

It’s important to rule out any other causes of hives in horses (such as physical causes and other diseases). Next, determining where on the body the hives are, what time of the year/season it is and the management of the horse (diet, housing etc) helps narrow down the allergen. Keeping a diary — when the signs started and what changes have occurred in the horse’s environment recently (feed, environment/housing, medications, recent injections or vaccinations) can be a huge help.

A physical exam is performed by your veterinarian to note what abnormalities are present and where on the body they occur. Skin scrapings, biopsies and skin cultures may be collected to rule out external parasites, bacterial or fungal infections and auto-immune diseases.

A re-breathing exam (to hear abnormal lower airway sounds by making the horse take deep breaths), endoscopy to investigate the source of nasal discharge and sampling of the lower airways (called a bronchoalveolar lavage or ‘lung wash’) investigate a respiratory issue. Intra-dermal skin testing can help identify an offending allergen. Food elimination trials need to be done for a prolonged period, where the horse is fed the simplest diet possible (ideally, hay). Once the signs have resolved, one new food can be reintroduced weekly to try to identify the trigger food.

Treatment and management

The goal for treating allergies is avoidance- identifying and excluding what is causing the allergy is paramount. For IBH, preventing access of biting insects to the horse by using special rugs, moving horses indoors especially at dawn and dusk to a fly-proof stable, and diligent use of insect repellent can help. For equine asthma, avoiding triggers such as pollen and dusty environments (bedding, ventilation, hay) is essential. Some horses may need complete pelleted feed when they can’t be on pasture. For seasonal allergies, predicting when the problem starts and commencing treatment just before can help minimise the severity of the allergy. In extreme cases, some horses may need to be moved to an entirely different location. The most common medication used for allergies is corticosteroids, which can be applied either topically, orally, via injection or inhaled. Corticosteroids work by dampening the body’s immune response to allergens. Adrenalin is used in cases of anaphylactic reaction. Anti-histamines are less reliable in horses compared to humans, but the anti-histamine hydroxyzine may help. Once an offending allergen has been identified, allergen-specific immunotherapy can be carried out to de-sensitise to the allergen. Lastly, diet supplementation with oil high in omega 3 fatty acids (found in canola, soya bean and fish oil) has been shown to help control seasonal allergies and equine asthma, by modulating the immune response.

Whilst most horses will likely develop the occasional allergic reaction and recover uneventfully, there is the potential for severe reactions to occur due to repeated exposure to allergens. If you have any concerns about allergic reactions in your horse, please seek advice from your veterinarian.

Sign up to our newsletter