Insect bite hypersensitivity and skin allergies in horses

Biting insects are a source of irritation for all of us, and they can prove to be particularly troublesome for our equine friends in their stable and paddock environments. Biting flies are the primary cause of itching and allergic skin reactions in horses ⁽¹⁾[DH1] . There are many species of biting flies in the UK, including Midges (Cullicoides species), Horse flies (tabanid species), Stable Flies (Stomoxys species) and black flies (Similium species)⁽²⁾. Horses with a hypersensitive (allergic) response to insect bites can suffer intense itchiness and are often driven to distraction with the urge to rub – resulting in hair loss, sores and chronically thickened skin⁽³⁾.  In extreme situations the horses’ quality of life can be severely impacted by this debilitating issue requiring ongoing fly control, often requiring medication and increased veterinary intervention to manage acute flare-ups. Alongside the management difficulties, frantic rubbing by the horse also frequently leads to significant self-injury, not to mention environmental damage to fences, buildings and rugs, which adds to the costs of daily care.

Reactions to fly bites

Most animals, including horses, will potentially develop localised swelling and itching at the site of a fly bite, but what happens if the horse is allergic to that biting fly? And what is going on to trigger this response? Usually, the trigger for the allergic response is found in the fly saliva, and it is likely that there is a genetic link to horses prone to these allergic reactions to fly bites, which is worth considering before planning to breed from your horse! Often it is noted that the horse reacts more severely as they get older, making management of this condition an unrelenting battle⁽¹⁾.

The allergic response within the body is thought to be a combination of type 1 and type 4 hypersensitivity reactions⁽⁴⁾. The type 1 hypersensitivity response is rapid and is caused by the allergy trigger (antigen) combining with specific antibodies (IgE) present in affected horses. The type 1 hypersensitivity response is typically associated with the development of urticaria (hives), which may spread over the whole body⁽⁶⁾. Type 4 hypersensitivity reactions show a delayed onset due to specific white blood cell stimulation⁽⁵⁾.

Allergic responses are typically first seen in horses aged 2-4 years, with a gradual increase in severity of the signs seen as the horse ages⁽⁴⁾. The most frequent hypersensitivity disorder seen in practice is commonly known as ‘Sweet Itch,’ caused by a hypersensitivity to midge bites (Cullicoides). With this condition, the mane and tail areas are frequently worst affected, with time this may extend to the ears and face. In some horses, the underside of the abdomen may also be affected with swelling, crusting and extreme itchiness evident. Chronic self-trauma can damage the skin, leading to thickened folds in the mane and tail regions. Some horses will become irritable or aggressive and may become difficult to ride or handle, weight loss can also occur in very agitated or restless horses⁽⁴⁾.

How do we diagnose an insect hypersensitivity?

The clinical appearance of the horse can be highly suggestive of an allergic skin disease, but how do we determine the true underlying cause? Evaluation of the environment, feed and consideration of the season may give the likely answers. However, intradermal skin testing is the gold standard test for allergen identification. This is not cheap and is also not readily available in general practice – it is usually carried out by a specialist dermatologist.[JW2]  In this process, tiny amounts of potential allergens are injected into the horse’s skin, and the area is then monitored, and any skin responses are measured to identify the likely allergen responsible. Alternatively, we can carry out a blood test to check IgE presence for a variety of possible antigens; this is easy to do in practice and is relatively inexpensive. However, the validity of the test is controversial and so the tests are difficult to recommend based on the scientific evidence available^(1,4).

Treatment and management of insect bite hypersensitivity

The usual treatment and management of insect hypersensitivities largely comprises two strategies – firstly to reduce contact with the allergen and, secondly, to control the severity of the symptoms seen⁽⁴⁾.

Management

Stabling at dawn and dusk is helpful to avoid midges and other biting flies in their most active periods. Reducing contact with flies can also be achieved by using fly rugs, and face and ear nets to help to create a physical barrier, and avoiding turnout near areas where flies gather, such as near standing water, dense vegetation or muck heaps⁽⁷⁾. Oil-based skin lotions applied to the horse can also create a barrier through which the flies are less likely to bite⁽¹⁾. Fly repellents in the form of spray or pour-on solutions should be applied regularly, and some fly-repellent cattle ear-tags can also be helpful in some cases when plaited into the mane⁽¹⁾. Regular use of an insecticidal shampoo used on the affected areas is useful, but very frequent application is required⁽³⁾. Lotions containing extracts of plant essential oils including camphor, lemongrass, may chang, peppermint and patchouli applied daily to the skin [JW3] have been suggested to improve symptoms of irritation⁽⁸⁾.

Treatment

In horses where symptoms of hypersensitivity persist, despite these management changes, then medication may also be required. It can help to feed a supplement containing fatty acids (specifically Omega-3 and Omega-6) which could help to reduce the itch response⁽⁴⁾. Other medications which may be used to control the itchiness or manage acute flare-ups include anti-histamines – which generally have minimal side effects, but are not effective enough as a sole treatment in many cases; or steroids – which are very effective at reducing itchiness but can have several side effects and should be used with care^(1,3,4). It’s important to note that it is a criminal offence for you to give any human medication to your horse without a veterinary prescription.[DH4] 

If the antigen triggering the allergic reaction can be identified, then it is possible to attempt some desensitisation by immunotherapy. This entails injecting a tiny amount of the allergen into the skin on a regular basis (every 1-2 weeks), gradually increasing the amount of allergen injected so that the body begins to tolerate its presence. This process can be useful to help control symptoms in some cases, however it has not provided a miracle cure and other management strategies are still likely to be required to manage the affected horses long term^(1,4,8).

The future – is there a vaccine available?

Studies have been carried out on a vaccine to try and reduce the symptoms of insect bite hypersensitivity and initial studies carried out in a group of Icelandic ponies was encouraging⁽⁹⁾. The vaccination is considered safe to use after these trials⁽⁸⁾. However, there is little evidence to show their effectiveness and as such they are not widely recommended at present.

References

• W Rosenkrantz, Arthropod Hypersensitivity (2003) Current Therapy in Equine Medicine (5^th Edition, Chapter 4.4 P184-186). N.Edward Robinson. Elsevier Saunders

•  Urquhart, GM. Armour, J. Duncan, JL. Dunn, AM. Jennings, FW (1996) Veterinary Parasitology (2^nd Edition, p145-158). Blackwell Science Ltd.
• R Pascoe, Skin (2011) Equine Clinical Medicine, Surgery and Reproduction (Chapter 12, p873-983) GA Munroe, JS Weese. Manson Publishing.
•  Petersen,A, Insect Hypersensitivites (2009) Current Therapy in Equine Medicine (6^th Edition Chapter 148 p678-680). N.Edward Robinson, KA Sprayberry. Elsevier Saunders
• Marwa K, Kondamudi NP. Type IV Hypersensitivity Reaction. 2023 Aug 12. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan–. PMID: 32965899.
• Scott, D. W. and Miller, W. H. (2011)Urticaria and angioedema. Equine Dermatology, 2nd edn. Elsevier Saunders, p. 281
• L Foil, C Foil, Fly Control (2003) Current Therapy in Equine Medicine (5^th Edition, Chapter 4.6 p191-195). N.Edward Robinson. Elsevier Saunders.
• Equine allergic skin diseases: Clinical consensus guidelines of the World Association for Veterinary Dermatology R. Marsella, S. White, V. A. Fadok, D. Wilson, R. Mueller, C.Outerbridge, W. Rosenkrantz. First published: 08 May 2023
• Fettelschoss-Gabriel A, Fettelschoss V, Thoms F, Giese C, Daniel M, Olomski F, et al. Treating insect-bite hypersensitivity in horses with active vaccination against IL-5. J Allergy Clin Immunol. 2018; 142: 1194–1205.e3.

Author: jackmillard