Insect bite hypersensitivity (IBH, Sweet itch, summer eczema) is an allergy to insect bites. It is the most common allergic skin disease in horses, and involves an allergic reaction to the saliva of various biting insects (Culicoides, Simulium, Stomoxys etc.). Both type I and type IV hypersensitivity reactions are involved.
Most attention is focused on Culicoides (false nut), and an allergen from these has now been identified. There are over 1000 Culicoides species globally.
Mostly in imported horses
There is a genetic predisposition for Icelandic horses. Symptoms are particularly seen in imported horses (disease prevalence 26% when born in Iceland, compared to 6.6% in Icelandic horses born in Sweden).
In a Swedish study with 441 Icelandic horses, about 15% of the horses were affected. Other pony breeds and Friesians, German Coldbloods, Arabians and Quarterhorses seem to be overrepresented, but the disease can affect all horses and at all ages.
There is no doubt that IgE antibodies play a role in insect hypersensitivity in horses. IgE antibodies against Culicoides can be found in all horses that have been exposed to the sting of a false nut. A study has shown that specific IgE antibodies against Culicoides could not be detected in horses out of season when the clinical symptoms were gone. IgE against Culicoides could also not be detected in horses in Iceland where Culicoides is not found.
Icelandic horses imported to Europe that developed IBH showed a high significant increase of IgE against Culicoides allergens during the second summer after importation. Allergen-specific IgE levels in imported Icelandic horses without symptoms of IBH were low during the same period. Allergen-specific IgE largely reflects the horse’s clinical condition, which can be used in the diagnosis of IBH (Ziegler et al).
The problems usually start at the age of 2-4. In Scandinavia, the season starts in spring (April) and lasts until autumn (October).
As a rule, papules and scaly eczema are seen in the mane and tail region and along the back line. Ears, the area between the jaws, chest and mid-abdomen may also be affected. Strong itching causes the horse to brush off its mane and tail hairs and inflict wounds on itself which can become secondarily infected. Urticaria and eosinophilic granulomas have also been described. The symptoms worsen year after year so that some horses never get their coat/hair back in winter.
Certain individuals can become so ill (skin infections, weight loss, behavioral change)
that they cannot be used in the summer (up to 50% reduction in use value occurs).
The diagnosis is made on the basis of a good history, clinical examination and response to insect control. Current differential diagnoses (atopy, feed allergy, contact allergy, drug side effect, scabies, lice, whipworm) must be ruled out.
A serological IgE test or intradermal test can be used to confirm the clinical diagnosis and identify which insect group the horse is most sensitive to. The test can also be useful in convincing the owner of the importance of effective insect control. The test result can be used to prepare allergen-specific immunotherapy.
Which test and when?
Intradermal test (requires extensive clinical experience):
- Artuvetrin skin test – 80% of clinically insect allergic horses show multiple reactions.
Serological IgE tests:
- Allercept test (Fc epsilon R1-alpha mast cell receptor): insect panel
- Artuvetrin Serum test (monoclonal antibody derived from recombinant horse IgE): allergy panel horse (also includes insects).
For both of these tests, the blood sample must be taken 4-5 weeks after the onset of symptoms. Out of season, insect IgE will drop drastically and there is therefore no point in taking such a test in winter.
Around 70% of horses with insect hypersensitivity are positive, but also 30%
of clinically healthy horses test positive during the insect season.
Treatment against insect hypersensitivity
This consists of avoiding contact with the insects, insect control, anti-itch treatment and possible allergen-specific immunotherapy.
Avoid contact with the insects and have good insect control:
Identify the insects the horse cannot tolerate (clinical symptoms + IgE test/ intradermal test) and ensure that the horse is not bitten by them.
Klegg (Tabanus) and Stingfly (Stomoxys) are diurnal and the horse should be stabled during the day.
Weevils (Simulium), mosquitoes (Culex) and weevils (Culicoides) are most active from late afternoon to early morning and the horse is stabled at this time. The horse must not stay near standing water. Move the horse to an area with less noise and more wind (mountain pasture or sea pasture). Good insect covers can help many horses.
The stable must be made “insect-proof” – the mosquito netting must be very fine-mesh and both the netting and the stable should be regularly sprayed with insect spray.
Strong fans help keep Culicoides away. Devices called “Mosquito Magnet” attract the blood-sucking insects with the help of CO2. Insect repellents must be applied frequently. Citronella has no effect.
Local treatment with products that have a mild itch-relieving and cooling effect (for example Ermidrà Shampoo and Ermidrà Balsam Spray).
If a skin infection occurs secondarily, the itching increases. The infection must be treated locally with shampoo, for example Clorexyderm Shampoo (chlorhexidine) or DermAcetic Shampoo (acetic acid).
Antihistamines work best as preventive treatment. Try hydroxyzine (Atarax) 1-2mg/kg 2-3 times daily. Side effects such as fatigue and changed behavior can be seen. Attention: teratogen! Antihistamines can have a better effect together with fatty acids and can also be combined with prednisolone. Cetirizine does not work in insect hypersensitivity.
Prednisolone has a very good anti-itch effect and may be needed for shorter periods. Try a starting dose of 0.8-2mg/kg in the morning until the itching is under control, then reduce to 0.25-1mg/kg in the morning. Thereafter, a reduction to the lowest possible dosage is attempted every other day.
The better the insect control, the less prednisolone is needed. Be aware of the risk of developing laminitis when using cortisone – especially in ponies. You can try to combine prednisolone with pentoxifylline (Trental) 10mg/kg twice/day.
Omega-3 fatty acids (Dr. Baddaky Omega-3) have a synergistic effect with antihistamines/steroids in the anti-inflammatory process.
In healthy horses, supplementation of fish oil in the feed can increase the proportion of fish fatty acids in plasma as well as the production of leukotriene b (Hall et al). Try with 5-10 ml of fish oil per 100 kg of body weight daily.
Allergen-specific immunotherapy with insect extracts is available and may be useful for younger horses. It is important to start such immunotherapy within 1-2 years after the symptoms have appeared. Good follow-up and simultaneous insect control can give a better effect. There are a number of published studies dealing with immunotherapy given to horses with insect hypersensitivity. In one study there was no effect, while in another 9 out of 10 horses got much better.
Recent research shows that ponies with insect hypersensitivity have a stronger Th2 response with associated cytokine production and that extract of recombinant Culicoides obsulentes can induce a Th1 response and IL-10-producing regulatory T cells (which reduces the allergic reaction).
A trial with a vaccine against equine IL-5 (a Th2 cytokine that recruits eosinophil cells to the tissue) produced a significant reduction of eosinophil cells and clinical lesions in 17 of 19 horses.