Emma Hales BVSc MRCVS, St David’s Equine Practice
Sweet itch is an extremely frustrating skin disorder with horses continually wanting to scratch every last piece of their mane and tail away. Affecting approximately 1 in every 20 horses, many of us struggle every year to control it.
Sweet itch is caused by a hypersensitivity reaction to the saliva of midge bites. This then leads to skin irritation causing the itchiness. Because the disorder is caused by midges, the disease is seasonal usually between March and November. We generally see horses mostly rubbing their mane, tails and faces causing hair loss and damage to the skin. Some owners notice their horses being quieter than normal, having a lack of energy due to the constant irritation. These signs alone at the right time of year are usually enough to make a diagnosis. However, if signs are not classical we can confirm the diagnosis with skin biopsies.
Learn more about midges:
The midges responsible belong to the Culicoides
The females require a blood feed before being able to mature their eggs.
Midges are unable to fly in wind or rain. They also don’t like bright sunlight hence why we see them out in force during twilight periods, on cloudy and calm days and also in forests away from direct light.
They breed in moist areas normally within herbage or decaying matter. Hence again why we see them in forests especially with nearby water sources.
When they bite they inject their saliva which is what the horse reacts to.
Midges can only fly approximately 100 metres.
Understanding the properties of midges gives us the best chance to be able to avoid them. So, as well as stabling horses at dawn and dusk, we should also consider keeping them in stables on cloudy and still days. If possible, problematic horses should be in paddocks on the top of hills where it is windy, with no nearby water source, well drained soil and no trees. This removes midges breeding areas and the conditions mean they are unable to fly there too.
Learn more about the hypersensitivity reaction:
The affected horse’s immune system over responds inappropriately to the saliva of the midge. In an attempt to breakdown the actually harmless proteins within the saliva they also attack and damage their own cells which ‘look’ similar to the proteins in the saliva.
The inflammation from this cellular damage causes the irritation which causes the itching. This is why just one bite from a midge can kick start a whole episode of sweet itch.
Unfortunately, the more a horse itches the more tissue damage and inflammation there is, leading to even more itching. This is what we call the itch-scratch cycle which escalates the problem even more.
It is expected that there is a genetic component to this disorder. For example, there is a higher prevalence in native breeds in comparison to thoroughbreds. However, we also know that the environment in which a horse grows up in is important. Icelandic ponies bred outside of Iceland have the same chance of developing sweet itch as other breeds. However, an Icelandic pony bred within Iceland is at a considerably higher risk of developing sweet itch if taken to another country. This is because Icelandic ponies bred in Iceland are completely naïve to Culicoides midges, and so their immune system has not developed a normal response to a midge bite. When exposed to the midges later in life the immune system is much more likely to respond inappropriately, leading to the hypersensitivity reaction.
This shows how important it is to prevent a midge biting so there’s no reaction and no start to the itch-scratch cycle. This means starting all the avoidance techniques early in the season before the first midge bites.
The more we can learn about the immune response, the more chance we have at developing a cure to stop the hypersensitivity reaction rather than just treating the symptoms of it. For example, exploring desensitisation methods, although not fully tested and therefore not available to use currently. There are reports of ringworm vaccinations in Switzerland having also controlled sweet itch as well.
Learn more about control techniques:
As we’ve already mentioned, avoid midges as much as possible by picking the most suitable field and stabling your horse during the risk times.
Protect your horse from the midges you can’t avoid. Special sweet-itch rugs and fly masks are better than repellants; close fitting, with a belly panel and tests have proven a zebra print is best!
If necessary, use multiple repellants with different ingredients to deter as many midges as possible. Ask your vet about using a mixture of benzyl benzoate and liquid paraffin which you can apply along the mane and on the tail head to repel and prevent them biting.
Even with the best avoidance and protection techniques possible you may still need treatment to reduce the reaction. Topical steroid medications act to break the itch-scratch cycle and we can use them in combination with the repelling techniques to control episodes. When the hypersensitivity reaction is more severe, systemic steroid medications act to reduce this reaction itself. Contact your vet to discuss these treatment options further.
Hopefully one day we’ll manage to cure sweet itch but until then learn more about the disorder and try to out-smart those midges!
St David’s Equine provides the equine veterinary services for Molecare Veterinary Services. Visit www.stdavids-equine.co.uk for more information.