Sweet Itch In Horses: Signs & Symptoms

horse in the pasture by the sea looking at the horizon

Summer means warmer temperatures and beautiful weather – an ideal combination for horse and handler alike. While cooler months hold their fair share of potential equine health issues, groomers, riders, and other experts often dread the wide range of horse skin conditions that summer foliage, insects, and sun exposure can cause. One of the more common discomforts of warm weather in the paddock is sweet itch in horses, which is also called summer itch among horse grooming experts. 

While equine sweet itch is both treatable and seasonal, it’s important to diagnose and address sweet itch as early as possible for the best chance of success. The most direct route to treatment is a diagnosis from an equine vet, rather than simple observation. This is because the welts and crusted sores from the condition can mimic other problems, such as ringworm or rain scald, and lead to unnecessary medication dosing and subsequent side effects.

What Causes Sweet Itch In Horses?

While the appearance of sweet itch can mimic a viral or bacterial infection, particularly as it progresses, it often starts with an annoying, invisible pest: the onchocerca parasite. These long, worm-like parasites live along the nuchal ligament, an area that follows the base of a horse’s neck. When their immature offspring, called microfilaria, travel to the sensitive underbelly of the horse, their presence causes the skin to redden and inflame, eventually triggering hair loss and open, weeping sores. 

These sores, in turn, attract biting flies of the Culicoides family – most commonly known to horse handlers as no-see-ums, gnats, or sand flies. When these flies bite sores already infected with microfilaria and add their saliva to the mix, the result can be a highly irritated area of skin that the horse will often attempt to rub or worry, making the area scab, blister, and loose hair as time goes on. Unfortunately, microfilaria are also all too happy to hitch a ride with a convenient biting fly, which means that one biting fly can potentially transfer sweet itch from an afflicted horse to a healthy one with relative ease, particularly within the same stable or paddock.

There is, however, thought to be a genetic element to how susceptible a horse may or may not be to this equine skin affliction. The flare-ups that differentiate a simple fly bite from an ongoing condition rest in a horse’s allergy – or lack thereof – to biting fly saliva. Just as some humans experience more painful mosquito bites than others, some horses are naturally genetically disposed to take fly bites in stride, with no unexpected allergic inflammation. 

What Does Sweet Itch Look Like?

From tree and fence-inflicted scrapes to actual viral or bacterial skin conditions, sweet itch may “blend in” for a while until it progresses to a more serious form. Generally, irritated areas will begin to appear on a horse’s belly line, moving up to the dock of the tail, rump, neck, or shoulders, though the rump area is more common. 

As sweet itch gets worse, the red, raised, and irritated areas of the skin will start to look blackened and wrinkled from exposure. These blotchy patches of callused-looking skin will spread throughout an affected area as the biting flies’ attacks do, causing additional flare-ups in a sensitive horse. If the rump, base of the tail, or back of the horse’s neck show clusters of these raised, blackened areas of skin in the summer, chances are they are afflicted with sweet itch. If they vanish abruptly in the winter, it’s an even safer bet that sweet itch was the culprit.

As the horse rubs, nips, or rolls against surfaces like walls or fence posts for relief, the hair over the affected area will fall out and leave behind raw skin and open, oozing sores. Because this raw skin attracts more biting flies, in turn, this painful, irritating cycle can repeat itself all summer unless it’s stopped with the right treatments and practices.  

What’s The Best Sweet Itch Treatment For Horses?

Even the most dedicated horse handler can’t keep every single biting fly away from their horse all summer. That said, making a stable and paddock area unwelcoming to these pests is both a great preventative and curative step. 

  • Because biting flies like wet, humid, and marshy conditions, ensuring proper drainage for any stables or exercise areas is a must. In areas where drainage isn’t ideal, try incorporating horse-safe materials like straw or sawdust to dry up the ground. 
  • When horses are outside, weather and temperature permitting, coverings like lightweight blankets and hoods can provide flies with less surface area access for biting. 
  • Promptly treating all open wounds with the appropriate medicine or topical treatments, as suggested by an equine vet, will help stave off the cycle of sweet itch and drive flies away. Use caution with injectable horse medicines, as long term use or overuse can cause other problems that may be even more concerning.
  • Timing outdoor exercise to avoid early morning and twilight hours, when flies are at their most active, will help limit the access these biting pests have to a horse. These hours will vary regionally depending on seasonal climate, but simple observation will usually guide handlers well.
  • Mounting tools like fly strips, large fans for air movement within the stables, and fly screens on any doors can also help limit biting insect exposure when horses are kept inside.
  • Spray or treat the perimeters of outdoor areas, as well as the areas around barn and stable doors, with horse-safe pesticides. While these measures won’t wipe out flies entirely, they can significantly cut back on the number of breeding insects.
  • Practices like diligent water hygiene in troughs and regularly checking feed for unwanted moisture will ensure that biting insects don’t make themselves comfortable in areas frequented by horses, whether indoors or out.
  • A balanced diet with proper ratios of Omega-3s, as well as regular supplementation with products like CBD oil for horses, can help support lower inflammation and, in turn, reduce the severity of skin issues like sweet itch. 

Can Humans Get Sweet Itch?

Summer offers plenty of skin-irritating opportunities for human handlers, ranging from sunburn and ticks to unpleasant poison ivy and poison oak rashes. Thankfully, sweet itch isn’t a “gift” that horses can pass along to their two-legged companions, even if they spend a lot of time riding together. While humans can definitely have their own sensitivities to the saliva of certain insects, such as the aforementioned mosquitos, sweet itch in horses isn’t transferable to humans the way a zoonotic virus would be.

That said, it’s always good practice for handlers to thoroughly wash up when moving horse to horse, particularly if they’ve treated any open sores. In addition to using biting flies to move from victim to victim, humans can also unwittingly transfer the sweet itch-triggering microfilaria between horses through touch or exposed materials – blankets, straps, saddles, and so on.  

What Are The Dangers Of Sweet Itch?

Old Kladruby Horse

Left untreated, any open-wound skin condition in horses can potentially lead to a worst-case scenario, complete with septic wounds or a blood infection. For severe or recurring sweet itch, overuse of corticosteroids (particularly injected medicines) can lead to other health concerns, such as laminitis of the hoof. These looming concerns underscore the importance of treating sweet itch as early and thoroughly as possible – preferably before it has a chance to spread on an individual horse or throughout an entire stable. 

A horse affected by sweet itch once will likely continue to have the same reaction every year, as biting insects become common in warmer weather. Once a horse has been diagnosed with this biting fly sensitivity, their handler can proactively treat, cover, and equip their stable(s) or pasture areas to minimize flies and stop the issue before it starts. Without this treatment, a horse may experience a great deal of distracting pain and discomfort and raised sores that may prevent proper saddling or equipment mounting. 

Sweet itch, despite its harmless-sounding name, spells trouble for any affected horse, especially as the summer heats up and flies multiply. The best way to keep this irritating condition at bay is to watch for early symptoms, block flies’ access to open wounds with topical creams, and routinely maintain good grooming practices. While keeping every risk factor out of the stable and pasture isn’t feasible, a few smart moves ahead of the summer heat will ensure that biting flies might just go hungry when it comes to a cherished equine companion.  

Sources Cited:

1)      “How to Stop Sweet Itch (Summer Itch) From Driving You and Your Horse Crazy!” Kentucky Performance Products, LLC | Horse Supplements (kppusa.com), (no published date), https://kppusa.com/2017/06/30/stop-sweet-itch-driving-horse-crazy/. Accessed June 4, 2021.

2)      Carson, Deirde M., BVSc, MRCVS & Rickets, Sidney W., LVO, BSc, BVSc, DESM, DipECEIM, FRCPath, FRCVS. “Sweet Itch in Horses.” VCA (vcahospitals.com), (no published date), https://vcahospitals.com/know-your-pet/sweet-itch-in-horses. Accessed June 4, 2021.

3)      Dr. Kellon, VMD. “Sweet Itch in Horses by Dr. Kellon.” Forageplus Talk (forageplustalk.co.uk), September 7, 2020, https://forageplustalk.co.uk/sweet-itch-in-horses-by-dr-kellon/. Accessed June 4, 2021.

4)      Clark, Aimi. “10 ways to beat sweet itch this summer.” Horse & Hound (horseandhound.co.uk), April 26, 2021, https://www.horseandhound.co.uk/features/how-to-manage-sweet-itch-530771. Accessed June 4, 2021.

5)      Bonner, Laurie; Freckleton, Melinda, DVM. “5 ways to prevent sweet itch.” EQUUS (equusmagazine.com), April 14, 2015, https://equusmagazine.com/diseases/prevention-sweet-itch-28009. Accessed June 4, 2021. 

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