Table of Contents

What is Sweet Itch?  And how to Treat Sweet Itch for Horses

Table of Contents

What is Sweet Itch?  And what does it do to horses?

Sweet itch is the common name for seasonal recurrent dermatitis.  This means it is an allergic reaction to the saliva of the Culicoides fly. A seasonal disorder, horses can display varying reactions depending on the body’s type of response and previous reaction history. Symptoms include uncontrollable itching to the point of hair removal and skin damage. While there is no cure for sweet itch, it can be managed by both preventative measures and treatment options depending on the severity of the reaction.

Horses are hypersensitive to insects

Did you know that hypersensitivity to insects is the most common allergy seen in horses? One of these allergies has its own name--Sweet Itch. Also known as summer itch, seasonal recurrent dermatitis is an allergic reaction to the saliva of Culicoides biting gnats or midges.  These gnats and midges are also referred to as a “no-see-ums.”

Some horse breeds, such as Icelandic horses, Shires, and Welsh ponies, are genetically predisposed to this condition. However, any horse can develop sweet itch after exposure to these insects. Like any allergic reaction, repeated exposure can result in escalated immune responses.

Standing water is breeding grounds for gnats

Culicoides gnats can be active from April until October, depending on the climate and weather conditions. Despite their small size, they can travel up to half a mile in search of food. The Culicoides insect breeds in standing water (ponds, puddles, and marshy areas) and are more active during dawn and dusk.

The biting gnats are typically classified into two types, “dorsal feeders” and “ventral feeders.” Dorsal feeders, the most common gnat, prefer to feed on the skin on the top half of the horse—the ears, poll, mane or top of the neck, withers, back, hindquarters, and tail. The less common ventral feeders tend to congregate on the horse’s face, chest, and belly.

How to Diagnose Sweet Itch

There are two different immune responses to the biting midge; IgE and Type 4 Hypersensitivity. These variations in reactions make it difficult to diagnose sweet itch with a traditional skin allergy test.

IgE Immune Response - Less Common

The IgE response is a more immediate reaction. Horses display reactions very quickly after being bitten (20 minutes to 24 hours). Horses responding in this manner may also develop a case of hives. Not all horses will have this response.

Type 4 Immune Response - More Common

Type 4 hypersensitivity is more common. In this case, the allergic reaction doesn’t appear until 48-72 hours after being bitten by the Culicoides midge.

These two, different immune responses make allergy skin testing difficult, as the window of time can vary significantly. Additionally, some horses are more generically “Fly bite sensitive” and would react to any insect; the particular reaction may not be indicative of the Culicoides midge specifically. Blood serum testing is not the best option either; skin and blood tests don’t always match up.

It’s important to keep in mind that allergic reactions can vary from year to year depending on how sensitive the horse is; many horses get more and more sensitive as time and number of exposures progress.

Horse Behavior & Symptoms of Sweet Itch

Horses will rub areas that have been bitten, often to the point of hair removal. They may rub on door frames, fences and fence posts, or trees and shrubs—really, anything that is available. Rubbing from the itching may cause

• open sores
• dry and thickened skin
• abnormal hair growth.

This condition should clear up during the winter months when biting flies are inactive.

The best way to diagnose this affliction is observing the clinical picture—are lesions seasonal? Seasons may vary depending on geographic location and annual weather patterns. Pay attention to how the lesions are distributed over the horse’s body. As discussed earlier, dorsal reactions are more common. Last, how does the horse respond to removing Culicoides from the environment? If the lesions begin healing, it’s a good sign as to the cause.

How to Treat Sweet Itch

Sweet itch can be treated in many different ways.

Short Term Treatments

To immediately relieve symptoms, horses diagnosed with sweet itch can be prescribed various medications depending on the circumstances. Antihistamines can help relieve itching, while antibiotics can treat or prevent infection from open wounds caused by the itching. Steroids can also be prescribed to assist with inflammation and itching. Corticosteroids can be injected; certain forms may provide relief for weeks instead of days.

It is important to consider cost and effect; antihistamine or corticosteroid creams can become expensive for long term use and are meant to relieve itching, not cure the condition itself.

Various skin care treatments, available over the counter, can also help alleviate symptoms. These include cool rinses, shampoos, and creams designed to sooth the skin and relieve itching.

Long Term Treatments

Longer term treatments include supplements to improve skin health, such as those with omega 3 fatty acids, or MSM. Supplements can take several weeks up to a month before results can be seen.

Allergy shots can also be administered to gradually desensitize the horse to the specific allergen.

How to Prevent Sweet Itch

The best way to prevent sweet itch is to reduce the number of biting flies and midges in the horse’s environment. While removing manure on a regular basis is important to overall insect control, midges do not breed in manure. Eliminating standing water is the most important factor in controlling biting gnat populations, as that is where they breed. 

Horses can also be protected from biting insects using a combination of fly sprays and fly protection, like a fly sheet, fly mask, and fly boots. It is important to distinguish between the different types of fly sprays; some are only insecticides, others are true repellents and will prevent the insect from landing.

Oil-based repellents generally last longer than water-based repellents. Even the best repellents usually need to be applied more than once per day to be effective.

Using mesh curtains on stall and barn openings can help keep insects out of the stable (however it must be fine mesh to keep out “no-see-ums” which are inherently small).

Additionally, stall fans may be helpful as moving air makes it more difficult for flying insects to land on the horse. Culicoides are weak-flying insects that can’t fly against a breeze, so fans can be quite effective4. Another factor to keep in mind is the feeding pattern of no-see-ums. Generally, biting midges like to feed at dawn or dusk, so keeping horses indoors during these times may help reduce exposure.

Conclusion

Sweet itch, while common, is both preventable and treatable. It’s always suggested to consult your veterinarian before beginning any new treatments.

Sources:

Smart Pack “Sweet Itch in Horses” by Dr. Lydia Gray. https://www.smartpakequine.com/content/Sweet-Itch

Kentucky Performance Products https://kppusa.com/2017/06/30/stop-sweet-itch-driving-horse-crazy/

VCA https://vcahospitals.com/know-your-pet/sweet-itch-in-horses

The Horse https://thehorse.com/157859/sweet-itch-itching-for-a-cure/