Originally thought to be a reaction to feeding sweet feed, "sweet itch" is a skin condition caused by a hypersensitivity reaction to insect bites, primarily the tiny Culicoides midge. Other names are Queensland Itch, summer eczema and equine insect-bite hypersensitivity. It commonly involves the midline of the belly, tail base and rump, but can also include other areas.
The reaction to the bites causes an intense itching that makes the horse rub himself raw. The midline of the belly often becomes swollen, oozing and crusted because of the allergic reaction. Expect hair loss.
With lesions involving the belly, or belly plus neck and shoulders, Onchocerciasis may also be a problem. Onchocerca is a long, thread-like parasite that lives as an adult in the nuchal ligament, the broad thick ligament running between the poll and the withers. The adults release immature forms called microfilaria, which travel under the skin to the tissues along the midline of the belly. Their presence there causes an irritation and eventual skin breakdown that attracts flies and Culicoides midges. The midges pick up the microfilaria and transmit them to the next horse. Reactions to the bites of Culicoides infected with Onchocerca may be more intense than normal due to the injected microfilaria. Secondary infections may develop.
Adult Onchocerca can't be treated with deworming drugs because they don't reach adequate concentration in the ligament. However, microfilaria are sensitive to ivermectin. A standard deworming dose is effective. Swelling and temporary worsening of the symptoms is commonly seen within four to 24 hours of treatment, but healing of lesions that aren't complicated by hypersensitivity to Culicoides will occur within seven days. Repeat the treatment at six- to eight-week intervals during the warm months to keep the skin clear of microfilaria.
To minimize the risk of secondary infections, wash the skin as needed to remove secretions and adherent dirt. Spot bathing can be done using a gentle soap, like Ivory or Corona equine shampoo (www.summitinds.com, 800-241-6996, $7/qt.).
The ideal topical for the skin will both help healing and keep the midges away. This small fly can be repelled by Campho-Phenique. Oils and petrolatum-based products help to physically prevent the midge bites by forming a barrier. (Be careful of anything that seems to further irritate the broken skin, such as many plant oils. Sensitivity/allergy may also develop after a period of use.)
For enhanced pain and itch control, either spray the cleansed area with Bactine, or mix liquid Zim's Crack Crème into your skin-care product. These products are readily available at most pharmacies. We've included our favorite equine-product choices in our chart.
Uckele's C4G Ointment is close to an all-in-one treatment. Shapley's M-T-G and Calm Coat are also highly effective. Sensitive horses might do best with Su-Per Healing Ointment. Sweet Itch Products Chart.