Sweet itch

From Horse
Sweet01.jpg
Classic 'sweet itch' in the neck region of a horse caused by stable flies

Sweet itch is a seasonal insect-bite allergy attributed to biting midges, Stomoxys spp and Tabanus spp horse flies.

The biting midges, “no-see-ums,” or punkies belong to the family Ceratopogondiae. The most common biting midges are Culicoides spp. They are associated with aquatic or semiaquatic habitats, eg, mud or moist soil around streams, ponds, and marshes. Biting midges are tiny gnats (1-3 mm long) and, like black flies, inflict painful bites and suck the blood of their hosts, both humans and livestock.

Clinical signs

Culicoides spp are vicious biters and can cause intense irritation and annoyance. In large numbers, they can cause livestock to be nervous and interrupt their feeding pattern. These gnats tend to feed on the dorsal or ventral areas of the host; feeding site preference depends on the species of biting gnat. They fly only in the warm months of the year and are most active before and during dusk. They feed often on the mane, tail, and belly of horses. Horses often become allergic to the bites, scratching and rubbing these areas, causing alopecia, excoriations, and thickening of the skin. This condition has several names, including culicoid hypersensitivity in Canada, Queensland itch in Australia, Kasen in Japan, sweat itch, and sweet itch. Because it is often seen during the warmer months of the year, it is also referred to as summer dermatitis. These flies also serve as the intermediate host for Ochocerca cervicalis; the microfilariae of this nematode are found in the skin of horses. Onchocerciasis is a nonseasonal dermatosis that is similar to sweet itch but usually is less pruritic and affects the head, neck, and belly. These flies also transmit the bluetongue virus in sheep and cattle[1].

Diagnosis

Like black flies and sand flies, biting midges are most often collected in the field and not found on the animals. In contrast to the clear, heavily veined wings of black flies, the wings of Culicoides spp are mottled. Identification is probably best left to an entomologist.

Treatment

Larvae may be attacked in their breeding grounds. Extension entomology personnel should be contacted for the latest approved recommendations. Bio Kill Stable Spray™, a modified permethrin, is approved for the spraying of stables and horseboxes to aid in the control of biting midges. A backpack or handheld bulk pesticide spray pack, turbo-blower, or fogger should be used. A fine spray should be produced under pressure, in the amount of 500-750 mL per stable (stable size: 3 m × 3.5 m to 4 m × 4 m). All surfaces in the stable should be sprayed. A reapplication 7-10 days later is needed. Thereafter, application every 3-4 wk should provide ample product buildup on the walls. A fan may be used in the stable to create air movement around the horses, because Culicoides spp are poor flyers. Fly repellent ear tags attached to the horse’s mane and tail (not approved in the USA); pyrethrum synergized with piperonyl butoxide, applied weekly; butoxypolypropylene glycol 800, applied daily; stable blankets; and fine screens on stable doors and windows have been used with mixed success.

References