Midges also transmit a range of infectious agents, including arboviruses (especially bluetongue virus) and the nematode Onchocerca. Horses can develop a hypersensitivity reaction to antigens in Culicoides saliva. This condition is known as sweet or Queensland itch, and can be a major clinical problem, in part because it is difficult to prevent the midges gaining access to the horses - they are so small that they can pass through standard insect screens.
There are more than 60 genera and 4000 species if biting midges (also known as punkies or no-see-ums). Culicoides is the primary genus in North America and there are more than 1000 species within the genus. Leptoconops and Lasiohelia are other genera in North America, the former in the west, the latter in the east.
Host range and geographic distribution
Adult female biting midges blood feed on a wide variety of mammals, including people, as well as birds. The females and the males also feed on nectar.
Among the various genera of biting midges, Culicoides is the most widely distributed around the world and is found in tropical and cooler areas, and up to approximately 4000 metres above sea level. Other midge genera have more limited distributions.
Life cycle - direct
Adult female Culicoides need a blood meal prior to laying eggs, which they do in damp soil adjacent to water or in moist, rotting vegetation. The larva hatches from the egg, moults through four instars, and then pupates. If the pupae are in water, they float to the surface before hatching to release the adult midges.
In warmer areas of the world, the life cycle (adult to adult) can be completed in a few weeks, but in cooler areas may take much longer. For example, eggs of one northern species (C. grisescens) may not hatch for months, allowing the flies to over-winter at this stage of the life cycle. This means that in cooler areas there is usually only one generation of Culicoides each year.
Life Cycle: Culicoides species
Pathology and clinical signs
The major problem for horses, and for other animal and bird hosts and for people, is the biting activity of the female midges. The flies feed by cutting and sponging. The flies can occur in swarms – preferring windless conditions, they are difficult or impossible to see, they are sufficiently small to penetrate normal mosquito screens, and the bites can be very painful and pruritic and can develop into papules and wheals. Midges find suitable animals to bite primarily on the basis of levels of carbon dioxide, and possibly other compounds produced by the host.
Culicoides species transmit Onchocerca cervicalis among horses, and midges can serve as vectors for arboviruses, protozoa, and other filarial nematodes of domestic animals and birds, and wildlife.
Queensland Itch or Sweet Itch or Summer Dermatitis
In some horses Culicoides is associated with a specific clinical condition – Queensland Itch – which results from a hypersensitivity to antigens presumably in the saliva of the insects. Similar reactions may occur in horses as a result of biting by black flies (Simulium species), stable flies (Stomxys calcitrans) and perhaps face flies (Haematobia irritans). There is evidence that affected horses have a genetic predisposition to the development of insect hypersensitivity, and some breeds (among them Arabians and Quarter Horses) are more commonly affected than others.
Queensland Itch (named for where it was first described) is the commonest cause of pruritic skin disease in horses around the world. In Canada, Queensland Itch has been reported from British Columbia, and also probably occurs elsewhere in the country.
Clinically, Queensland Itch is a seasonal problem, occurring at times of the year, and times of the day, when the adult flies are most active. Clinical signs often worsen as the horse ages. The most characteristic clinical sign is pruritus, and there may also be crusted papules. The distribution of the pruritus and the lesions matches the preferred biting sites of the local species of midges. Lesions may be primarily dorsal (mane, rump and tail head), ventral (ventral thorax and abdomen, axillae and groin), or both areas may be affected. The lesions may spread to affect almost any part of the body, other than the flanks.
Many affected horses will attempt to relieve the itching by rubbing and biting, and this may lead to self-trauma and secondary bacterial infection.
Treatment and control
Various topical pyrethroid-based products as well as one product containing dichorvos have efficacy claims against the adult flies, but they may have to be used daily or weekly. Treatment of uncomplicated midge bites is usually based on topical medications to relieve the pruritus. Treatment of hypersensitivity is more complex, and often depends on the use of systemic medications for the pruritus.
Additional information on the products mentioned is available from the Compendium of Veterinary Products (Twelfth Edition, 2011), or from the manufacturers.
Biting by midges can be minimized by housing susceptible horses during periods of the day when the flies are active. But midges can penetrate normal mosquito screening, so smaller mesh material should be used. Spraying the screens with an insecticide may be helpful, as may a spray-mist applicator of insecticide inside the housing. Also, adult biting midges do not usually fly far from their breeding grounds, so it would be helpful if areas suitable for egg laying and development are removed, or if the horses are kept well away from them.
Public health significance
Kleider N et al. (1984) Culicoides hypersensitivity in the horse: 15 cases in southwestern British Columbia. Canadian Veterinary Journal 25: 26-32.