An insect is an invertebrate animal of the class Insecta, characterised by a three-part body (head, thorax, abdomen), three pairs of jointed legs, and, in most adult forms, one or two pairs of wings. Hundreds of insect species directly affect horse health and welfare through biting, blood-feeding, laying eggs in tissue, and acting as intermediate hosts for parasites and pathogens.
Stable flies (Stomoxys calcitrans) and horse flies (Tabanus spp.) inflict painful bites that cause horses to stamp, bunch, and lose condition. Culicoides midges trigger sweet itch, a hypersensitive dermatitis that produces intense pruritus along the mane and tail. Mosquitoes vector West Nile virus, Eastern and Western equine encephalitis — core targets of the vector-borne disease vaccines. Bot fly larvae (Gasterophilus spp.) are ingested after the horse licks eggs from its coat; the larvae then attach to the stomach lining, making regular deworming to control bot fly larvae the primary control. Integrated pest management on a horse property combines manure removal to eliminate breeding sites, fly traps, permethrin-based repellents, and fine-mesh fly rugs during peak insect season.
Further Reading
Insects as a class are described in the insect article on Wikipedia. The role insects play as vectors and irritants in equine skin conditions is covered in the Merck Veterinary Manual section on Skin Disorders of Horses.