This worldwide problem is caused by inflammatory mediators or toxic substances present in the saliva of various hematophagous insects. It typically affects dogs, cats, and horses. Clinical signs are characterized by small papules and wheals with central hemorrhagic crusts that can progress to multiple small ulcers. Lesions are found on the apexes of the pinnae of cats and dogs with erect ears or on the folded surfaces of the pinnae of dogs with flopped ears. The causative insect can vary with the season and environment and include, among others, mosquitoes (Aedes spp, Culex spp), the stable fly (Stomoxys calcitrans), and black flies (Simulium spp). Mosquitoes can also cause a hypersensitivity reaction in cats characterized by inflammatory lesions on the pinnae, face, and feet (see Allergy). The rabbit flea (Spilopsyllus cuniculi) is found mainly in Europe and Australia and can be transmitted to dogs and cats. It adheres tightly to the skin of the host and typically affects the tip of the pinna, where it may cause dermatitis. In horses, bites of the stable fly, black flies, and Culicoides spp can cause a hypersensitivity reaction or severe dermatitis that results in lesions on the dorsal and/or ventral trunk and face in addition to the pinna. Treatment includes fly repellents, controlling the fly population with environmental clean up (manure, compost, etc), and insecticides. Topical or oral short-acting glucocorticoids may be necessary to reduce the inflammation and pruritus in severe cases.