Insect Bite Dermatitis Affecting the Pinna in Animals

Insect bite dermatitis in animals is a worldwide problem caused by inflammatory mediators or toxic substances present in the saliva of various hematophagous insects. It typically affects dogs, cats, and horses. Lesions are found at the bite site; the pinnae are commonly affected, as are the neck, head, and abdomen.

The bites of various insects (eg, flies, mosquitoes, fleas) can cause dermatitis, and the causative insect can vary with the season and environment.

In addition to causing dermatitis, the bites of mosquitoes (Aedes spp, Culex spp) can also cause a hypersensitivity reaction in cats, which is characterized by inflammatory lesions on the pinnae, face, and paws.

The characteristic clinical signs of insect bite dermatitis are small papules and wheals with central hemorrhagic crusts that can progress to multiple small ulcers. Lesions are found on the apex of the pinna in cats and dogs with erect ears or on the folded pinnal surfaces in dogs with flopped ears.

In horses, bites of stable flies (Stomoxys calcitrans), black flies (Simulium spp), and biting midges (Culicoides spp) can cause a hypersensitivity reaction or severe dermatitis that results in lesions on the dorsal and/or ventral trunk and face, as well as the pinna.

Treatment in horses primarily involves insect control measures, including use of fly repellents, adjustment of turnout times to avoid peak insect feeding times, control of fly populations via environmental cleanup (manure, compost, etc), and insecticide use. Topical or oral short-acting glucocorticoids might be necessary to decrease inflammation and pruritus in severe cases. The use of therapeutic face masks with ears can decrease insect exposure.

The rabbit flea (Spilopsyllus cuniculi), found mainly in Europe and Australia, can affect dogs and cats by adhering tightly to the skin of the tip and margins of the pinna, where it can cause dermatitis. The rabbit flea is an important vector of diseases, including myxomatosis.