Arthropods commonly cause dermatitis of the pinnae either through direct damage from the bite of the parasite or as a result of hypersensitivity. Ticks can cause irritation at the site of attachment and may be found on the pinna or in the ear canal. The spinous ear tick (Otobius megnini), found in the southwestern USA, south and central Americas, southern Africa, and India, is a soft-shelled tick, the larval and nymphal forms of which parasitize the external ear canal of horses, cattle, sheep, goats, deer, rabbits, cats, and dogs.
Clinical signs include head shaking, head rubbing, or drooped pinnae.
Treatment in large animals involves both the animal and the environment. As many ticks as possible should be mechanically removed with forceps, and the coat sprayed or dipped with acaricides. Acaricide choices include diazinon, pyrethroids, carbaryl, and avermectins (ivermectin, doramectin, eprinomectin). These products can be applied with a high-pressure sprayer or as pour-ons. Longterm use of acaricides may result in selection of resistant ticks and environmental pollution.
Vaccines are an environmentally friendly alternative to acaracides and although they have been shown to be relatively effective against tick infestations in Australia, Cuba, Mexico, and other Latin American countries, they are not able to completely control infestation or transmission of pathogens transmitted by ticks. These vaccines are not yet available in the USA.
Gotch ear, described in cattle, horses, mules, and a goat, results from infestation of the pinna with the Gulf Coast tick, Amblyomma maculatum. Adult ticks prefer feeding on animal ears, and when ticks are present in sufficient numbers the pinna becomes edematous, erythematous, and crusting at the tick-attachment sites. Curling of the tip of the ear and necrosis may occur.
Spotted fever group rickettsiae, such as Rickettsia parkeri, have been suggested to cause the skin lesions, because ticks removed from the pinna of animals with gotch have been, in some cases, positive for Rickettsia spp by PCR. However, efforts to find the organisms in skin samples from affected areas have failed. The two main differential diagnoses for gotch ear are a bite from another tick and trauma to the ear. Removal of ticks and treatment of any secondary infection are curative.
