Tick Bite Pinnal Dermatitis in Animals

Like several other arthropods, ticks commonly cause dermatitis of the pinna, either through direct damage from the tick bite or as a result of hypersensitivity. Ticks can cause irritation at the site of attachment, which can be on the pinna or in the ear canal.

The spinous ear tick (Otobius megnini), found in the southwestern US, South America, Central America, southern Africa, and India, is a soft-shelled tick whose larval and nymphal forms parasitize the external ear canal of horses, cattle, sheep, goats, deer, rabbits, cats, and dogs.

Clinical signs of tick bite pinnal dermatitis include the following:

• head shaking

• head rubbing

• drooped pinnae

• rarely, aural hematoma (1)

Treatment of tick bite pinnal dermatitis in large animals primarily involves ectoparasite control (see Tick Control), which involves treating both the animal and the environment. As many ticks as possible should be mechanically removed from the patient with forceps, and the coat should be sprayed or dipped with an acaricide.

Acaricide choices include the following:

• diazinon

• pyrethroids

• carbaryl

• avermectins (ivermectin, doramectin, eprinomectin)

These products can be applied with a high-pressure sprayer or as pour-ons. Alternatively, ear tags impregnated with abamectin can be affixed to a safety collar to treat and prevent infestations (2). Long-term use of acaricides can result in selection pressure for resistance in ticks, as well as environmental pollution.

Vaccines are an environmentally friendly alternative to acaricides, and although they have been relatively effective against tick infestations in Australia, Cuba, Mexico, and other Latin American countries, they cannot completely control infestation or transmission of pathogens transmitted by ticks. These vaccines are not yet available in the US.

Gotch ear, described in cattle, horses, mules, and a goat (3), results from infestation of the pinna with the Gulf Coast tick, Amblyomma maculatum. Adult ticks of this species prefer feeding on the ears of their hosts. When ticks are present in sufficient numbers, the pinna becomes edematous, erythematous, and crusted at the tick-attachment sites. Curling of the tip of the ear and necrosis can also occur.

It has been suggested that the spotted fever group rickettsiae (eg, Rickettsia parkeri), which are transmitted by tick bite, might cause the skin lesions of gotch ear, because ticks removed from the pinnae of animals with gotch ear have, in some cases, tested positive for these bacteria by PCR assay. However, efforts to find Rickettsia spp in skin samples from affected pinnae have failed.

A differential diagnosis of gotch ear primarily includes a bite from another tick and trauma to the ear.

Removal of ticks and treatment of any secondary infection are curative of gotch ear.