Otodectic mange (otoacariasis)

Otodectic mange or otoacariasis is a parasitic infestation of the external auditory canal caused by Otodectes cynotis, a cosmopolitan psoroptid mite with an oval body about 460-530 µm long and a long, conical rostrum. The limbs are long and terminate with foot-like structures which, at their extremity, have a cup-shaped suction pad (pulvillus) which the parasite uses to move rapidly between the waxy debris in the ear. The mite shows marked sexual dysmorphism: the male has four pairs of long limbs, which exceed the body both anteriorly and posteriorly, and poorly developed abdominal lobes. The female, on the other hand, has three pairs of limbs since the fourth pair is atrophied, while the abdominal lobes are well developed. The third pair of limbs of the female mite do not have pulvilli but have long bristles (Fig. 1).

The life-cycle of Otodectes cynotis is conducted entirely on the host (permanent parasite) within the external auditory canal. The female lays oval-ellipsoid eggs which hatch rapidly in about 4 days, giving rise to a larva that feeds actively for about 1 week before evolving through two successive instars (protonymph and deutonymph) (Fig. 2). Mating, which can often be observed during a microscopic examination, involves the male and a deutonymph.

The male attaches to the nymph using its copulatory suction pads and only if the nymph transforms into a female will reproduction occur; otherwise, the deutonymph will give rise to a male mite (Fig. 3). The life-cycle, from egg to adult mite, lasts about 3 weeks, while the adult parasite can live on the host for about 2 months.

Also known as auricular mange, otoacariasis is extremely common in young animals, readily affecting both dogs and cats, although the prevalence is considerably higher in the feline species than in the canine one. Contagion of humans who have been in contact with infested animals has been reported rarely and is manifested as papular lesions mainly located on the torso and arms.

The parasites live in the auditory canal where, feeding on epidermal debris, they stimulate the secretion of variable amounts of wax. The development of hypersensitivity phenomena, demonstrated in cats, explains the intense pruritus that often accompanies this infestation. It should, however, be noted that the intensity of the pruritus is not proportional to the number of mites present in the auditory canal.

CLINICAL SIGNS

The clinical signs of otodectic mange are a pruriginous, erythematous-ceruminous otitis, usually bilateral, characterized by the formation of a typical dry, dark brown wax that resembles coffee grounds, even though the wax can sometimes be pale-coloured (Fig. 4, video 1). The otitis is often complicated by secondary infections caused by yeasts and/or bacteria (Fig. 5). Lesions of the auricle and peri-auricular region may be seen in some affected animals; these lesions (haematomas of the external ear, alopecia, erosions, ulcers, scabs) are self-inflicted (Figs. 6 and 7). Less frequently there are extra-auricular skin lesions on the temples, head, face and neck; these papules and self-inflicted lesions are related to hypersensitivity phenomena sometimes due to the ectopic localisation of the mites (Fig. 8). In some cats, mites can be found in the external auditory meatus in the absence of clinical signs (asymptomatic carriers); this could be explained by the lack of hypersensitivity reactions.

DIAGNOSIS

The method of choice for finding Otodectes cynotis is observation of auricular secretions under a microscope. These secretions are collected from the external auditory meatus using a cotton-tipped swab. The material collected is then immersed in an abundant quantity of vaseline oil, covered with a coverslip and observed under a microscope at a low magnification. The samples must be taken before applying wax-dissolving products and before cleaning the ears.

To increase the sensitivity of this examination, it is advisable to take several samples, collecting some material also from the horizontal part of the auditory canal using the cone of the otoscope as a guide to introduce the swab. The diagnosis of otodectic mange is made from observing the parasite or its eggs under a microscope (Fig. 9). Usually a large number of mites are visible by microscopy, although in some subjects several samples may have to be taken in order to see a single parasite or egg.

Sometimes the mites can be seen macroscopically or during an otoscopic examination as small, very mobile, white dots (Figs. 10 and 11, video 2).

A superficial skin scrape can be carried out to find mites in the rare cases that these have localised on the skin away from the ear.

DIFFERENTIAL DIAGNOSES

• Yeast-induced waxy otitis (dog and cat)
• Sarcoptic mange (dog)
• Feline scabies (cat)
• Allergies (cat)
• Dermatophytoses (cat)

TREATMENT

The treatment of otodectic mange is based on both topical and systemic acaricidal drugs. Before starting the specific antiparasite treatment it is advisable to wash out the ear, which mechanically removes a large number of parasites as well as the excess wax induced by their presence. All the animals in contact with an infected one must be treated because of the very contagious nature of this disease and also because of the possible presence of asymptomatic carriers.

In Italy there are numerous compounds available for topical and systemic use which have been registered for the treatment of otodectic mange in dogs and cats. In traditional topical therapy, products containing acaricides (tetramethrin, phenothrin, carbaryl, dichlorophen, pyrethrum and thiabendazole) are introduced directly into the auditory canal every day for at least 3 weeks. If microencapsulated permethrin, which releases the active principle gradually, is used, the product can be introduced every 7 days; the treatment must be repeated at least three times.

Systemic treatment involves the use of spot-on selamectin, given twice at a 30-day interval, and moxidectin (available in combination with imidacloprid), also given twice at a 28-day interval. Systemic treatment has the advantage of being effective against any ectopic, cutaneous collections of Otodectes.

There are also alternative protocols using drugs that have not been registered for the treatment of otoacariasis, such as ivermectin 1% (0.2-0.4 mg/kg injected subcutaneously at least twice at a 14-day interval or per os, in which case at least three doses should be given, with a dose administered every 7 days) and spot-on fipronil (instilling a few drops directly into the auditory canal at intervals of 15-30 days).

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