Tackling feline ear disease, dermatophytosis


This is an exciting time in feline dermatology. Many new diseases have been discovered, and new medications are available for treatment. The days of giving a Depo-Medrol injection to treat any itchy cat, and labeling those that fail to respond as "psychogenic" hopefully are over! In this article, I will discuss feline ear mites and dermatophytosis, two of the most common infectious diseases seen in feline practice. Practical diagnostic and the latest treatments will be given for each disease, to assist you in your work-up and clinical management of these cases.

Feline ear disease


Photo. 3: Otodectes cynotis, the ear mite of dogs and cats.
Ear mites are the most common cause of otitis in cats, especially in kittens. Otodectes cynotis, the ear mite of dogs, cats, and ferrets, causes otitis with a classic "coffee ground-like" appearance of brown-black granular discharge in the ear canals. O. cynotis is a white, fast-moving psoroptid mite (Photo 3) with four pairs of legs and a three-week life cycle. Adults have a lifespan of two months.

Clinical signs are caused by irritation of the ear canal epithelium due to mite feeding behavior as they ingest epidermal debris and tissue fluid. Most cats show moderate to severe otic discharge and pruritus but this can be variable. Some cats with a large amount of discharge will have little otic pruritus, while those with minor discharge can be very pruritic. Mites are commonly found elsewhere on the body, including the neck, rump and tail. These ectopic mites may cause pruritus and dermatitis, which may resemble fleabite hypersensitivity, food hypersensitivity, or atopy. Therefore, ear mite infestation is a differential diagnosis for any pruritic cat. Ear mites are highly contagious to dogs, cats and ferrets, so all contact animals must be treated. These mites are zoonotic and can cause a temporary papular dermatitis in people.

Diagnosis of ear mites is straight forward, and involves taking a cotton swab of ear canal debris and rolling it onto a glass slide with a drop of mineral oil added to spread material evenly. The mites are large and often readily apparent. Often mites can be seen on otoscopic examination as moving white specks among the otic discharge.


Photo 2: Microsporum canis macroconidia showing canoe-like shape and fungal hyphae.
Treatment begins by cleaning the ear canals with mineral oil or a cerulytic agent. There are numerous otic medications available for the treatment of ear mites. All contact animals must be treated. Otic parasiticides such as Tresaderm® (neomycin sulfate-thiabendazole-dexamethasone), which has ovicidal and adulticidal activity, or various antibiotic-antifungal-corticosteroid otic ointments which smother the mites, such as Otomax®, are effective if used for 14-21 days in combination with three to four weekly whole body applications of flea sprays to kill ectopic mites. A newer otic parasiticidal product, Acarexx®, which contains 0.01 percent ivermectin, has been found to be effective against O. cynotis adults, eggs and larvae. In multiple pet households, or when otic products are ineffective or difficult to apply, systemic ivermectin (1 percent Ivomec bovine solution) is very effective at 0.3mg/kg subcutaneously every two weeks for two treatments or orally once a week for three to four weeks. Due to a higher rate of adverse neurologic reactions, ivermectin should not be used in kittens less than 16 weeks of age. Selamectin (Revolution®) or fipronil (Frontline Top Spot®) applied once a month for two treatments is also very effective in the treatment of feline Otodectes infestations. If there is secondary bacterial or yeast infection, or a large amount of otic discharge, then a concurrent topical otic medication such as Tresederm® should be used after ear cleaning. Failure to identify and treat ear mites in a timely fashion may lead to permanent ear canal damage, with chronic otitis externa and otitis media. In addition, the ear canals' normal "self-cleaning" mechanism, whereby canal epithelial cells slide over each other to push out accumulated cerumen and debris, may be damaged. This failure of self-cleaning often leads to chronic ceruminolith (wax ball) formation deep within the ear canal later in life.

Dermatophytosis (Ringworm) Feline dermatophytosis is a fungal infection caused by the zoonotic dermatophyte Microsporum canis (Photo 2). This organism lives in the superficial keratinized layer of the skin, hair and claws. These fungi can only infect hairs in the anagen (growth) phase. After exposure to a dermatophyte, damage to the stratum corneum is needed for infection to occur through invasion of anagen hair follicles. M. canis is transmitted via direct contact with infected cats or kittens, or indirectly via contact with infected hair. Infection may also occur via contact with fungal elements on grooming tools, cat carriers, bedding or in the infected cat's environment (house dust, furnace filters, heating vents, carpets, drapery, floors and furniture). Arthospores of M. canis in the environment can remain infectious for up to 18 months! Asymptomatic carrier cats may occur, with passive carriage of arthospores on the haircoat acquired from an infected cat or contaminated environment. In naturally occurring dermatophytosis, up to a six-week incubation period is possible from the time of exposure to infection. In healthy kittens or cats, dermatophytosis generally is self-limiting, and often will resolve without treatment in three months. Young kittens are predisposed to this infection due to the presence of a still-developing cell-mediated immune system. Persian and Himalayan cats are predisposed to developing more severe, resistant generalized dermatophytosis, possibly due to a fungal-specific defect in cell-mediated immunity. Any disease, medication, or physiological state that weakens the host immune response can increase the risk of infection.


Photo 4: Human dermatophytosis infection.
Examples include FeLV/FIV infection, iatrogenic or naturally-occurring hyperadrenocorticism, diabetes mellitus, neoplasia, and pregnancy/lactation. One of the worst cases of dermatophytosis that I ever saw was in a cat that had been treated with monthly Depo-Medrol® injections for one year. About 50 percent of people exposed to symptomatic or asymptomatic carrier cats develop signs of dermatophytosis. Children and elderly people are predisposed, as are people on chemotherapy or other immunosuppressive medications (see Photo 4). Clinical signs of dermatophytosis in cats can be very variable, thus any cat with a dermatitis or hair loss should have a fungal culture performed as part of a routine work-up for skin disease.