Image Quiz: Dermatology—An itchy fox terrier
Infection with Demodex injai is correct!
Demodex injai is easily differentiated from D. canis as it is one-and-a-half to two times longer than D. canis. It has also recently been described as the cause of facial pruritus and lip dermatitis in a case series of Shih tzus. Demodex folliculorum is the human demodex mite, and D. gatoi is the feline demodex mite.
This large, long-bodied follicular mite seems to most commonly affect terriers, and infection is characterized by greasy truncal seborrhea. Unlike Demodex canis, D. injai are not found in large numbers, so they can be difficult to find on skin scrapings. Multiple scrapings with microscopic evaluation of all fields under low power using the 4X or 10X objective are necessary for diagnosis. Adult-onset demodicosis is usually due to an underlying immunosuppressive condition, and, in this case, hypothyroidism was diagnosed. Prior chronic glucocorticoid therapy for atopic dermatitis was likely also a contributing factor.
Treatment options for D. injai infection are the same as for D. canis infection (daily oral ivermectin or milbemycin, or topical amitraz every one to two weeks). Treatment is continued until one month beyond a negative skin scraping result.
This dog was treated by discontinuing the glucocorticoid therapy and intiating thyroxine supplementation and topical amitraz. The demodicosis resolved with therapy and did not recur. The pruritus resolved with a hypoallergenic diet trial and flared on subsequent diet challenge, documenting the diagnosis of later-onset food allergy complicating prior atopic dermatitis.