Silene Young, DVM
The next time a chronically itchy cat comes into your clinic, don't forget to rule out demodectic mange.
The next time a chronically itchy cat comes into your clinic, don't forget to rule out demodectic mange. While demodicosis in cats and dogs is not thought to be pruritic, the rules are changing. The more recently identified Demodex species in cats — and dogs — Demodex gatoi, has a significantly different presentation than that of Demodex cati. Here's a comparison of the two species:
Infection with D. gatoi is often pruritic, resulting in alopecia from excessive grooming in the areas a cat can reach, such as the limbs, flanks and ventrum. It mimics other pruritic skin disorders in cats, such as allergic or psychogenic disease, and is often symmetrical. In addition to alopecia, cats may present with hyperpigmentation, scaling, excoriations or crusting.
In comparison to D. cati, the more recently identified D. gatoi is a shorter mite that lives in the stratum corneum, much more superficially than D. cati, which lives in the hair follicles. Microscopically, D. gatoi it is about half the length of D. cati and appears more rounded at the tail end.
Demodex gatoi appears to be more prevalent in parts of the southern United States such as Texas and Florida; it is diagnosed less frequently in other parts of the country. Paul Bloom, DVM, Dipl. ACVD, Dipl. ABVP, from the Allergy, Skin and Ear Clinic for Pets in Livonia, Mich., reports treating very few cats per year in Michigan, but he says that his colleagues in the southeastern United States see it much more frequently.
Despite the contagious nature of the D. gatoi mite, the geographic prevalence does not appear to be widening. And even though practitioners from areas such as Texas and Florida might have D. gatoi higher on their differential diagnosis list, the mite should be on the differential list for any pruritic, alopecic cat.
A cat with D. gatoi infection may respond to steroids, which help control the pruritus, but will often make repeat visits to the clinic for incompletely resolved clinical signs. This presentation is a tip-off to consider looking for D. gatoi. Current experience appears to indicate that steroid usage does not exacerbate the disease, in contrast to steroid usage with D. cati.
In addition, the D. gatoi mite appears to be contagious, so the index of suspicion should increase if there are multiple cats in a household with similar clinical signs. However, cats can harbor D. gatoi without significant pruritus.
As with any dermatologic case presentation, a minimum database including deep and superficial skin scrapes, cytology and acetate tape preparations can help diagnose or potentially rule out demodectic mange in cats.
Diagnostic testing for D. gatoi mites is significantly different than that of the follicular D. cati mite. Demodex gatoi may be best found on superficial skin scrapes, and Bloom recommends scraping unaffected areas with broad coverage since cats often groom off the mites they can reach. In addition, he recommends performing skin scrapes on all in-contact cats, as the mites may be more likely found on a nonpruritic housemate. In contrast, D. cati diagnosis would require deep skin scrapes in the affected areas. There are also reports that both species of Demodex may be found in the feces of affected cats because of their frequent grooming behavior.
Since the mites are commonly removed during grooming by the cat, multiple scrapes, conducted at five or six different times, are recommended. Repeated false negative skin scrape results are common with D. gatoi, so if there is still suspicion after multiple negative skin scrape results, diagnosis can be confirmed by response to treatment. It has also been reported that D. gatoi can be found on acetate tape preparations, which might be helpful in a multiple cat household situation.
No evidence indicates that these mites can survive in the environment (e.g., cat beds, carpeting), so appropriate treatment of all in-contact cats should resolve the infestation when it occurs.
The primary challenge for practitioners trying to manage D. gatoi mites is finding them, says Bloom. Cats with D. gatoi infection present like a typical allergy case, and since the mite is difficult to find on scrapes, the disease may go undiagnosed for some time. Empirical treatment of cats may be indicated to rule out D. gatoi as part of the diagnostic plan.
Then, of course, there is the treatment. The recommendation is to use 1.6 percent to 2 percent lime sulfur dips every five to seven days for four to six weeks on all in-contact cats. A pump sprayer has been recommended to ease the stress on the cats,1 and placing an Elizabethan collar to prevent ingestion of the dip is also suggested.
Demodex cati does not have any known geographic prevalence, is not contagious and is occasionally but not as commonly pruritic as its superficial cousin D. gatoi. Also, as with Demodex canis in dogs, D. cati may be present in small numbers as normal flora in healthy cats.
Dermal lesions associated with D. cati tend to be localized to the head, pinnae, neck and eyelids with crusting, alopecia and the appearance of a miliary dermatitis. Comedones, seborrhea, pustules and papules may also be present. In addition, D. cati infection can present as an otitis, and the mites can be identified in the ear canals.
Cats with the less common follicular mite, D. cati, often have an underlying metabolic illness or are immunocompromised. Disorders such as diabetes mellitus, neoplasia, feline leukemia virus, feline immunodeficiency virus and hyperadrenocorticism should be ruled out in cats with D. cati infection. Some breed predisposition for generalized demodicosis with D. cati has also been noted in Siamese and Burmese cats.
Many cats with D. cati mites will experience resolution with control of their underlying metabolic disease, so identifying and managing this is critical. Nevertheless, treatment of the demodicosis may also be necessary. As with D. gatoi, 1.6 percent to 2 percent lime sulfur dips every five to seven days for four to six weeks is a recommended and effective treatment; 0.0125 percent to 0.025 percent amitraz applied every four to seven days has also been used to treat demodectic mange in cats but carries a significant toxicity risk and is generally not recommended.
Other treatments for Demodex species in cats have been discussed, including subcutaneous doramectin injections given weekly for up to three treatments, selamectin applied topically for multiple treatments and ivermectin given orally for varying time periods. However, for both safety and efficacy reasons, most veterinary dermatologists appear to recommend lime sulfur dips as the best treatment option.
Silene Young, DVM, is a freelance medical writer and editor in Costa Mesa, Calif.
1. Tater KC, Patterson AP. Canine and feline demodicosis. Vet Med 2008;103(8):444-461.