Demodicosis in 2010 (Proceedings)

Canine Demodicosis

Currently there are 3 morphologically separate species of follicularly oriented demodex mites in the dog: Demodex canis, Demodex injae, and Demodex®. The unnamed species has a very short tail but is not Demodex gatoi of the cat. Although the mites have a different morphology, they all can produce the same clinical disease, namely a parasitic folliculitis with a frequent secondary staphylococcal infection. The basic tenant of dermatology "Under every pyoderma, hair loss condition, or greasy seborrhea, the demodex mite might lurk " is still valid. This applies mostly in puppies but adults dogs can also develop spontaneous disease. Notice that pruritus is not in the sentence. Dogs with demodicosis can be itchy but this typically is a lesional pruritus where the animal itches at pre-existing skin lesion. The pruritus in these animals is typically associated with a secondary staphylococcal or Malassezia infection and not the mites themselves. In the past few years, there have been multiple case reports of dogs with facial and pedal pruritus, mimicking that seen in allergic skin disorders, that is associated with Demodex injae infection. With resolution of the demodicosis, the dog's allergies disappear. If these dogs are examined away for the areas they are itching,one usually finds a mild greasy seborrheic condition which should prompt a skin scraping.

The clinical presentations of canine demodicosis are as they always were: local demodicosis, juvenile-onset (3-18 months of age) demodicosis, and adult-onset (≥4 years of age) demodicosis. Cases diagnosed in dogs 18- 48 months old probably had demodicosis since puppyhood. It is important to differentiate these conditions since localized demodicosis has, in the author's opinion, no genetic basis and will always resolve spontaneously. The author is unaware of any cases that have relapsed later in life.

Localized Demodicosis

There is no uniformly accepted standard which defines exactly what localized demodicosis is. In the author's opinion this is a disease where the animal has no more than 4 small localized areas of alopecia, erythema, and scaling. These lesions typically are seen on the face and/or forelimbs and rarely are infected. Bilateral involvement of the ear canals without any other skin lesions is also a localized condition. Cases which have more than 4 discrete spots, have an entire body region like the head involve, or have pododemodicosis of all 4 feet should not be categorized as localized demodicosis but rather as a localized presentation of generalized demodicosis. This differentiation is important because dogs with generalized demodicosis, even in its most mild form, have a genetic predisposition to this disease. They should not be used for breeding and might experience a relapse later in life. The ease of differentiating localized from generalized disease depends on the duration of the clinical signs before presentation. It typically takes a puppy 4 to 8 weeks to develop all of its localized spots to their maximum extent. If the case is presented with lesions this old, the odds of the dog developing additional lesions is small and the diagnosis of localized demodicosis can be made. If the spots were just recognized, a re-examination in a month will be needed before an accurate assessment can be made.

The cutaneous lesions will always resolve spontaneously unless there is a significant secondary infection. Topical treatments with benzoly peroxide gel, Goodwinol™ ointment, or any of the otic parasiticides have been reported to be effective. The specifics of the case dictate which if any these treatments might be appropriate. Otic localized demodicosis is rare and requires treatment since these dogs typically develop a ceruminous otitis externa with the likelihood of a secondary Malassezia or bacterial infection. Ear cleaning with the application of a topical parasiticide is needed. Topical otic avermectins are not effective.

Generalized Demodicosis

Simply put - everything that isn't localized is generalized. All of these dogs have a genetic predisposition to this disease and should not be used for breeding even if their active disease self-cures or responds completely to treatment. Dogs with onset of disease in puppyhood probably have an increased genetic predisposition towards the disease. Dogs with adult-onset of disease also have a genetic predisposition but require some additional immunologic insult to allow the mites to begin their proliferation. In the author's practice, indolent lymphoma is the most common triggering disease in these dogs. Cases with Cushing's disease, hypothyroidism (?), neoplasia, etc are also recognized. In these cases, treatment will be unrewarding unless the underlying condition can be corrected.

In the USA, there are only 2 currently licensed products for the treatment of generalized demodicosis in the dog. One is the FDA-registered product, Mitaban®, and the other is the EPA-registered product, ProMeris®. Mitaban® is applied as a dip every 14 days at the licensed strength of 250 ppm. Because the dog's hair coat can plug the hair follicle orifice, dogs should be clipped frequently and need to be bathed before dipping if the skin is dirty. The reported success of the licensed treatment varies between 0 and 98%. Since this is an FDA-registered product, extra label usage is permitted. Dipping more frequently, weekly in most instances, with an increased concentration: 500, 750, or 1000 ppm, gives better results and consistent cure rates around 95% are attainable. This product has many limitations for its use and should be researched before dispensing it to a client or using it in-house. ProMeris® for dogs is a relatively new monthly spot application product for the control of fleas and ticks. Fleas are controlled by metaflumizone and the ticks are dealt with by amitraz. Because of the inclusion of amitraz, the product is also licensed for the treatment of generalized demodicosis. Since amitraz can kill all arachnids, it could be useful in the treatment of scabies or cheyletiella but it is not labeled for those indications. In a recent study where the ProMeris® was dispensed to 24 dogs (13 with juvenile onset, 11 with adult-onset) who could not be cured with other treatments, ~ 93% of the juvenile-onset dogs had an excellent response (negative skin scrapings) while 45% of the adult-onset cases had this response. In this study, the ProMeris® was applied at a 14 day interval which is double its licensed rate of application. The investigators in this study warn of the product's use in households with asthmatic individuals and saw a pemphigus-like drug reaction. The latter has been seen by others and is more common in the Labrador retriever.

Since the late 80s, the most effective treatments for generalized demodicosis in the dog centered on the use of an oral ivermectin or avermectin. Unless the dog had a pharmacogenetic (ABCB1-1Δ (MDR1) gene) abnormality, the various avermectins were safe and efficacy could approach 95%. Since the pharmacogenetic abnormality is not just restricted to Collies and is found in mixed-breed dogs, genetic testing should be performed on dogs before oral medications are used. Commonly used products include ivermectin: 0.3-0.6 mg/kg q24-48h; milbemycin: 1.5-2 mg/kq q24h; or doramectin: 0.2-0.6 mg/kg sq. Treatment is continued until multiple skin scrapings at the same office visit are negative for any stage or part of a demodex mite and then for an additional 30 to 60 days. If the scrapings are done accurately, relapse rates of less than 5% should be expected. These cases can be controlled long term with the continual application of the product.

Advantage-Multi® (10% moxidectin, 2.5% imidacloprid) is licensed in Europe for the treatment of generalized demodicosis with a monthly application. In tough cases, the product, as licensed, has not lived up to the expectations of dermatologists. Several studies have shown that more frequent application increases the efficacy of the product. The numbers of cases reported are too small to make firm conclusions but a ~50% cure rate can be expected with application every 14th day. Cases not cured at that frequency may respond to weekly application. This product is not licensed for this application in the US but since it is an FDA-registered, not an EPA-registered, product, it can used with informed consent for this treatment. The author has had success in the long term control of dogs that could not be cured with other products.

Feline Demodicosis

Three morphologically separate species of demodex mites occur in cats: Demodex cati, Demodex gatoi, and Demodex ®.D. cati is a convential follicularly-oriented mite while D. gatoi is a surface organism. The unnamed species more closely resembles D. Gatoi and probably shares a similar pathogenicity.

Cats with Demodex cati demodicosis have some underlying immunosuppressive condition with FIV infection being most common. This condition is very rare in the cat and is very unrewarding to treat. D gatoi infection is much more common and actually is a contagious disease. A typical history is one of an indoor-outdoor cat who develops a steroid-resistant pruritus. Facial, pedal, and inguinal localization appears to be most common but signs can appear anywhere. Since the mite does not live in the hair follicle, the hair loss seen is due to the cat's licking or pulling on the hairs in the area. A secondary folliculitis is uncommon unless the cat has received steroid treatments. As in most pruritic conditions the itching becomes more intense with advancing time. Another fairly common presentation is where a strictly indoor cat develops the pruritic dermatitis described above. In this case, the infection was introduced into the household by one of the indoor-outdoor catmates or by a new cat, typically from a shelter, introduced into the house.

The diagnosis of D. gatoi infection is much more difficult then diagnosing D. cati or any other follicular demodex mite infection. In the latter situation, the mite is always there because its follicular habitat prevents the animal from licking/scratching it off. D. gatoi lives in the surface keratin layer and is easily licked off. Scrapings must be done in areas where there is minimal trauma. The best area is in the transition zone from normal to abnormal. In some cases, the best place to scrape is another cat in the household who isn't so itchy. In some cases, especially those with the most chronic histories, the mite cannot be demonstrated and response to treatment makes the diagnosis. Systemic ivermectins or avermectins are not effective in curing this infection because of the mites' surface localization. The application of Frontline® will reduce the cat's pruritus but may not eradicate the mite because of issues with the surface translocation of the active ingredient, fipronil. The spray may perform better. Currently, the only product with consistent efficacy is lime sulfur applied weekly for 3 to 6 weeks.