Canine Demodicosis: Overview & Update

Canine demodicosis is an inflammatory condition of the skin caused by increased numbers of Demodex spp mites that are normally present in low numbers in the hair follicles and sebaceous glands.¹ At least 2 different species of mites (ie, D canis, D injai) are responsible for canine demodicosis. D cornei was once considered a separate species, but recent literature suggests it is a variant of D canis.²

Clinical signs include patchy to diffuse alopecia, erythematous papules, pustules, and comedones. Some patients, especially those infested with D injai, may have severe seborrhea oleosa.³ Secondary infections with bacteria and/or yeast are common and may result in more severe lesions.¹ (See Figure 1.)

FIGURE 1

Adult-onset demodicosis with severe, methicillin-resistant deep pyoderma with hemorrhage and resulting hemorrhagic crusts. Figure courtesy of Katherine A. Doerr-Siegfried, DVM, DACVD

Presentation can be localized or generalized. Localized demodicosis typically is limited to less than 6 patchy alopecic regions on the head or limbs,¹ whereas the generalized form (see Figures 2 and 3) can affect an entire body region or 3 or more paws.¹

FIGURE 2

Adult-onset generalized demodicosis with secondary Malassezia spp dermatitis and bacterial pyoderma; generalized demodicosis affects an entire body region or 3 or more paws.1 Figures courtesy of Katherine A. Doerr-Siegfried, DVM, DACVD

FIGURE 3

Closer view of patient in Figure 2; note the diffuse comedones

Dogs may be affected at any age. Juvenile-onset demodicosis typically presents in patients younger than 1 year of age, and presentation is usually localized.¹ It is suspected that some cases are due to a defect in the immune system that may be genetically linked, as evidenced by certain breed predispositions. However, immunosuppression caused by internal parasites or malnutrition may also predispose younger patients to a proliferation of Demodex spp mites.^4,5 Patients that show clinical signs of adult-onset demodicosis are often older than 4 years of age. Fifty percent of these patients have an underlying condition—typically a disease that suppresses the immune system and predisposes them to demodicosis.⁶ 

Demodicosis can occur in any canine breed, but some breeds are more frequently diagnosed.^1,4

Diagnosis

Canine demodicosis is diagnosed through microscopic observation of mites in deep skin scrapings (see Figure 4) and/or trichograms.¹ Deep skin scrapings are obtained by scraping the skin in the direction of the haircoat with a dulled surgical blade or a spatula until capillary oozing is obtained. The deep skin scraping or trichogram is positive when fusiform eggs, 6- or 8-legged larvae, 8-legged nymphs, or 8-legged adults are seen.¹ D canis adult mites typically are 40 × 250-300 µm with a moderately long tail. D injai adult mites are approximately 40 × 350 µm with a longer tail.¹ When pododemodicosis or chronic fibrosing lesions are present, or the patient is a shar-pei, mites may be difficult to obtain by skin scraping and a skin biopsy may be required for diagnosis.¹

FIGURE 4

Adult D canis mite seen on a deep skin scraping. Figure courtesy of Katherine A. Doerr-Siegfried, DVM, DACVD

In patients with adult-onset generalized demodicosis, additional diagnostics (eg, CBC, serum chemistry profile, thyroid function tests) may be required to identify underlying diseases that can suppress the immune system (eg, hypothyroidism, hyperadrenocorticism, malnutrition, neoplastic disease).⁷

Treatment

Localized demodicosis may resolve without treatment within 2 months.¹ In breeding dogs, treatment should be withheld initially to determine whether the condition will resolve on its own or progress to generalized demodicosis. If progression occurs, the dog should not be used for breeding because it may pass the disease to subsequent generations.¹

In the United States, amitraz dip is the only product licensed to treat demodicosis.⁸ Extra-label treatments such as macrocyclic lactones (eg, avermectins, milbemycins) are used more often than amitraz because administration is easier and more convenient.^9,10 These products should not be used in breeds susceptible to the ABCB1-1Δ (MDR1) gene mutation.^1,11 Examples of macrocyclic lactones include oral ivermectin or doramectin, topical moxidectin, and oral milbemycin. More recently, the isoxazoline class of parasiticides (eg, afoxolaner, flura-laner, lotilaner, sarolaner) has shown promise as an extra-label option.^12-14

Treatment Options for Canine Demodicosis

*The only labeled treatment is amitraz every 2 weeks. The treatment plans recommended in this table are extra-label but are supported by research and the author’s personal experience.

Patients should be monitored with monthly deep skin scrapings from the 3 to 5 most severely affected areas, with treatment continuing for 30 days after the second negative monthly deep skin scrapings.⁵ A skin scraping is considered negative when no dead or live mites are observed. A patient is not considered “cured” until 1 year past the last negative deep skin scraping.¹ Juvenile-onset demodicosis has a higher chance of being cured if the patient has no underlying health concerns. Patients with adult-onset demodicosis may require chronic therapy unless an underlying disease process is discovered and treated.¹

Conclusion

The veterinary team should be knowledgeable about this disease for the sake of clients and patients. (See Demodicosis Talking Points.) Demodex mites are natural inhabitants of dogs’ skin and sebaceous glands. A weakened immune system can allow mite populations to increase, resulting in alopecia, erythema, and scabbing. Localized and generalized forms of canine demodicosis can occur, and onset can start at less than 1 year of age (ie, juvenile-onset) or older than 4 years of age (ie, adult-onset). Localized demodicosis may resolve without treatment, whereas adult-onset demodicosis is often associated with an underlying condition that requires concurrent therapy.