Demodex (mange mite)

Species

Canine
Demodex canis
Demodex injai
Demodex sp.

Feline
Demodex cati
Demodex gatoi
Demodex sp.

Overview of Life Cycle

Most Demodex spp. are considered normal mammalian fauna.
Neonates are thought to typically acquire mites from the dam via direct skin-to-skin contact, but most individual animals do not develop clinical disease.
All stages of the life cycle (eggs, larvae, nymphs, adults) reside within the lumen of hair follicles and within sebaceous gland ducts; some species are more commonly found in the stratum corneum.
Development from egg to adult takes approximately 20 to 35 days and is completed entirely on the host.

Stages (see images on right)

Six-legged larvae hatch from fusiform-shaped eggs and undergo several molts to become eight-legged nymphs and ultimately adults.
Adults are eight-legged, slender, and elongated mites; their appearance is often described as cigar-shaped.
In the canine Demodex species, the length of adult mites ranges from 180 to 210 µm for D. canis, 330 to 370 µm for D. injai, and 90 to 140 µm for the unnamed species.
In the feline Demodex species, the length of adult mites ranges from 181 to 219 µm for D. cati and 81 to 115 µm for Demodex gatoi.

Disease

Although most Demodex spp. are considered normal mammalian fauna, overgrowth of mites may be associated with development of patchy hair loss or mild to severe dermatitis in dogs and (less commonly) in cats.
In cases of demodicosis caused by the unnamed canine Demodex sp. or D. gatoi in cats, disease is thought to be caused by the infestation itself rather than an overgrowth of mites.

Dogs

Canine demodicosis may be localized or generalized and both forms may present in either juvenile or adult dogs.

Pruritus does not occur in uncomplicated infestations; however, pruritus can be seen with secondary bacterial pyoderma.

In dogs, localized demodicosis—characterized by a mild, nonpruritic, patchy alopecia on the head or limbs—usually develops in puppies less than 6 months of age. Most cases of juvenile-onset localized demodicosis resolve spontaneously without treatment.

Generalized demodicosis in dogs is a moderate to severe disease that in most cases is attributable to an overgrowth of mites believed to occur because of an underlying systemic disease or immune defect. With the unnamed canine Demodex sp., however, the infestation itself is thought to cause disease.

Physical examination findings include alopecia (which may involve several large to coalescing areas of affected skin), erythema, and often secondary superficial or deep pyoderma. Lymph nodes may be enlarged, and when pyoderma is present, pruritus may develop.

Cats

Cats with localized demodicosis develop alopecia and crusts as well as some scaling around the face, neck, and eyelids, with varying degree of hyperpigmentation.

Generalized demodicosis due to overgrowth of D. cati is usually associated with underlying systemic disease (e.g., diabetes, Cushing's disease, neoplasia, feline immunodeficiency virus).

Cats infested with D. gatoi are pruritic and may excessively lick or groom affected areas. In cats with D. gatoi, dermatitis is not associated with underlying disease.

Prevalence

Canine demodicosis (demodectic mange) due to D. canis is a common skin disease in the dog; disease due to D. injai or other Demodex sp. in dogs appears to be very rare.
Feline demodicosis due to D. cati is rare, and feline demodicosis due to D. gatoi is even less common, although cases are reported from the southeastern United States.

Host Associations and Transmission Between Hosts

Demodex spp. are host-adapted mites of mammals. Mites have not been shown to cross-infest among dogs, cats, and humans.
Neonates are thought to acquire mites from their dam via direct skin-to-skin contact during nursing.
Transmission of mites may also occur during direct contact between older animals, but demodectic mange is not contagious as most animals that develop generalized demodicosis are thought to have an underlying immune defect (see Disease).
Demodex gatoi is a superficial species reported from cats and is thought to be contagious among cats.

Site of Infestation

Demodex canis and D. cati infest hair follicles and sebaceous glands.
Mites may occasionally be reported from other tissues (e.g., lymph node, intestinal wall, kidney, thyroid gland) following dissemination via blood or lymphatic drainage.
Early studies of nursing neonatal puppies have found D. canis mites initially within the skin of the face, and then over time mites are transferred throughout the skin of the entire body. Mites are not found in the skin of stillborn puppies or puppies born by Caesarian that are not allowed to nurse.
Localized demodicosis in dogs most commonly develops on the head or limbs; the lesions of generalized canine demodicosis may develop anywhere on the body.
Demodex gatoi infestation in cats is most frequently reported from the groin, ventral chest, and sometimes limbs.

Pathogenesis

The immunopathogenesis of demodicosis is not fully understood, and in most cases an underlying cause is not identified. Although a responsible condition is not always identified, many cases of generalized demodicosis appear to be the direct result of underlying diseases that compromise the immune system. Excessive cortisone, chemotherapy, and underlying cancer or diabetes have all been associated with the development of generalized demodicosis in individual animals. Accordingly, dogs and cats with generalized demodicosis should be carefully evaluated for potential underlying disease states.
No specific deficits in innate or humoral immunity have been identified in dogs with generalized demodicosis. However, some studies suggest that cellular immunity may be compromised in some individuals that go on to develop generalized demodicosis.

Diagnosis

Demodicosis due to D. canis, D. injai, and D. cati is diagnosed by microscopic examination of deep skin scrapes from affected areas of alopecia (Link to Images).
Alternatively, in uncooperative dogs or sensitive areas in which skin scrape is difficult (e.g., feet, interdigital region), hairs may be plucked from an affected area and placed in mineral oil on a slide for microscopic examination.
Because the unnamed canine Demodex sp. and D. gatoi in cats reside within the stratum corneum, superficial skin scraping or tape impression offers a better method for detecting these mites (Link to Images). Because the pruritus associated with D. gatoi infestations leads to removal of the mites by grooming, they often are difficult to find.
In rare cases of “occult demodicosis,” i.e., no mites are observed with either the skin-scraping or hair-pluck techniques, a skin biopsy may demonstrate Demodex mites. The mites (or mite fragments) can be seen within the lumen of the hair follicles or (rarely) within the sebaceous glands/ducts, depending on the type of mite. This technique may be necessary in Demodex cases involving the feet and in the Chinese Shar Pei.

Treatment

Dogs—Localized Demodicosis

Most cases of localized demodicosis resolve spontaneously without treatment.
If treatment is desired, a rotenone-based insecticide ointment (Goodwinol) has been approved. Although the ointment is miticidal, localized irritation may occur.

Dogs—Generalized Demodicosis

Generalized demodicosis may require extended, aggressive therapy to resolve disease.
Comprehensive treatment should include use of an effective miticide, evaluation for any underlying disorders and appropriate treatment when found, antibiotic therapy when pyoderma is present, and spaying of female dogs to prevent recurrence during subsequent heat cycles.
Amitraz dip (Mitaban®) at 250 ppm every 2 weeks is the only approved miticidal treatment for generalized demodicosis in the United States.

Hair should be clipped throughout treatment; dogs should be allowed to air-dry or should be dried with a blow dryer after each application.
Use of a benzoyl peroxide shampoo prior to the application of amitraz dip is recommended; dogs should not be shampooed between applications.
Some practitioners recommend weekly dips or a more concentrated formulation (e.g., 500 ppm) in refractory cases or to clear dogs more rapidly.
Side effects may occur and are detailed on the product label.
Key safety recommendations when using amitraz dip include application of the product in a well-ventilated area and use of protective gloves and a mask during application.
Asthmatics and diabetics should exercise particular caution when handling dogs that have been treated with amitraz.

Other miticidal treatments not labeled in the United States include high-dose oral ivermectin, oral milbemycin oxime, and topical moxidectin.

Ivermectin injectable may be given orally at escalating doses using 100 µg/kg increments. Begin with 100 µg/kg for 3 days followed by 200 µg/kg for 3 days followed by 300 µg/kg. Some practitioners recommend remaining at the 300-µg/kg dose whereas others recommend continuing to increase the dose every 3 days to 600 µg/kg. It may take up to 33 weeks to resolve lesions and eliminate mites. Continue treatment for 1 to 2 months after two consecutive negative skin scrapings.Milbemycin oxime (Interceptor®) has also been used daily at doses ranging from 0.5 to 2 mg/kg. Doses are escalated gradually, building to a final dose of 1.5 to 2.0 mg/kg (Holm, 2003). Continue treatment for 1 to 2 months after two consecutive negative skin scrapings; the treatment course ranges from 60 to 300 days (Mueller, 2004).
Some dogs, particularly herding breeds such as Collies, Shetland Sheepdogs, Border Collies, Australian Shepherds, and Old English Sheepdogs, may have mutations in their MDR1 genes and thus have increased risk of avermectin/milbemycin toxicity. The escalating dose regimen should be discontinued if any of the following signs are observed: mydriasis, mild salivation, significant changes in behavior, ataxia, or seizure. Practitioners can determine whether a dog has the MDR1 mutation by sending a cheek scraping to the Washington State University at Pullman College of Veterinary Medicine Veterinary Clinical Pharmacology Lab for analysis (http://www.vetmed.wsu.edu/depts-VCPL/test.aspx).
Moxidectin/imidacloprid topical (Advantage Multi®) is marketed in Europe under the trade name Advocate® and carries a label claim in Europe for treatment of D. canis infestation (Heine et al, 2005).

Cats

No products are labeled for demodicosis in cats.
Lime sulfur dips have been reported effective. Dips should be performed weekly for 6 weeks with 3.1% solution (4 ounces per gallon of water).
Ivermectin has been used once weekly at 0.3 mg/kg orally for four consecutive weeks. Side effects may occur.
Amitraz has been used in cats at a 0.0125 to 0.025% solution every 5 to 7 days for 4 to 6 weeks. Side effects may occur.

Control and Prevention

Metaflumizone/amitraz (ProMeris® for dogs) is a monthly product approved for the control of Demodex spp. that can cause demodectic mange.
For intact female dogs that develop generalized demodicosis, spaying is recommended because they may experience relapse of disease in subsequent heat cycles.
The development of demodecosis was long believed to have a genetic predisposition, and as a result, some veterinarians discourage breeding affected animals. However, the hereditary nature of demodecosis has not been clearly demonstrated.

Public Health Considerations

Demodex mites are host-adapted; there is no zoonotic potential in either canine or feline demodicosis.

References

Heine J, Krieger K, Dumont P, Hellmann K. Evaluation of the efficacy and safety of imidacloprid 10% plus moxdectin 2.5% spot-on in the treatment of generalized demodicosis in dogs: results of a European field study. Parasitol Res Suppl 1:S89-96, 2005.
Holm BR. Efficacy of milbemycin oxime in the treatment of canine generalized demodicosis: a retrospective study of 99 dogs (1995–2000). Vet Dermatol 14:189-95, 2003.
Mueller RS. Treatment protocols for demodicosis: an evidence-based review. Vet Dermatol 15:75-89, 2004.