Demodex

General information

Demodex mites are strictly host-specific ectoparasites of numerous mammals and of humans. So far, there have been three species each described in dogs and cats (dogs: particularly Demodex (D.) canis, rarely D. injai and D. cornei; cats: especially D. cati, but also D. gatoi and an unamed species).

The entire development of demodex mites takes place in the hair follicles, the sebaceous and apocrine glands of the host. They cannot survive very long in the environment. Transmission mainly occurs postpartum while nursing. Demodex mites belong to the physiological skin fauna, but are facultative pathogenic. In dogs, there is often a low number of mites present without any clinical symptoms (prevalence up to 85%), but demodicosis is rare. Nevertheless, it is one of the most frequent dermatoses in dogs (especially young dogs), in cats, however, it is very rare.

In dogs, lesions generally start in the face or on the forelegs and spread from there. The localised form affects a few well-defined skin areas and most notably occurs in young dogs. The skin areas are often hairless and may also be scaly. Comedones are typical as well. In general, itching only occurs in case of secondary bacterial infections.

If more than four lesions are present, an entire body region or at least two paws are affected and if it continuously worsens without treatment, it is referred to as generalised demodicosis. There are usually secondary bacterial infections present and alopecia appears with follicular papules up to furunculosis, focal ulcerations and fistula tracts. Most of the time, there is no itching, but sometimes intense pain. Fever, anorexia, lethargy, lymphadenopathy and sepsis may occur and might be fatal if not treated. Special forms are podo- and otodemodicosis.

Favourable factors for mass reproduction of mites include, e.g., endoparasitosis, malnutrition, cortisone treatment, neoplasia, hypothyroidism or hyperadrenocorticism. There is a genetic predisposition in young dogs (juvenile generalised demodicosis). These dogs should be excluded from breeding.

In cats, demodicosis particularly occurs if systemic diseases, such as diabetes mellitus, FIV, FeLV or neoplasia, are present, and most notably causes alopecia and crusts on the head and neck. Itching is also possible. D. gatoi is a mite that dwells rather on the surface and lives in the stratum corneum (not in the hair follicles). It has a short, broad body. D. gatoi is considered a primarily pathogenic parasite and is highly contagious. D. cati, in contrast, has the elongated morphology characteristic of Demodex mites, lives in the hair follicles and is part of the cutaneous fauna of cats.

It seems that hypersensitivity to the mites can cause severe pruritus even if only a few D. gatoi mites are present (similar to sarcoptic mange in dogs). How intense the itching is varies between cats; asymptomatic carriers have been described as well. The disease is characterised by pruritic lesions, self-induced alopecia, miliary dermatitis, excoriation, erosions and ulceration. Any part of the body may be affected, but the abdomen, inner thighs, flanks and forelegs appear to be the most common sites.

Demodicosis caused by D. gatoi should be included in the differential diagnosis of any cat with pruritus.