Dermatophilus infections

STEPHANIE SPECK

Bundeswehr Institute of Microbiology, Department of Virol­ogy and Rickettsiology, Munich, Germany

Dermatophilosis, also named streptothricosis, lumpy wool disease and contagious dermatitis, is an acute, subacute or chronic exudative to proliferative skin disease affecting various domestic and wild animal species, as well as humans.

Dermatophilus congolensis, is an aerobic, Gram-positive actinomycete bacterium with an unusual fungus-like life cycle and morphology. The agent is thought to be main­tained in small foci of infection on a carrier animal or within scab particles in dust. Dormant immature zoospores that are resistant to desiccation and high temperature are generally transferred among hosts. When mature, these spores are motile by multiple flagella and are infective. The mature spores may initiate infection in damaged skin. In smears from lesions, D. congolensis is usually seen as fila­mentous and branching hyphae up to 0.3—0.6 μm long and 0.6—1.0 μm wide. These hyphae consist of coccoid bacterial cells arranged longitudinally, in a symmetrical chain leading to the unique morphology (‘ rolls of coins’ ) of D. congolensis'⁴⁶.

Dermatophilosis affects a variety of domestic and wild animal hosts, including terrestrial and aquatic mammals and reptiles. Birds are generally resistant to dermatophilo- sis. The disease has a worldwide distribution and is most prevalent in humid, tropical and subtropical regions but has also been described from countries with temperate climates⁽⁴⁷⁾. In continental Europe, only a few cases of dermatophilosis in wildlife have been reported, including in Alpine chamois (Rupicapra rupicapra) from Switzer- land⁽⁴⁸⁾, Austria and Italy⁽⁴⁹⁾, and roe deer ( Capreolus capre- olus) from Switzerland¹-⁵⁰).

The epidemiology of dermatophilosis is still uncertain. In tropical climates, the disease has a seasonal pattern, occurring during wet periods. Accordingly, the morbidity and mortality rate of the disease vary depending on the season⁽⁴⁷⁾. Elsewhere, such patterns are less evident⁽⁵¹⁾. Der- matophilus congolensis is not highly pathogenic p er se, and predisposing factors (e.g. rainfall, humidity, damp coat hair, cuts and traumatic abrasions of the skin, ectopara­sites) are considered to favour infection, eventually leading to severe disease. Normal skin seems to be resistant to infection. Dermatophilus can be transmitted mechanically by contact with infected animals, via ectoparasites (ticks, flies, biting flies, lice, mites) and also through intradermal inoculation by contaminated thorn bushes⁽⁴⁷⁾. More severe lesions of dermatophilosis associated with tick infestation (Amblyomma variegatum, Ixodes ricinus) have been described in cattle⁽⁴⁷⁾ and roe deer⁽⁵⁰⁾ but not in Alpine chamois.

Following entry to the epidermis, localized growth of

D. congolensis induces erythema, hyperplasia of epidermal cells, exudation and formation of crusts. In chronic disease, the lesions spread widely, with extensive exudates and crusts, which become scabs. Affected areas are usually found on the back, tail, carpal and tarsal regions but might also appear on udder, scrotum, neck and head.

Histologically the lesions are mainly restricted to the upper layers of the epidermis. The stratum corneum is usually markedly thickened and the underlying dermis is superficially infiltrated with neutrophils and mononuclear inflammatory cells. Hyperkeratosis, hyperplasia of the basal layers of the epidermis, degeneration of epidermal cells as well as oedema, neutrophilic infiltration and micro­abscesses of the dermis have been described in Alpine chamois⁽⁴⁹⁾.

Clinical signs and lesions can vary in severity and the extent of the skin area affected, involving a small number of discrete spots, large confluent areas or virtually the entire skin. In Alpine chamois, clinical signs were exuda­tive dermatitis and paintbrush lesions consisting of focal matting of hair⁽⁴⁸⁾ as well as proliferative dermatitis⁽⁴⁹⁾.

The unique clinical appearance of D. congolensis allows a strong presumptive diagnosis of dermatophilosis on the direct examination of stained smears. Microscopic investigation can be made of detached crusts if the material is moist. The smear can be stained by Gram, Giemsa or Wright’s stain. Giemsa is a better stain to show the characteristic morphology of the bacterium. Typical dark purple stained segmenting filaments and coccoid spores arranged in rouleaux form are seen. Skin biopsies and crusts can be processed routinely for histology, stained similarly and examined. Isolation and identification of the organism is required if the classical morphology is not seen by direct microscopy¹-⁵¹). Dermatophilus congolensis is com­paratively easy to culture, but Haalstras method for primary isolation of D. congolensis should be used to over­come growth of contaminant organisms from scab material(⁴6).