Diseases, Epidemiology, and Pathogenesis

Dermatophilus congolensis is an obligate parasite of animals and affects many species, causinggeneralized exudative dermatitis in livestock. Dermatophilosis occurs in cattle in tropical and subtropical regions (Figure 9-2); disease in sheep is especially common in areas of high rainfall. Temperate breeds of cattle are more severely affected than tropical breeds. Economic losses derive from effects on production of beef, mutton, milk, hides, skins, and wool, and are especially notable in relation to cattle production in West and Central Africa and in the Caribbean, and sheep production in Australia.

FIGURE 9-2 Bovine dermatophilosis.

(Courtesy Raymond E. Reed.)

The life cycle of the organism involves germination of cocci and production of hyphae that elongate through the epidermis and then undergo transverse and longitudinal division to produce filaments that release cocci. Motile zoospores develop from cocci and are released from wet crusts to establish new sites of infection on the same animal or new hosts.

The infection is spread by contact (direct orvia fomites) or by biting insects. Establishment of infection is affected by host genetic factors, and there is good evidence for effects of immunosuppression by Amblyomma variegatum ticks. Skin infections in sheep occur worldwide, and are basically mycotic dermatitis. Clinical disease is characterized by matting of hair or wool, and scaband crust formation, which can be substantial in chronic cases and lead to loss of hair or even skin. Crust forms by about 4 days after experimental inoculation and lesions resolve without treatment in about 4 weeks. Dermatitis of wool-covered areas is commonly called “lumpy wool.” Infection can also involve the face and scrotum, and the sometimes severe ulcerative dermatitis of lower legs and feet is called “strawberry footrot.” In its more severe forms, infection may result in deaths. Moist conditions promote disease development in sheep, and the same can be said of the disease in cattle, goats, and horses. “Rain rot” involves the superficial layers of skin and is characterized by the formation of crusts and scabs, varying in size from quite small to about 2.5 cm in diameter. In advanced cases, smaller lesions may coalesce, leaving large areas of skin affected. When examined microscopically, crusts have a palisaded appearance, with layers of keratinocytes, serous exudate, and neutrophil infiltrates, suggesting cycles of epidermal proliferation, invasion by D. congolensis, exudation, and influx of neutrophils. Intensity of cellular infiltration correlates with lesion severity.

Various factors, including pre-rainy season malnutrition, predispose to D. congolensis infection. Infection begins at sites where skin defenses are compromised by intense rainfall or by mechanical trauma, and wet weather facilitates dispersal of the organism from affected to healthy skin. Biting flies cause skin damage and initiate inflammatory exudation, which encourages bacterial growth and attracts flies that mechanically vector D. congolensis to new hosts.

Dermatophilus congolensis is hemolytic, and produces phospholipases and proteolytic enzymes. Infectivity has been correlated with production of extracellular proteases; most isolates produce these enzymes, with isolate-to-isolate variation in the number of enzymes and the quantity of each produced. Studies with inhibitors suggest that these are serine proteases; their alkaline pH optima may enable them to function in lesion development. Bovine skin is normally pH 5.6, but it becomes alkaline in the face of humoral components of the inflammatory response. Lipases and proteases may play a role in penetration of the epithelial barrier, including perhaps nutrient acquisition, inactivation of host inflammatory proteasecascades, or hydrolysis of cytokines and other immune effector molecules.

A progressive, chronic form of bovine dermatophilosis occurs in animals infested with the ixodid tick Amblyomma variegatum; ticks anddisease have similar seasonal and geographic distribution; tick control reduces the prevalence of disease. This association probably involves more than simple transmission of D. congolensis by A. variegatum, especially in that molting of larvae and nymphs between hosts augurs against mechanical transfer of D. congolensis. It seems likely that development of progressive disease is related to immunosuppression by factors in tick saliva.

Dogs, rabbits, deer and other ungulates, foxes, seals, and pigs are affected by scabs and hair loss. Cats are often infected by way of puncture wounds, and the resulting abscesses involve the subcutis, muscles, and lymph nodes, with chronic draining fistulas. The organism is also associated with ulcerative lymphangitis in cattle and ulcerative and hyperkeratotic skin diseases in lizards and alligators (Figure 9-3), and has been isolated sporadically from various skin diseases in monkeys, ground squirrels, and polar bears. Several cases acquired from animals have occurred in humans. Dermatophilus chelonae is a newly characterized organism isolated from skin lesions in chelonids (turtles and tortoises).

FIGURE 9-3 Dermatophilus congolensis infection in a lizard.

(Courtesy Raymond E. Reed.)

Diagnosis

Diagnosis is based on the finding of morpholog-ically compatible organisms in stained smearsfrom clinical materials; conventional or fluorescent antibody stains may be useful. Gram-positive, branching segmented hyphae are seen, with hyphal elements larger and less regularly shaped than those of members of the genera Streptomyces and Nocardia. After 24 hours of incubation on solid media, D. congolensis produces tiny, grayish white round colonies that pit and adhere to the agar. Colonies may become orange after 2 to 5 days, and they are frequently β-hemolytic. Stains reveal segmenting, longitudinal and transverse filamentsand coccoid spores that are often deep purple and in packets. Septation of hyphal elements results in formation of zoospores that are motile by way of polar flagella. Differentiation is by standard phenotypic assays (Table 9-1).

TABLE 9-1 Differentiation of Dermatophilus species

Immunodiagnosis of D. congolensis infections has been based upon crude and partially purified antigens derived from various morphologic phases of in vitro cultivated organisms. Results of such assays are often poorly reproducible, perhaps because of variation in preparation methods or among strains used as the antigen source.

Dermatophilus chelonae produces small, dry, adherent, umbonate colonies with two zones of hemolysis (an inner β and an outer α) after 72 hours of incubation on solid media. This species has a characteristic putrid odor.