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Dermatophilosis
(Dermatophilus
congolensis)
Dermatophilus congolensis is the bacterium that can cause
the disease mud fever and rain scald in equines. In temperate climates,
sheep are also susceptible to the disease, as are cattle in the
tropics (Lloyd, 1993). D. congolensis is a Gram-positive, opportunistic
actinomycete that produces superficial crusting of the epidermis
(Rosser, 1995; Scott et al, 1995).
Taking advantage of warm, dark, moist skin regions, infection is
usually present along side trauma to the area and commonly affects
the heels and lumbar-sacral region (Hayes, 2002).
The bacteria can be transmitted by arthropods, common in the tropics,
however equine exposure to the disease is usually limited to soil
transmission (Rosser, 1995; Lloyd, 1993).
During its infective stage, D. congolensis, as a motile coccus,
swims in films of water on the skin surface. It is chemotactically
attracted towards carbon dioxide diffusing through the skin, which
is believed to lead it to areas where the stratum corneum is thin
or damaged, increasing susceptibility to infection. It germinates
to produce a branching filament, which invades the living epidermis
(Lloyd, 1993).
The immune system responds with early mast cell degranulation, vasodilation
and an influx of dendritic cells, which accumulate and differentiate
beneath the infected epidermis. A concomitant dermal invasion by
neutrophils and T and B-lymphocytes lead to epithelial infiltration
and scab formation. Hypertrophy of the epidermis also indicates
keratinocyte involvement in the host response (Sasiak et al, 1996).
Scab formation lifts the organism away from its nutrient source,
stimulating maturation of the filaments, which become transversely
and longitudinally septete. Within each septum a coccus is formed
and remains dormant until the scab is wet, stimulating motility
and its escape from the scab (Lloyd, 1993).
An immune response to D. congolensis may involve immune responses
mediated by CD1 antigen presentation and gamma delta T cells. These
form part of the antigen receptor complex, in association with lymphocytes
(Tizard, 2000).
Sera from animals that are disease-free but in contact with those
infected with dermatophilosis have been found to contain antigens
(Ag) recognised by sera from naturally exposed and experimentally
infected animals. These may be involved in immunity against D.
congolensis (Makinde & Gyles, 1999).
Predisposition to the infection is predominantly due to environmental
conditions, with animals incurring prolonged exposure to the wet,
at a higher risk (Hayes, 2002). Therefore, the disease is more commonly
found in animals that live out through the winter months, usually
native or heavy breeds. Also, Rosser (1995) found that animals with
white haired and white skinned areas, such as white socks, are more
susceptible to the infection.
For more information on D.congolensis and Mudfever
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