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Dermatophilosis
(Dermatophilus
congolensis)
Dermatophilus congolensis causes dermatophilosis characterized
by acute or chronic dermatitis involving the superficial layers
of the skin (Ikram & Hill, 1991) causing the disease mud fever
and rain scald in equines (Hayes, 2002). It is also referred to
as rain rot, dew poisoning and cutaneous streptothricosis having
a worldwide distribution.
It is a gram-positive, non-acid-fast, branching filamentous aerobic
of facultative anaerobic actinomycete. Where it exists in nature
is unknown (Schlipf, 1997).
Although it may affect many animal species, it is seen most frequently
in horses, cattle, sheep and goats. In temperate climates, especially
in tropical areas, it can severely affect productivity in sheep
and cattle (Biberstein, 1999).
Ambrose et al (1998) reported D. congolensis produces a number
of extra cellular alkaline serine proteases, suggesting the presence
of host-specific variation between isolates and to a lesser extent
between isolates from the same host species.
The bacteria take advantage of warm, dark moist skin areas and so
infection is normally present along side injury to the region usually
affecting the heels and lumber-sacral areas (Hayes 2002).
The infections involve the superficial layers of the epidermis and
are characterized by the formation of crusts or scabs varying in
the size (Carter et al, 1995).
Its size and chemical composition distinguish it as a bacterium,
but it forms a branched mycelium, which fragments into motile coccoid
elements as they differentiate into multiflagellated zoospores resembling
the spores of a fungus (Hall, 1980; Biberstein, 1999).
Deposited zoospores respond to a carbon dioxide gradient diffusing
into deeper cell layers, leading to regions where the stratum corneum
is thin or damaged. Upon germination the spores sprouts a germ tube,
which elongates and thickens, dividing both diagonally and longitudinally
forming a strand several layers thick invading the living epidermis
(Biberstein, 1999).
As the scab forms it lifts the organism away from the nutritional
supply, resulting in maturation of the filaments, which become transversely
and longitudinally sepate. 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).
Immunity to Dermatophilus congolensis might involve non-classic
responses mediated by CD1 antigen presentation and gamma delta T
cells (Ambrose et al. 1999).
Large numbers of neutrophils and lymphocytes penetrate the epidermis
and entered the infected surface region. Within the underlying dermis
there is an accumulation of dendritic cells immediately below the
infected epidermis and evidence of mast cell deregulation (Lloyd,
1993).
D. congolensis is promoted by wet conditions, during heavy
or persistent rainfall and is more common in autumn and winter and
so is predominantly due to environmental conditions. Animals exposed
to long periods of wet conditions damaging the stratum corneum,
are at a higher risk (Knottenbelt & Pascoe, 1999) Therefore
the infection is commonly found in the equine exposed to constant
winter environments (Hayes, 2002). Generally breeds with long feathery
legs are often more susceptible, usually native or heavy breeds
(Johnston, 1994).
White-skinned areas appear to be more susceptible to D. congolensis
and so more commonly found on white distal limbs (Knottenbelt &
Pascoe, 1999).
Some horses with decreased immune function can suffer increased
incidence or severity of the disease (MacDonald & Carter, 1997).
For more information on D.congolensis and Mudfever
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