here - Australian College of Veterinary Scientists

congolensis is not easy to isolate, particularly from older, longstanding lesions.
However, typical organisms may be seen in wet preparations made from crusts after
rehydration and maceration on slides, prior to staining. Dermatophilosis is often a
problem of a number of horses in a yard and may be endemic, with increased
numbers of affected animals noted in the winter months. The spores are persistent
in the environment and careful attention to management practices is an important
part of control measures. Ideally an area should be set aside for topical treatment of
affected animals and crusts carefully disposed of and the area disinfected. Systemic
antibiotic therapy is usually effective in cases not responding to topical antibacterial
agents, but should be continued beyond clinical cure.
Dermatophytosis can be caused by a number of fungal organisms. Identification of
the causal agent may be helpful in identifying the original source (other horses, cattle,
cats, sylvatic species, geophilic). There is also a zoonotic risk to be considered. Scalp
brushes provide a useful tool for sampling animals for culture, equivalent to the
Mackenzie toothbrush technique employed in small animals. Management of this
condition is covered later.
Pastern folliculitis may be a primary pyoderma, but mixed growth of organisms is
frequently obtained on swabs. Tissue samples obtained by biopsy may well be a
better material for culture and provide a truer picture of the major pathogen
involved. However, cytological examination often provides sufficient information to
initiate antibacterial therapy. Management of bacterial folliculitis is the subject of a
later presentation.
Chorioptic mange is the major parasitic cause of skin lesions of the distal limb. The
parasite exhibits a preference for heavy breeds of horse. An epidemiological study
conducted by the author has shown that the presence of feathers is not a
prerequisite, but the actual nature of the nutritional or other factors provided by
such horses is unknown. Management of this condition is covered later.
Harvest mite larvae (“chiggers”) can parasitise the lower limbs of horses grazed on
pastures inhabited by the free-living adult mites or ridden through infested woods or
trails. The larval stage requires a blood meal and any convenient mammal may be
bitten. The larva is only attached for 48-72 hours, but in some horses lesions of
pruritus, crusting, erythema and scaling may persist beyond detachment of the
parasite, suggesting a hypersensitivity component to the lesions. Fipronil has been
demonstrated to be effective in the management of trombiculidiasis in dogs, and
would be appropriate for the condition in equine patients too, as an off-label use.
Photosensitivity may affect the distal limbs of horses with white socks. The cause
may be due to ingestion of primary photosensitising agents, such as hypericin in
Hypericum spp. of plants (St John’s Wort). Photocontact dermatitis may occur. In the
case of primary photocontact or systemic photosensitisation, one would expect
several of a group of horses to be affected, although photoallergic reactions may
occur, which would be limited to sensitised individuals. Often it is difficult to
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ACVSC Proceedings Dermatology Chapter Science Week 2005