CLINICAL EXAMINATION
A full clinical examination to assess both the general health status and the skin is necessary in most cases. Ensure that the animal is adequately restrained and that you have sufficient light. Work systematically down each body region, beginning at the head and ending at the tail and perineal region. Be sure to include all aspects of the feet including the coronary band and the frog. The skin may need to be cleaned to observe some lesions. In some instances, sedation may be necessary.
A record of the distribution and severity of primary and secondary lesions should be kept. Forms including a horse outline make this much easier (Figure 1.3).
It may be helpful to visit and examine the paddocks and exercise areas used.
Figure 1.3 Example of an examination form for recording distribution and nature of lesions in equine dermatology cases.
DIAGNOSTIC TESTS
The history and clinical examination should enable you to formulate a list of differential diagnoses. It may help to create a problem list, identifying the relevant historical features and predominant clinical signs, categorising them as contagious or non‐contagious, and allocating the disease within the following groups, which form the basis for the problem‐orientated approach in this book:
Pruritic
Crusting and scaling
Ulcerative and erosive
Nodular or swollen
Alopecia/hair coat changes
Pigmentary disorders
A diagnostic plan can then be constructed, diagnostic procedures selected, and samples collected. Sample collection may include the following techniques.
Hair plucks
Useful to determine whether the lesions of alopecia or hypotrichosis are due to self‐inflicted damage (fractured hair shafts, split ends) indicating that the condition is pruritic, or due to abnormal hair growth (absence of anagen roots, abnormal catagen roots), and to examine for dermatophytes and for parasite eggs.
Choose fresh, unmedicated lesions.
For suspected dermatophytosis, where cultures are required, first lightly clean the areas to be sampled with 70% alcohol (to reduce contaminant organisms).
Tissue or epilation forceps can be used to grasp gently and pull out hairs from the periphery of the lesion.
Samples for microscopy can be placed on adhesive tape wrapped around a microscope slide and mounted in a drop of liquid paraffin just prior to examination.
Samples for fungal culture submission should be held in paper or sterile, non‐airtight containers to prevent a humid environment that might support the growth of saprophytic organisms.
Crusts
Useful for cytological examination looking for bacterial organisms (particularly Dermatophilus) and for submission for fungal and bacterial culture.
Choose a fresh, unmedicated lesion.
Impression smears of the underside of freshly removed crusts, stained with a rapid Romanowsky‐type stain (e.g. Diff‐Quik, Hemacolor, Rapi‐Diff, Speedy‐Diff) or Gram’s stain, can provide a quick method of diagnosis for dermatophilosis.
Crusts can be collected and held in paper envelopes or sterile containers for transport to the laboratory.
Dried crusts can be emulsified in a drop of sterile saline on a slide, warmed to allow rehydration of material, prior to air‐drying and fixing (heat fixation for Gram’s stain, methanol/ethanol fixation for rapid‐differentiating Romanowsky‐type stains) for cytological examination and identification of bacterial and fungal organisms.
Coat brushings
These allow for examination for surface‐living external parasites and dermatophytes where the lesions are diffuse or extensive. Scrapings are better for deeper resident mite infestations.
Use a sterile scalp brush or new toothbrush to brush firmly over the lesions (Mackenzie brush technique; Figures 1.4a and b). Place the brush in a paper envelope to protect it prior to submission for dermatophyte culture.
A scalp brush or wooden tongue depressor can be used to collect debris directly into a sterile Petri dish for external parasites. Material should be examined promptly as chorioptic mange mites are highly motile and easily lost from sample containers.
Figure 1.4 (a) A coarse‐toothed brush (e.g. 90 mm Denman scalp brush) facilitates sampling of large areas of skin and coat. The collected hair can be removed and examined, or the teeth may be embedded in a fungal culture medium as illustrated. (b) Here Microsporum canis has been isolated using this technique.
Skin scrapings
Skin scrapings can be performed for detection of external parasitic diseases such as chorioptic mange, larval stages of harvest mites, demodicosis (rare), or for dermatophyte culture and cytology.
If necessary, remove hair over areas to be sampled by careful clipping.
Use a wooden tongue depressor for superficial sampling or a large, curved scalpel blade if deeper samples are required, with the sharp edge blunted to reduce the risk of injuring the horse or operator.
Moisten the sample site or the collection tool with liquid paraffin (more useful for examination for mites), water or normal saline (for dermatophytes).
Gently scrape crusts, scales, and associated hair so that the material accumulates on the blade or tongue depressor. Transfer onto a microscope slide with more liquid paraffin, or with potassium hydroxide solution if collected in aqueous medium, which allows clearing of debris and easier identification of pathogens.
Deeper scrapings are needed for suspected demodicosis, deep enough to cause capillary
