1
The diagnostic approach

A structured approach is essential. Vital information is obtained during the history-taking process and sufficient time must be allowed for this. Accurate information on husbandry is particularly important. Clinical examination must include systemic and skin components. The process is illustrated with flow diagrams (Figures 1.1 and 1.2).

TAKING THE HISTORY

The approach (Figure 1.1) is similar to that adopted in other species.Points to include are:

• Breed, age, sex, origin:
  • Consider these aspects carefully; in many conditions, these simple data will have an important impact on your diagnostic considerations.
• History of skin problems in related animals.
• Type of husbandry and use:
  • Length of time owned.
  • Use - competitions, general riding, breeding, racing.
  • Feeding regimen.
  • Periods spent in stable or at pasture.
  • Type of stable and bedding - stable hygiene, contamination.
  • Conditions in paddocks - mature meadow pasture or new grass ley, proximity of water, trees.
  • Seasonal changes in management.
  • Routine health care procedures - vaccination, deworming.

    Figure 1.1 Taking the history. Components and the sequence of the history taking process. Analysis of the history should enable the clinician to construct an initial list of differential diagnoses that may help to focus the clinical examination along particular diagnostic lines. It may enable the diagnostic process to be abbreviated where a likely diagnosis is indicated, or it may point towards the need for a more detailed approach.

  • Grooming procedures - sharing of grooming kit, tack, grooms.
  • Equipment used in contact with horse - boots, bandages, saddle cloths, rugs.
  • Contact with other horses, other species - opportunities for disease transmission.
  • History of the current problem.
    • First signs, progression, response to treatment and management changes.
    • Seasonal effects.
    • Previous episodes of disease.
    • Results of any diagnostic tests.
    • Current or recent therapy - includes questions about use of over-the-counter and non-veterinary products.
    • Evidence of transmission - lesions in other horses, other species, humans.
  • General health - concurrent or previous conditions.

Figure 1.2 Clinical examination and diagnostic procedures. A thorough general and dermatological examination should be carried out unless the history points clearly towards a diagnosis. Examination coupled with history enable a list of differential diagnoses to be drawn up, formulation of a diagnostic plan and the selection of appropriate tests and sites to be sampled, and/or therapeutic trials.

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 ooze.
• Sample several sites, collect plenty of material, and divide amongst several slides to make thin suspensions, which are quicker and easier to examine efficiently.

Surface adhesive tape samples

An alternative method for obtaining surface material, including Oxyuris equi eggs, surface-living ectoparasites, hair fragments, exfoliated cellular material, and surface microorganisms for direct microscopical examination or after staining; it is less...