Discussion

Initial visual inspection of inflammatory masses should allow description and subdivision into different lesion types. This is the first step to restricting the list of differential diagnoses so that a more specific diagnosis is made easier and a further diagnostic protocol is chosen. Solid, rounded masses are either papules (<1 cm) or nodules (>1 cm), fluid filled lesions are either pustules (pus-filled) or vesicles (filled with clear fluid), and flat-topped lesions are either plaques (coalesced papules) or wheals (circumscribed oedema). Additional observed changes might further help differential diagnosis such as scaling, crusting, alopecia etc... The main alternative diagnoses for inflammatory 'lumps and bumps' in horses comprise neoplasms (e.g. sarcoid, melanoma and lymphoma) and cysts (e.g. dermoid cysts).

Papules:
These generally reflect infective dermatoses including dermatophytosis, bacterial dermatitis (e.g. Staphylococus aureus) and dermatophilosis. There is very little that looks similar to these diseases other than perhaps a papular noninfective urticarial rash.
Hair plucks should be collected from papular lesions and this is best performed with rubber-covered artery forceps. Margins of the lesions offer the highest diagnostic rate for dermatophytosis. Prior cleaning of the sample area with alcohol followed by air drying may help remove some commensals and secondary invaders. Direct microscopic identification of dermatophytes can be difficult and negative findings during direct microscopy does not rule out dermatophytosis. Hair plucks can be examined in mineral oil or 10 - 20% potassium hydroxide. Specific stains such as chlorazol black E is preferred by some or Calcofluor white is a product contained in washing powders that may bind to fungal cell walls and fluoresces under ultraviolet light and can also be used as a further aid to microscopic identification. The best samples to examine under the microscope for a diagnosis of dermatophytosis are pieces of broken hairs that may be slightly thicker and/or distorted than neighbouring hairs. The hair bulb area appears to be prominently affected in most cases and a swelling, roughening, fraying and irregular outline of the hair is suggestive. Definitive findings in dermatophyte infections are hyphae and arthrospores in the hair shaft but these are seen only in 50 - 60% of cases at most.
Fungal culture is a more reliable diagnostic test and all suspected dermatophyte samples should be further cultured to help confirmation and identification of the species. Dermatophyte growth is best obtained from hair plucks following alcohol spraying of the skin to reduce contaminants. Dermatophyte test medium and Sabouraud's dextrose agar are the media of choice for dermatophytes and both should ideally be used.
Bacteriological examination of skin disease in horses is complicated by the fact that skin is never normally a sterile site and therefore results are frequently complicated by contaminants and commensals leading to confusion as to which bacteria are relevant and which are not. Pustules can be punctured and aspirated/swabbed although these are not commonly encountered in equine skin diseases. Bacteriology taken from underneath crusts can be helpful but are also strongly subject to contamination. Skin biopsies can also be subject to culture.
Impression smears may be taken directly from fresh scabs of suspected dermatophilosis cases. Place 1 - 2 drops of saline onto a slide, clip excess hair from the scab, place underside of scab onto slide and allow to macerate for a few minutes, then remove scab from slide. Allow slide to air dry, then heat fix and stain with Methylene blue, Gram's, Giemsa or DifQik. Demonstration of Gram-positive branching of filaments of parallel rows of cocci (zoospores) is diagnostic although culture can be helpful in confirming this.

Nodules:
As a general rule to obtain a specific diagnosis in cases of nodular skin disease then skin biopsy is the technique of choice. Skin biopsies can provide useful additional information and are most valuable if performed early in the course of disease. It may be helpful to consult the pathologist early in the course of disease to advise on appropriate sampling techniques. Sample a large enough area (minimum 6 mm punch biopsy), possibly include a normal edge and use an appropriate fixative (10% formalin, minimum x5 volume). Do not shave or scrub the lesion prior to biopsy but rather sterilise around the lesion. Infiltrate only a minimum of local anaesthetic or use a ring block to minimise artefacts caused by local anaesthetic infusion. There are various types of sampling techniques including: Punch biopsy - probably the most commonly used technique, ideally no less than 6 - 7 mm diameter; Wedge biopsy - include a small rim of normal tissue along with the lesion; Excision biopsy - excise entire lesion/nodule with an appropriate margin for the type of tissue to be biopsied; Fine needle aspirate - may be useful for fluid filled cavities but of limited value in solid masses.
The commonest causes of inflammatory skin nodules in horses include:
- equine collagenolytic granuloma
- viral papillomas
- mastocytoma
- amyloidosis
- abscesses
- habronemiasis
- panniculitis
- phaeohyphomycosis

Wheals:
These are key featues of urticaria - a condition which may or may not have an allergic basis. Where allergy is suspected then further investigation is warranted.
For dietary allergens, a 4 week elimination diet such as hay only (or hay plus a single hard feed component) and subsequent weekly challenges can be useful.
For inhaled allergens, intra-dermal skin testing remains the gold standard. It requires some experience in the performing and interpretation of the test along with access to specific allergens which can be expensive. Drugs can affect test results and the patient should not have received corticosteroids, anti-histamines or NSAIDs for at least
14 days and preferably 28 days prior to testing.
Serum ELISA, RAST (Radioallergosorbent test) and RIA allergy tests are easy to perform as they require the clinician only to take a blood sample and send it off to one of a number of commercial laboratories. Theoretically the extent of allergy is correlated with the amount of antigen-specific IgE. However, numerous published papers have failed to support the use of these test and results need to be interpreted with caution.

Pustules and vesicles:
These are infrequently seen in horses but may indicate immune mediated disease such as pemphigus vulgaris.