There are various methods for sampling the skin and the best option(s) will depend on the clinical differentials and nature of the lesions, as well as costs:
(1) Skin Scraping:
For the identification of burrowing mites (uncommon in horses)
- Using a 22 scalpel blade areas are directly shaved/scraped into sterile containers.
- Mineral oil can be applied to the skin if only looking for mites, but this will compromise examination for ringworm spores and is discouraged
(2) Skin Brushings:
- Using a stiff brush and a petri dish or other clean container, the skin is groomed into the dish.
- Mites/lice may be observed directly (harvest mite, forage mites, poultry mites, and lice). Crust, scale and hairs will also be harvested and can be examined.
(3) Hair Pluckings:
For the identification of dermatophytes and dermatophilosis (rain scald).
- Hairs should be plucked using haemostats from the margins of lesions. The hairs should be placed in a sterile container; these samples can be used for direct microscopy and culture.
(4) Sellotape Strips:
- These are most useful for identifying Oxyuris eggs from the perineal/tail region.
- After adhesion to the skin surface, the tape is removed and stuck onto a slide with a drop of mineral oil.
- The tape can also be stained with stains such as Diff-Quick.
(5) Skin Biopsy:
Biopsies are often the most useful technique to investigate skin disease. The skin often reacts to different aetiologies in a stereotypic pattern and an aetiological diagnosis is not always possible but, even when a specific diagnosis cannot be achieved, they often help to eliminate certain clinical differentials. For tumours, they may help confirm the type of tumour and margins of excision.
- Collecting multiple biopsies will often increase the likelihood of an accurate diagnosis.
- In general, do not shave or scrub the area prior to biopsy.
- Sedation and local or regional anaethesia is usually required.
- Avoid infiltrating the region to be biopsied directly with anaethetic
(a) Shave biopsy:
- The epidermis is shaved off parallel to the surface of the skin. No sutures are required.
- This can be useful for conditions affect only the outer surface of the skin and there is concern regarding trauma to deeper structures, but in most cases the diagnostic information obtained is limited and this form of biopsy is not encouraged.
(b) Punch biopsy:
- Sterile 4,6,8mm biopsy punches are available. Larger samples are less likely to be affected by crush artefact and are more likely to be representative.
- Use a 25g needle to remove the sample from the skin (rather than forceps that can induce crush artefact).
- Biopsies from multiple areas is often useful.
- There is usually no requirement to suture the biopsy site.
(c) Wedge biopsy:
- Abnormal and normal skin can be more easily included in one sample and this is often useful for histological examinations.
- An elliptical incision is made down to the subcutis. The biopsy site can be cleaned and sutured after biopsy.
Biopsies should be immersed in 10% formalin fixative (10x biopsy volume) and submitted to the laboratory. If an infectious cause is suspected, then fresh tissue can also be submitted for culture (if deep dermal/subcutaneous infection is suspected, the surface of the skin can be sterilised prior to collecting the sample to reduce surface contaminants).
Fine needle aspirates can provide information in some cases, although the level of diagnostic information can be limited. Aspirates may confirm the presence of suppurative inflammation and causative organisms, or may help identify seromas or haematomas. Melanomas may yield pathognomonic pigmented material. The yield of cells from sarcoids is usually low and the proliferating fibroblasts can rarely be distinguished cytologically from active fibroblasts in other lesions (eg granulation tissue).
- A 23 g needle is usually suitable with a 2-5ml syringe.
- Pre-label the slide (on the side that the cells will be smeared)
- Air dry the slides (no need to use heat or fixatives).
Whatever type of sample you are submitting - it is always extremely useful to submit photos of the lesions to the laboratory (firstname.lastname@example.org), indicating the location of sampling if possible, and your clinical differential diagnoses on the submission form.
Please do contact the laboratory (01638 663017) to discuss optimal sampling.