How to perform a skin scrape

01 February 2014
Practical
4 mins read
02 February 2014
Volume 5 · Issue 1

Abstract

A skin scrape is a fairly quick and simple diagnostic technique that all veterinary nurses should be able to perform competently. It is also an inexpensive diagnostic aid, that is easy to undertake, and if assessed in-house, can produce rapid results which permit prompt treatment of affected animals. Competency in these skills will improve management of skin cases and will free up valuable time for the veterinary surgeon to continue with a busy clinic.

A skin scrape is a fairly quick and simple diagnostic technique that all veterinary nurses (VN) should be able to perform competently. It is an inexpensive diagnostic aid that is primarily used for identification of ectoparasites. Skin scrapes are easy to undertake, and if assessed in-house, can produce rapid results which permit prompt treatment of affected animals. As skin scrapes are the most frequently used tests in veterinary dermatology (Miller et al, 2013), assigning this responsibility to VNs can assist with the efficient running of busy veterinary practices. Skin scrapes are usually well tolerated and can be performed in the conscious animal during consultation. For fractious animals or for sampling of sites that are painful or technically difficult to sample (interdigital skin, pinnae) it may be worth considering sedation to ensure representative samples are obtained.

Skin scrapes are used to diagnose ectoparasites, such as Demodex spp, Sarcoptes spp and Cheyletiella spp mites. However, it must be remembered that the sensitivity of this diagnostic test is not 100% and if a negative result is obtained, particularly for suspected Sarcoptes spp infestation, a strict ectoparasite treatment trial should be performed to completely eliminate disease.

Where to sample

Selecting a sample site can be difficult. The chosen site should be an area of active disease, i.e. erythematous papules in cases of sarcoptic mange, scale in cases of cheyletiellosis and alopecia, erythema or comodones in cases of demodicosis (Table 1). VNs must be able to distinguish between skin lesions resulting from the disease process and secondary trauma, as excoriated areas should be avoided (Patel and Forsythe, 2008). Several scrapes should be taken to give the best chance of ectoparasite identification (Littlewood 2002).


Ectoparasite Where to scrape
Sarcoptes Lateral elbow, lateral hock and pinnae
Demodex Areas of alopecia, comodones
Otodectes Head and neck (ectopic infestations)
Cheyletiella Dorsum, areas of dandruff/scale

Equipment needed

  • Microscope (compound)
  • Microscope slides
  • Coverslips
  • Pencil
  • Liquid paraffin/potassium hydroxide (KOH)
  • Scalpel blade (preferably size 10, but a smaller size 15 can be used for difficult areas)
  • Curved scissors/clippers — if using clippers take care not to disrupt the skin surface
  • Gloves.

    • Liquid paraffin versus potassium hydroxide

    All skin scrapes should be mounted in mineral oil, e.g. liquid paraffin or potassium hydroxide, depending on preference and what the sample is being examined for (Table 2). In the author's opinion, liquid paraffin is preferable as it can be placed directly onto the patient's skin. If KOH is used the patient's skin must be scraped dry, which makes it difficult to collect the sample. Liquid paraffin and KOH cannot be mixed as the two will emulsify.


    Action KOH Liquid paraffin
    Can it be applied to the skin NO YES
    Does it kill mites YES NO
    Does it breakdown keratin YES NO
    Is heat required YES NO

    KOH is lethal to all mites, so in conditions such as demodecosis, where it is important to establish if the mites are alive, liquid paraffin should be used to evaluate if treatment is working.

    10% KOH is used to help breakdown keratin, which can be particularly useful in cases that have crusted lesions. It must be remembered that KOH is caustic to the skin so care must be taken when used. A lower concentration of KOH can be used but will take longer for the breakdown of keratin (Littlewood, 2002) (Table 2).

    Superficial versus deep skin scrapes

    Ectoparasites live at different levels in the skin and it is important to know the differences to determine the depth of skin scrape required to reach the parasite during sampling.

    Superficial — Cheyletiella is a surface-dwelling mite, so superficial skin scrapes should be performed. No capillary ooze is necessary. Superficial scrapings can also be used to diagnose ectopic Otodectes infestation, where mites are living outside the ear canals.

    Deep — Demodex canis and Demodex injai live in the hair follicle and Sarcoptes scabiei are burrowing mites, therefore deep skin scrapes are required to identify these parasites. Scraping the skin until capillary ooze is obtained is essential to ensure the epidermis (where the mites reside) has been removed.

    Conclusion

    Performing and assessing skin scrapes are valuable skills which every veterinary nurse can learn. Competency in these skills will improve management of skin cases and will free up valuable time for the veterinary surgeon to continue with a busy clinic. Investment in a good microscope and ensuring it is well maintained and accessible for use at any time is important. Poor maintenance and storage of microscopes will deter their use and mean samples are more likely to be submitted to a laboratory. This will increase the risk of false negative results as samples may become

    Step-by-step guide to taking a skin scrape

  • Prepare all equipment required ready for use (Figure 1)
    Figure 1.
  • Select area to be sampled
  • Clip hair if necessary, being careful not to disrupt the skin surface (Figure 2)
    Figure 2.
  • For suspected demodicosis cases squeeze the skin between thumb and fingers (this will squeeze any Demodex mites out of the hair follicle) (Figure 3)
    Figure 3.
  • Place a small amount of liquid paraffin or equivalent onto a microscope slide (Figure 4)
    Figure 4.
  • Collect a small amount of liquid paraffin (if using) onto the scalpel blade and carefully spread the liquid paraffin over the area to scrape or place a drop directly onto the skin. The area can be scraped dry, but using the liquid paraffin helps to collect the sample (Figure 5)
    Figure 5.
  • Holding the skin taut, angle the blade at 45° to the skin and scrape in the direction of hair growth while applying moderate pressure. This helps any mites in the hair follicle to be ‘squeezed’ out (Figure 6)
    Figure 6.
  • After a few scrapes, use the scalpel blade to ‘scoop’ up the sample and place onto the microscope slide (Figure 7)
    Figure 7.
  • Carry on scraping until capillary ooze is achieved, if a deep scrape is required (Figure 8)
    Figure 8.
  • Collect all material onto a microscope slide and ensure an even spread, to ease microscopic examination (Figure 9)
    Figure 9
  • Place a coverslip on top of the slide (Figure 10)
    Figure 10.
  • Label the microscope slide, to enable identification of the sampled area (Figure 11)
    Figure 11.
  • Dispose of scalpel blade safely
  • Examine the whole sample in a systematic fashion using a low power objective (×4 or ×10). Ensure the iris diaphragm is closed and the light intensity is lowered as this will provide better contrast and assist ectoparasite identification damaged or degrade in transit and it will delay the time to diagnosis, meaning some patients with debilitating skin disease will have to wait longer for treatment.

    • Key Points

  • Skin scrapes are an easy and simple technique that all veterinary nurses can become competent in.
  • Performing skin scrapes in-house can speed up diagnosis.
  • Sampling representative area/lesions is essential in achieving a diagnosis.
  • Equipment maintenance is important in the efficient and effective assessment of skin scrapes.
    • skin scrape
    capillary ooze