Sampling techniques

Equipment required

    • blunted No. 10 scalpel blade or laboratory spatula
    • clippers (to clip the hair prior to skin scraping)
    • glass slides
    • sterile container
    • cotton buds
    • scotch tape
    • mosquito forceps
    • mineral oil
    • coverslips

Ectoparasites sampling – skin scrapings

General guidance

      • When sampling for ectoparasites use the predilection sites for each suspected species to help guide the areas you sample from.
      • Clip the area of skin prior to sampling where possible and apply a small amount of mineral oil to the site.
      • Scrape briskly in the direction of the hair growth on the skin until scale and surface material is dislodged (if taking a superficial scrape), or until capillary oozing is observed (for a deep skin scrape).
      • Aim to be at about a 45 degree angle with at least 3-5 sites sampled.
      • Place collected material onto the slide, add more oil and then apply a coverslip.
      • Remember some mites can be found on tape strips or on ear swabs.

Sarcoptes

      • Can be found on both superficial and deep skin scrapings.
      • The skin area to be scraped should be approximately 10×10 cm.
      • The prime, predilection sites for these mites are the hocks, elbows and ear pinnal margins.

Demodex

      • Demodex mites live in hair follicles, therefore their identification requires deeper skin scrapings.
      • It helps to first gently squeeze the skin between thumb and forefinger, as this extrudes mites from the hair follicles.
      • Then scrape briskly in the direction of hair growth on the skin until capillary oozing is observed.
      • Approximately 2-3 different locations should be scraped.
      • In areas which are difficult to scrape (for example puppies with peri-ocular lesions) you can pluck the hairs instead.

Dermatophytes

If Wood’s lamp positive take samples from the edge of the fluorescing area.

3 sample options are available to submit:

          1. Hair plucks including hair roots (approx. 30-100 hairs if possible) plucked using a pair of haemostat forceps.
          2. Skin scales collected and placed into a sterile pot.
          3. Mackenzie’s tooth brush technique (especially for latent infections). Using a new toothbrush, hair and scale are collected on the bristles by brushing the hair coat for 30-60 seconds, paying particular attention to lesional skin. The shaft of the brush can be cut off and the entire head of the brush submitted to the laboratory in the pot

Cheyletiella

Flea combing or tape strips can be helpful as these mites are very superficial.

The history of a pet rabbit at home, or having been in contact with a rabbit, markedly increases the chance of likelihood these mites.
The predilection site is dorsal aspect with scaling seen.

 Trombiculae

Ears and feet, both of which get close to the ground, are likely places to find these mites.
When feeding these mites create a ‘mineralised straw’ to suck the blood out which will remain as a hard nodule for a couple of weeks.
This nodule is very itchy so even after successful treatment it is normal for these cases to remain pruritic after successful treatment for a while.

Otodectes cynotis

The predilection sites are the preauricular skin, head, neck and tail base.

General tips

Avoid scraping areas that are excessively crusted or excoriated, as this may lead to false-negative results.

Bacteria and Yeast sampling – cytology

Skin cytology samples can be obtained in a variety of ways, depending on the type of lesion present. It’s important to select the correct technique for the lesion presented.

Direct impression smears – Sampling pustules and papules

      • If a pustule is found it should be carefully opened with a fine needle.
      • Take four or five impressions by directly pressing the glass slide onto the lesion.
      • Move the slide slightly before each impression to help avoid too thick a build-up of material on the slide.
      • Be gentle when taking direct impressions, otherwise cellular damage will render the cytology uninterpretable.
      • Where a direct impression is not possible, material may be transferred from the lesion to the slide by the use of a cotton bud.

Indirect impression smears – Sampling crusting lesions

      • Impressions of the underside of crust can be diagnostically valuable,  particularly in cases of suspected pemphigus foliaceus.
      • Use a microscope slide edge to raise the edge of a crust, and then obtain an impression smear of the exudate under the crust.

Tape strips – Sampling moist or greasy lesions (or lichenified skin)

      • Firmly press a microscope slide onto the surface of the lesion to make an impression smear sample.
      • Acetate tape impressions can be used to sample dry, lichenified lesions.
      • Best tape to be used is Scotch tape, because it is clear and non-laminated. Sellotape is laminated and may wrinkle off/delaminate in the fixative used when staining making it difficult to examine under microscope.
      • A piece of tape about 50% longer than a microscope slide is used. The middle of the tape is pressed repeatedly onto the area to be sampled several times to collect surface cells and debri.
      • Then wrap the tape around both ends of the slide to hold it firmly in position.

Tricky sites – Sampling interdigital lesions 

      • Samples can be obtained by pressing the interdigital web directly onto a slide, swabbing the interdigital web with a cotton bud and spreading the debri onto a slide or by making acetate tape impressions.
      • If paronychia is present, debri can be collected from the nail beds by using the wooden/plastic end of a cotton bud. The debri is then smeared onto a microscope slide.

Ear cytology

      • Samples of cerumen or pus can be collected from the vertical ear canals using cotton buds (one per ear).
      • The swab should be gently rolled so a thin amount of debri is left on the slide.

Charcoal swab (bacterial culture & sensitivity testing)

    • Swabs can be taken from the vertical ear canal in a similar way to obtaining an ear cytology sample.
    • The best lesion to sample is an intact pustule, which is opened with a fine needle and the discharge is collected onto the tip of the a bacterial swab.
    • If there are no intact lesions then a swab from the underside of a crust or any other exudative lesion may be used.
    • If the lesions are dry, the swab may be moistened with sterile saline and rubbed on the lesional skin several times.
    • In deep pyoderma, swabs may be inserted into draining sinus tracts or the lesion gently squeezed to exude pus, which is collected onto the swab.
    • Try to source these sinus tract ends rather than end up sampling what may be a secondary infection elsewhere on the skin.
    • Deeper tissue samples may be harvested for culture using a biopsy punch or elliptical incision.
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