The skin carries a large number of microorganisms and provides the first line of the defence for the body, therefore making it common to encounter some sort of skin pathology. These can broadly be subdivided into infectious (bacterial or fungal), and neoplastic.
Common clinical signs associated with infectious skin infections are crusts, papules, abscesses, and draining tracts, which can be pruritic or painful, localised or widely diffused.
It is often impossible to make a differential diagnosis of fungal vs bacterial infections based solely on a visual assessment, therefore resolution of the disease relies on accurate diagnosis and treatment.
Diagnostic techniques
Whenever a skin infection is suspected, a bacteriological/fungal test and antibiotic susceptibility test should be performed on a tissue/fluid sample. These are particularly indicated in cases with deep pyoderma or pyoderma that is non-responsive to general antiseptic therapy.
In order to collect a sample, the skin around a pustule should be cleaned with 70% alcohol, then allowed to air dry.
The pustule is then opened with a sterile needle and the contents absorbed with a sterile swab (ideally a swab that is stored in transport gel). Additionally, a skin biopsy can be taken at the same site.
The entire scab should be submitted in cases suspected to be affected with dermatophilosis.
Biopsies
These can be done in different ways:
- Needle aspiration (with a 19-21G needle attached to a 5-10ml syringe)
- Punch biopsy (with a 18G needle or larger)
- Punch biopsy (with bespoke 4, 6, or 8mm round cutting instruments)
- Trucut biopsy (a bespoke long double sleeved needle for deep tissue)
- Incision biopsy (with a scalpel blade)
- Excision biopsy (with a surgical kit)
- Biopsy tissue is stored in 4% formaldehyde for transportation.
Other types of sampling
Skin grooming or combing — to collect hairs and ectoparasites. Store the collection in a Petri dish.
Skin scrape — using a blade to "scratch" the skin and collect hairs, crusts and (if present) parasites. These can then be placed in culture mediums or under a microscope, by attaching them to adhesive then onto a slide. Store the material collected in a Petri dish for trasnportation.