Emily Rätsep, Amanda Mansz
Animal Health Laboratory, University of Guelph, Guelph, ON.
AHL Newsletter 2025;29(1):24.
This article is an update of a prior AHL Newsletter article (Dec. 2003;7(4):39) by DeLay et al: ‘Show me some skin - obtaining useful information from skin biopsies’.
Dermatologic disease can be very frustrating for everyone involved, from patient and client to the treating veterinarian and pathologist. In many species, these can present with a variety of relatively non-specific clinical signs, including hair loss, pruritis and variable changes involving the skin surface such as crusting, scaling or reddening of the skin. Histology can play a helpful, if not critical, role in diagnosing the underlying cause. However, there are several important things to consider when taking a biopsy for diagnostic purposes.
Time of biopsy: Sample a variety of lesions in varying degrees of progression. Lesions can change over the course of the disease process; it helps to know how the disease progresses. Often as the barrier function is lost during the progression of the skin disease, the histological and gross appearance becomes complicated by a secondary superficial bacterial infection. Therefore, either an early disease process is obscured by secondary inflammation, or the results of that chronic inflammation (scarring, etc.) can mask the lesion, resulting in a non-specific diagnosis. Sampling from earlier in the disease course (prior to secondary infection), or sampling after an initial course of antimicrobial therapy is helpful as it reduces the secondary skin lesions. Primary skin lesions include macule/patch, papule/plaque, pustule, vesicle/bulla, wheal, nodule or cyst. Lesions that may be considered primary or secondary include alopecia, scale, crust, follicular casts, comedo and pigmentary abnormalities. Secondary lesions include epidermal collarette, scar, excoriation, erosion/ulcer, fissure, lichenification and callus. Sampling a variety of stages is key!
Treatment considerations: Glucocorticoid treatment can change the appearance of the skin and thereby affect diagnosis. While early antimicrobial therapy can allow for reduction of the secondary bacterial infection that complicates interpretation of the histological lesions, glucocorticoid treatment can greatly affect the appearance of the skin. Therefore, there should be a period of 4-6 weeks between the last glucocorticoid treatment and skin biopsy to allow for the effects of the therapy to fade. The duration of treatment and date of last treatment should be included in the submission history
Dermatologic history: Lesion distribution can allow for differentiation among specific disease processes. Remember to include the owner’s chief complaint, the animal’s age, breed, sex and coat colour, and always include the lesion distribution in your clinical history. Dermatological diseases tend to have specific anatomic distributions, and the ability to correlate the histological findings with the gross location is very helpful in diagnosis. Depending on the disease, you may see preferential targeting of face, nasal planum, mucocutaneous junctions, dorsal back, etc. Providing this information in the clinical history allows the pathologist to correlate the histological findings with the known disease targets. It is also important to provide information regarding any previous treatments and the results of these therapies, as these can impact the histological features. Photographs can always be included with case submissions, and are always welcomed.
Multiple biopsies (Fig. 1): A minimum of 6 biopsies is recommended, especially for most equine and food animal dermatological cases. In many dermatological diseases, the histological appearance can vary depending on when the sample is taken during the development of the lesion (the appearance can also vary depending on the area of the body affected), or if there are secondary skin changes that occur in more chronic disease processes (i.e., secondary bacterial infections). Therefore, multiple biopsies from the various affected sites are recommended, as this gives the pathologist a better picture of the spectrum of lesions observed. This is especially important in large animals with a greater skin surface area (i.e. equine and other farm animals).
Talk to us: If the report does not match the clinical appearance or there is an inappropriate/inadequate response to therapy, please reach out to us. While histology can provide a great deal of information about dermatological diseases, it is the summation of the diagnostic work up that helps provide a diagnosis. The most useful diagnoses can be made when the microscopic lesions are interpreted in association with the clinical picture. Including your list of differential diagnoses is a helpful way of sharing with us what you are seeing clinically, and at the very least, will allow us to rule in/out your disease of concern.
Resubmissions are sometimes necessary: Sometimes the initial biopsy does not capture the full picture due to many of the considerations described above. If the histologic results do not appear to match the clinical picture, a resubmission can sometimes provide more diagnostic information.
While skin diseases can be challenging to work up, a few small changes in sample selection and submission can go a long way toward providing a diagnosis. Our hope is that these recommendations and the quick tips listed below (Table 1) will help you to maximize the diagnostic return on your future skin biopsy submissions.
Table 1. Quick tips for maximizing the diagnostic utility of skin biopsies for histopathologic evaluation
Figure 1. Appropriate skin biopsy selection. A minimum of 6 biopsies from variable sites are recommended. The red circle represents a sample from the leading edge of the lesion, progressing from normal-haired skin (without too much normal skin) to abnormal crusted and alopecic skin. The blue circles are regions of thick crusting (primary or secondary lesion) representing a stage of disease that may differ from the green circle where skin is alopecic (primary or secondary lesion). The purple circle is the mucocutaneous junction which can be an important location for various disease processes.
Reference
1. Miller WH, et al. Diagnostic methods. In: Muller & Kirk's Small Animal Dermatology, 7th ed. Miller WH, Griffin CE and Campbell, KL, eds. Saunders-Elsevier, 2013:57-107.