This is one condition that can only be diagnosed with a skin biopsy. Note the depigmentation of the skin of the muzzle and part of the nose. This is a form of skin cancer where malignant lymphocytes infiltrate the epidermis of the skin. Lesions can be seen elsewhere, particularly mucucutaneous junctions of the anus, vulva or prepuce and can also cause ulcerations in the oral cavity.
Low power view of a skin biopsy from the above dog. See the pale band at the lower level of the epidermis. This is caused by infiltration of the epidermis by abnormal lymphocytes.
High power view of skin biopsy showing infiltration of the epidermis with abnormal lymphocytes
The same dog after 12 weeks on chemotherapy. The cutaneous lesions as well as other lesions such as ulcerations on the tongue had resolved. Treatment does not appear to increase life expectancy but can make the animal much more comfortable and appear almost normal.
Skin biopsy is an extremely useful tool in veterinary dermatology. Although many skin conditions can look similar to the naked eye, under the microscope there can be many differences in the appearance of the skin which can often allow a dermatohistopathologist give a definitive diagnosis or at least rule hone down the diagnosis to a few separate conditions.
Dermatohistopathology is a subject in its own right and there are very few pathologists in the UK that a veterinary dermatologist would trust. This is just one reason why you may want to use the services of a dedicated veterinary dermatologist- some general histopathologists do not have the breadth and depth of knowledge necessary in this discipline. I have been using the same dermatohistopathologist for the last 16 years and trust her judgement .
I will resort to histopathogy particularly in the investigation of hair loss, in suspected keratinisation disorders where there is a lot of scale production, when an autoimmune disorder such as pemphigus foliaceus is suspected or when a tumour is removed (to confirm diagnosis and stage it) or I suspect a neoplastic process in the skin such as epithliotrophic lymphoma (often mistaken as an allergic or scaling disorder).
One range of diseases where histopathology is of little use is allergic skin diseases. A skin biopsy taken from an animal with food allergy, atopy or flea allergy will often come back with a diagnosis of “superficial perivascular dermatitis” which will be of little diagnostic use.
Skin biopsy is performed either under general anaesthesia or under sedation and local anaesthesia. Most cases will involve taking several punch biopsies, where a small piece of full thickness skin between 4-8mm diameter (depending in the site and the size of punch used) is removed and placed in fixative solution. The skin biopsies are then mailed to the lab, where the tissue is processed , the biopsy embedded in paraffin wax and sections one cell thick are sliced of and mounted on a microscope slide and stained with special stains prior to being read by the dermatohistopathologist.
Sometimes a larger, ellipse of skin is removed if it is felt it would provide the pathologist with the information necessary to give a diagnosis.
When punch biopsies are taken, the wound is either left open to heal naturally or sutured with one or two stitches.
Another reason for taking biopsies is for tissue culture in cases of suspected deep skin infection with either bacteria or fungi. I recently diagnosed a case of MRSP by this method.
The pathologists report on a skin biopsy can sometimes be two pages long, with a description of what is seen, then a “morphological diagnosis” where a diagnosis of the pathological processes is given and then a “clinical diagnosis” with further information on possible treatment that may be of use. To the clinician.
This vary rare condition was seen in a Devon Rex cat that was reported to have lost its coat at 12 weeks of age. The hair follicles are abnormal and are plugged with abnormal keratin (comedomes) and are not producing normal hair. The skin is thickened and there where numerous nodules from which an oily pus could be expressed.
Close up view, showing the abnormal thickend "hair" which is thick plugs of abnormal keratin sprouting out of dilated hair follicles.
Close up showing follicular plugging (comedomes) and oily pus coming from skin.
Histopathology of skin biopsy specimen showing massively dilated hair follicle and infiltrate of inflammatory cells below it in the dermis.
