What is a crust?
Crusts on a pet’s skin are an accumulation of dried exudate, skin cells, pus, serum, and blood.
Most often the crusts you will see on a dog’s skin are secondary lesions, due to pyoderma. However, these crusts have a primary cause. It is important to get a good medical history and thorough physical exam (what is the distribution of these crusts) to find that primary cause.
Make sure to rule out ectoparasites, like fleas, and mites, like demodex and sarcoptes. Also, it is important to look for infections like dermatophytes.
If the pet is pruritic, then the most common underlying or primary causes are allergies, like food or environmental. If there is no pruritus, then disorders like endocrine and metabolic diseases are important to rule out. Remember that disorders like Cushing’s can cause a dog to get recurrent pyodermas. Should the pet continue to develop crust lesions, then biopsy of the skin is necessary to rule out disorders like primary keratinization disorders, nutritional disorders, autoimmune and neoplastic causes.
The picture here is of a 11-year-old M/N German Shepherd with an approximate one-year history of these crust lesions developing on the skin associated with minimal pruritus. There was no prior history of skin issues throughout this pet’s life. No endocrine disorder was present on bloodwork. The referring veterinarian treated for pyoderma and allergies with no complete resolution.
We took biopsies of the skin and histopathology revealed pemphigus foliaceus, an autoimmune skin disorder. Pemphigus can be variably pruritic and can confuse a general practitioner with pyoderma and allergies. One key finding on exam for this pet was subtle crusting on the pinna and muzzle, not common in allergies. In pemphigus, the primary lesion is a pustule that is usually transient, the pustule progresses to an erosion and then crusts, that can have a yellow color.
The purpose of this post is to show that crusts on the skin can be more than just a pyoderma. And should you find yourself with crust lesions on a patient that are not going away, then consider a biopsy.
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