When it comes to skin disease in dogs and cats, the physical exam should include a check of every nook and cranny.
One the most challenging things about veterinary dermatology is that a lot of conditions look very similar. That’s because there are only a limited number of reaction patterns in the skin. How are you supposed to tell what’s what?
A dermatologic exam is one part of the puzzle and should include your entire patient from stem to stern:
You’re looking at the sclera, conjunctiva and periocular region. Many atopic animals will have conjunctivitis or episcleritis, which could end up causing self-induced trauma, blepharitis, erythema, hypotrichosis and excoriations. What else do you think of when I say "periocular"? Demodicosis should come to mind.
This includes palpating the canals and examining the concave and convex surfaces, as well as margins of the pinnae. If crusting is noted on the ear margins, Sarcoptes scabiei should be on your differentials list and a pinnal-pedal reflex should be checked. Ear margin dermatitis is also seen in vasculitis, drug reactions, solar dermatitis and immune-mediated diseases like lupus and pemphigus foliaceus.
Generalized lymphadenopathy accompanies any type of mange but will be more localized when dealing with infections.
Not many conditions will cross over from skin to the nasal planum, so if crusting is present, consider mucocutaneous pyoderma or an immune-mediated condition like lupus or pemphigus foliaceus. If there is a loss of pigment in the nasal planum, along with a smoothing out of the normal cobblestone appearance, you’ll be leaning toward diskoid lupus erythematosus.
Hypotrichosis on the dorsal muzzle is a subtle hallmark of hypothyroidism and may or may not be accompanied by facial edema, causing a tragic expression. If there is crusting of the lip margins, this could be mucocutaneous pyoderma or an immune-mediated disease. Some immune-mediated skin diseases also have oral lesions, so take a peek while you’re there. Cats can also have oral lesions with eosinophilic granuloma complex and may have an accompanying indolent ulcer on the upper lip or a swollen (“pouty”) lower lip.
Examine under the collar, at the ventral neck fold and down both forelegs. Erythema and evidence of barbering are often seen on the cranial aspect of forelimbs at the flexure sites of elbows in patients with environmental allergies. Ask the owner if they’ve seen their furry friend do any “corn cobbing” or nibbling down the legs. Continue to palpate lymph nodes as your exam progresses caudally.
Other classics for environmental allergies include erythema of the interdigital spaces, caudal surfaces of the carpus, axillae, inguinal region and ears. When these areas are inflamed, they create the perfect conditions for secondary bacterial and fungal infections. Performing cytology is necessary for appropriate treatment. Even after 20 years of working in dermatology, I am still surprised by findings on cytology—you can never be sure what’s going on until you examine your samples under the microscope!
Patients with atopy often have secondary infections on the ventrum, while patients with adverse cutaneous food reactions will often have secondary lesions over the dorsum. Food-allergic patients also often have perianal erythema with or without adverse gastrointestinal signs that can include halitosis, burping, borborygmus, flatulence, scooting, anal gland issues, pica, vomiting and abnormal stools (very hard, soft, diarrhea, voluminous, multiple bowel movements per day). Unfortunately, many sensitive patients have a combination of food and environmental allergies.
A moth-eaten appearance can indicate folliculitis, which can be a result of a bacterial infection or even demodicosis. Take a good look at the hairs. If follicular casts are evident, your differentials could include vitamin-A–responsive dermatoses, primary seborrhea, sebaceous adenitis, demodicosis or even dermatophytosis. Is the coat brittle or dry? Is it greasy? Does it epilate easily? Any conditions involving the hair follicle or sebaceous gland could be involved.
Are there areas of alopecia? Are there lesions? If these are symmetric, consider an internal disease process such as allergies or immune-mediated, metabolic or endocrine disorders. Asymmetric lesions move your differentials to infections, ectoparasites or neoplasia. Are there any comedones (blackheads)? Comedones are also seen with endocrine disorders as well as vitamin-A-responsive dermatoses, demodicosis and dermatophytosis. They can also be seen in animals that are genetically predisposed, like schnauzers or Mexican hairless.
Look for patterns—many conditions have hallmark patterns. Examples: Extremities are affected in vasculitis; face and feet in demodicosis; ear margins, hocks and elbows for Sarcoptes; dorsum for cheyletiellosis; and head and paws for pemphigus.
Are flakes fine or large? Very large scaling is seen in ichthyosis, while smaller scaling can occur in seborrhea, follicular dysplasia or following chronic inflammation. Scaling with pruritus is seen with cheyletiellosis, Demodex injai or cutaneous T-cell lymphoma. Scaling without pruritus can be seen with dermatophytosis or keratinization disorders. Bear in mind that there will be increased scaling for the first couple of months in a patient just started on thyroid supplementation.
Thin or atrophied skin can be seen with Cushing’s syndrome or in patients on long-term corticosteroids. Myxedema can be seen with hypothyroidism. Thickened or lichenified skin is seen in patients with chronic inflammation. Hyperpigmentation is also seen as a footprint of inflammation. On the opposite end, hypopigmentation is seen with vitiligo and, more concerning, with lupus.
Are there papules, pustules or epidermal collarettes? All of these are seen with pyoderma but also in immune-mediated skin diseases. Pustules that are a result of a bacterial skin infection usually only span one hair follicle. Larger pustules spanning more than one follicle are seen with immune-mediated disease. Bacterial infections usually affect the trunk and may extend down legs, while immune-mediated diseases will affect the head and paws followed by the trunk. Always check mucocutaneous membranes. If affected, your differentials can include harsher forms of pemphigus with a poorer prognosis.
Don’t forget to examine every paw, including claw beds, nails and the interdigital spaces. Crusting of foot pads can be seen with many things, including pemphigus, lupus, vasculitis, zinc-responsive dermatosis and hepatocutaneous syndrome (often with fissuring). Swollen footpads in cats are seen with plasma cell pododermatitis. If more than one claw is affected in a dog, consider symmetric lupoid onychodystrophy. Crusting of ungual folds is often seen in cats with pemphigus.
Hyperkeratosis of footpads and the nasal planum can be seen with distemper, zinc-responsive dermatosis, hepatocutaneous syndrome, pemphigus and lupus. This can also be seen with idiopathic nasodigital hyperkeratosis and hereditary nasal parakeratosis of Labrador retrievers. Breed predilections play a big role in narrowing in on a differential diagnosis as well as age and response to previous treatments.
It's a lot, but if you put all the pieces of the puzzle together (patient signalment, history, physical findings and diagnostics), you can’t go wrong.
Diagnostic methods. In: Miller W, Griffin C, Campbell K. Muller and Kirk’s Small Animal Dermatology. 7th ed. St. Louis: Elsevier Mosby, 2013.
Differential diagnoses. In: Hnilica K, Patterson A. Small Animal Dermatology: A Color Atlas and Therapeutic Guide. 4th ed. St. Louis: Elsevier Mosby, 2016.
Jennie Tait, AHT, RVT, VTS (Dermatology – Charter Member) works at Yu of Guelph Veterinary Dermatology with locations in Ontario, Canada.