Skin disease accounts for up to 25% of all small animal veterinary consultations, making it the most common reason for veterinary visits after wellness care. Despite this frequency, dermatology cases generate significant frustration due to the stepwise, often time-consuming diagnostic process. Use the Dermatology AI for lesion analysis and the Microscope AI for cytology interpretation.
A thorough history is the most undervalued diagnostic tool in veterinary dermatology. Key questions include:
Onset and duration: Acute (<2 weeks) suggests infection, parasites, or contact reaction. Chronic (>6 weeks) suggests allergy, endocrinopathy, or autoimmune disease.
Pruritus level (1-10 scale): 0-3 = non-pruritic (consider endocrine, neoplastic); 4-6 = moderate (common with secondary infection); 7-10 = severe (allergy, scabies, Malassezia dermatitis). Pruritus that preceded lesions suggests allergy; lesions that preceded pruritus suggest infection or parasites.
Seasonality: Seasonal pruritus suggests atopic dermatitis (pollen allergens) or flea allergy dermatitis. Non-seasonal pruritus may indicate food allergy, Demodex, or scabies.
Diet: Current diet, treats, supplements, and any previous diet trials. A true elimination diet trial requires a novel or hydrolyzed protein source for a minimum of 8 weeks.
Flea/tick prevention: Product used, compliance, all pets in household treated. Flea allergy dermatitis remains the most common allergic skin disease worldwide.
Contagion: Other pets or humans affected? Scabies, dermatophytosis, and Cheyletiella are contagious to household contacts.
Describe lesions using standardized dermatologic terminology:
Primary lesions (directly caused by disease): macule (flat color change <1 cm), patch (flat >1 cm), papule (raised <1 cm), plaque (raised >1 cm), pustule (pus-filled), vesicle (fluid-filled), wheal (edematous), nodule (deep, >1 cm), and tumor (large nodule).
Secondary lesions (result from disease progression or self-trauma): scale, crust, excoriation, erosion, ulcer, lichenification, hyperpigmentation, alopecia, and comedo.
Distribution pattern: The location of lesions is as diagnostically important as the lesions themselves.
| Distribution Pattern | Top Differentials |
|---|---|
| Ventral (axillae, groin, ventral abdomen) | Atopic dermatitis, contact dermatitis, Malassezia dermatitis |
| Dorsal lumbosacral (tail base) | Flea allergy dermatitis |
| Pinnae, elbows, hocks, ventral chest | Scabies (Sarcoptes) |
| Periocular, perioral, pedal (paws) | Atopic dermatitis, demodicosis |
| Trunk, bilaterally symmetric alopecia | Endocrinopathy (hypothyroidism, Cushing's) |
| Nasal planum, pinnae, mucocutaneous junctions | Autoimmune (pemphigus, lupus), zinc-responsive dermatosis |
| Focal, well-demarcated circular alopecia | Dermatophytosis (ringworm), bacterial folliculitis |
Five diagnostic tests should be performed on virtually every dermatologic case before advanced diagnostics:
1. Skin scraping (superficial): Scrape with mineral oil, 3-5 sites, looking for Sarcoptes, Cheyletiella, Demodex (superficial scraping is less sensitive for Sarcoptes; treat empirically if clinical suspicion is high).
2. Skin scraping (deep): Squeeze skin fold, scrape until capillary bleeding. Looking for Demodex canis/injai/gatoi. Multiple sites. Negative deep scrapings in a dog with appropriate lesions do not rule out demodicosis; consider skin biopsy in breeds with thick skin (Shar-Pei).
3. Impression cytology (tape strips/direct smears): Assess for bacteria (cocci vs rods), Malassezia yeast (peanut-shaped organisms), and inflammatory cells (neutrophils vs eosinophils). This is the single most important test in veterinary dermatology for identifying secondary infections.
4. Trichogram: Pluck hairs and examine microscopically. Assess hair shaft integrity (dermatophyte-infected hairs show fragmented, swollen shafts), stage of growth cycle, and Demodex mites in the hair follicle.
5. Wood's lamp examination: Only ~50% of M. canis strains fluoresce apple-green under Wood's lamp. Negative does not rule out dermatophytosis. Positive is strongly suggestive. DTM (dermatophyte test medium) culture remains the gold standard for dermatophyte diagnosis.
Warning: Do NOT prescribe antibiotics or antifungals for dermatologic cases without first performing cytology. Treating based on appearance alone leads to diagnostic delay, owner frustration, antimicrobial misuse, and failure to identify the underlying cause.
Follow a systematic rule-out process:
Rule out ectoparasites first: Treat all dogs in the household with isoxazoline-class parasiticides (fluralaner, sarolaner, afoxolaner) for a minimum of 8-12 weeks regardless of scraping results. This simultaneously treats Demodex, Sarcoptes, fleas, and most mites. Response to treatment is diagnostic.
Rule out secondary infections: Treat all bacterial and yeast infections identified on cytology. Many "allergic" dogs are actually pruritic due to secondary Staphylococcal pyoderma or Malassezia overgrowth. Resolution of infection may dramatically reduce pruritus, changing the clinical picture.
Elimination diet trial: Once parasites and infections are controlled, initiate an 8-week strict elimination diet using a novel protein (e.g., venison, rabbit, kangaroo) or hydrolyzed protein diet. No treats, table scraps, flavored medications, or supplements outside the diet. If pruritus resolves, food allergy is confirmed; provocation challenges identify specific triggers.
Allergy testing: If environmental allergy is suspected after ruling out food allergy, intradermal testing (IDT) or serum allergen-specific IgE testing guides allergen-specific immunotherapy (desensitization). IDT is considered more specific but requires specialist equipment and sedation. Serum testing is more accessible in general practice.
Skin biopsy is indicated when: lesions are unusual or do not fit common differential diagnoses, autoimmune disease is suspected (pemphigus foliaceus, discoid lupus erythematosus), neoplasia is suspected, the case has not responded to appropriate empiric therapy, and nodular or ulcerative disease is present. Submit multiple (3-6) 6mm punch biopsies from representative lesions, including lesion margins. Avoid sampling traumatized, crusted, or secondarily infected areas when possible.
Refer to a veterinary dermatologist when: the case has not responded to a systematic diagnostic workup, severe or refractory demodicosis is present, intradermal allergy testing and immunotherapy are desired, unusual or suspected autoimmune skin disease is present, or the owner and clinician are frustrated by lack of progress. Early referral reduces client frustration and may be more cost-effective than prolonged empiric treatment.
The Dermatology Specialist can help plan the diagnostic workup, interpret complex results, and determine whether referral is appropriate for a specific case.
- A thorough dermatologic history (onset, pruritus level, seasonality, diet, flea prevention, contagion) is the foundation of diagnosis.
- Lesion distribution is as important as lesion morphology: ventral = allergy, dorsal lumbosacral = FAD, pinnae/elbows = scabies.
- Impression cytology is the single most important test in veterinary dermatology; never prescribe antibiotics or antifungals without it.
- Follow the diagnostic algorithm: rule out parasites, then infection, then food allergy, then test for environmental allergy.
- Elimination diet trials must be strict, 8 weeks minimum, with no treats or flavored medications outside the diet.
- Refer early to a dermatologist when the systematic workup is not yielding results; it is more cost-effective than prolonged empiric therapy.