Three Equine Pastern Infections Vets Commonly Confuse (and How to Tell Them Apart)

Based on a Zarasyl CE webinar presented by Dr. Stephen White, DACVD, Distinguished Professor Emeritus, UC Davis School of Veterinary Medicine

Lower-limb infections in horses tend to blur together clinically. Staph pyoderma, dermatophyte infection, and dermatophilosis can all produce circular, crusting lesions on the distal extremities, and in practice they're frequently misidentified as one another. Here's how each one actually differs, and what to reach for diagnostically.

Staph Pyoderma

Staph pyoderma can closely mimic both photosensitization and photo-aggravated vasculitis on visual exam alone, which makes cytology the fastest way to sort it out. A simple tape prep or impression smear showing large numbers of neutrophils along with cocci points toward a bacterial pyoderma rather than a vascular or photosensitivity-driven process. In many of these cases, a topical antibacterial alone is sufficient without needing to escalate to systemic therapy.

Dermatophyte vs. Pyoderma: The Circular Lesion Problem

Both dermatophyte infections and bacterial pyoderma are hair follicle infections, and both tend to spread outward, producing a similarly circular pattern of alopecia. Visually, they can be genuinely difficult to tell apart, and it's common for lesions assumed to be dermatophyte infections to actually be pyoderma instead.

The more reliable path is to pluck hair from the margin of the lesion and send it for fungal culture. Cytology alone is a less useful tool here. Dermatophyte hyphae aren't reliably visible on a standard tape prep, and identifying them under the microscope even when present takes real practice.

Dermatophilosis: The Three-Ingredient Infection

Dermatophilosis, caused by Dermatophilus congolensis, needs three things present at once to establish an infection: moisture, trauma (which can be as simple as a high-pressure hose accidentally hitting the horse, or tick and arthropod bites), and a carrier source in the environment, which is sometimes just crust that has fallen to the ground from a previously affected animal.

Cytologically, this one has a distinctive signature: characteristic branching, "railroad track" pattern cocci-like organisms are visible on a tape prep or impression smear, which makes it one of the easier diagnoses to confirm quickly at the microscope. Most cases respond well to topical antibacterial treatment, with a chlorhexidine and miconazole combination being a commonly used option; miconazole is technically an antifungal but carries antibacterial activity as well, which is part of why that pairing works for both dermatophyte and dermatophilosis presentations.

The Practical Workflow

For any circular, crusting distal limb lesion, a reasonable sequence looks like this: start with cytology to check for the railroad-track pattern of dermatophilosis or heavy neutrophil/coccus presence suggesting pyoderma, and if neither is conclusive, move to a fungal culture from plucked margin hairs to rule dermatophyte in or out. This keeps the workup fast for the more common bacterial causes while still catching fungal disease that cytology alone would miss.

Worth Ruling Out in the Right Geography

Corynebacterium pseudotuberculosis is a less common but more serious differential, particularly relevant in regions where it's endemic. It can present with draining tracts on the limbs and, in more advanced cases, internal abscessation, and diagnosis requires biopsy and culture rather than cytology alone. Treatment is typically a prolonged antibiotic course guided by culture and sensitivity results.

Bringing This Knowledge to the Field

This field guide comes from a continuing education webinar Zarasyl hosted with Dr. White. Practical, exam-room-ready reference content like this is exactly what we aim to bring to DVMs and surgical teams, and we're proud to make it available to the wider veterinary community.