7 Equine Dermatology: Diagnoses and Treatments

Objectives of Equine Dermatology

• Recognize and list major dermatoses, including pruritic, scaling, and pigment disorders, and formulate diagnostic plans.
• Understand the lifecycle, signs, diagnosis, and treatment of parasitic skin diseases.
• Compare dermatophilosis and dermatophytosis.
• Differentiate between equine sarcoid, habronemasis, pythiosis, squamous cell carcinoma, and exuberant granulation tissue.
• Recognize the types and treatment options for equine sarcoids.
• Compare and contrast equine dermal neoplasia, including prognosis.

Evaluation of a Dermatology Patient

• History includes duration, seasonality (allergies, insects), individual vs. herd (immune vs. infectious), zoonotic evidence, and past treatments.

Physical Examination

• Assess lesion type and configuration: pruritus, excoriation, pustules, lichenification, pigmentation, maculopapular eruption.
• Note lesion distribution: dorsal, ventral, nonpigmented skin, mane, and tail.
• Evaluate the horse's overall health for systemic illness signs.

Diagnostic Approach

• Skin scrapes are performed to identify mites, indicating mange.
• Cultures are obtained to identify fungal and bacterial infections.
• Skin biopsies can help diagnose immune-mediated dermatoses.
  • Avoid excessive skin preparation before biopsy.
  • Collect crusts in scaling/crusting cases.
  • Take multiple samples from different lesions, especially early or demarcated ones, using a 6 mm punch biopsy or elliptical resection.
  • Close the skin with 1-2 cutaneous sutures.
• Mince preps can help identify Onchocerca.
  • Macerate a fresh 6mm punch biopsy sample in warm saline and incubate for 30 minutes.
  • Microscopic examination can reveal moving microfilaria.

Pruritic Dermatoses

• Presence of broken hairs, excoriation, hemorrhagic crust, and alopecia.

Lice (Pediculosis)

• Common ectoparasite, host-specific, more prevalent in winter.
• Sucking louse: Haematopinus asini affects mane and tail and feeds on blood.
• Biting louse: Damalinia equi affects the dorsolateral trunk and feeds on epidermal debris.
• Transmission is through direct or indirect contact (tack, brushes, bedding).
• Clinical signs include a patchy, rough hair coat, pruritus, alopecia, anemia, and debilitation.
• Diagnosis involves demonstrating nits attached to hair.
• Treatment includes Ivermectin, topical pyrethrin or lime-sulfur (repeated), disinfecting tack, and using commercial flea sprays.

Mange (Acariasis)

• Sarcoptes scabiei var equi (head mange) causes intense pruritus on the head and neck, burrowing into the superficial epidermis, resulting in scaling, crusting, excoriation, and lichenification; it's very uncommon and highly contagious, reportable outside of North America.
• Psoroptes equi (body mange) causes intense pruritus on the mane and tail, spreading to the trunk; it's very uncommon, highly contagious, and reportable outside of the US.
• Chorioptes (leg mange) is relatively common, especially in Draft horses, causing intense pruritus on legs and perineum, pastern dermatitis, scaling, hemorrhage, oozing serum, lichenification, and hypersensitivity to insect saliva.
• Trombicula (chiggers) are common but hard to find.
  • They are larvae of free-living mites found in grasses, forests, or swamps, and are most common in late summer and fall.
  • They cause wheals and papules with orange, red, or yellow dots (larvae) in the center.
  • Infections are usually self-limiting but may require lime-sulfur or prednisone.

Diagnosing and Treating Mange

• For diagnosis, ideal skin scrape sites are non-traumatized areas near new lesions, using mineral oil and deep scrapes with a surgical blade.
• Skin biopsy rarely identifies mites.
• Ivermectin is a treatment.

Ticks

• They suck blood, transmit diseases, and cause tick paralysis (primarily in foals) and urticaria.
• They are most common in spring and summer on ears, perineum, axilla, distal limbs, mane, and tail.
• Frontline spray is an effective treatment for legs, mane, and tail.

Onchocerciasis (Onchocerca cervicalis)

• Adult nematodes live in the ligamentum nuchae and produce microfilaria that migrate to the ventral midline and head, with Culicoides as the intermediate host.
• Causes leukoderma, alopecia, lichenification, excoriation, and ulceration.
• Commonly affects horses over 4 years old.
• Skin scrapings are ineffective for diagnosis, but histopathology reveals superficial eosinophilic perivascular cuffing; minced skin preparations are more reliable.
• Current anthelmintics do not kill adult worms; periodic ivermectin treatment helps manage exacerbations.
• Pretreatment with prednisolone can prevent reactions to dying microfilaria.

Insect Hypersensitivity

• Flying insects like Culicoides, black flies, horn flies, deer & horse flies, stable flies & mosquitoes can cause hypersensitivity.
• Culicoides (no-see-ums, midges, Queensland Itch) feed at dusk to dawn in pasture.
• Black flies feed in the morning and evening.
• Horn, Deer/Horse & Stable flies feed in bright sunlight and are associated with round bales.
• Hypersensitivity results in pruritic dermatitis, alopecia, crusts, and scaling, especially on the mane, tail, and pectoral region, leading to self-trauma.
• Skin scraping is uninformative.
• Diagnosis is often based on history, season, and lesion distribution; skin biopsy shows superficial eosinophilic perivascular dermatitis and edema.
• Prevention involves knowing fly types, using fans, housing horses in screened areas during peak feeding times (dusk, night, dawn), and applying topical residual insecticides (pyrethrins).
• Reduce inflammation using prednisolone, dexamethasone, or hydroxyzine.
• Immunotherapy/hyposensitization can help horses with atopy, especially if started young (under 4 years old).

Miscellaneous Causes of Pruritus

• Food allergies (potatoes, malt, beet pulp, alfalfa).
• Contact allergies and irritant dermatitis.
• Cutaneous drug reactions (penicillin, sulfa-based antimicrobials).
• Tail rubbing is related to pinworms or needing sheath/mammary glands cleaning and can be due to illegal alcohol injection.

Scaling and Crusting Dermatoses

• Key dermatoses causing scaling and crusting

Dermatophilosis (Dermatophilus congolensis)

• Gram-positive, facultative anaerobic actinomycete that can't penetrate intact skin; causes rain scald or rain rot.
• Forms crusted, moist mats of hair resembling paintbrushes; under fresh crusts, the skin is ulcerated, moist, and yellow to pink, primarily on the dorsal area, needing chronic moisture and macerated skin for transmission through zoospores.
• Carrier animals serve as reservoirs and stay viable for 42 months.
• Diagnose using impression smears to identify branching, multiseptate hyphae with transverse and horizontal cocci (railroad tract formation), bacterial culture, and histopathology.
• Treat by bathing with antibacterial shampoo (chlorhexidine) with sedation for crust removal, systemic penicillin for severe cases, and environmental improvements.

Dermatophytosis

• Dermatophytosis poses a zoonotic concern (Trichophyton equinum, Trichophyton metagrophytes, Microsporum gypseum, and Microsporum canis).
• Contagious skin disease, transmitted by direct contact or fomites; predisposed by debilitation, moisture, and stress.
• Clinical signs: alopecia, crusted papules with circumferential spread on the face, trunk, and pectoral region; pain and pruritus vary based on secondary bacterial infection.
• Diagnosis: Wood’s lamp is rarely successful, skin scrape isn't successful, fungal culture is performed using a dermatophyte test medium, and histopathology detects septate hyphae or oval spores; perform fungal cultures on hairs from lesion periphery.
• Treatment is self-limiting in 1-6 months, contagious with zoonotic potential, and usually treated with topical antifungals; systemic antifungals are costly.

Pemphigus Foliaceus

• Autoimmune skin disease (Type II hypersensitivity) where antibodies target keratinocyte glycocalyx, disrupting intercellular cement.
• Results in non-pruritic scaling, crusting, alopecia, rare subcorneal pustules, and painful skin; lesions start on the head and spread.
• Histopathology reveals subcorneal acantholysis and neutrophilic inflammation; direct immunofluorescence highlights intercellular cement.
• Young horses have an excellent prognosis with prednisolone (1 mg/kg, SID) for a cure.
• Adults are difficult to treat and may need long-term glucocorticoids

Distal Limb Dermatitis

• Also known as mud fever, dew poisoning, or greasy heel, is an infection of the lower leg.
• Often caused by muddy, wet conditions and involves Staph, Dermatophilosis, or fungi.
• Treatment: change environment, bandage, and oral antibiotics.

Chronic Progressive Lymphedema

• Is similar to distal limb dermatitis in draft horses but can progress to a large, abnormal leg.
• Genetics and impaired distal limb lymphatics play a role.
• Secondary infections and parasites are involved.
• Is a manageable problem.

Photosensitization

• Causes crusting and inflamed skin on white areas.
• Primary photosensitization is caused by ingesting certain plants.
• Liver disease can also cause it.

Nodular Skin Disease

• Includes diseases that form nodules on the skin

Urticaria

• Type I hypersensitivity reaction.
• Causes wheals, piloerection, and dermal edema.
• The origin of topical, inhaled, or ingested allergens is usually hard to identify.
• Treat with hydroxyzine HCL, prednisolone, dexamethasone, or epinephrine if there's respiratory distress (10 ml, IV 1:10,000 units).

Nodular Necrobiosis

• Etiology is unknown
• Eosinophilic granulomas form as single or multiple, haired, raised, firm dermal masses (0.5-10 mm); nonpainful, nonpruritic.
• Diagnosed based on collagen degeneration with eosinophilic inflammation.
• Treat with intralesional triamcinolone (not exceeding 20 mg).

Papillomatosis

• Caused by papilloma virus in young horses (12-24 months).
• Presents as multiple, raised, dermal, nonhaired, nonpainful, nonpruritic lesions.
• Diagnosed through visual inspection.
• Treatment: benign neglect, crushing lesions to stimulate antigen response, or immunostimulant therapy (EqStim) to clear in weeks to months.

Sporothrix schenkii (Sporotrichosis)

• Caused by dimorphic aerobic fungus; zoonotic, enters via traumatic wounds.
• Presents as papules in a chain along one limb; firm nodules form and ulcerate along a chain with interspersed draining tracts, often on the medial, proximal limb, with brown to red thick exudates.
• Diagnosed via cytologic exam (cigar-shaped yeast, Wright Giemsa), culture, and biopsy (nodular, granulomatous dermatitis, yeast rarely seen).
• Manage with sodium iodide (20%) IV or oral iodide (EDDI 20 mg/kg, SID); iodides can cause dry skin, tearing, and abortion.

Corynebacterium pseudotuberculosis

• Results in 2 different syndromes
• Ulcerative Lymphangitis: a single limb (often hind) shows chains of nodules that abscess, ulcerate, and drain. Affected limb cycles from swollen painful to periods of remission; the regional lymphatics are corded and dilated on ultrasound.
• Pigeon Breast or Dryland Distemper: deep muscle abscess
• Diagnose with bacterial culture and sensitivity (ultrasound-guided aspirate); treatment involves antibiotics, bandaging, and has a poor prognosis for normal leg function; lance and drain pigeon breast (usually avoid antibiotics); it's contagious, and organisms grow in soil (isolate).

Habronemiasis

• Larvae of Habronemia spp or Draschia megastomata cause hypersensitivity.
• Lesions appear seasonally and regress spontaneously in winter.
• Lesions are on legs, ventrum, eye, prepuce, urethral process, or existing wounds.
• Causes ulcerated, granulomatous, pruritic, and hemorrhagic lesions.
• Wounds may contain calcified granules.
• Diagnosed via signs, skin scraping can be unreliable while histopathology may reveal necrotic larvae.
• Treat with surgical excision, steroids, wound care, fly control, and ivermectin.

Pythiosis

• Pythium insidiosum causes pythiosis.
• Aquatic fungus invades macerated skin after contact with contaminated water in Gulf Coast States.
• Forms circular, ulcerated, granulomatous masses with serosanguinous exudates on legs and ventral midline.
• Lesions are intensely pruritic, and hemorrhage is due to self-trauma; hallmark kunkers develop within draining tracts.
• Differentiate from habronemiasis, SCC, and exuberant granulation tissue.
• Diagnose by biopsy (immunohistochemistry), culture, and cytologic evaluation.
• Treat through surgical excision, topical ketoconazole, DMSO, rifampin (phycofixer), and regional antifungal therapy, but the prognosis is poor.

Dermal Neoplasia

• Includes neoplasias that affect the skin

Sarcoid Skin Tumor

• Most common neoplasia in horses.
• Viral etiology (bovine papilloma virus) and genetic susceptibility.
• Three forms: flat/occult, verrucous, and fibroblastic
• Diagnosis - histopathology: Fibroblastic proliferation of the dermis with concurrent epidermal hyperplasia. Collagen in the derm is whorled, tangled or herringbone pattern; individual cells are spindle-shaped, fusiform to stellate.
• Treatment: BCG, chemotherapy (Cistplatin, 5-FU), Xterra, mistletoe extract, chyrotherapy, radioactive isotope implants, surgical removal with medical adjuncts, electrochemical treatment with cisplatin.

Squamous Cell Carcinoma

• Commonly manifests in Appaloosas, Belgians, Paints, or previous burn sites.
• Lesions are ulcerated, hemorrhagic, and nodular, found on ocular area, vulva, and prepuce.
• Genital SCC is linked to a novel equine papilloma virus.
• Diagnose via histopathology and treat with topical or intralesional 5 fluorouracil.

Melanoma

• Occurs in aged, grey horses, more common in horses with genetics of a black horse, more severe in various breeds; many are benign, but some progress.
• Lesions are firm, nodular, dermal, hairless, and black; located on the perineum, prepuce, and other places.
• Diagnosed via clinical signs and fine needle aspirate.
• Rarely requires treatment, but surgical resection may be needed if disruptive; cimetidine, irradiated autologous vaccine, and canine vaccines are treatment options.

Disorders of Pigmentation

• Includes vitiligo, leukoderma, and Reticulated leukotrichia

Fading Arab Syndrome

• Also called vitiligo, is the development of depigmented skin without trauma.
• Presents as annular areas of depigmentation around the eyes, muzzle, anus.
• It's heritable (breed flaw) and may respond to copper supplementation.

Leukoderma

• Is depigmentation resulting from trauma or disease: pressure sores, warts, onchocerciasis, burns.
• There is no treatment.

Reticulated Leukotrichia

• Is an inherited disorder in Quarter Horses.
• Clinical signs begin in yearlings, with dorsal distribution of linear crusting lesions in a reticulated pattern.
• New hair growth is white; considered a breed flaw.
• Miniature horses have a similar pattern (lacing) without crust stage.

Miscellaneous Conditions

• Includes Fistulous withers and Ehler Danlos

Fistulous Withers

• Is a chronic bacterial infection of supraspinous bursa from Streptococcus zooepidemicus or Brucella spp.
• Diagnose through bacterial culture, serology for brucellosis, and radiograph of dorsal spinous processes.
• Prognosis is poor.
• Treat with en bloc resection, long-term antibiotics, or euthanasia for Brucellosis, due to human health risk.

Ehler Danlos Syndrome

• Also called hyperelastosis cutis, is a heritable, autosomal recessive condition in Quarter Horses.
• Yearlings have fragile skin, failure to heal, and loose skin with pockets of serum.
• Located on the axilla, tuber coxae, saddle sores, and bite wounds.
• Histopathology may not be helpful.
• Affects cornea, ligaments, and tendons as well.
• Diagnose through signalment, clinical signs, and genetic testing performed at UCDavis and Cornell.
• Often, horses are euthanized, and sunlight worsens lesions.