Equine External Parasites Study Guide

Overview and Clinical Importance

External parasites (ectoparasites) represent a significant cause of dermatologic disease in horses worldwide. These organisms live on or within the skin surface and cause pathology through direct tissue damage, hypersensitivity reactions, blood loss, and transmission of infectious agents. External parasites commonly affecting horses include lice, mites, ticks, and various biting flies. Understanding the identification, life cycles, clinical presentations, and treatment options for these parasites is essential for NAVLE success and clinical practice.

Recognition of ectoparasite infestations is critical because they cause significant welfare concerns including intense pruritus, self-trauma, secondary infections, anemia in severe cases, and transmission of vector-borne diseases such as equine piroplasmosis, Lyme disease, and equine infectious anemia.

Equine Lice (Pediculosis)

Pediculosis is infestation with lice and represents one of the most common causes of pruritus in horses. Two species of lice infest horses: Haematopinus asini (sucking louse) and Damalinia equi (also called Werneckiella equi or Bovicola equi, the biting/chewing louse). Lice are host-specific obligate parasites that complete their entire life cycle on the horse.

Comparison of Equine Lice Species

Life Cycle and Epidemiology

The louse life cycle is completed entirely on the host in approximately 3 weeks. Female lice cement eggs (nits) to hair shafts. Eggs hatch into nymphs, which undergo three molts before reaching adulthood. Adult lice survive only a few days off the host. Transmission occurs through direct horse-to-horse contact or via fomites (grooming equipment, tack, shared blankets).

Seasonal Pattern: Lice infestations are most prevalent during late winter and early spring when horses have longer hair coats and are housed in close proximity. Infestations typically decrease in summer as coats shed and UV exposure increases.

Risk Factors: Overcrowding, poor nutrition, debilitation, immunocompromise (e.g., Pituitary Pars Intermedia Dysfunction/Cushing's disease), and inadequate grooming predispose horses to heavier infestations.

Clinical Signs

• Pruritus: Intense itching, rubbing against objects, self-trauma
• Alopecia: Patchy hair loss from rubbing, especially mane, tail, and flanks
• Coat Changes: Scurfy, dull, 'moth-eaten' appearance
• Skin Changes: Excoriations, crusting, secondary bacterial infection
• Nits: Visible eggs cemented to hair shafts (shiny, pale, oval)
• Anemia: Possible with heavy sucking lice infestations

Diagnosis

Diagnosis is made by direct visualization of adult lice or nits on the hair coat. Part the hair in predilection sites (mane, tail, flanks) and examine carefully. A hand lens or magnifying glass aids visualization. Skin scrapings are not typically necessary but may reveal lice. Careful examination is essential as biting lice in particular may be present in small numbers.

Treatment

Key Principle: Treatment must be repeated at 10-14 day intervals because most insecticides do not kill eggs (nits). At least 2-3 treatments are typically required to break the life cycle.

Management: Treat all in-contact horses simultaneously. Clean and disinfect grooming equipment, blankets, and tack. Improve nutrition and address underlying health issues.

Equine Mites (Mange)

Mange refers to skin disease caused by parasitic mites. Several mite species can affect horses, with Chorioptes bovis (chorioptic mange) being the most common in North America. Other species include Sarcoptes scabiei var. equi (sarcoptic mange - rare and reportable), Psoroptes equi (psoroptic mange - rare and reportable), and Demodex equi/caballi (demodectic mange - very rare).

Chorioptic Mange (Leg Mange, Feather Mites)

Chorioptes bovis (formerly C. equi) is a surface-dwelling, non-burrowing mite that feeds on skin debris and lymph. It is the most common form of mange in horses and is particularly prevalent in draft breeds with heavy feathering (Clydesdales, Shires, Belgian Draft, Friesians).

Life Cycle: The entire life cycle (egg to adult) occurs on the host and is completed in 2-3 weeks. Mites can survive off the host for up to 69 days under favorable conditions, which complicates environmental control.

Predilection Sites: Distal limbs from fetlock to pastern, especially the pastern cavity and around the heel bulbs. May extend to ventral abdomen and tail base in severe cases.

Clinical Signs: Intense pruritus causing foot stamping, leg biting, and rubbing legs together. Affected horses exhibit papules, crusting, scaling, alopecia, and skin thickening (lichenification). A moist dermatitis of the fetlock ('greasy heel' or 'scratches') may develop. Clinical signs are typically worse in winter and may improve during summer.

Comparison of Equine Mite Species

Diagnosis and Treatment of Mange

Diagnosis: Superficial skin scrapings from the margins of lesions, examined under microscopy. Tape preparations or skin brushings can also be used for surface mites. Chorioptes is relatively easy to find; Sarcoptes may require multiple deep scrapings (biopsy may be needed if scrapings are negative but clinical suspicion is high).

Management: Clip feathering to improve treatment penetration and visualization. Treat all in-contact horses. Environmental decontamination is challenging due to prolonged mite survival (up to 69 days). Reinfection is common without comprehensive herd treatment.

Equine Ticks and Tick-Borne Diseases

Ticks are important ectoparasites of horses that cause direct damage through blood feeding and serve as vectors for several significant diseases. Multiple ixodid (hard) tick species affect horses in North America, with the most clinically important being Ixodes scapularis (black-legged/deer tick), Amblyomma americanum (lone star tick), and Dermacentor variabilis (American dog tick).

Major Tick Species Affecting Horses

Major Tick-Borne Diseases of Horses

Equine Granulocytic Anaplasmosis (EGA)

Caused by Anaplasma phagocytophilum, transmitted by Ixodes scapularis (East) and I. pacificus (West). This is the most common tick-borne disease diagnosed in horses. Clinical signs include fever (up to 104°F), lethargy, limb edema, ataxia, icterus, and petechiation. Diagnosis is by PCR, serology (acute/convalescent titers), or visualization of morulae in neutrophils. Treatment with oxytetracycline (7 mg/kg IV q24h) typically results in rapid improvement.

Lyme Disease (Borreliosis)

Caused by Borrelia burgdorferi, transmitted by Ixodes species. Clinical disease in horses is less common than serological exposure suggests. Signs are variable and may include chronic weight loss, shifting leg lameness, behavioral changes, uveitis, and neurologic signs. Diagnosis is challenging; serologic tests (Lyme Multiplex assay) detect exposure but not necessarily active disease. Treatment typically involves prolonged tetracycline therapy.

Equine Piroplasmosis (EP)

Caused by Theileria equi and Babesia caballi, transmitted by various Dermacentor, Rhipicephalus, and Hyalomma species. This is a REPORTABLE disease in the continental US. Clinical signs include fever, anemia, icterus, hemoglobinuria, and potentially death. Most US cases are associated with iatrogenic transmission (contaminated needles) rather than tick bites. Diagnosis by cELISA or PCR; positive horses are quarantined.

Tick Prevention and Treatment

• Daily tick checks: Examine horses after turnout, especially in wooded areas. Focus on chest, axilla, groin, ears
• Prompt removal: Remove attached ticks within 24 hours to reduce disease transmission risk
• Topical repellents: Pyrethroid-based sprays (permethrin) applied per label; reapply after rain/sweat
• Environmental management: Keep pastures mowed, reduce brush/woodland edges, separate horses from deer habitat
• Note: Fipronil is effective but NOT labeled for horses; amitraz can cause toxicity in horses

Biting Flies and Related Parasites

Various fly species cause significant problems for horses through direct tissue damage, nuisance, allergic reactions, and disease transmission. Key categories include biting flies (horse flies, deer flies, stable flies, black flies, biting midges) and non-biting flies (house flies, face flies) that can transmit parasites and pathogens.

Culicoides Midges and Insect Bite Hypersensitivity

Culicoides species (biting midges, no-see-ums, punkies) are tiny (1-3 mm) blood-feeding insects that cause Insect Bite Hypersensitivity (IBH), also known as Sweet Itch, Queensland Itch, Summer Eczema, or Culicoid Hypersensitivity. This is a Type I and IV hypersensitivity reaction to salivary proteins injected during feeding.

Clinical Signs: Intense pruritus leading to severe self-mutilation. Affected horses rub the mane, tail head, and dorsal/ventral midline, causing alopecia, excoriations, and lichenification. Signs are seasonal (worse in warm months when midges are active) and progressive (worsening with each exposure year).

Management: Stable horses during peak midge activity (dawn and dusk). Use fans (midges are weak fliers). Apply repellents. Use fly sheets with belly coverage. Eliminate standing water breeding sites. Medical management includes antihistamines (limited efficacy), corticosteroids (for severe cases), and allergen-specific immunotherapy.

Exam Focus: Icelandic horses exported from Iceland (where Culicoides are absent) have a very high incidence of IBH because they lack prior exposure and tolerance. Classic presentation: horse with severe rubbing of mane and tail base, worse in summer, pattern suggesting dorsal/ventral midline distribution.

Cutaneous Onchocerciasis

Onchocerca cervicalis is a filarial nematode transmitted by Culicoides midges. Adult worms live in the ligamentum nuchae and produce microfilariae that migrate to the dermis. The prevalence in endemic areas (e.g., Kentucky, Gulf Coast) may exceed 85% in older horses.

Clinical Signs: Many infected horses are asymptomatic. Clinical disease results from hypersensitivity to dying microfilariae and manifests as non-seasonal dermatitis affecting the ventral midline, face, neck, withers, and chest. Lesions include alopecia, scaling, depigmentation, and crusting. Unlike IBH, lesions are typically less pruritic. Ocular involvement (uveitis, keratitis) may occur.

Diagnosis: Skin biopsy from the ventral midline (minimum 6 mm) macerated in saline to identify microfilariae. Response to ivermectin treatment supports diagnosis.

Treatment: Ivermectin (200 mcg/kg PO) kills microfilariae but NOT adult worms. Death of microfilariae may cause temporary worsening of clinical signs (hypersensitivity reaction) - pretreat with NSAIDs (flunixin meglumine) if needed. Periodic retreatment is required as adults continue producing microfilariae. Routine ivermectin deworming has significantly reduced onchocerciasis prevalence.

Summary of Biting Flies