Equine Internal Parasites Study Guide

Overview and Clinical Importance

Internal parasites represent one of the most significant health challenges in equine medicine, affecting horses worldwide regardless of management system. These parasites cause substantial economic losses through decreased performance, poor body condition, colic, and death. Understanding the biology, pathogenesis, diagnosis, and treatment of equine parasites is essential for the NAVLE and clinical practice.

The primary internal parasites of clinical concern in horses include: large strongyles (Strongylus spp.), small strongyles (cyathostomins), ascarids (Parascaris spp.), tapeworms (Anoplocephala perfoliata), pinworms (Oxyuris equi), and bots (Gasterophilus spp.). The emergence of widespread anthelmintic resistance has transformed parasite control strategies from interval-based deworming to evidence-based targeted selective treatment.

Large Strongyles (Strongylus Species)

Three species of large strongyles infect horses: Strongylus vulgaris (the most pathogenic), Strongylus edentatus, and Strongylus equinus. While effective anthelmintics have significantly reduced their prevalence, S. vulgaris is considered a re-emerging pathogen due to changes in deworming practices and anthelmintic resistance concerns.

Strongylus vulgaris - The Bloodworm

Life Cycle and Migration

Prepatent period: 6-7 months (the longest of all equine strongyles). Eggs are passed in feces and develop through L1 to L3 (infective stage) in the environment within 1-2 weeks under optimal conditions. Horses ingest L3 larvae while grazing.

Unique migration pattern: After ingestion, L3 larvae penetrate the intestinal wall and enter small arterioles. They migrate up the arterial system to the cranial mesenteric artery (CMA) and its root adjacent to the aorta. Larvae remain in the arterial system for 3-4 months, causing verminous arteritis. They then return via the arterial lumen to the cecum and colon where they mature into adults.

Pathogenesis

Verminous arteritis: Larval migration causes endothelial damage, inflammation, thrombus formation, and arterial wall thickening. The CMA and its branches (ileo-ceco-colic artery) are most commonly affected. Pathological changes include intimal proliferation, medial necrosis, adventitial fibrosis, and thromboembolism.

Clinical consequences: Thromboembolic colic occurs when blood clots or arterial damage compromises intestinal blood flow. This can lead to non-strangulating intestinal infarction. Aberrant migration to renal, iliac, or rarely cerebral arteries can cause organ-specific pathology.

Clinical Signs

• Recurrent colic (ranging from mild cramping to severe, surgical colic)
• Weight loss and poor body condition
• Fever during acute larval migration
• Dull demeanor and anorexia (duration greater than 24 hours suggestive)
• Intermittent hind limb lameness (iliac artery involvement)
• Death from intestinal infarction or rupture

Other Large Strongyles

Small Strongyles (Cyathostomins)

Cyathostomins (small strongyles) are the MOST PREVALENT internal parasites of horses worldwide, with infection rates of 89-100% in grazing horses. Over 50 species exist, with approximately 10 species commonly found. While most infections are subclinical, larval cyathostominosis represents a life-threatening syndrome with mortality rates of 40-70%.

Life Cycle

Prepatent period: Variable, ranging from 6 weeks to greater than 2 years (unique feature). Eggs passed in feces develop to L3 on pasture. After ingestion, L3 larvae penetrate the cecal and colonic mucosa where they encyst and undergo hypobiosis (arrested development). This encysted stage (early L3 or EL3) can persist for months to years before emerging to complete development.

Larval Cyathostominosis

Definition: Acute, severe typhlocolitis caused by mass synchronous emergence of hundreds of thousands of encysted larvae from the intestinal wall. This releases accumulated excretory/secretory products, triggering massive inflammation.

Risk Factors and Timing

• Age: Young horses (less than 5 years) most commonly affected
• Season: Late winter/early spring in temperate climates (larvae emerge from winter hypobiosis)
• History: Recent deworming may trigger synchronous emergence
• Grazing history: Heavy pasture exposure during previous season

Clinical Signs

• Acute, severe diarrhea (often watery and profuse)
• Rapid weight loss
• Subcutaneous edema (ventral, limbs) due to hypoproteinemia
• Colic
• Fever
• Depression and inappetence

Laboratory Findings

• Hypoalbuminemia (hallmark finding)
• Hyperglobulinemia
• Elevated serum amyloid A (SAA)
• Neutrophilia or neutropenia
• FEC may be LOW or ZERO (disease caused by larval stages, not adults)

Treatment

Moxidectin (400 mcg/kg PO) is the drug of choice for larval cyathostominosis - it is effective against encysted larvae. Combine with corticosteroids (dexamethasone 0.05-0.1 mg/kg IV) to reduce inflammation from larval death. Intensive supportive care with IV fluids, plasma transfusions for hypoproteinemia, and nutritional support is essential.

Ascarids (Parascaris Species)

Parascaris equorum and Parascaris univalens (now considered the predominant species globally) are large roundworms primarily affecting foals and young horses. Adult horses develop immunity and rarely harbor significant infections. Ascarids have emerged as a major concern due to widespread macrocyclic lactone resistance.

Life Cycle

Prepatent period: 10-16 weeks (79-110 days). Eggs passed in feces embryonate in the environment and become infective (containing L2 larva) in 2-4 weeks. Eggs have extremely thick shells and can survive in the environment for years, resistant to most disinfectants.

Hepatotracheal migration: After ingestion, L3 larvae hatch in the intestine and penetrate the gut wall. They enter the portal circulation and migrate to the liver (causing focal necrosis and hemorrhage), then via the hepatic vein to the lungs. In the lungs (14-17 days post-infection), larvae break out of pulmonary capillaries into alveoli, migrate up the trachea, are coughed up and swallowed, and mature into adults in the small intestine.

Clinical Signs by Stage

Ascarid Impaction

This is the most serious complication of ascarid infection. Median age: 5 months (coincides with peak worm burden). Risk factors include recent deworming (72% of cases dewormed within 24 hours), heavy worm burden, and use of highly effective anthelmintics causing rapid worm death.

Diagnosis: Transabdominal ultrasound shows dilated small intestinal loops with hyperechoic masses (worms) visible. Worms may be recovered via nasogastric tube in severe cases.

Tapeworms (Anoplocephala perfoliata)

Anoplocephala perfoliata is the most common and clinically significant equine tapeworm, found in 14-81% of horses depending on geographic location. It has been associated with specific types of colic, particularly affecting the ileocecal junction.

Life Cycle

Indirect life cycle requiring an oribatid mite (intermediate host). Gravid proglottids release eggs into feces. Oribatid mites ingest eggs and cysticercoid larvae develop within the mite. Horses become infected by ingesting mites while grazing. Adult tapeworms develop at the ileocecal junction in 4-6 weeks. Prepatent period: 6-10 weeks.

Pathogenesis

Tapeworms attach to the mucosa at the ileocecal junction using their scolex with four suckers. Heavy infections (greater than 20-100 worms) cause: mucosal ulceration and erosion, chronic inflammation, thickening of the intestinal wall, damage to the enteric nervous system (ENS) affecting motility, and fibrosis of the ileocecal valve.

Associated Colic Types

Diagnosis

Challenge: Standard fecal flotation has LOW sensitivity (less than 40%) because tapeworm eggs are shed irregularly in proglottid packets. Modified techniques using centrifugation and sedimentation-flotation improve detection to 60-92% sensitivity.

Serum/saliva ELISA: Antibody tests detect exposure but do not confirm current active infection. Useful for herd-level assessment.

Egg morphology: D-shaped eggs with pyriform apparatus containing oncosphere (hexacanth larva).

Pinworms (Oxyuris equi)

Oxyuris equi is a common cause of perianal pruritus and tail rubbing in horses. While not highly pathogenic, pinworms cause significant irritation and cosmetic damage. They have a direct life cycle with a prepatent period of approximately 5 months.

Life Cycle and Pathogenesis

Adults live in the large intestine (cecum and colon). After mating, female worms migrate to the anus and deposit eggs in a sticky, yellow-gray gelatinous material on the perianal skin. This material desiccates, causing intense pruritus ani. Eggs become infective (containing L3) in 4-5 days and horses become infected through ingestion of eggs from contaminated environment, fomites (stall walls, fences, grooming equipment), or mutual grooming.

Clinical Signs

• Intense tail rubbing against walls, fences, posts ("rat tail" appearance)
• Perianal alopecia and excoriations
• Yellow-white gelatinous material around anus
• Secondary bacterial skin infections
• Restlessness and irritability

Diagnosis

Standard fecal flotation is NOT reliable because eggs are deposited on skin, not in feces. Use the "Scotch tape" (cellophane tape) test: Press adhesive tape firmly to perianal skin, then attach to a glass slide for microscopic examination. Alternatively, scrape perianal area with a tongue depressor in mineral oil.

Bot Flies (Gasterophilus Species)

Gasterophilus species are not true worms but fly larvae (myiasis) that parasitize the equine stomach. Three species are significant: G. intestinalis (most common), G. nasalis, and G. haemorrhoidalis. Generally considered benign but can cause gastric ulceration in heavy infections.

Life Cycle

Adult bot flies are bee-like, do not feed, and live only 1-2 weeks. Females deposit 150-1,000 yellowish eggs on horse hair: G. intestinalis on forelegs and shoulders, G. nasalis under jaw, G. haemorrhoidalis on lips. Eggs hatch when stimulated by licking/moisture/warmth. L1 larvae enter the mouth and burrow into tongue and gums for 3-4 weeks, then migrate to the stomach where they attach to the mucosa for 8-10 months. Mature L3 larvae detach, pass in feces, and pupate in soil for 3-5 weeks before emerging as adult flies.

Clinical Significance

Most infections are subclinical. Heavy burdens may cause oral irritation (salivation, head shaking), gastric ulceration at attachment sites near the margo plicatus, rarely gastric perforation or esophageal impaction. Adult bot fly activity (buzzing, egg-laying) causes significant behavioral disturbance in horses during summer/fall.

Anthelmintic Classes and Resistance

Understanding anthelmintic drug classes, their spectra of activity, and resistance patterns is essential for evidence-based parasite control. No new anthelmintic classes have been introduced for horses in over 40 years, making resistance management critical.

Anthelmintic Drug Classes

Fecal Egg Count Reduction Test (FECRT)

Gold standard for detecting anthelmintic resistance. Collect fecal samples at Day 0 (pre-treatment) and Day 14 (post-treatment). Calculate percent reduction. Thresholds for resistance: Benzimidazoles and pyrantel less than 90% reduction = resistance suspected. Macrocyclic lactones less than 95% reduction = resistance suspected.

Egg Reappearance Period (ERP)

An early indicator of developing resistance. Expected ERPs: Ivermectin 8-10 weeks, Moxidectin 12-16 weeks. Shortened ERPs (4-6 weeks) indicate developing macrocyclic lactone resistance in cyathostomins, even when FECRT shows greater than 95% efficacy.

Diagnostic Methods

AAEP Parasite Control Guidelines

The American Association of Equine Practitioners (AAEP) recommends evidence-based, targeted selective treatment (TST) rather than interval-based rotational deworming. Key principles include using fecal egg counts to stratify horses as low, medium, or high shedders, performing annual FECRT to monitor anthelmintic efficacy, treating all horses 1-2 times yearly (baseline), and targeting high shedders with additional treatments.

Age-Based Recommendations

Foals (Birth to Weaning)

• First treatment at 2-3 months: Target ascarids with benzimidazole (fenbendazole or oxibendazole)
• Second treatment at 4-6 months: Repeat benzimidazole for ascarids
• FEC at weaning to determine if ascarids or strongyles predominate
• AVOID macrocyclic lactones in young foals due to widespread ascarid resistance

Weanlings and Yearlings

• Third treatment at 9 months: Target strongyles and tapeworms
• Consider ivermectin + praziquantel or moxidectin + praziquantel
• Continue monitoring with FEC; may need additional treatments

Adult Horses

• Perform FEC to stratify: Low shedders (less than 200 EPG), Medium (200-500 EPG), High (greater than 500 EPG)
• ALL horses: Treat 1-2x yearly (typically spring and fall)
• High shedders: Additional targeted treatments based on FEC monitoring
• Include praziquantel for tapeworms at least once yearly (fall)
• Treat for bots after first hard frost