Endoparasites of Horses

Endoparasites of Horses

Learning Outcomes

  • Explain the mode of infection of common endoparasites of horses.

  • Describe the location within the host and life cycle of common endoparasites of horses.

  • Explain diagnostic techniques for the identification of endoparasites in horses.

  • Identify treatment measures for endoparasites in horses.

  • Explain preventive and control measures for endoparasites in horses.

Overview of Endoparasites in Horses

Nematodes

  • Parascaris equorum (Roundworm)

  • Cyathostomes (Small Strongyles)

  • Strongylus spp. (Large Strongyles)

    • Strongylus vulgaris

    • Strongylus edentatus

    • Strongylus equinus

  • Oxyuris equi (Pinworm)

  • Strongyloides westeri (Threadworm)

  • Habronema spp. and Draschia megastoma (Summer Sores)

  • Onchocerca spp. (O.cervicalis)

  • Thelazia lacrymalis (Eyeworm)

Cestodes

  • Anoplocephala spp. (Tapeworm)

  • Paranoplocephala mamillana (Tapeworm)

Protozoa

  • Giardia assemblage E

  • Eimeria leuckarti

  • Sarcocystis neurona

Insects

  • Gastrophilus spp. (Bots)

In-depth Analysis of Nematodes

Parascaris equorum

  • Phylum: Nematoda

  • Common Name: Roundworm, Ascarids

  • PPP (Pre-Patent Period): 12 weeks

  • Adults: 28-50 cm long, resides in the small intestine.

Clinical Signs

  • Associated with migrating larvae (in liver and lungs).

  • Symptoms in foals/weanlings include:

    • Potbelly

    • Unthrifty appearance

    • Intermittent colic

    • Constipation or diarrhea

    • Coughing.

  • Adult horses typically show mild to no clinical signs but remain infected and a source of eggs.

Mode of Infection and Life Cycle

  • Eggs are shed in feces, approximately 150,000 eggs daily.

  • Eggs develop into L1 and L2.

  • Infective ova (L2) are ingested on pastures and stalls.

  • Larvae migrate from the liver to the lungs and then to the trachea (L3).

  • They are coughed up and then swallowed.

  • Eggs are sticky and remain viable in the environment for months to years, even in cold climates. They stay infective for up to 10 years!

Diagnosis

  • Techniques include standard vial or centrifugal flotation.

  • Eggs appear as rough, brown, thick-walled oval to round structures with a single-celled zygote, measuring approximately 90 – 100 µm.

Treatment

  • Include pasture management, steam cleaning in stables, and fenbendazole usage. Resistance has been observed in some areas against moxidectin and ivermectin.

  • Not zoonotic.

Cyathostomes (Small Strongyles)

  • Phylum: Nematoda

  • Common Name: Small strongyle

  • Most abundant nematode parasites of horses; over 40 species.

  • Adults: Approximately 25 mm; reside in the large intestine and cecum, penetrating the mucosa.

  • PPP: 40 days.

Clinical Signs

  • Unthrifty appearance, poor performance, and intermittent colic, especially prevalent in horses younger than 2 years; potential for diarrhea.

Life Cycle and Transmission

  • Eggs are passed in feces. A larva develops within each egg (L1, L2), which then hatches.

  • L3 (infective stage) develops in about 7 days and moves onto grass.

  • Horses ingest larvae, which may:

    • Hypobiosis as L3, or

    • Migrate into intestinal mucosa to develop into adults, laying eggs.

Hypobiosis

  • Some cyathostomes undergo hypobiosis, an arrested development of L3s in the intestinal mucosa of infected horses.

  • This stops the life cycle during harsh environmental conditions for eggs and larvae, typically occurring in late fall and early winter in Ontario.

Diagnosis

  • Fecal flotation: Eggs are approximately 90 x 50 µm with smooth thin walls surrounding an 8-16 cell morula; egg size varies by species.

Treatment

  • Resistance: Observed against pyrantel pamoate and fenbendazole.

  • Recommended to use moxidectin for adults and encysted larvae.

  • Selective treatment for infected horses is advised, as most horses over 4 years show resistance to reinfection.

  • Not zoonotic.

Importance of Cyathostomes

  • Cyathostomes are increasingly significant due to the lack of effective dewormers against encysted larval stages, alongside their capacity to develop resistance against most anthelmintic drug classes.

  • Control: Proper use of anthelmintics and maintenance of the environmental conditions minimizing survival and development of free-living stages.

Strongylus spp. (Large Strongyles)

  • Phylum: Nematoda

  • Species Include:

    • Strongylus vulgaris: Adult is 2 cm long, known as the bloodworm.

    • Strongylus edentatus: Adult 4-5 cm long.

    • Strongylus equinus: Adult 5 cm long.

Biology and Pathogenicity

  • PPP: 6-11 months; adults are destructive feeders residing in the large intestine/cecum.

  • Larvae migrate through blood vessels and abdominal organs, causing significant damage (e.g., to blood vessels, pancreas, liver).

  • Particularly pathogenic in foals and yearlings.

Clinical Signs

  • Significant issues arise from larval migration:

    • Colic due to disrupted blood supply.

    • Anemia, weight loss or inadequate weight gain, hind-limb lameness, and potentially death.

Diagnosis

  • Fecal flotation: Eggs are about 90 x 50 µm, resembling small strongyle eggs with 8 to 16 cell morula structure.

Treatment

  • For larvae: Ivermectin or moxidectin.

  • For adults: Moxidectin, ivermectin, fenbendazole, and pasture management.

Comparative Prevalence

  • Small strongyles (Cyathostomes) are currently the dominant equine intestinal parasites in Canada, having replaced large strongyles such as Strongylus vulgaris, now rare due to routine deworming practices. Resistance to dewormers in small strongyles presents a primary concern today.

Oxyuris equi (Pinworm)

  • Phylum: Nematoda

  • Common Name: Pinworm

  • Adults: Females up to 15 cm, males less than 1 cm long, residing in the colon, cecum, and rectum.

  • PPP: 5 months; more common in horses younger than 18 months.

Clinical Signs

  • Pruritus of the perianal region leads to tail rubbing.

  • Not zoonotic.

Lifecycle

  • Gravid females migrate to the anus, laying eggs in sticky clumps on the perineal skin. After laying, the female exits anus and dies.

  • Infective larvae develop within each egg, dropping to the ground after 4-5 days.

  • Eggs may adhere to feeding troughs, stables, etc. Horses ingest larvated eggs, which hatch in the small intestine and migrate to the large intestine.

Diagnosis

  • Indicators: Tail rubbing against surfaces, scotch tape test across the anus, gentle scraping of the perianal skin using a tongue depressor.

  • Fecal flotation reveals Oxyuris equi egg characteristics:

    • Measuring approximately 90 x 40 µm with an operculum, quickly becoming infective (in 4-5 days) and easily transported through the air.

Control and Treatment

  • Maintenance of a clean stall environment is crucial. Dewormers include fenbendazole, pyrantel pamoate, ivermectin, and moxidectin (with noted resistance in some areas).

Strongyloides westeri (Threadworm)

  • Phylum: Nematoda

  • Common Name: Threadworm

  • Adults: Slender, hair-like worms, about 10 mm in length, found in the small intestine.

  • PPP: 7-10 days; highest prevalence in young, suckling foals. Typically not an issue in adult horses.

Clinical Signs

  • Primarily affects young foals, leading to symptoms like diarrhea, coughing, and weight loss.

Transmission

  • Modes Include:

    • Transmammary via milk.

    • Skin penetration.

    • Ingestion of infective larvae.

Lifecycle

  • Larvated eggs are excreted in feces. Larvae develop within eggs, then hatch, infecting horses through skin penetration or ingestion. Larvae migrate through the vasculature, into the trachea, coughed up, swallowed, and complete development as adult females in the small intestine.

  • Lactating mares transfer larvae to colostrum and milk, serving as a major infection source for suckling foals.

Diagnosis

  • Fecal flotation is necessary, emphasizing the use of fresh feces to locate larvated eggs approximately 50 x 35 µm in size.

Treatment

  • Includes ivermectin and oxibendazole. Deworming of mares within 24 hours post-parturition is advised (ivermectin).

  • Not zoonotic.

Habronema spp. & Draschia megastoma

  • Phylum: Nematoda

  • Species: Habronema muscae, H. microstoma, Draschia megastoma

  • Common Names: Stomach worms.

  • Vectors and Intermediate Hosts: House fly (Musca domestica) and stable fly (Stomoxys calcitrans).

  • Adults: Reside in the stomach; larvae may migrate to conjunctiva and skin.

  • PPP: 2 months.

Life Cycle

  • Adult worms shed eggs in feces. Eggs hatch in feces, and larvae are ingested by fly larvae (maggots), developing further within an intermediate host. Habronema and Draschia larvae migrate to the fly's mouthparts and are transferred to the horse when flies feed on wounds, lips, eyes, and nostrils or when horses ingest the fly.

Clinical Signs

  • Infections cause conjunctivitis due to larvae from flies, summer sores that delay wound healing, and gastritis from adults in the stomach.

Diagnosis

  • Difficult due to eggs disintegrating rapidly in feces; methods include gastric lavage or stomach content examination.

  • Egg Characteristics: Oblong, delicately-shelled, measuring between 50-80 µm x 10-20 µm; skin scraping or biopsy can identify larvae.

Treatment

  • Essential to control flies and administer ivermectin for lesions or orally.

Onchocerca spp.

  • Phylum: Nematoda

  • Common Names: Skin Nodular worm, neck threadworm.

  • Definitive Hosts: Horses (specifically O. cervicalis, O. reticulata); other Onchocerca species infect livestock.

  • Intermediate Hosts: Culicoides spp. (biting midges).

  • Adults: 7-30 cm long, narrow and hair-like; reside in the nuchal ligament or tendons of forelegs.

  • PPP: 6-12 months.

Mode of Transmission

  • Adult worms inhabit ligaments and tendons, generating microfilariae that migrate to the dermis, which are ingested by Culicoides spp. or other biting insects. Following maturation in the intermediate hosts, infective larvae are passed to definitive hosts through blood meals.

Clinical Signs

  • Include lameness due to ligament and tendon inflammation, skin lesions from microfilariae (often worse in summer), and potential involvement in periodic ophthalmia (eye disorders).

Diagnosis

  • Microfilariae may be diagnosed through skin biopsies.

Treatment and Prevention

  • Treatment is challenging but may include ivermectin or moxidectin. Prevention of Culicoides exposure is crucial. Not zoonotic.

Thelazia lacrymalis (Eyeworm)

  • Phylum: Nematoda

  • Common Name: Eyeworm; T. lacrymalis usually infects horses.

  • Vector and Intermediate Host: Face fly (Musca autumnalis).

  • Adults: Tiny, measuring 3-10 mm, inhabit the eyes and surrounding tissues including the lacrimal gland and tear ducts.

Clinical Signs

  • Chronic conjunctivitis, cloudy eyes, excessive tearing, and potential corneal ulcers.

Life Cycle and Mode of Infection

  • Face flies feed on secretions from horse eyes, ingest larvae that become infective within 2-4 weeks. Deposited infective L3 larvae mature into adults within 10 weeks, subsequently laying larvae in eye secretions, perpetuating the cycle.

Diagnosis

  • Inspection of eyes may reveal worms; often adult worms are not observed. Microscopic examination of tears may identify eggs or larvae.

Treatment

  • Difficult to achieve, may involve mechanical removal, topical organophosphates, or oral fenbendazole. Preventive measures include fly control and using fly masks.

  • Zoonosis: Not known for this species.