Trichostrongylus (Hair Worm)

Trichostrongylus (Hair Worm): Morphology, Life Cycle, Pathogenesis, Clinical Signs, Treatment & Control

Trichostrongylus is a genus of small, hair-like gastrointestinal nematodes belonging to the family Trichostrongylidae. These parasites infect a wide range of domestic and wild animals, particularly ruminants. They inhabit the abomasum or small intestine and can cause significant economic losses due to poor growth, weight loss, reduced productivity, and gastrointestinal disease.

Important species include Trichostrongylus axei, which primarily affects the abomasum, and Trichostrongylus colubriformis, which parasitizes the small intestine. This article covers the morphology, taxonomy, important species, life cycle, pathogenesis, clinical signs, diagnosis, treatment, and control of Trichostrongylus in veterinary parasitology.

Morphology

• Members of the genus Trichostrongylus are small, slender, hair-like worms that are pale reddish-brown in color.
• The buccal capsule, leaf crown, and teeth are absent.
• A distinct excretory pore is situated in a conspicuous ventral notch at the anterior end.
• The male bursa has well-developed lateral lobes, whereas the dorsal lobe is indistinct.
• Spicules are pigmented and may be equal, unequal, or dissimilar.

Taxonomical Classification

• Kingdom: Animalia
• Phylum: Nematoda
• Class: Chromadorea (Secernentea)
• Order: Rhabditida (formerly Strongylida)
• Suborder: Strongylina
• Superfamily: Trichostrongyloidea
• Family: Trichostrongylidae
• Genus: Trichostrongylus
• Common Name: Hair Worm, Bankrupt Worm

Important Species of Trichostrongylus

• Trichostrongylus axei – Stomach Hair Worm (affects the abomasum of ruminants and the stomach of horses and pigs)
• Trichostrongylus colubriformis – Intestinal Hair Worm
• Trichostrongylus vitrinus – Intestinal Hair Worm
• Trichostrongylus capricola – Goat Hair Worm
• Trichostrongylus probolurus – Intestinal Hair Worm of Small Ruminants
• Trichostrongylus rugatus – Intestinal Hair Worm of Sheep and Goats
• Trichostrongylus longispicularis – Hair Worm of Small Ruminants
• Trichostrongylus tenuis – Cecal Hair Worm of Grouse and Other Birds

Difference Between Trichostrongylus axei and Trichostrongylus colubriformis

Trichostrongylus axei

• Common Name: Stomach Hair Worm, Black Scour Worm
• Host: Sheep, goats, cattle, horses, pigs, and humans.
• Predilection Site: Abomasum of sheep, goats, and cattle; stomach of horses, pigs, and humans.
• Eggs: Eggs are oval, thin-shelled, and contain segmented yolk material.

Trichostrongylus colubriformis

• Common Name: Intestinal Hair Worm
• Host: Sheep, goats, and cattle.
• Predilection Site: The anterior part of the small intestine of sheep, goats, cattle, and camels. It has also been reported in rabbits, pigs, humans, and dogs.
• Eggs: Eggs are oval, thin-shelled, and contain segmented yolk material.

Life Cycle

Eggs are passed in the feces of the host (the bionomics of the larvae are similar to those of Strongyle spp.). The infective stage is found on grass blades during the early morning and late evening. Infection of the definitive host occurs through the ingestion of infective larvae along with vegetation.

Following ingestion, exsheathment occurs in the intestine. The infective larvae penetrate the abomasal and intestinal mucosa, where L3 molts to L4 in about 5 days, and L4 molts to L5 after an additional 5 days. Finally, L5 returns to the lumen of its predilection site (abomasum/stomach or small intestine) and reaches maturity. The prepatent period is 20–25 days.

Pathogenesis

In the stomach of horses and the abomasum of cattle, the worms burrow into the mucosa, causing hemorrhage and necrosis. The histotrophic phase (larval migration) occurs in the stomach, abomasal epithelium, and dilated gastric glands, resulting in mucosal hyperemia, catarrhal inflammation, necrosis, and epithelial ulceration.

In calves, larval parasites embedded in the gastric mucosa cause “raised plaque”-like lesions consisting of grayish, flat areas with sharply demarcated borders that resemble “ringworm lesions.” In horses, hypertrophic gastritis with pedunculated polypoid lesions is associated with protein-losing gastropathy and hypoalbuminemia in heavy infections. In sheep, acute inflammation of the abomasal mucosa and death may occur.

In the small intestine, pathogenesis depends on the age and nutritional status of the host.

All larval stages occur within tunnels. L3 larvae burrow superficially into the intestinal crypts, causing severe villous atrophy and decreased brush border enzyme activity.

The lesions are associated with protein-losing enteropathy and hypoalbuminemia. Loss of serum protein leads to anorexia and alters gastric secretion.

Calcium and phosphorus absorption is reduced. Hypophosphatemia and decreased selenium intake are observed. Growth rate and production decline, resulting in stunted growth. Reduced deposition of protein, calcium, and phosphorus has also been reported.

Clinical Signs

In sheep and goats, young animals are commonly affected. Heavy infection with Trichostrongylus axei causes gastrointestinal disturbances. If sheep and goats acquire a heavy infection over a short period, it may result in sudden death, leg weakness, mild anemia, variable appetite, emaciation, dry skin, and an inability to stand shortly before death.

Alternating diarrhea and constipation may occur. Diarrheic feces are dark in color; therefore, the parasite is commonly known as the “Black Scour Worm.” Diagnosis is made by fecal examination.

Treatment

• Albendazole: 7.5 mg/kg PO in sheep and cattle; 10 mg/kg PO in goats (avoid use during the first trimester of pregnancy)
• Fenbendazole: 5–10 mg/kg PO as a single dose
• Levamisole: 7.5 mg/kg PO or SC as a single dose
• Ivermectin: 0.2 mg/kg SC or PO as a single dose
• Moxidectin: 0.2 mg/kg PO or SC as a single dose

Supportive therapy, including fluid therapy, correction of electrolyte imbalance, nutritional support, and treatment of anemia or hypoproteinemia, may be required in severely affected animals.

Control

Control is based on strategic deworming, rotational grazing, pasture management, avoiding overstocking, and routine monitoring of parasite burden by fecal egg count or fecal egg count reduction test (FECRT). Anthelmintic selection should be based on local resistance patterns.