Trichostrongylus axei: Abomasal Hairworm of Cattle and Sheep (Gastric Parasitism)

Etiology and Taxonomic Position

Trichostrongylus axei (Cobbold, 1879) is a slender, reddish-brown nematode belonging to the family Trichostrongylidae. It is the smallest of the abomasal trichostrongylids in ruminants, measuring 3 to 6 mm in length. The species is distinguished by its synlophe (longitudinal cuticular ridges) and the morphology of the copulatory bursa in males. T. axei is unique among the genus Trichostrongylus in its predilection for the abomasum and proximal duodenum, whereas other species (e.g., Trichostrongylus colubriformis) typically inhabit the small intestine. This nematode is a common cause of gastric parasitism in cattle and sheep, and it also infects goats, horses, and various wild ungulates.

The life cycle is direct, with no intermediate host. Adult worms reside in the gastric mucosa, where they cause mechanical and inflammatory damage. The prepatent period ranges from 18 to 21 days. Eggs are passed in feces, develop to first-stage larvae (L1) within the egg, hatch, and molt to second-stage (L2) and third-stage infective larvae (L3) on pasture. The L3 is the only stage capable of infecting the host. After ingestion, exsheathment occurs in the rumen or abomasum, a process that has been studied in detail for ovine and bovine isolates [1].

Epidemiology and Host Range

Trichostrongylus axei has a cosmopolitan distribution, with higher prevalence in temperate and subtropical regions where pasture contamination is sustained. The parasite is particularly important in mixed grazing systems where cattle and sheep share pastures, as cross-infection occurs readily. The abomasal hairworm is also a significant pathogen in horses, causing gastritis and diarrhea.

Epidemiological factors include:

• Pasture contamination: L3 larvae can survive on pasture for several months, especially under cool, moist conditions.
• Host immunity: Young animals (lambs, calves) are most susceptible; older animals develop partial immunity but remain reservoirs.
• Seasonality: In temperate climates, peak transmission occurs in spring and autumn when larval survival is optimal.
• Mixed grazing: Sheep and cattle grazing together increase the risk of cross-species transmission.

The parasite is often found in mixed infections with other abomasal nematodes such as Ostertagia ostertagi and Haemonchus contortus. For a comparative perspective on related trichostrongylids, see the article on Trichostrongylus colubriformis: The Bankrupt Worm of Sheep and Cattle – Pathogenesis and Pasture Management.

Life Cycle and Transmission

The life cycle of T. axei is typical of trichostrongylid nematodes. Adult females produce eggs that are shed in feces. Under favorable environmental conditions (temperature 15–25°C, adequate moisture), eggs embryonate and hatch within 24–48 hours. The L1 and L2 stages feed on bacteria in the fecal pellet. The L3 stage is ensheathed and non-feeding, surviving on herbage until ingested by a grazing animal.

Upon ingestion, the L3 larva undergoes exsheathment in the rumen or abomasum. The kinetics of exsheathment differ between ovine and bovine isolates, as demonstrated by Hertzberg et al. [1] using both in vivo and in vitro models. In that study, ovine-derived T. axei L3 exsheathed more rapidly in sheep rumen fluid than in cattle rumen fluid, suggesting host-specific adaptation. This has implications for cross-species transmission dynamics.

After exsheathment, the L3 penetrates the gastric mucosa and molts to L4 within 3–4 days. The L4 stage emerges from the mucosa and molts to the adult stage. Adults reside on the mucosal surface, feeding on tissue fluids and epithelial cells. The prepatent period is approximately 18–21 days.

The following Mermaid diagram illustrates the life cycle:

graph TD
    A[Adult worms in abomasum], > B[Eggs in feces]
    B, > C[L1 larvae in fecal pellet]
    C, > D[L2 larvae]
    D, > E[L3 infective larvae on pasture]
    E, > F[Ingestion by host]
    F, > G[Exsheathment in rumen/abomasum]
    G, > H[L3 penetrates mucosa]
    H, > I[L4 in mucosa]
    I, > J[Adult worms on mucosal surface]
    J, > A

Pathogenesis and Pathology

The pathogenic effects of T. axei are primarily due to mechanical irritation and inflammatory responses in the abomasal mucosa. Adult worms cause erosion of the superficial epithelium, leading to loss of parietal cells and reduced acid secretion. This results in elevated abomasal pH, impaired protein digestion, and increased gastrin secretion. The inflammatory infiltrate consists of eosinophils, lymphocytes, and plasma cells.

Gross pathological findings include:

• Hyperemia and edema of the abomasal mucosa.
• Petechial hemorrhages at sites of worm attachment.
• Mucosal thickening and rugal hypertrophy in chronic infections.
• Occasionally, ulceration and necrosis in heavy burdens.

Histologically, there is villous atrophy, crypt hyperplasia, and infiltration of inflammatory cells. The damage is reversible after worm expulsion, but chronic infections can lead to permanent loss of functional mucosa.

The pathophysiological consequences include:

• Protein-losing enteropathy (hypoalbuminemia).
• Reduced weight gain and poor feed conversion.
• Diarrhea (often mild to moderate).
• Anemia (less severe than with Haemonchus contortus).

For comparison, the pathology of other abomasal parasites is discussed in the article on Fasciolosis in Cattle and Sheep: Liver Fluke Diagnosis via Coproantigen ELISA, Pooled PCR, and Anthelmintic Resistance to Triclabendazole.

Clinical Signs

Clinical disease is most commonly observed in young lambs and calves, especially when grazing contaminated pastures. The severity depends on the worm burden, host age, nutritional status, and concurrent infections.

Signs include:

• Reduced appetite and weight loss.
• Diarrhea (soft to watery feces, sometimes with mucus).
• Rough hair coat and dull appearance.
• Submandibular edema (bottle jaw) in severe cases due to hypoalbuminemia.
• Mild anemia (packed cell volume may decrease but rarely below 20%).
• Decreased milk production in lactating ewes and cows.

In adult animals, subclinical infections are common, with reduced productivity being the main economic impact. Mixed infections with Ostertagia or Cooperia species can exacerbate clinical signs.

Diagnostics

Diagnosis of T. axei infection relies on a combination of clinical history, fecal examination, and postmortem findings. Molecular methods are increasingly used for species-specific detection.

Coprological Examination

The standard method is fecal egg count (FEC) using a modified McMaster technique. T. axei eggs are thin-shelled, ellipsoidal, and measure approximately 80–100 µm by 40–50 µm. They are morphologically indistinguishable from eggs of other trichostrongylids (e.g., Ostertagia, Cooperia). Therefore, egg counts reflect total strongyle egg output, not species-specific burden.

Larval culture and differentiation are required for species identification. Feces are cultured for 7–10 days at 25°C, and L3 larvae are recovered by Baermann technique. T. axei L3 are characterized by a long, pointed tail with a distinct constriction and a sheath that extends beyond the tail tip. Morphometric keys are available.

Molecular Diagnostics

PCR-based assays targeting the internal transcribed spacer (ITS) regions of ribosomal DNA allow specific detection of T. axei in fecal samples or larval pools. Real-time PCR can quantify egg output and differentiate species in mixed infections. These methods are particularly useful for epidemiological studies and for detecting anthelmintic resistance.

Postmortem Examination

At necropsy, the abomasum is opened and washed, and the contents are sieved. Adult worms are recovered from the mucosal surface and identified under a stereomicroscope. The abomasal mucosa can also be digested in pepsin-HCl to recover larvae.

Exsheathment Kinetics as a Diagnostic Tool

The study by Hertzberg et al. [1] demonstrated that the exsheathment rate of T. axei L3 differs between ovine and bovine isolates when exposed to rumen fluid from the respective host species. This phenomenon can be exploited in experimental settings to assess host adaptation, but it is not used in routine diagnostics.

The following table summarizes key diagnostic methods:

For a broader perspective on diagnostic approaches in livestock, see the article on Cooperia oncophora: Cattle Nematode in Calves on Pasture – Epidemiology and Anthelmintic Control.

Treatment and Anthelmintic Resistance

Anthelmintic treatment is the mainstay of control. Several drug classes are effective against T. axei:

• Macrocyclic lactones (ivermectin, doramectin, moxidectin): Highly effective against adult and larval stages.
• Benzimidazoles (albendazole, fenbendazole, oxfendazole): Effective but resistance is emerging.
• Imidazothiazoles (levamisole): Effective against adults.
• Amino-acetonitrile derivatives (monepantel): Effective, but resistance has been reported in related species.

Dosing should be based on accurate body weight. Underdosing promotes resistance. Resistance in T. axei is less documented than in Haemonchus contortus or Teladorsagia circumcincta, but cases of reduced efficacy to benzimidazoles and macrocyclic lactones have been reported. Fecal egg count reduction tests (FECRT) should be performed to monitor efficacy.

For a discussion of resistance in related parasites, see the article on Liver Fluke (Fasciola hepatica) in Sheep: Anthelmintic Resistance Diagnosis and Herd-Level Management.

Control and Pasture Management

Integrated control strategies are essential to reduce reliance on anthelmintics and delay resistance.

Pasture Management

• Rotational grazing: Move livestock to clean pastures before larvae become infective.
• Mixed grazing with horses or other species can dilute contamination, but cross-infection with T. axei occurs between cattle and sheep.
• Resting pastures for 6–8 weeks during warm, dry weather reduces larval survival.
• Hay or silage cropping removes infective larvae.

Selective Treatment

Targeted selective treatment (TST) based on clinical signs, FEC, or performance indicators (e.g., weight gain, body condition score) reduces selection pressure for resistance. Only animals with high egg counts or poor condition are treated.

Vaccination

No commercial vaccine is available for T. axei. Research into recombinant antigens is ongoing but not yet field-ready.

Biological Control

Nematophagous fungi (e.g., Duddingtonia flagrans) can reduce larval numbers on pasture when fed to livestock. This approach is experimental but promising.

For a comprehensive overview of control in sheep, refer to the article on Sheep Internal Parasites: Winter Management, Parasite Resistance in Dorpers, and Human Health Risks.

Conclusion

Trichostrongylus axei is a significant cause of gastric parasitism in cattle and sheep worldwide. Its small size and predilection for the abomasum distinguish it from other trichostrongylids. Diagnosis requires species-specific methods such as larval culture or PCR. Control relies on integrated pasture management and judicious anthelmintic use to preserve efficacy. Continued research into host-parasite interactions, including exsheathment kinetics [1], will inform future control strategies.

References

[1] Hertzberg H, Huwyler U, Kohler L, et al. Kinetics of exsheathment of infective ovine and bovine strongylid larvae in vivo and in vitro. Parasitology. 2002. URL: https://pubmed.ncbi.nlm.nih.gov/12166522/