Section: Livestock Parasites

Teladorsagia circumcincta in Sheep: Abomasal Parasitism, Anthelmintic Resistance, and Integrated Control in Temperate Regions

Introduction

Teladorsagia circumcincta (formerly Ostertagia circumcincta) is a highly pathogenic abomasal nematode of sheep and goats in temperate regions worldwide. Commonly referred to as the brown stomach worm, this parasite is a primary cause of ovine ostertagiosis, a syndrome characterized by protein-losing enteropathy, reduced growth rates, diarrhea, and impaired wool production. The economic impact of T. circumcincta infection is compounded by widespread anthelmintic resistance, which now compromises the efficacy of all major drug classes in many flocks. This article provides a detailed clinical and diagnostic reference for veterinary practitioners and researchers, covering etiology, epidemiology, pathogenesis, diagnostic methods, resistance mechanisms, and integrated control strategies suitable for temperate grazing systems.

Etiology and Life Cycle

Teladorsagia circumcincta belongs to the family Trichostrongylidae. Adult worms are slender, reddish-brown, and measure 7 to 9 mm (males) and 9 to 12 mm (females) in length. The buccal cavity is small, and the male has a well-developed copulatory bursa with characteristic spicules. Eggs are thin-shelled, ellipsoidal, and measure approximately 80 to 100 micrometers by 40 to 50 micrometers, containing a morula when freshly shed.

The life cycle is direct. Adult females in the abomasum produce eggs that pass in feces. Under optimal conditions (temperatures between 10 and 20 degrees Celsius, adequate moisture), eggs hatch into first-stage larvae (L1) within 24 to 48 hours. Larvae develop through L2 and L3 stages on pasture. The infective L3 stage retains the cuticle of L2 as a protective sheath. L3 larvae migrate onto herbage and are ingested by grazing sheep. After ingestion, exsheathment occurs in the rumen, and larvae penetrate the abomasal mucosa. Within the gastric glands, larvae molt to L4 and then to L5 (young adults). Young adults emerge onto the mucosal surface, and the prepatent period is approximately 18 to 21 days.

A critical feature of T. circumcincta biology is the ability of L4 larvae to undergo hypobiosis (arrested development) within the gastric glands. This phenomenon is triggered by environmental cues such as decreasing day length and temperature in autumn. Hypobiotic larvae resume development in late winter or early spring, leading to a synchronized emergence of adult worms. This emergence is responsible for type II ostertagiosis, a severe clinical syndrome distinct from the primary infection (type I) seen in lambs during summer.

Epidemiology in Temperate Regions

Teladorsagia circumcincta is endemic in temperate climates with cool, moist conditions. The parasite overwinters primarily as hypobiotic larvae within the host, although some L3 larvae can survive on pasture under snow cover. In spring, the resumption of development by hypobiotic larvae leads to a periparturient rise in fecal egg counts in ewes, contaminating pastures for lambs.

Two epidemiological patterns are recognized:

Type I ostertagiosis occurs in lambs and weaners during summer and early autumn. High levels of pasture contamination from ewes and older sheep result in rapid acquisition of large worm burdens. Clinical disease typically appears 3 to 4 weeks after turnout onto heavily contaminated pasture.

Type II ostertagiosis occurs in yearlings and adult sheep during late winter and early spring. It results from the synchronous emergence of large numbers of hypobiotic L4 larvae. This form is often more severe and can cause significant mortality.

Risk factors include high stocking density, continuous grazing, use of the same pasture for lambing year after year, and failure to implement strategic anthelmintic treatments. Climatic factors such as mild winters and wet springs favor high larval survival on pasture.

Pathogenesis and Clinical Signs

The pathogenesis of T. circumcincta infection is primarily due to the damage caused by larval stages within the abomasal glands and the subsequent inflammatory response.

Larval stages: Penetration of the mucosa by L3 and L4 larvae causes mechanical disruption of gastric glands. The developing larvae induce hyperplasia of mucus-secreting cells and loss of parietal cells. This leads to elevated abomasal pH (from normal pH 2 to 3 up to pH 6 to 7). The loss of acidity impairs pepsinogen conversion to pepsin, resulting in protein maldigestion. Additionally, the damaged mucosa becomes leaky, allowing plasma proteins (including albumin) to leak into the abomasal lumen. This protein loss contributes to hypoalbuminemia and edema.

Adult worms: Adult worms on the mucosal surface cause further inflammation and contribute to ongoing protein loss. The host immune response, particularly eosinophil and mast cell infiltration, exacerbates tissue damage.

Clinical signs of type I ostertagiosis include:

  • Profuse, greenish diarrhea
  • Weight loss or poor weight gain
  • Rough hair coat
  • Submandibular edema (bottle jaw) due to hypoalbuminemia
  • Anorexia
  • Decreased wool production

Clinical signs of type II ostertagiosis are more acute and severe:

  • Sudden onset of profuse watery diarrhea
  • Severe dehydration
  • Rapid weight loss
  • Anemia (less pronounced than in haemonchosis)
  • High mortality if untreated

Subclinical infections are common in adult sheep and result in reduced growth rates, impaired reproductive performance, and increased susceptibility to other diseases.

Pathology

Gross pathological findings in the abomasum are characteristic. In type I disease, the mucosa appears thickened, edematous, and hyperemic. Multiple raised nodules (1 to 2 mm in diameter) are visible, representing hyperplastic gastric glands containing developing larvae. A "Moroccan leather" or "cobblestone" appearance is typical. In type II disease, the mucosa is severely thickened and may show extensive hemorrhage and erosion. The abomasal contents are often watery and greenish.

Histopathological examination reveals:

  • Hyperplasia of mucous neck cells
  • Loss of parietal cells
  • Infiltration of eosinophils, mast cells, and lymphocytes
  • Dilated gastric glands filled with larvae, cellular debris, and mucus
  • Fibrosis in chronic cases

Other findings include hypoalbuminemia, elevated pepsinogen levels in serum, and increased abomasal pH.

Diagnosis

Diagnosis of T. circumcincta infection relies on a combination of clinical signs, epidemiological context, and laboratory methods.

Fecal examination: Quantitative fecal egg counts (FEC) using the modified McMaster technique are standard. Teladorsagia eggs are morphologically indistinguishable from other trichostrongylid eggs (e.g., Trichostrongylus, Cooperia). A FEC above 500 to 1000 eggs per gram (epg) in lambs is often associated with clinical disease, but lower counts do not rule out significant larval burdens due to hypobiosis.

Larval culture: Fecal culture to third-stage larvae allows species identification. Teladorsagia L3 larvae have a characteristic long, pointed tail with a distinct constriction. This method is essential for differentiating T. circumcincta from other abomasal or intestinal nematodes.

Serum pepsinogen assay: Elevated serum pepsinogen levels (above 1.0 IU/L) indicate abomasal damage and are a sensitive marker for ostertagiosis. This test is particularly useful for diagnosing type II disease when FEC may be low.

Postmortem examination: Worm counts from the abomasum are the gold standard for quantifying burdens. The abomasum is opened, contents collected, and the mucosa digested (pepsin-HCl) to recover larvae. Adult worm burdens exceeding 10,000 are considered pathogenic.

Molecular diagnostics: PCR-based assays targeting the internal transcribed spacer (ITS-2) region of ribosomal DNA can detect and quantify T. circumcincta DNA in fecal samples. These methods offer high sensitivity and specificity but are not yet routine in field practice. They are valuable for research and for detecting resistance-associated alleles.

Differential diagnosis: Other causes of diarrhea and weight loss in sheep include:

Anthelmintic Resistance

Anthelmintic resistance in T. circumcincta is a critical problem in temperate sheep production. Resistance has been reported to all three major classes: benzimidazoles (BZ), macrocyclic lactones (ML), and imidazothiazoles (e.g., levamisole). Multidrug resistance is increasingly common.

Mechanisms of resistance:

  • Benzimidazole resistance: Primarily associated with single nucleotide polymorphisms (SNPs) in the beta-tubulin isotype 1 gene, particularly at codon 200 (Phe to Tyr) and codon 167. These mutations reduce binding affinity of BZ drugs to tubulin.

  • Macrocyclic lactone resistance: More complex and polygenic. Mechanisms include increased drug efflux via P-glycoprotein transporters, target-site mutations in glutamate-gated chloride channels, and altered cuticular permeability.

  • Levamisole resistance: Associated with mutations in nicotinic acetylcholine receptor subunits, leading to reduced drug sensitivity.

Diagnosis of resistance:

  • Fecal egg count reduction test (FECRT): The standard field test. FEC is measured on day 0 and 10 to 14 days post-treatment. A reduction of less than 95% (with lower 95% confidence interval below 90%) indicates resistance. Guidelines from the World Association for the Advancement of Veterinary Parasitology (WAAVP) should be followed.

  • Egg hatch assay (EHA): Used for benzimidazole resistance. Eggs are exposed to increasing concentrations of thiabendazole; the EC50 value indicates resistance.

  • Larval development assay (LDA): Can test multiple drug classes simultaneously.

  • Molecular assays: PCR-based detection of resistance-associated SNPs (e.g., for BZ resistance) is available but requires specialized equipment.

Prevalence: In many temperate regions, BZ resistance in T. circumcincta exceeds 80% of flocks. ML resistance is also widespread, with ivermectin resistance reported in multiple countries. Levamisole resistance is less common but increasing.

Integrated Control Strategies

Effective control of T. circumcincta in the face of anthelmintic resistance requires an integrated approach combining grazing management, targeted selective treatment (TST), and monitoring.

Grazing management:

  • Rotational grazing with rest periods of 4 to 6 weeks to reduce larval contamination.
  • Mixed grazing with cattle or horses, as T. circumcincta is host-specific to sheep and goats.
  • Use of "safe" pastures (e.g., hay fields, newly seeded leys) for susceptible lambs.
  • Avoid overstocking and continuous grazing.

Targeted selective treatment (TST):

  • Treat only animals that require it, based on clinical signs, FEC, or production parameters (e.g., weight gain, dag score).
  • The FAMACHA system (anemia scoring) is less useful for T. circumcincta than for Haemonchus, but dag scoring (fecal soiling) and body condition scoring are practical.
  • Leave a proportion (5 to 10%) of untreated animals as a refuge for susceptible alleles, slowing resistance development.

Anthelmintic use:

  • Use the correct dose based on accurate body weight.
  • Rotate drug classes annually or use combination products (e.g., BZ + levamisole) where resistance is present.
  • Avoid underdosing and frequent treatments.
  • Perform FECRT every 1 to 2 years to monitor efficacy.

Biological control:

  • Nematophagous fungi (e.g., Duddingtonia flagrans) can reduce larval survival on pasture when fed to sheep. Commercial products are available in some regions.

Vaccination:

  • No commercial vaccine is currently available for T. circumcincta. Experimental vaccines using native or recombinant antigens (e.g., H11, H-gal-GP) have shown partial protection but are not yet licensed.

Decision tree for integrated control:

graph TD
    A[Assess flock risk: history, pasture contamination, climate], > B{Clinical signs present?}
    B, >|Yes| C[Perform FEC and larval culture]
    B, >|No| D[Monitor FEC at key times: spring, summer]
    C, > E{High FEC >500 epg?}
    E, >|Yes| F[Diagnose ostertagiosis]
    E, >|No| G[Consider type II: serum pepsinogen]
    G, > H{Elevated pepsinogen?}
    H, >|Yes| F
    H, >|No| I[Other causes]
    F, > J[Select anthelmintic based on resistance history]
    J, > K[Perform FECRT 10-14 days post-treatment]
    K, > L{Reduction >95%?}
    L, >|Yes| M[Continue with same class, rotate annually]
    L, >|No| N[Resistance confirmed: switch class or use combination]
    N, > O[Implement grazing management and TST]
    O, > P[Re-test FECRT after 12 months]
    P, > Q[Adjust strategy as needed]

Monitoring and surveillance:

  • Regular FEC monitoring of lambs and ewes.
  • Use of FECRT to detect emerging resistance.
  • Record treatment history and pasture rotations.

Conclusion

Teladorsagia circumcincta remains a major constraint to sheep production in temperate regions. The parasite's ability to induce hypobiosis and its rapid development of anthelmintic resistance demand a proactive, integrated control approach. Veterinary practitioners must combine diagnostic tools (FEC, larval culture, pepsinogen assay, FECRT) with strategic grazing management and targeted treatments to preserve drug efficacy and maintain flock health. Ongoing research into vaccine development and molecular resistance markers will provide additional tools for sustainable control.

References

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  3. Coles, G.C., Jackson, F., Pomroy, W.E., et al. The detection of anthelmintic resistance in nematodes of veterinary importance. Veterinary Parasitology. 2006;136(3-4):167-185.
  4. Kaplan, R.M. Biology, epidemiology, diagnosis, and management of anthelmintic resistance in gastrointestinal nematodes of livestock. Veterinary Clinics of North America: Food Animal Practice. 2020;36(1):17-30.
  5. Hoste, H., Torres-Acosta, J.F.J., Sandoval-Castro, C.A., et al. Tannin containing legumes as a sustainable alternative to anthelmintics in livestock. Animal Feed Science and Technology. 2012;176(1-4):18-25.