Gastrointestinal Parasites in Sheep: Worms and Control

By Dr. Zubair Khalid, DVM, MS, PhD · 2026-06-06

Key Takeaways

• Gastrointestinal parasitism in sheep is primarily caused by nematodes (e.g., Haemonchus contortus, Teladorsagia circumcincta), cestodes (Moniezia spp.), and protozoa (Eimeria spp.), with nematodes being the most economically significant.
• Diagnosis relies on fecal egg counts (FEC) using the McMaster technique, larval culture for species differentiation, and the FAMACHA system for targeted treatment of Haemonchus-induced anemia.
• Anthelmintic resistance is a critical issue, particularly in H. contortus, T. circumcincta, and T. colubriformis, necessitating the use of fecal egg count reduction tests (FECRT) to monitor drug efficacy and guide treatment strategies.
• Integrated control involves a combination of targeted selective treatment (TST), strategic pasture management (e.g., rest periods, alternate grazing), and genetic selection for parasite resistance and resilience.
• Clinical signs range from subclinical production losses and ill-thrift to severe anemia (bottle jaw with H. contortus), diarrhea (e.g., Trichostrongylus, Nematodirus), and mortality, with pathology varying by parasite predilection site.
• Emerging diagnostic tools include molecular diagnostics (PCR) for species identification and detection of anthelmintic resistance polymorphisms, while commercial vaccines for gastrointestinal nematodes are not yet widely available.

Introduction

Gastrointestinal parasitism remains one of the most economically burdensome disease complexes affecting sheep flocks worldwide. The term "worms sheep get" encompasses a diverse array of helminths (nematodes, cestodes, trematodes) and protozoa that inhabit the alimentary tract [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. Among these, nematodes of the abomasum and small intestine are the most prevalent, causing subclinical production losses, clinical disease, and mortality in naive or immunocompromised animals. Effective control requires integrated strategies that combine targeted diagnostics, rational anthelmintic use, pasture management, and genetic selection for resistance. This article provides a detailed, publication-grade review of the major gastrointestinal parasites of sheep, with emphasis on their biology, diagnosis, treatment, and sustainable control.

Etiology of Ovine Gastrointestinal Parasitism

The parasites responsible for gastrointestinal disease in sheep belong to three major taxonomic groups: nematodes (roundworms), cestodes (tapeworms), and protozoa (coccidia, cryptosporidia). The following sections detail the most significant taxa.

Worms Sheep Get: A Taxonomic Overview

The principal nematodes include Haemonchus contortus (barber pole worm), Teladorsagia circumcincta (brown stomach worm), Trichostrongylus colubriformis and Trichostrongylus axei (black scour worms), Nematodirus battus (thread-necked worm), Cooperia curticei, Oesophagostomum columbianum (nodular worm), Chabertia ovina (large-mouthed bowel worm), and Bunostomum trigonocephalum (hookworm) [<a href="#ref-2">2</a>, <a href="#ref-3">3</a>]. Cestodes are represented by Moniezia expansa and Moniezia benedeni [<a href="#ref-4">4</a>]. Protozoan parasites include Eimeria spp. (coccidiosis) and Cryptosporidium parvum.

Table 1 summarizes the principal nematode genera, their predilection sites, and key epidemiological features.

Life Cycle Overview

All strongyle nematodes share a direct life cycle: adults inhabit the gastrointestinal tract, eggs are passed in feces, larvae develop to the infective third stage (L3) on pasture, and ingestion of L3 completes the cycle. Nematodirus battus requires prolonged chilling and mass egg hatch, leading to spring outbreaks. Moniezia spp. require an intermediate oribatid mite host [<a href="#ref-4">4</a>]. Protozoan parasites such as Eimeria have a direct, fecal-oral cycle with endogenous multiplication in enterocytes.

Epidemiology

The distribution and intensity of parasitic infections are influenced by climate, pasture management, host immunity, and stocking density. Haemonchus contortus thrives in warm, humid environments and is a major cause of anemia and hypoproteinemia. Teladorsagia circumcincta is predominant in cooler temperate regions and is associated with loss of appetite and poor growth. Nematodirus battus causes acute diarrhea and weight loss in lambs during spring pasture emergence. Periparturient ewes experience a rise in fecal egg counts (the periparturient rise) due to immunosuppression and contribute heavily to pasture contamination.

Clinical Signs and Pathology

Clinical manifestations vary with parasite species, burden, and host age.

Nematodes

Haemonchus contortus: Acute or peracute anemia, submandibular edema (bottle jaw), pale mucous membranes, depression, and sudden death in heavily infected lambs. Postmortem findings include hemorrhagic abomasitis and adult worms visible on mucosa (barber pole pattern).

Teladorsagia circumcincta: Weight loss, reduced feed intake, mild diarrhea, hypoproteinemia, and ill-thrift. Pathology shows abomasal hyperplasia and increased pH due to disruption of parietal cells.

Trichostrongylus colubriformis and Trichostrongylus axei: Profuse watery diarrhea, dehydration, inappetence, and poor wool quality. Enteritis, villous atrophy, and catarrhal inflammation are observed.

Nematodirus battus: Acute-onset diarrhea, dehydration, abdominal pain, and rapid weight loss in lambs aged 6 to 12 weeks. Pathology reveals duodenal inflammation and villous atrophy.

Oesophagostomum columbianum: Nodular lesions in the large intestinal wall, chronic diarrhea, and tenesmus.

Chabertia ovina: Acute hemorrhagic colitis, diarrhea with mucus, and emaciation.

Bunostomum trigonocephalum: Anemia, hypoproteinemia, and diarrhea due to blood feeding in the small intestine.

Cestodes

Moniezia spp. usually cause subclinical infection, but heavy burdens can cause ill-thrift, unthriftiness, and occasional intestinal obstruction [<a href="#ref-4">4</a>]. Tapeworm segments are visible in feces.

Protozoa

Eimeria spp. cause coccidiosis in lambs, with signs including non-hemorrhagic diarrhea, tenesmus, dehydration, and reduced growth. Cryptosporidium parvum leads to watery diarrhea in neonatal lambs and is zoonotic.

Diagnosis

Accurate diagnosis underpins effective control.

Fecal Egg Count (FEC)

The modified McMaster technique (sensitivity approximately 50 eggs per gram) is the standard quantitative method. Differential egg identification is possible based on morphology (size, shape, number of blastomeres). Nematodirus eggs are large and polar-distinct. Strongyloides eggs are thin-shelled and embryonated. Moniezia eggs are triangular with a pyriform apparatus. Eimeria oocysts are identified by size, shape, and sporulation characteristics.

Larval Culture and Differentiation

Third-stage larvae can be cultured from feces and identified to genus using morphometric keys. This is essential for distinguishing between Haemonchus, Teladorsagia, and Trichostrongylus.

FAMACHA System

Anemia-based clinical scoring for Haemonchus contortus allows targeted treatment. Ovine conjunctival color is scored from 1 (red, non-anemic) to 5 (pale, severely anemic). Sheep with scores 4 or 5 are treated selectively, reducing anthelmintic use.

Biochemical and Immunological Markers

Serum pepsinogen levels increase with abomasal damage (e.g., Teladorsagia infection). Serum albumin and total protein reflect protein-losing enteropathy. Commercial ELISA kits for coproantigen detection of Fasciola hepatica are available, but no pan-nematode antigen test is widely used for field diagnosis.

Molecular Diagnostics

PCR-based assays targeting ribosomal ITS-1 and ITS-2 sequences enable species-specific identification of nematodes and detection of anthelmintic resistance polymorphisms (e.g., beta-tubulin for benzimidazole resistance) [<a href="#ref-3">3</a>, <a href="#ref-5">5</a>]. Pooled fecal samples can be used for herd-level screening.

Treatment and Anthelmintic Resistance

Anthelmintic Classes

Table 2 lists the major classes and their mechanisms.

Anthelmintic resistance is a global crisis, particularly in H. contortus, T. circumcincta, and T. colubriformis. Resistance to multiple classes (multi-drug resistance) is common in many regions. The fecal egg count reduction test (FECRT) is the standard for diagnosing resistance, using a reduction threshold of less than 95% for the drug class.

Treatment Strategies

• Targeted selective treatment (TST): Use FAMACHA to treat only anemic animals, preserving refugia of susceptible worms.
• Strategic deworming: Administer treatments at times of low pasture contamination (e.g., pre-lambing, before turnout).
• Combination therapy: Use two or more classes simultaneously to delay resistance, but only when resistance to individual drugs is confirmed.

Integrated Control Strategies

Sustainable control requires reducing pasture contamination and minimizing selection for resistance.

Pasture Management

• Rest periods: Allow pasture to rest for 6 to 12 weeks to reduce L3 larval numbers, though survival depends on climate.
• Alternate grazing: Co-graze with cattle or horses, as nematodes are largely host-specific. Cattle can consume sheep nematode larvae, which do not develop further.
• Rotational grazing: Move sheep to clean pasture (newly established or hayed) after treatment.

Genetic Selection

Breeding for resistance (ability to suppress egg output) and resilience (ability to tolerate infection without production loss) is gaining traction. Breeds such as Red Maasai, St Croix, and Dorper show variable resistance. Estimated breeding values for FEC are available in some countries.

Nutritional Support

Protein supplementation enhances immune function and resilience to nematodes, reducing fecal egg counts and clinical effects.

Vaccination

No commercial vaccine exists for gastrointestinal nematodes in sheep. Experimental antigens such as H. contortus H11 and H-gal-GP have shown efficacy but remain undeveloped commercially.

Diagnostic and Control Workflow

graph TD
 A["Sheep flock with clinical signs"] --> B["Collect fecal samples"]
 B --> C["Perform FEC and larval culture"]
 C --> D{"Identify predominant species"}
 D -->|"Haemonchus"| E["Use FAMACHA scoring"]
 D -->|"Teladorsagia/Trichostrongylus"| F["Consider serum pepsinogen"]
 D -->|"Nematodirus"| G["Check season and age"]
 E --> H{"Anemia score 4-5?"}
 H -->|"Yes"| I["Administer effective anthelmintic"]
 H -->|"No"| J["Monitor without treatment"]
 F --> K{"FEC > 500 epg?"}
 K -->|"Yes"| I
 K -->|"No"| J
 G --> L{"Lamb age 6-12 weeks?"}
 L -->|"Yes"| I
 L -->|"No"| J
 I --> M["Perform FECRT 10-14 days post-treatment"]
 M --> N{"Reduction > 95%?"}
 N -->|"Yes"| O["Continue same class"]
 N -->|"No"| P["Switch to alternative class or combination"]
 O --> Q["Implement pasture rotation and refugia"]
 P --> Q
 Q --> R["Monitor FEC at intervals"]

Conclusion

Gastrointestinal parasitism in sheep remains a formidable challenge to global small ruminant production. An integrated approach combining accurate diagnosis, targeted and rational anthelmintic use (including TST and combination therapy), strategic pasture management, and genetic selection is essential to preserve drug efficacy and ensure flock health. Ongoing research into vaccine development and molecular markers of resistance will further refine control strategies.

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References

Disclaimer*: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.

Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.