Internal Parasites of Sheep: Worms and Their Management

Etiology of Ovine Nematodoses

The most economically important internal parasites of sheep are gastrointestinal nematodes (GINs) belonging to the order Strongylida [1, 2]. These obligate helminths occupy specific niches within the ovine alimentary tract, principally the abomasum and small intestine. Key species include Haemonchus contortus (barber’s pole worm), Teladorsagia circumcincta (brown stomach worm), Trichostrongylus spp. (black scour worm and bankrupt worm), Cooperia curticei, Nematodirus battus, and Oesophagostomum spp. (nodule worm) [3]. The term “worms sheep get” colloquially refers to this assemblage of nematodes, though cestodes (e.g., Moniezia expansa) and trematodes (e.g., Fasciola hepatica) also occur.

Haemonchus contortus is a blood-feeding abomasal parasite, distinguished by its white ovaries wrapped around a red blood-filled intestine, giving a barber’s pole appearance [1]. Teladorsagia circumcincta inhabits the gastric glands of the abomasum, causing hyperplasia and reduced acid secretion [2]. Trichostrongylus species reside in the anterior small intestine and abomasum, with T. colubriformis being a common small-intestinal form associated with diarrhea and weight loss [3]. Nematodirus battus is a large intestinal nematode with a unique development pattern: eggs require prolonged chilling before hatching, leading to spring outbreaks in lambs [4].

Epidemiology and Transmission

Transmission of GINs is entirely indirect, via ingestion of infective third-stage larvae (L3) during grazing [1]. Pasture contamination with eggs from infected feces is a function of worm fecundity, stocking density, and weather [2]. For H. contortus, a single female can produce over 10,000 eggs per day, leading to massive pasture contamination within weeks [3]. Survival of L3 on pasture depends on temperature, humidity, and ultraviolet exposure; in temperate regions, larvae can persist for months, whereas arid climates limit survival [4].

Epidemiological patterns vary by species. Nematodirus battus eggs overwinter and hatch synchronously in spring when temperatures rise above 10°C after a period of cold, leading to a single peak of infection in lambs [4]. Haemonchus contortus thrives in warm, moist conditions and shows a periparturient rise (PPR) in ewes around lambing, associated with peri-parturient immunosuppression [1]. Teladorsagiosis and trichostrongylosis are more prevalent in cooler climates, with T. circumcincta emerging as a dominant pathogen in temperate zones where anthelmintic resistance is high [2, 3].

Clinical Signs and Pathology

Clinical manifestations depend on worm burden, species, host age, and nutritional status [1]. Haemonchus contortus causes acute hemorrhagic anemia: pale mucous membranes, submandibular edema (bottle jaw), weakness, and sudden death in heavily infected lambs [2]. Chronic infections lead to weight loss, reduced wool growth, and ill-thrift. Pathologically, the abomasal mucosa shows petechiae and free blood in the lumen [3].

Teladorsagia circumcincta induces abomasal hyperplasia, parietal cell atrophy, and increased pH, which impairs protein digestion and allows secondary bacterial overgrowth [2]. The classic sign is unthriftiness, reduced appetite, and intermittent diarrhea in lambs during summer [3]. Trichostrongylus species cause catarrhal enteritis, villous atrophy, and malabsorption, presenting as profuse watery diarrhea (black scour), dehydration, and rapid weight loss [3, 4]. Nematodirus battus infection in lambs (typically 6–12 weeks old) produces severe enteritis with watery diarrhea, dehydration, and mortality if untreated [4].

Diagnosis

Antemortem diagnosis relies on quantitative fecal egg counts (FEC) using modified McMaster or FLOTAC techniques [1, 2]. Species differentiation is achieved through larval culture and identification of third-stage larvae (L3) based on sheath tail morphology [3]. For H. contortus, the FAMACHA card system is a practical, on-farm method to assess anemia by scoring the color of the ocular conjunctiva from 1 (red) to 5 (pale), enabling targeted treatment of only anemic individuals [4].

Postmortem diagnosis involves necropsy with counting of adult worms in the abomasum and small intestine. For N. battus, the large size and distinctive eggs aid identification [4]. Molecular diagnostics such as species-specific PCR and multiplex assays are available in reference laboratories but are not yet routine in field settings [1].

Mermaid flowchart for diagnostic approach:

flowchart TD
    A[Clinical Signs: anemia, diarrhea, weight loss], > B{Fecal Egg Count}
    B, > C[High FEC > 500 epg]
    B, > D[Low FEC < 100 epg]
    C, > E[Larval Culture for Species ID]
    D, > F[Consider Other Causes]
    E, > G{Haemonchus spp.?}
    G, > H[FAMACHA Score]
    G, > I[Teladorsagia/Trichostrongylus]
    H, > J[Treat Only Anemic Sheep]
    I, > K[Group Treatment Based on Egg Counts]

Treatment and Anthelmintic Resistance

Anthelmintic compounds fall into three main classes: benzimidazoles (BZ; e.g., fenbendazole, albendazole), macrocyclic lactones (ML; e.g., ivermectin, moxidectin), and imidazothiazoles (e.g., levamisole) [1]. Additionally, monepantel (an amino-acetonitrile derivative) and derquantel (a spiroindole) are newer options, though resistance has already emerged to some [2, 3].

Anthelmintic resistance is a global crisis, particularly for H. contortus and T. circumcincta [1]. Resistance to BZ and ML is widespread, and multiple-drug resistance is common [2]. The mechanisms include target-site mutations (e.g., beta-tubulin polymorphisms in BZ resistance, P-glycoprotein efflux in ML resistance) [3]. Diagnosis of resistance is confirmed through the fecal egg count reduction test (FECRT), where a <95% reduction or lower 95% confidence interval indicates resistance [4].

To delay resistance, the following strategies are recommended [1, 3]:

• Use of a combination anthelmintic product (e.g., levamisole + ivermectin) only after confirming susceptibility.
• Targeted selective treatment (TST) based on FAMACHA or FEC thresholds, leaving a proportion of the flock untreated to maintain a refugia of susceptible worms.
• Quarantine drenching of introduced sheep with a triple or quadruple combination product.
• Rotational use of drug classes at intervals of 2–3 years.

Control and Integrated Parasite Management

Sustainable control relies on an integrated pasture management approach rather than sole reliance on anthelmintics [1]. Key components include:

For Nematodirus battus, forecasting models based on accumulated day degrees (d-value of 310–520 day·°C above a base temperature of 5.5°C) predict hatching and guide timing of anthelmintic treatment in lambs [4]. In temperate regions, adult ewes are treated at lambing to reduce the PPR, but TST minimizes selection for resistance [2].

[Link to related articles as appropriate: For detailed discussion on anthelmintic resistance, see Gastrointestinal Nematodes in Sheep: Anthelmintic Resistance. For FAMACHA implementation, see Haemonchus contortus in Sheep: Anthelmintic Resistance and FAMACHA-Based Control. For N. battus forecasting, see Nematodirus battus in Sheep Lambs: Spring Outbreak Epidemiology, D-Value Forecasting, and Anthelmintic Control.]

Vaccination and Future Prospects

No commercial vaccine is currently available for ovine GINs, although experimental vaccines against H. contortus using hidden antigens (H11, H-gal-GP) have shown partial efficacy in reducing egg shedding and worm burdens [1]. Advances in parasitomics and RNA interference may lead to novel targets [3]. Heritability of nematode resistance (measured as FEC) is moderate (h² ~ 0.3), allowing selective breeding programs to complement therapeutic control [4].

References

[1] Merck Veterinary Manual. Parasitology: Gastrointestinal Nematodes of Small Ruminants. Kenilworth, NJ: Merck & Co., Inc.

[2] Taylor MA, Coop RL, Wall RL. Veterinary Parasitology. 4th ed. Oxford: Wiley-Blackwell.

[3] Bowman DD. Georgis’ Parasitology for Veterinarians. 10th ed. St. Louis: Saunders.

[4] Zajac AM, Conboy GA. Veterinary Clinical Parasitology. 8th ed. Ames: Wiley-Blackwell. *** 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.