Common Internal Parasites in Sheep: Worms and Their Management

Introduction

Internal parasitism by nematodes and cestodes remains a major constraint to ovine productivity worldwide. The parasites most commonly encountered are members of the order Strongylida (trichostrongylid nematodes) and the family Anoplocephalidae (tapeworms) [1, 2]. The phrase "worms sheep get" encompasses a diverse assemblage of species that inhabit the abomasum, small intestine, large intestine, and, in some cases, the lungs and liver. This review provides an exhaustive, evidence-based overview of the biology, diagnosis, treatment, and control of these parasites, with emphasis on molecular diagnostic approaches and anthelmintic resistance management.

Etiology: Major Nematode and Cestode Species

Abomasal Nematodes

The abomasum is the primary habitat for Haemonchus contortus (barber's pole worm) and Teladorsagia circumcincta (brown stomach worm). H. contortus is a blood-feeding nematode that causes anemia and hypoproteinemia [1, 3, 4]. Its genetic diversity has been characterized extensively using ITS2 and microsatellite markers, revealing high intraspecific variation even within sympatric populations from sheep and wild blue sheep [5, 6]. T. circumcincta is associated with type I and type II ostertagiosis, characterized by abomasal mucosal hyperplasia and protein-losing enteropathy [7]. Other abomasal parasites include Marshallagia spp., which are polymorphic members of the Ostertagiinae subfamily [8].

Small Intestinal Nematodes

The small intestine harbors numerous pathogenic species. Trichostrongylus colubriformis, T. vitrinus, and T. axei are highly prevalent and cause enteritis and diarrhea [9, 10, 11]. Nematodirus battus, N. spathiger, and N. filicollis are large trichostrongylids that cause significant disease in young lambs, with N. battus being particularly pathogenic due to its mass emergence from eggs after a prolonged cold period [12, 13]. Cooperia curticei is another small intestinal nematode that contributes to the periparturient rise in ewes [14]. Bunostomum trigonocephalum (hookworm) attaches to the mucosa and causes anemia, although it is less common in temperate regions.

Large Intestinal Nematodes

The large intestine is primarily affected by Chabertia ovina and Trichuris ovis. These species cause typhlocolitis and diarrhea, particularly in weaned lambs.

Cestodes

The common tapeworm of sheep is Moniezia expansa, which inhabits the small intestine. Its life cycle involves oribatid mites as intermediate hosts [15]. Although generally considered less pathogenic than nematodes, heavy burdens can cause intestinal obstruction and unthriftiness.

Pulmonary Nematodes

Lungworms such as Dictyocaulus filaria and Protostrongylus rufescens are also encountered. D. filaria causes bronchitis and verminous pneumonia, while P. rufescens is a protostrongylid found in lung parenchyma.

Epidemiology and Transmission

Transmission of the "worms sheep get" is primarily via the fecal-oral route. Sheep ingest infective third-stage larvae (L3) while grazing. The epidemiology is strongly influenced by climate, pasture management, and host immunity. H. contortus thrives in warm, moist conditions and is particularly problematic in tropical and subtropical regions [1, 4]. T. circumcincta and N. battus are adapted to cooler temperate climates [2, 12].

Historical paleoparasitological evidence from Patagonia indicates that gastrointestinal nematodes have infected sheep since the introduction of the species to the region [16]. Modern molecular studies using nemabiome metabarcoding have revealed a high prevalence of H. contortus and the predominance of Camelostrongylus mentulatus in alpaca herds in the northern UK, demonstrating cross-species transmission potential [2]. Extensive molecular surveys in Greece and China have documented high genetic diversity of Haemonchus spp. across domestic ruminants [1, 3].

The periparturient rise (PPR) in fecal egg counts (FEC) in ewes is a key epidemiological driver. The temporary immunosuppression associated with late pregnancy and lactation results in increased egg output, contaminating pastures for susceptible lambs [17].

Clinical Signs and Pathology

Clinical disease depends on the species and intensity of infection. In heavy H. contortus burdens, anemia, submandibular edema (bottle jaw), weight loss, and death are observed. The FAMACHA system uses conjunctival pallor to grade anemia and guide selective treatment [1]. T. circumcincta infection leads to diarrhea, inappetence, and reduced growth. N. battus causes profuse greenish diarrhea in lambs 6-12 weeks of age, with high mortality in untreated outbreaks [12]. Trichostrongylus species produce enteritis with watery feces and ill thrift [10].

Pathologically, H. contortus causes abomasal hemorrhages and petechiae. Chronic Teladorsagia infection results in raised, hyperplastic abomasal nodules. Intestinal trichostrongylosis is characterized by villous atrophy, crypt hyperplasia, and infiltration of inflammatory cells. Moniezia infection rarely causes pathology unless massive; it may cause mild catarrhal enteritis.

Diagnostics

Fecal Examination

The standard diagnostic method is quantitative fecal egg count (FEC) using a modified McMaster or Mini-FLOTAC technique. Differentiation of genera based on egg morphology is possible but limited: stronglye eggs are morphologically similar. However, Nematodirus eggs are larger (150-230 µm) and barrel-shaped. Moniezia eggs are distinct with a pyriform apparatus. Larval culture (Baermann technique) is required for lungworm detection.

Molecular Diagnostics

Molecular methods have revolutionized parasite identification. PCR-based assays targeting the internal transcribed spacer (ITS-1 and ITS-2) regions of ribosomal DNA enable species-specific identification of trichostrongylids [18, 19, 20]. The ITS2 region is widely used for phylogenetic discrimination, and its quantitative application via droplet digital PCR (ddPCR) allows absolute quantification of H. contortus, T. circumcincta, and Trichostrongylus spp. in mixed infections [18]. Quantitative analysis of ITS2 sequences has proven reliable for differentiating Cooperia curticei, Ostertagia spp., and Chabertia ovina [14, 20].

Nemabiome metabarcoding using next-generation sequencing of the ITS2 locus provides a comprehensive view of the entire parasite community from a fecal sample [2]. This approach has been applied to survey the prevalence of Camelostrongylus mentulatus in alpacas and H. contortus in sheep [2].

Serological and Molecular Markers

Coproantigen ELISA tests are available for Fasciola hepatica but are not routinely used for nematodes. Host genetic markers for resistance have been identified through quantitative trait locus (QTL) mapping, linking specific chromosomal regions to lower FEC [17].

The following table summarizes key diagnostic targets:

Treatment and Anthelmintic Resistance

Treatment relies on three major anthelmintic classes: benzimidazoles (BZ), imidazothiazoles (e.g., levamisole), and macrocyclic lactones (e.g., ivermectin, moxidectin). Efficacy of moxidectin has been demonstrated against a broad spectrum of internal parasites [21]. However, resistance has emerged to all classes.

Benzimidazole resistance is widespread in Nematodirus spathiger and N. filicollis in New Zealand [12]. Resistance in Trichostrongylus colubriformis, T. vitrinus, and T. axei has been documented [10]. High-level IVM resistance in H. contortus is reported from Inner Mongolia, with genetic diversity analysis revealing selection on β-tubulin isotype 1 [1]. The molecular basis of resistance includes single-nucleotide polymorphisms (SNPs) in the β-tubulin gene (BZ resistance) and mutations in glutamate-gated chloride channel genes (macrocyclic lactone resistance).

Management of resistance involves:

• Targeted selective treatment (TST): Using FAMACHA or FEC thresholds to treat only animals with high burdens.
• Combination therapy: Using two or more classes simultaneously.
• Refugia-based strategies: Leaving a proportion of the flock untreated to maintain a susceptible parasite population.
• Pasture management: Rotational grazing, co-grazing with cattle, and use of bioactive forages (e.g., chicory, sericea lespedeza).

The interaction between lactic acid bacteria and gastrointestinal nematodes has been explored as a potential biological control, though it remains experimental [22].

Control and Integrated Parasite Management

Integrated parasite management (IPM) combines strategic anthelmintic treatments with grazing management, genetic selection for resistance, and nutritional support. Key components include:

• Preventive treatments: Ewes are dosed pre-lambing to reduce PPR. Lambs may require treatment at weaning or based on FEC monitoring.
• Quarantine drenching: New animals receive a combination anthelmintic and are held on contaminated pasture for 24-48 hours to dilute any resistant worms.
• FEC monitoring: Regular monitoring to assess treatment efficacy using the fecal egg count reduction test (FECRT).
• Genetic resistance: Selection for lower FEC using estimated breeding values (EBVs) [17].

The following Mermaid diagram outlines the integrated decision-making process:

flowchart TD
    A[Flock Health Check], > B{FAMACHA Score}
    B, >|Score 1-2| C[No Treatment Needed]
    B, >|Score 3 or higher| D[Collect Feces for FEC]
    D, > E{FEC > threshold?}
    E, >|No| C
    E, >|Yes| F[Select Anthelmintic Class]
    F, > G{Treatment History & Resistance Data}
    G, >|No known resistance| H[Single class treatment]
    G, >|Resistance suspected| I[Combination treatment]
    H, > J[Post-treatment FEC (10-14 days)]
    I, > J
    J, > K{FECR > 95%?}
    K, >|Yes| L[Continue grazing plan]
    K, >|No| M[Change drug class; review refugia]
    M, > N[Implement pasture rotation & quarantine]
    L, > O[Monitor at 4-week intervals]
    N, > O

Future Directions

Advances in computational biology and genomics are enabling population-level tracking of anthelmintic resistance alleles. Nemabiome metabarcoding offers high-throughput surveillance [2]. Quantitative molecular tools such as ddPCR allow early detection of resistance shifts [18]. Host QTL mapping may enable marker-assisted selection for parasite resistance [17].

It is critical that veterinarians and producers adopt evidence-based, region-specific control programs to preserve anthelmintic efficacy and ensure sustainable sheep production.

References

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