Gastrointestinal Parasites in Small Ruminants: Focus on Worms in Sheep

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

Key Takeaways

• Gastrointestinal nematodes, particularly Haemonchus contortus, are the most economically damaging parasites in sheep, causing anemia, hypoalbuminemia, and reduced productivity.
• Diagnosis relies on coprological methods like fecal egg counts (FEC) using McMaster or Mini-FLOTAC, with molecular techniques (nemabiome sequencing) offering enhanced species identification and resistance monitoring.
• Anthelmintic resistance to benzimidazoles, imidazothiazoles, and macrocyclic lactones is widespread, necessitating targeted selective treatment (TST) based on FEC or FAMACHA© scores to preserve drug efficacy.
• Integrated parasite management (IPM) strategies, including pasture rotation, nutritional support, and quarantine treatments, are crucial for controlling worm burdens below pathogenic thresholds.
• Clinical signs range from subclinical ill-thrift and reduced wool production to acute anemia and edema (bottle jaw) in severe cases, with risk factors including age, nutritional status, and management system.
• Hematological indicators like Packed Cell Volume (PCV) and the FAMACHA© system are practical tools for assessing anemia and guiding treatment decisions, reducing the selection pressure for resistant parasites.

Introduction

Gastrointestinal (GI) parasitism in small ruminants, particularly sheep, represents a persistent constraint to productivity and welfare worldwide [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. The term worms sheep get encompasses a diverse assemblage of nematodes, cestodes, and trematodes that inhabit the alimentary tract, with nematodes (roundworms) being the most prevalent and economically damaging [<a href="#ref-3">3</a>, <a href="#ref-4">4</a>]. Economic losses arise from reduced weight gain, decreased milk production, impaired wool quality, mortality, and the cost of control measures [<a href="#ref-5">5</a>, <a href="#ref-6">6</a>]. The global prevalence of GI parasites in sheep typically exceeds 70% in most surveys, with strongyle-type eggs dominating coprological findings [<a href="#ref-1">1</a>, <a href="#ref-7">7</a>].

This article provides a detailed, evidence-based review of the etiology, epidemiology, clinical pathology, diagnostic approaches, therapeutic interventions, and integrated control strategies for GI worms in sheep, drawing exclusively on peer-reviewed literature from the past two decades.

Etiology and Parasite Diversity

The major helminth groups affecting sheep belong to the phyla Nematoda, Cestoda, and Platyhelminthes (Trematoda). Among nematodes, the order Strongylida includes the most pathogenic genera: Haemonchus, Teladorsagia (formerly Ostertagia), Trichostrongylus, Cooperia, Nematodirus, Oesophagostomum, and Chabertia [<a href="#ref-8">8</a>, <a href="#ref-9">9</a>]. The abomasal blood‑feeding nematode Haemonchus contortus is globally recognized as the single most important parasite of sheep due to its high fecundity, pathogenicity, and propensity for developing anthelmintic resistance [<a href="#ref-10">10</a>, <a href="#ref-11">11</a>].

Other nematodes include Strongyloides papillosus (small intestine), Trichuris ovis (cae cum), and Bunostomum trigonocephalum (small intestine) [<a href="#ref-12">12</a>, <a href="#ref-13">13</a>]. Cestodes are represented by Moniezia expansa and Moniezia benedeni in the small intestine, while trematodes such as Fasciola hepatica (liver fluke) and Dicrocoelium dendriticum (lancet fluke) affect the hepatobiliary system but are often detected in fecal surveys alongside GI worms [<a href="#ref-14">14</a>, <a href="#ref-15">15</a>]. Protozoan parasites, particularly Eimeria spp., frequently co‑occur but are not the focus of this article except as coinfecting agents [<a href="#ref-16">16</a>].

Recent molecular surveys using nemabiome deep‑amplicon sequencing have refined species identification. In Grenada, the predominant species in sheep were H. contortus (42%), Trichostrongylus colubriformis (38%), and Oesophagostomum columbianum (12%) [<a href="#ref-1">1</a>]. Similar species assemblages have been reported from Myanmar, Nigeria, and Albania [<a href="#ref-6">6</a>, <a href="#ref-9">9</a>, <a href="#ref-17">17</a>].

Epidemiology

Global Prevalence Patterns

Prevalence estimates for GI worms in sheep vary widely by geographic region, management system, and diagnostic method. Table 1 summarizes key prevalence data from representative studies.

Table 1. Prevalence of gastrointestinal helminths in sheep from selected surveys.

In Africa and the Middle East, mixed infections are the rule rather than the exception, with more than 80% of positive sheep harboring at least two parasite genera [<a href="#ref-2">2</a>, <a href="#ref-3">3</a>]. The high prevalence in tropical and subtropical regions reflects favorable climatic conditions for free‑living larval stages and continuous exposure on pasture [<a href="#ref-19">19</a>].

Risk Factors

Multiple host and environmental factors influence infection levels. Age is a significant determinant; lambs and juveniles often exhibit higher prevalence but lower egg counts than adults, likely due to developing immunity [<a href="#ref-2">2</a>, <a href="#ref-20">20</a>]. In Gabon, juveniles had higher prevalence but lower parasite loads than adults, while pregnant females carried approximately 2.5‑fold higher burdens than non‑pregnant ewes [<a href="#ref-2">2</a>].

Sex differences are inconsistent across studies. Some surveys report higher prevalence in females, attributed to periparturient immunosuppression and stress [<a href="#ref-3">3</a>, <a href="#ref-20">20</a>], while others find no significant sex effect [<a href="#ref-10">10</a>]. Body condition score is inversely correlated with strongyle egg counts: animals with poor body condition have significantly higher fecal egg counts (FEC) and are at greater risk of clinical disease [<a href="#ref-21">21</a>, <a href="#ref-22">22</a>].

Management system exerts a strong influence. Sheep reared under extensive (pasture‑based) systems have higher parasite diversity and prevalence than those under intensive (confined) systems [<a href="#ref-23">23</a>]. Seasonal rainfall patterns drive larval availability on pasture; peaks in FEC are commonly observed during the rainy season [<a href="#ref-4">4</a>, <a href="#ref-13">13</a>].

Clinical Signs and Pathology

The clinical expression of GI parasitism depends on the parasite species, burden, host immunity, and nutritional status. H. contortus infection causes acute or chronic anemia, submandibular edema (bottle jaw), weakness, and death in heavily parasitized lambs [<a href="#ref-10">10</a>]. The pathophysiological mechanism is direct blood‑feeding by adult worms, each consuming approximately 0.05 mL of blood per day, leading to iron‑deficiency anemia and hypoalbuminemia [<a href="#ref-8">8</a>, <a href="#ref-11">11</a>].

Teladorsagia circumcincta and Trichostrongylus spp. induce a protein‑losing enteropathy, resulting in diarrhea, weight loss, and reduced growth rates [<a href="#ref-9">9</a>, <a href="#ref-24">24</a>]. Oesophagostomum columbianum causes nodular lesions in the large intestine, which may abscess and lead to chronic ill‑thrift [<a href="#ref-1">1</a>, <a href="#ref-5">5</a>]. Nematodirus battus infection in young lambs can cause profuse watery diarrhea and sudden death, particularly when large numbers of larvae emerge synchronously from the intestinal mucosa [<a href="#ref-25">25</a>, <a href="#ref-26">26</a>].

Chronic subclinical parasitism is far more common than acute disease in managed flocks. It manifests as depressed feed conversion efficiency, reduced wool production, and impaired reproductive performance [<a href="#ref-2">2</a>, <a href="#ref-4">4</a>]. Necropsy findings in sheep that die from haemonchosis include pale mucous membranes, hydropericardium, and massive numbers of adult worms in the abomasum [<a href="#ref-12">12</a>].

Diagnosis

Accurate diagnosis is essential for targeted treatment and resistance monitoring. Conventional coprological techniques remain the mainstay, but molecular methods are increasingly employed for species identification.

Coprological Methods

Table 2. Comparison of common coprological diagnostic techniques for GI worms in sheep.

The Mini‑FLOTAC technique has been shown to detect more helminth eggs than the McMaster method in both sheep and goats, with a weak concordance between the two methods, suggesting Mini‑FLOTAC is superior for detecting low‑intensity infections [<a href="#ref-11">11</a>]. For rapid field screening, qualitative flotation remains widely used [<a href="#ref-27">27</a>].

Molecular Diagnostics

Nemabiome analysis using deep amplicon sequencing of the ITS‑2 region of ribosomal DNA allows simultaneous identification and quantification of multiple nematode species from pooled fecal samples [<a href="#ref-1">1</a>]. This approach has revealed cryptic species diversity and is invaluable for resistance monitoring. Real‑time PCR assays targeting specific species (e.g., H. contortus, T. colubriformis) are also available for research and diagnostic laboratories [<a href="#ref-6">6</a>, <a href="#ref-8">8</a>].

Hematological Indicators

Packed cell volume (PCV) is a practical proxy for anemia caused by blood‑feeding nematodes. Sheep with severe strongyle infection (FEC >1200 EPG) have significantly lower PCV than those with mild or moderate infection [<a href="#ref-20">20</a>]. The FAMACHA© system, based on ocular mucous membrane color, is a validated field tool for identifying anemic sheep that require targeted anthelmintic treatment, thereby reducing selection for resistance [<a href="#ref-10">10</a>].

Treatment and Anthelmintic Resistance

Anthelmintics remain the primary intervention for controlling GI worms in sheep. The three major drug classes are benzimidazoles (e.g., albendazole, fenbendazole), imidazothiazoles/tetrahydropyrimidines (e.g., levamisole, morantel), and macrocyclic lactones (e.g., ivermectin, moxidectin) [<a href="#ref-21">21</a>, <a href="#ref-28">28</a>]. However, resistance to all these classes is widespread and increasing.

Evidence of Resistance

Surveys from Ethiopia reported resistance to albendazole in 32.9%, levamisole in 41.7%, and ivermectin in 13.3% of trials, with multiple‑drug resistance documented in four studies [<a href="#ref-23">23</a>]. In the United States, resistance to all three major classes has been confirmed in H. contortus on many farms [<a href="#ref-21">21</a>]. Resistance is most commonly detected in Haemonchus, Oesophagostomum, Trichostrongylus, and Trichuris species [<a href="#ref-23">23</a>, <a href="#ref-29">29</a>].

Mechanisms and Management

Resistance arises through repeated use of the same drug class, underdosing, and frequent mass treatments. To delay resistance, the following strategies are recommended:

• Use targeted selective treatment (TST) based on FEC or FAMACHA© score rather than whole‑flock blanket dosing [<a href="#ref-10">10</a>].
• Combine drugs from different classes (e.g., benzimidazole + levamisole) when resistance to a single class is suspected [<a href="#ref-21">21</a>].
• Maintain a refugia population of susceptible worms by leaving a proportion of untreated animals [<a href="#ref-10">10</a>, <a href="#ref-21">21</a>].
• Practice quarantine treatment of newly purchased animals with a combination product to prevent importation of resistant isolates [<a href="#ref-8">8</a>].

Ethnoveterinary Alternatives

In resource‑limited settings, plant‑based remedies are sometimes employed. A survey in Burkina Faso documented 32 medicinal plant species used by the BWA community to treat GI parasites in small ruminants [<a href="#ref-7">7</a>, <a href="#ref-22">22</a>]. While some plant extracts show in vitro anthelmintic activity, rigorous efficacy and safety data are lacking, and ethnoveterinary practices should complement rather than replace evidence‑based deworming.

Control Strategies

Integrated parasite management (IPM) combines grazing management, nutritional support, diagnostic monitoring, and judicious anthelmintic use to maintain worm burdens below pathogenic thresholds [<a href="#ref-15">15</a>, <a href="#ref-18">18</a>].

Grazing Management

• Pasture rotation with rest periods of 4-6 weeks reduces larval contamination [<a href="#ref-26">26</a>].
• Mixed or alternate grazing with cattle or horses exploits host specificity of most sheep nematodes [<a href="#ref-13">13</a>].
• Avoid overstocking, which increases contamination pressure [<a href="#ref-2">2</a>].

Nutritional Enhancement

Sheep with adequate protein and energy intake are more resilient to parasitism. Supplementary feeding can reduce FEC and mitigate production losses [<a href="#ref-20">20</a>]. Copper and cobalt supplementation may support immune function [<a href="#ref-21">21</a>].

Biological Control

The use of nematophagous fungi (e.g., Duddingtonia flagrans) to trap and destroy free‑living larvae on pasture is a developing strategy, though commercial products remain limited in availability [<a href="#ref-15">15</a>].

Diagnostics‑Driven Decision Making

Regular FEC monitoring allows timely intervention and evaluation of drug efficacy via the fecal egg count reduction test (FECRT). A reduction of <95% with the lower 95% confidence interval below 90% indicates resistance [<a href="#ref-21">21</a>].

The following decision tree outlines a rational approach to managing GI worms in sheep flocks.

graph TD
 A["Flocks with clinical signs or known risk"] --> B["Collect fecal samples (10-15 animals)"]
 B --> C{"Perform FEC using McMaster or Mini-FLOTAC"}
 C -->|"Mean EPG > 500"| D["Assess anemia via FAMACHA or PCV"]
 C -->|"Mean EPG < 200"| E["No immediate treatment; monitor monthly"]
 D -->|"Anemic (FAMACHA 4-5, PCV <20%)"| F["Targeted treatment with effective anthelmintic class"]
 D -->|"Non-anemic"| G["Consider TST; treat only high-shedding individuals"]
 F --> H["Post-treatment FEC after 10-14 days"]
 H --> I{"FECRT < 95%?"}
 I -->|"Yes"| J["Resistance suspected; switch class or use combination therapy"]
 I -->|"No"| K["Class effective; continue rotation strategy"]
 J --> L["Perform molecular diagnostics (nemabiome) to identify resistant species"]
 L --> M["Adjust grazing and quarantine protocols"]
 E --> N["Reassess in 4-6 weeks or after rainfall"]
 G --> K

Conclusion

Gastrointestinal nematodes remain a major threat to sheep health and productivity worldwide. The diversity of worms sheep get is vast, but a few highly pathogenic species, particularly Haemonchus contortus, dominate losses. Accurate diagnosis using modern coprological and molecular tools, combined with targeted selective treatments and grazing management, is essential to preserve anthelmintic efficacy. Resistance is an escalating crisis that demands a shift from calendar‑based deworming to evidence‑driven, integrated parasite control programs. Future research should focus on expanding nemabiome surveillance, developing novel anthelmintics, and validating ethnoveterinary approaches under controlled conditions.

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