Gastrointestinal Parasites of Sheep: Clinical Management and Control

Introduction

Gastrointestinal (GI) parasitism represents a primary constraint to sheep production systems worldwide, causing substantial economic losses through reduced weight gain, decreased milk and wool yield, impaired reproductive performance, and increased mortality [1, 2, 3]. The burden of these infections is particularly pronounced in extensive grazing systems where animals are continuously exposed to contaminated pastures [4, 5]. The parasites that infect the ovine alimentary tract comprise a taxonomically diverse assemblage of nematodes, cestodes, trematodes, and protozoans, each with distinct life cycles, pathogenic mechanisms, and epidemiological profiles [6, 7, 8]. Effective clinical management requires a thorough understanding of these pathogens, their interactions with the host, and the environmental and management factors that modulate transmission dynamics [9, 10].

Etiology and Parasite Diversity

Nematodes (Roundworms)

The most clinically and economically significant GI parasites of sheep are the nematodes of the order Strongylida, principally those inhabiting the abomasum and small intestine [11, 12, 13]. Haemonchus contortus, the barber's pole worm, is a blood-feeding abomasal nematode that causes severe anemia and hypoproteinemia in susceptible animals [14, 15, 16]. Teladorsagia circumcincta (formerly Ostertagia circumcincta) and Trichostrongylus axei parasitize the abomasum and proximal duodenum, inducing abomasitis and mucosal hyperplasia [17, 18, 19]. Trichostrongylus colubriformis and Cooperia curticei inhabit the small intestine and are associated with enteritis and malabsorption [20, 21, 22]. Nematodirus battus is a highly pathogenic nematode of lambs, with a unique egg-hatching biology that leads to spring outbreaks in temperate regions [23, 24, 25]. Strongyloides papillosus, though less pathogenic, is prevalent in young lambs and can cause dermatitis and enteritis [26, 27, 28].

Large intestinal nematodes include Oesophagostomum columbianum and Chabertia ovina, which produce nodular lesions and colitis, respectively [29, 30, 31]. Trichuris ovis (whipworm) inhabits the cecum and colon, causing typhlocolitis in heavy burdens [32, 33, 34, 35].

Cestodes (Tapeworms)

The most common ovine cestode is Moniezia expansa (and less frequently M. benedeni), which resides in the small intestine [1, 2, 3]. These parasites utilize oribatid mites as intermediate hosts and are most prevalent in lambs and yearlings [4, 5, 6]. Avitellina species are also encountered in certain geographic regions [7, 8, 9].

Trematodes (Flukes)

Fasciola hepatica, the liver fluke, is a significant trematode pathogen of sheep, causing fasciolosis characterized by hepatic necrosis, fibrosis, cholangitis, and anemia [10, 11, 12]. Paramphistomum cervi and related species (rumen flukes) parasitize the rumen and reticulum, with immature stages causing enteritis during migration [13, 14, 15]. Dicrocoelium dendriticum, the lancet fluke, occurs less frequently but can be regionally important [16, 17, 18].

Protozoa

Coccidiosis, caused by pathogenic Eimeria species such as E. crandallis, E. ovinoidalis, E. bakuensis, E. faurei, and E. ahsata, is a major enteric disease of lambs [19, 20, 21]. Cryptosporidium parvum and Giardia duodenalis are zoonotic protozoans that cause diarrhea in young lambs and have public health implications [4, 22, 23, 24].

Epidemiology and Risk Factors

Prevalence and Geographic Distribution

The prevalence of GI parasites in sheep varies widely across geographic regions and production systems [25, 26, 27]. In a study in Sulaymaniyah province, Iraq, a high prevalence of GI parasites was documented in imported sheep [1]. In southern Ethiopia, an overall prevalence of 59.11% was reported in sheep, with strongyle-type eggs being the most common [2]. In the Minna Modern Abattoir, Nigeria, 63.16% of sheep examined harbored GI parasites, with Haemonchus spp. being the most prevalent [3]. In Kafrelsheikh governorate, Egypt, the overall prevalence was 50%, with protozoan infections (29.02%) and helminths (37.05%) both common [4]. In Gondar, Ethiopia, the prevalence was 59.11% in sheep, with strongyle spp. at 22.14% [5].

In Gwagwalada, Nigeria, prevalence was influenced by breed and management factors [6]. In Lesotho, Merino sheep showed nematode prevalence of 53.9% and coccidia prevalence of 46.5% in one district, and 65.0% and 38.2% respectively in another [8]. In Rawalpindi and Islamabad, Pakistan, prevalence was also reported to be high [9, 11]. In Upper Dir, Northern Pakistan, overall prevalence was 64% with Haemonchus (28.2%) and Eimeria crandallis (23.3%) being most common [10].

In Kajiado North, Kenya, an overall prevalence of 91.3% was recorded, with Strongylus species eggs in 80% and Eimeria oocysts in 60.8% of samples [13]. In Cuajinicuilapa, Mexico, prevalence was 89% in female sheep [14]. In the Maekel region of Eritrea, Haemonchus sp. (27.2%) and Eimeria sp. (19.79%) were most common [15]. In semi-arid Rajasthan, India, field studies confirmed significant burdens [23]. On Estonian islands, GI parasites were also prevalent [24]. In district Sialkot, Pakistan, point prevalence studies confirmed endemicity [26]. In Kaduna State, Nigeria, 45.2% of sheep were infected with nematode parasites [28]. In the central plain zone of Punjab, the epidemiology was characterized by mixed infections [17].

Host Risk Factors

Age is a major determinant of infection risk. Lambs and juvenile sheep (<1 year) typically harbor higher parasite burdens than adults, due to a combination of naive immunity and behavioral factors [5, 8, 10]. The periparturient relaxation of immunity in ewes leads to a periparturient rise in fecal egg counts, contributing to pasture contamination [35]. Sex has been variably associated with infection; some studies report higher prevalence in females [5, 10], while others find no sex effect [8]. Breed-specific resistance has been documented; for example, Red Maasai sheep show greater resistance to GI nematodes compared to other breeds [13].

Environmental and Management Risk Factors

Pasture contamination, stocking density, and grazing management are critical determinants of transmission [7, 10]. Communal grazing and mixed-species grazing increase exposure to contaminated pastures [7, 15]. Housing systems with poor drainage and hygiene also elevate risk [10, 15]. Irrigation and rainfall patterns drive the free-living stages of nematodes, with peaks in transmission occurring during warm, wet seasons [13, 35]. Anthelmintic treatment frequency and regime strongly influence infection prevalence; twice-per-year treatment and the use of multi-drug combinations reduce infection levels [4, 10].

Clinical Signs and Pathology

General Manifestations

GI parasitism in sheep produces a spectrum of clinical signs depending on parasite species, burden, host age, and nutritional status [12, 18, 24]. Common signs include weight loss or poor weight gain, diarrhea or pasty feces, submandibular edema (bottle jaw), pallor of mucous membranes, reduced appetite, and weakness [3, 6, 14]. In young lambs, acute disease with high mortality can occur, particularly with Nematodirus battus or Eimeria species [23, 35].

Anemia and Hypoproteinemia

Haemonchus contortus is the principal cause of anemia in sheep, due to the blood-feeding activity of adult worms in the abomasum [15, 16, 18]. Each worm can consume 0.05 mL of blood per day, leading to severe blood loss, pallor, weakness, and exercise intolerance [21, 27, 31]. The FAMACHA system, which scores ocular mucous membrane color, is a practical field tool for detecting anemia and guiding selective treatment [21, 27, 31].

Enteritis and Malabsorption

Trichostrongylus colubriformis and T. vitrinus cause villous atrophy, crypt hyperplasia, and enteritis in the small intestine, resulting in protein-losing enteropathy, diarrhea, and reduced nutrient absorption [20, 22, 29]. Nematodirus battus infection in lambs causes acute, profuse diarrhea, dehydration, and sometimes sudden death [23, 24, 25]. Strongyloides papillosus can cause enteritis and, in heavy infections, respiratory signs due to larval migration [26, 27, 28].

Colitis and Nodular Lesions

Oesophagostomum columbianum larvae encyst in the wall of the large intestine, forming caseous or fibrous nodules that can obstruct gut passage and cause chronic wasting [28, 29, 30]. Chabertia ovina causes hemorrhagic typhlocolitis with diarrhea, tenesmus, and mucus in the feces [31, 32, 33].

Coccidiosis

Ovine coccidiosis, primarily caused by Eimeria crandallis and E. ovinoidalis, produces watery to hemorrhagic diarrhea, tenesmus, dehydration, and weight loss in lambs, typically between 3 and 8 weeks of age [19, 20, 21]. Pathology includes destruction of intestinal epithelium, villous atrophy, and inflammatory infiltration of the lamina propria [19, 20, 21].

Fasciolosis

Acute fasciolosis, caused by Fasciola hepatica, results from massive migration of immature flukes through the liver parenchyma, causing hepatic necrosis, hemorrhage, and sudden death [10, 11, 12]. Chronic fasciolosis is characterized by anemia, hypoalbuminemia, weight loss, and hepatic fibrosis with cholangitis [10, 11, 12].

Diagnostic Approaches

Fecal Examination Techniques

Quantitative fecal examination using the McMaster technique is the cornerstone of GI parasite diagnosis, providing fecal egg counts (FEC) expressed as eggs per gram (EPG) of feces [8, 13, 14]. The McMaster method uses a counting chamber with a known volume, allowing accurate quantification of nematode eggs, cestode eggs, and coccidial oocysts [8, 13, 14]. The sensitivity of the McMaster technique is generally 50 EPG when using a 2-gram sample and standard flotation solutions [8]. For detecting low-intensity infections or specific parasites, sedimentation techniques (e.g., for trematode eggs) and flotation with different specific gravity solutions are employed [4, 15, 18].

Larval Culture and Differentiation

To identify nematodes to genus or species level, coprocultures are performed, allowing third-stage larvae (L3) to be recovered and identified by morphological features [13, 18, 22]. The Baermann technique is used to recover larvae of Strongyloides and lungworms from fresh feces [13, 18, 22]. Larval culture is essential for distinguishing between Haemonchus, Teladorsagia, Trichostrongylus, and other genera that produce similar strongyle-type eggs [13, 18, 22].

Advanced Diagnostic Technologies

Molecular diagnostics, including conventional PCR and real-time PCR, facilitate species-specific detection and quantification of GI parasites from fecal samples [15, 21, 30]. These assays can differentiate between closely related species and detect the presence of anthelmintic resistance alleles [22, 30]. Fluorescent lectin binding has been investigated as a method to distinguish nematode eggs of different species based on surface carbohydrate differences [29]. High-throughput sequencers enable the characterization of the entire parasite community (parasitome) from pooled fecal samples and can detect mixed infections with high sensitivity [21, 30, 31].

Clinical Pathology

Hematological parameters such as packed cell volume (PCV), hemoglobin concentration, and total plasma protein provide indirect evidence of parasitism, particularly for blood-feeding species like Haemonchus contortus [21, 27, 31]. Eosinophilia is a common finding in helminth infections and can be used as a supportive indicator [21, 27, 31].

Treatment and Anthelmintic Strategies

Anthelmintic Classes

The primary drug classes used for treating GI nematodes in sheep are the benzimidazoles (e.g., fenbendazole, albendazole, oxfendazole), the macrocyclic lactones (e.g., ivermectin, moxidectin, abamectin), and the imidazothiazoles/tetrahydropyrimidines (e.g., levamisole, morantel) [22, 25, 27]. Fenbendazole, administered at 5.0 mg/kg body weight, showed 100% efficacy against Ostertagia, Trichostrongylus, Cooperia, and Oesophagostomum, and 95.3% against Haemonchus in a classic study [25]. Oxyclozanide and tetramisole combinations have been used for trematode and nematode control, though reduced responsiveness has been observed [34].

Anthelmintic Resistance

Resistance to all major anthelmintic classes is now widespread and represents the most significant threat to sustainable sheep production [22, 27, 30, 31]. Benzimidazole resistance in GI nematodes of sheep has been documented on sheep farms in the Czech Republic, with prevalence ranging from 25% to 98% on affected farms [22]. Resistance is mediated by mutations in the beta-tubulin gene (e.g., F200Y, E198A, F167Y) for benzimidazoles, and by P-glycoprotein efflux and glutamate-gated chloride channel alterations for macrocyclic lactones [22, 30, 31]. The fecal egg count reduction test (FECRT) remains the gold standard for detecting anthelmintic resistance on farms [22, 27, 31].

Selective Targeted Treatment

The FAMACHA system, which uses ocular mucous membrane color to identify anemic sheep, enables selective treatment of animals with high Haemonchus burdens, reducing the number of treatments and slowing the development of resistance [21, 27, 31]. This approach is most effective in regions where H. contortus is the dominant pathogen [21, 27, 31].

Alternative and Adjunctive Therapies

Trace element supplementation (e.g., copper, cobalt, zinc) has been shown to modulate parasite burdens in grazing sheep, likely through enhancement of immune function [16]. Phytomineral preparations have been investigated as anthelmintic alternatives, with some evidence of efficacy against GI nematodes [19]. Biological control using nematophagous fungi such as Duddingtonia flagrans has been evaluated; while the fungus can survive passage through the GI tract and reduce larval recovery in feces, its efficacy under field conditions has been inconsistent [20].

Control and Integrated Parasite Management

Pasture Management

Strategic grazing management is central to controlling the free-living stages of GI nematodes [4, 7, 10, 13]. Rotational grazing, cross-grazing with cattle or horses (which do not share the same nematode species), and prolonged rest periods reduce larval contamination [13, 33]. The timing of grazing to avoid periods of peak larval availability on pasture is critical [13, 23, 35].

The "Worms Sheep Get" Focusing on Nematodes

The most clinically important worms sheep get are the gastrointestinal nematodes discussed throughout this review, principally Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus colubriformis, Nematodirus battus, and Cooperia curticei [7, 11, 13]. These species have direct life cycles, with eggs passed in feces developing through four larval stages (L1 to L3) on pasture, and the infective L3 being ingested by grazing sheep [13, 18, 23]. Understanding the environmental requirements for egg hatching and larval survival (temperature, moisture) is essential for predicting transmission risk [13, 23, 35].

Integrated Control Framework

An integrated parasite management (IPM) program combines strategic anthelmintic use, grazing management, genetic selection for resistance, and nutritional support [27, 30, 31]. The following table summarizes key components:

Biological and Genetic Control

Biological control utilizing nematophagous fungi such as Duddingtonia flagrans has shown potential for reducing larval contamination on pasture, though practical application remains challenging [20]. Genetic selection for resistance to GI parasites is a sustainable long-term control strategy [21, 27, 30, 31]. Heritability estimates for fecal egg count (FEC) range from 0.00 to 0.46, and genomic selection using single-step GWAS has identified candidate genes associated with immune function that influence resistance [21, 27, 30]. The identification of quantitative trait loci (QTL) on multiple ovine chromosomes supports the feasibility of marker-assisted selection for parasite resistance [21, 27, 30, 31].

Monitoring and Surveillance

Regular FEC monitoring (every 4-8 weeks during the grazing season) allows early detection of rising parasite burdens and guides treatment decisions [8, 13, 22]. Pooled fecal sampling from sentinel lambs provides a cost-effective herd-level surveillance tool [13, 22]. Anthelmintic efficacy should be evaluated annually via FECRT to detect emerging resistance [22, 27, 31].

flowchart TD
    A["Fecal sample collection (fresh, pooled or individual)"], > B["McMaster egg count (EPG)"]
    B, > C{"EPG > threshold?"}
    C, >|"Yes: >500 EPG strongyles"| D["FECRT for resistance detection"]
    C, >|"Yes: >2000 EPG coccidia"| E["Treat with coccidiostat"]
    C, >|"No: low egg count"| F["Monitor and retest in 4 weeks"]
    D, > G{"FECR >95%?"}
    G, >|"Yes: susceptible"| H["Treat with target drug class"]
    G, >|"No: resistant"| I["Switch to alternative drug class"]
    H, > J["Move to clean pasture post-treatment"]
    I, > J
    J, > K["Maintain untreated refugia (10-20%)"]
    K, > L["Genetic selection for resistance (low EBV for FEC)"]
    L, > M["Integrated management: rotation, nutrition, hygiene"]
    M, > A

Conclusion

Gastrointestinal parasitism remains a primary constraint to sheep health and productivity across all production systems [1, 35]. The diversity of parasite species, their complex life cycles, and the widespread development of anthelmintic resistance demand an integrated, evidence-based approach to clinical management [22, 27, 31]. Effective control relies on accurate diagnosis, strategic and selective treatment, pasture management, genetic improvement for resistance, and ongoing surveillance [4, 7, 13, 21]. As anthelmintic resistance continues to erode the efficacy of available drugs, sustainable parasite management will increasingly depend on the integration of these complementary strategies [27, 30, 31]. The longitudinal dynamics of co-infecting parasites in sheep populations underscore the need for year-round monitoring and tailored interventions [35].

References

[1] Mohammed AA. Prevalence of haemoprotozoan and gastrointestinal parasites of sheep imported from Syria into Sulaymaniyah province of Iraq. Annals of Parasitology. 2021. URL: https://www.semanticscholar.org/paper/4d57a94f7c4f1365531d56fbacab4a57c257d1e6

[2] Sebro E, Kebamo M, Abebe A. Prevalence of Gastrointestinal Parasites of Sheep and Goats in An-Lemo, Hadiya Zone Southern Ethiopia. Indian Journal of Science and Technology. 2022. URL: https://www.semanticscholar.org/paper/071bc431fdd3ee20880bf1cfccbe02ea3eebddb2

[3] Otuu CA, Hassan S, Urama AC, et al. Prevalence of gastrointestinal parasites of sheep and goats slaughtered in Minna Modern Abattoir, Niger State, Nigeria. Journal of Animal Science and Veterinary Medicine. 2019. URL: https://www.semanticscholar.org/paper/54214600783cb35acec8b9d6e12aa8e5ecf15351

[4] Sultan K, Elmonir W, Hegazy Y. Gastrointestinal parasites of sheep in Kafrelsheikh governorate, Egypt: Prevalence, control and public health implications. 2016. URL: https://www.semanticscholar.org/paper/9c4bd554334e553244fc0df45123c44e5da14554

[5] Fayisa O, Duguma A, Temesgen M, et al. Gastrointestinal parasites of sheep and goat in and around Gondar town, Northwest, Ethiopia. Biotehnologija u stocarstvu. 2020. URL: https://www.semanticscholar.org/paper/429ea0d5123aceaa2f62d74069ab768040ca1676

[6] Jegede O, Adejoh AA, Obeta S, et al. Gastrointestinal Parasites of Sheep and Goats in Gwagwalada Area Council, Federal Capital Territory, Abuja, Nigeria; with a special reference to sex, breed and age. 2015. URL: https://www.semanticscholar.org/paper/5e0c938871ec25557967e85064ca8b424cd416c6

[7] Mahlehla M, Molapo S, Phoofolo M, et al. Awareness and control methods of gastrointestinal parasites of merino sheep among farmers from different agro-ecological zones of Lesotho. Veterinary World. 2021. URL: https://www.semanticscholar.org/paper/ae1c1e1e0e1bd99764ed27498f064c8331259037

[8] Mahlehla M, Molapo S, Phoofolo WM, et al. Prevalence and Faecal Egg Counts of Gastrointestinal Parasites of Merino Sheep in Lesotho. World's Veterinary Journal. 2021. URL: https://www.semanticscholar.org/paper/06b7b96f1be1236e2b4f956d6a9d0ccd4e7f6286

[9] Asif M, Azeem S, Asif S, et al. Prevalence of Gastrointestinal Parasites of Sheep and Goats in and around Rawalpindi and Islamabad, Pakistan. 2008. URL: https://www.semanticscholar.org/paper/754a7da3dfaa417a70f568c56614917d7791893d

[10] Fadladdin Y, Shah MZ, Khan K, et al. Diversity, Prevalence, and Risk Factors of Gastrointestinal Parasites in Sheep Herds Grazing in Upper Dir district, Northern Pakistan. Egyptian Journal of Veterinary Sciences. 2025. URL: https://www.semanticscholar.org/paper/d3b33938ca5c60f39888ba5136091ff61fce20ec

[11] Gadahi J, Arshed MJ, Ali Q, et al. Prevalence of Gastrointestinal Parasites of Sheep and Goat in and around Rawalpindi and Islamabad, Pakistan. 2009. URL: https://www.semanticscholar.org/paper/8bec36bca304843574a9c939079109f2a03a62e9

[12] Almalaik AHA, Bashar A, Abakar A. Prevalence and Dynamics of Some Gastrointestinal Parasites of Sheep and Goats in Tulus Area Based on Post-Mortem Examination. 2008. URL: https://www.semanticscholar.org/paper/7832a54d9c82183b4ef88d4c7e34350e337a6561

[13] Mokhothu MJ, Ngugi RK, Mwenji BM, et al. Prevalence and Risk Factors of Gastrointestinal Parasites in Sheep in Kajiado North Sub-County, Kenya. International Journal of Research and Innovation in Applied Science. 2025. URL: https://www.semanticscholar.org/paper/8c3f1da729b7558bf2a4cd2dab013e81cb181a71

[14] Moran-Montesinos A. Prevalence of Gastrointestinal Parasites in Backyard Sheep in the Municipal Seat of Cuajinicuilapa, Guerrero, Mexico. Journal of Veterinary Research and Clinical Care. 2025. URL: https://www.semanticscholar.org/paper/ed054d03fb93a514cd7615d0b55edaef6806b562

[15] Araya H, Maloba F, Joshua M, et al. Prevalence of Gastrointestinal Parasites in Sheep, Goats and Zoonotic Helminths in Maekel Region, Eritrea. Asian Journal of Advances in Agricultural Research. 2025. URL