Internal Parasites of Sheep: Worms, Diagnosis, and Management
By Dr. Zubair Khalid, DVM, MS, PhD ·
Key Takeaways
- Internal parasites, primarily nematodes like Haemonchus contortus and Teladorsagia circumcincta, significantly impact ovine productivity through subclinical losses, clinical disease, and mortality, particularly in lambs and periparturient ewes.
- Diagnosis relies on a multi-faceted approach including fecal egg counts (FEC) using methods like McMaster or Mini-FLOTAC, Baermann sedimentation for lungworms, and sedimentation for liver fluke eggs, complemented by hematological assessment (e.g., FAMACHA system for anemia) and serological tests (ELISA for Fasciola hepatica, Toxoplasma gondii).
- Anthelmintic resistance is a critical global threat, necessitating strategic drug use and monitoring via fecal egg count reduction tests (FECRT); available drug classes include benzimidazoles, macrocyclic lactones, imidazothiazoles, salicylanilides, amino-acetonitrile derivatives, and spiroindoles.
- Integrated Parasite Management (IPM) is paramount, combining targeted selective treatment (TST) based on FEC or FAMACHA scores, strategic grazing management (pasture rotation, mixed-species grazing), genetic selection for parasite resistance, and robust biosecurity measures like quarantine drenching.
- Clinical signs vary by parasite, with H. contortus causing anemia and hypoproteinemia, T. circumcincta leading to abomasal inflammation and diarrhea, and Fasciola hepatica inducing acute or chronic fasciolosis with liver damage and anemia.
Introduction
Internal parasites represent a major constraint to ovine productivity and welfare globally [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. The term "worms sheep get" encompasses a diverse assemblage of helminths and protozoa that inhabit the gastrointestinal tract, respiratory system, liver, and other tissues [<a href="#ref-1">1</a>, <a href="#ref-3">3</a>]. These infections cause subclinical production losses, clinical disease, and mortality, particularly in lambs and periparturient ewes [<a href="#ref-4">4</a>]. The economic impact arises from reduced weight gain, decreased wool production, impaired fertility, and costs associated with treatment and control [<a href="#ref-4">4</a>, <a href="#ref-5">5</a>]. Understanding the biology, epidemiology, and pathophysiology of these parasites is essential for designing effective diagnostic and management programs [<a href="#ref-6">6</a>, <a href="#ref-7">7</a>].
Etiology and Classification
The internal parasites of sheep are taxonomically diverse and include nematodes (roundworms), cestodes (tapeworms), trematodes (flukes), and protozoa [<a href="#ref-1">1</a>, <a href="#ref-8">8</a>].
Nematodes
Nematodes are the most prevalent and economically significant internal parasites of sheep [<a href="#ref-1">1</a>, <a href="#ref-9">9</a>]. The major genera include:
- Haemonchus contortus: The barber's pole worm, a blood-feeding abomasal nematode [<a href="#ref-1">1</a>, <a href="#ref-10">10</a>].
- Teladorsagia circumcincta: The brown stomach worm, an abomasal parasite causing type I and type II ostertagiosis [<a href="#ref-1">1</a>].
- Trichostrongylus spp.: Including T. colubriformis and T. axei, small intestinal and abomasal parasites respectively [<a href="#ref-1">1</a>, <a href="#ref-11">11</a>].
- Nematodirus spp.: Including N. battus and N. filicollis, intestinal parasites with a unique egg development requirement [<a href="#ref-1">1</a>].
- Cooperia curticei: A small intestinal nematode [<a href="#ref-1">1</a>].
- Oesophagostomum columbianum: The nodule worm, a large intestinal parasite [<a href="#ref-1">1</a>, <a href="#ref-9">9</a>].
- Chabertia ovina: A large intestinal nematode [<a href="#ref-1">1</a>].
- Dictyocaulus filaria: The large lungworm, a bronchial parasite [<a href="#ref-1">1</a>].
- Protostrongylus rufescens and Muellerius capillaris: Small lungworms residing in lung parenchyma and bronchioles [<a href="#ref-1">1</a>].
Cestodes
Cestodes of sheep include:
- Moniezia expansa and Moniezia benedeni: Intestinal tapeworms requiring oribatid mites as intermediate hosts [<a href="#ref-1">1</a>].
- Taenia multiceps (larval stage: Coenurus cerebralis): Causes gid or sturdy, a neurological disease [<a href="#ref-1">1</a>].
- Echinococcus granulosus (larval stage: hydatid cyst): A zoonotic cestode forming cysts in viscera [<a href="#ref-1">1</a>].
Trematodes
Trematodes include:
- Fasciola hepatica: The liver fluke, causing fasciolosis [<a href="#ref-1">1</a>, <a href="#ref-12">12</a>].
- Dicrocoelium dendriticum: The lancet fluke, a bile duct parasite [<a href="#ref-1">1</a>].
- Paramphistomum spp.: Rumen flukes [<a href="#ref-1">1</a>].
Protozoa
Protozoan parasites include:
- Eimeria spp.: Coccidia causing coccidiosis, primarily in lambs [<a href="#ref-1">1</a>].
- Cryptosporidium parvum: A zoonotic protozoan causing enteritis in lambs [<a href="#ref-1">1</a>].
- Toxoplasma gondii: A protozoan causing abortion in ewes [<a href="#ref-1">1</a>].
Epidemiology
The prevalence and intensity of internal parasites are influenced by climatic conditions, management practices, host immunity, and parasite biology [<a href="#ref-2">2</a>, <a href="#ref-13">13</a>]. Seasonal patterns are well documented, with peak transmission typically occurring during warm, moist periods [<a href="#ref-13">13</a>, <a href="#ref-14">14</a>]. In temperate regions, larvae survive on pasture over winter and contaminate spring grazing [<a href="#ref-13">13</a>, <a href="#ref-15">15</a>]. In tropical and subtropical regions, transmission may occur year-round with peaks during rainy seasons [<a href="#ref-2">2</a>, <a href="#ref-16">16</a>].
Grazing management strategies significantly affect parasite exposure [<a href="#ref-15">15</a>]. Mixed-species grazing with cattle can reduce pasture contamination for sheep-specific parasites [<a href="#ref-15">15</a>]. Stocking density, pasture rotation, and the use of clean or rested pastures are critical determinants of infection risk [<a href="#ref-7">7</a>, <a href="#ref-15">15</a>].
Host factors include age, nutritional status, and immune competence [<a href="#ref-4">4</a>, <a href="#ref-17">17</a>]. Lambs are highly susceptible due to naive immune systems [<a href="#ref-4">4</a>]. Periparturient ewes experience a relaxation of immunity, leading to increased egg shedding, a phenomenon known as the periparturient rise [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. Trace element deficiencies, particularly cobalt and copper, have been associated with increased susceptibility to parasitism [<a href="#ref-17">17</a>, <a href="#ref-18">18</a>, <a href="#ref-19">19</a>].
Clinical Signs and Pathology
Clinical manifestations depend on the parasite species, burden, and host factors [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>].
Gastrointestinal Nematodes
Haemonchus contortus is a blood-feeding parasite causing anemia, hypoproteinemia, submandibular edema (bottle jaw), and death in severe cases [<a href="#ref-1">1</a>, <a href="#ref-10">10</a>]. The pathogenesis is directly related to blood loss, with each worm consuming approximately 0.05 mL of blood per day [<a href="#ref-1">1</a>]. Teladorsagia circumcincta causes abomasal inflammation, reduced feed intake, and protein-losing enteropathy, leading to weight loss and diarrhea [<a href="#ref-1">1</a>]. Trichostrongylus spp. cause enteritis, diarrhea, and reduced growth rates [<a href="#ref-1">1</a>, <a href="#ref-11">11</a>]. Nematodirus battus is particularly pathogenic in lambs, causing severe diarrhea and dehydration [<a href="#ref-1">1</a>].
Lungworms
Dictyocaulus filaria causes parasitic bronchitis, characterized by coughing, dyspnea, and reduced exercise tolerance [<a href="#ref-1">1</a>]. Muellerius capillaris and Protostrongylus rufescens are less pathogenic but can cause chronic cough and pneumonia in heavy infections [<a href="#ref-1">1</a>].
Liver Fluke
Fasciola hepatica causes acute or chronic fasciolosis [<a href="#ref-1">1</a>, <a href="#ref-12">12</a>]. Acute disease results from massive migration of immature flukes through the liver parenchyma, causing hemorrhage, hepatitis, and sudden death [<a href="#ref-1">1</a>, <a href="#ref-12">12</a>]. Chronic disease is characterized by bile duct hyperplasia, fibrosis, anemia, and hypoalbuminemia [<a href="#ref-1">1</a>, <a href="#ref-12">12</a>].
Coccidia
Eimeria spp. cause coccidiosis, primarily in lambs 3 to 8 weeks of age [<a href="#ref-1">1</a>]. Clinical signs include diarrhea (often with blood or mucus), tenesmus, dehydration, and weight loss [<a href="#ref-1">1</a>].
Diagnosis
Accurate diagnosis is fundamental to effective management [<a href="#ref-20">20</a>, <a href="#ref-21">21</a>]. Diagnostic methods include clinical examination, fecal analysis, hematology, serology, molecular techniques, and postmortem examination [<a href="#ref-20">20</a>, <a href="#ref-21">21</a>, <a href="#ref-22">22</a>].
Fecal Examination
Fecal egg counts (FEC) are the cornerstone of nematode diagnosis [<a href="#ref-21">21</a>]. The McMaster technique is a quantitative flotation method that estimates eggs per gram (EPG) of feces [<a href="#ref-21">21</a>]. The Mini-FLOTAC method offers improved sensitivity and is recommended for low-intensity infections [<a href="#ref-21">21</a>]. Centrifugal flotation techniques provide higher sensitivity than simple flotation [<a href="#ref-21">21</a>].
For lungworm diagnosis, the Baermann sedimentation technique is used to recover first-stage larvae [<a href="#ref-21">21</a>]. For Fasciola hepatica, sedimentation techniques are required to detect eggs, which are large and operculated [<a href="#ref-21">21</a>].
Hematology
Packed cell volume (PCV) and hemoglobin concentration are useful indicators of anemia caused by H. contortus [<a href="#ref-1">1</a>]. The FAMACHA system, a clinical scoring method based on conjunctival color, is a practical tool for identifying anemic sheep in the field [<a href="#ref-1">1</a>].
Serology
Commercial ELISA kits are available for detecting antibodies to Fasciola hepatica and Toxoplasma gondii [<a href="#ref-1">1</a>]. Coproantigen ELISA tests detect fluke antigens in feces and are useful for diagnosing active fasciolosis [<a href="#ref-1">1</a>].
Molecular Diagnostics
PCR-based assays, including conventional PCR, real-time PCR, and high-throughput sequencing, enable species-specific identification of parasites [<a href="#ref-1">1</a>]. These methods are particularly valuable for detecting mixed infections and for confirming anthelmintic resistance [<a href="#ref-1">1</a>].
Postmortem Examination
Necropsy with worm counts from the gastrointestinal tract and other organs provides definitive diagnosis and quantification of parasite burdens [<a href="#ref-1">1</a>, <a href="#ref-23">23</a>].
Treatment
Anthelmintic therapy is the primary means of controlling parasitic infections [<a href="#ref-10">10</a>, <a href="#ref-24">24</a>, <a href="#ref-25">25</a>]. Drug classes include:
- Benzimidazoles (e.g., albendazole, fenbendazole): Inhibit microtubule polymerization [<a href="#ref-1">1</a>, <a href="#ref-26">26</a>].
- Macrocyclic lactones (e.g., ivermectin): Potentiate glutamate-gated chloride channels [<a href="#ref-1">1</a>, <a href="#ref-10">10</a>].
- Imidazothiazoles (e.g., levamisole): Nicotinic acetylcholine receptor agonists [<a href="#ref-1">1</a>, <a href="#ref-25">25</a>].
- Salicylanilides (e.g., closantel): Uncouple oxidative phosphorylation, effective against blood-feeding parasites [<a href="#ref-1">1</a>].
- Amino-acetonitrile derivatives (e.g., monepantel): Novel mode of action targeting acetylcholine receptors [<a href="#ref-1">1</a>].
- Spiroindoles (e.g., derquantel): Nicotinic antagonist [<a href="#ref-1">1</a>].
Anthelmintic resistance is a global threat to sheep production [<a href="#ref-1">1</a>, <a href="#ref-7">7</a>]. Resistance has been reported to all major drug classes [<a href="#ref-1">1</a>, <a href="#ref-7">7</a>]. The fecal egg count reduction test (FECRT) is the standard method for detecting resistance [<a href="#ref-21">21</a>]. Guidelines for conducting FECRT have been updated to improve accuracy [<a href="#ref-21">21</a>].
Control and Management
Integrated parasite management (IPM) combines strategic anthelmintic use, grazing management, genetic selection, and monitoring [<a href="#ref-6">6</a>, <a href="#ref-7">7</a>, <a href="#ref-14">14</a>].
Grazing Management
Pasture rotation, mixed-species grazing, and the use of clean or rested pastures reduce larval contamination [<a href="#ref-7">7</a>, <a href="#ref-15">15</a>]. Co-grazing with cattle can lower sheep parasite burdens because most sheep nematodes are host-specific [<a href="#ref-15">15</a>].
Targeted Selective Treatment
The FAMACHA system allows for targeted treatment of only anemic sheep, reducing selection pressure for resistance [<a href="#ref-1">1</a>]. Targeted selective treatment (TST) strategies treat only animals with high FEC or clinical signs [<a href="#ref-1">1</a>].
Genetic Selection
Breeding for parasite resistance is a long-term strategy [<a href="#ref-1">1</a>]. Some sheep breeds, such as the Red Maasai and certain wool breeds, exhibit genetic resistance to H. contortus [<a href="#ref-1">1</a>].
Nutritional Management
Adequate protein and trace element nutrition supports immune function and resilience to parasitism [<a href="#ref-17">17</a>, <a href="#ref-18">18</a>, <a href="#ref-19">19</a>].
Biosecurity
Quarantine drenching of introduced animals with a combination of anthelmintics from different classes is recommended to prevent introduction of resistant parasites [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>].
Diagnostic and Management Decision Flowchart
flowchart TD
A["Clinical Signs: Anemia, Diarrhea, Weight Loss, Cough"] --> B["Fecal Sample Collection"]
B --> C{"Quantitative FEC"}
C -->|"High EPG"| D["Identify Genus via Larval Culture or PCR"]
C -->|"Low EPG"| E["Consider Baermann for Lungworms or Sedimentation for Flukes"]
D --> F{"Anthelmintic Resistance Suspect?"}
F -->|"Yes"| G["Conduct FECRT"]
F -->|"No"| H["Select Anthelmintic Based on Genus"]
G --> I["Change Drug Class or Use Combination Therapy"]
H --> J["Administer Treatment"]
J --> K["Post-Treatment FEC at 10-14 Days"]
K --> L{"Reduction < 95%?"}
L -->|"Yes"| M["Confirm Resistance, Adjust Strategy"]
L -->|"No"| N["Monitor and Implement IPM"]
E --> O["Specific Diagnosis: Lungworm or Fluke"]
O --> P["Targeted Treatment"]
P --> N
M --> N
N --> Q["Grazing Management, FAMACHA, Genetic Selection"]
Q --> R["Re-evaluate at Next Season"]Conclusion
Internal parasites of sheep, particularly gastrointestinal nematodes, remain a significant challenge to global sheep production [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>, <a href="#ref-4">4</a>]. Effective management requires a comprehensive understanding of parasite biology, epidemiology, and the mechanisms of anthelmintic resistance [<a href="#ref-1">1</a>, <a href="#ref-7">7</a>]. Diagnostic tools, including FEC, FAMACHA, and molecular assays, are essential for informed decision-making [<a href="#ref-20">20</a>, <a href="#ref-21">21</a>]. Integrated parasite management, combining strategic anthelmintic use with grazing management, genetic selection, and nutritional support, is critical for sustainable control [<a href="#ref-6">6</a>, <a href="#ref-7">7</a>, <a href="#ref-14">14</a>]. Continued research into parasite biology, resistance mechanisms, and novel control strategies is necessary to mitigate the impact of these pathogens [<a href="#ref-1">1</a>, <a href="#ref-7">7</a>].