# [Gastrointestinal Nematodes in Sheep](/knowledge/parasites/livestock-parasites/gastrointestinal-nematodes-in-sheep-clinical-guide): Epidemiology, Clinical Signs, and Management

## Key Takeaways

- Gastrointestinal nematodes (GINs) are a significant economic constraint in sheep production, causing reduced growth, decreased yield, and increased mortality through direct life cycles involving ingestion of infective L3 larvae from pasture.
- Key pathogenic species include *Haemonchus contortus* (causing anemia and bottle jaw), *Teladorsagia circumcincta* (associated with mucosal inflammation and hypobiosis syndromes), and *Trichostrongylus colubriformis* (leading to diarrhea and hypoproteinemia).
- Epidemiological drivers include climate (optimal 15-25°C with moisture), grazing management, and host factors like the periparturient rise (PPR) in ewes, which increases pasture contamination for lambs.
- Diagnosis relies on fecal egg counts (FEC) using the McMaster technique, larval culture for species identification, and the FAMACHA scoring system for estimating anemia due to *Haemonchus contortus*.
- Anthelmintic resistance to Benzimidazoles (BZ), Levamisole (LM), and Macrocyclic Lactones (ML) is widespread, necessitating integrated control strategies such as pasture rotation, selective treatment based on FAMACHA/FEC, and quarantine drenching of new stock.
- Integrated control emphasizes pasture management (e.g., 6-8 week rest periods), selective treatment to maintain refugia, and genetic selection for parasite resistance, alongside judicious use of available anthelmintic classes.

---

Gastrointestinal nematodes (GINs) represent a major constraint to sheep production globally, causing substantial economic losses through reduced weight gain, decreased wool and milk yield, increased mortality, and costs associated with treatment and control [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. These parasitic roundworms inhabit the abomasum and intestines, and their life cycles are primarily direct, with infection occurring via ingestion of third-stage larvae (L3) from contaminated pasture [<a href="#ref-1">1</a>]. Understanding the epidemiology, clinical presentation, and evidence-based management of these parasites is essential for maintaining flock health and productivity while mitigating the growing problem of anthelmintic resistance [<a href="#ref-2">2</a>, <a href="#ref-3">3</a>].

## Types of [Worms Sheep Get](/knowledge/bacteria/livestock-parasites/worms-sheep-gastrointestinal-nematode-control): Major Gastrointestinal Nematodes

The most clinically and economically important GINs of sheep belong to the order Strongylida, with a few species from other orders also relevant. The following table summarizes the primary species, their predilection sites, and key pathogenic features.

| Species | Predilection Site | Pathogenicity | Key Clinical Features |
|-----|----------|--------|------------|
| *[Haemonchus contortus](/knowledge/parasites/livestock-parasites/haemonchus-contortus)* | Abomasum (barber’s pole worm) | Highly pathogenic (blood-feeding) | Anemia, submandibular edema (bottle jaw), sudden death [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>] |
| *[Teladorsagia circumcincta](/knowledge/parasites/livestock-parasites/teladorsagia-circumcincta-sheep-abomasal-worm-anthelmintic-resistance)* | Abomasum | Moderate (mucosal inflammation) | Type I (summer) and Type II (winter) hypobiosis syndromes, diarrhea, reduced growth [<a href="#ref-1">1</a>, <a href="#ref-5">5</a>] |
| *[Trichostrongylus colubriformis](/knowledge/parasites/livestock-parasites/trichostrongylus-colubriformis-bankrupt-worm-sheep-cattle)* | Small intestine | Moderate (villous atrophy) | Diarrhea (brown scour), weight loss, hypoproteinemia [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>] |
| *[Trichostrongylus axei](/knowledge/parasites/livestock-parasites/trichostrongylus-axei-abomasal-hairworm)* | Abomasum/small intestine | Moderate | Anorexia, diarrhea, gastritis [<a href="#ref-1">1</a>, <a href="#ref-7">7</a>] |
| *Nematodirus battus* | Small intestine | High in lambs | Spring outbreak in lambs, profuse watery diarrhea, dehydration [<a href="#ref-1">1</a>, <a href="#ref-8">8</a>] |
| *Cooperia curticei* | Small intestine | Low to moderate | Mild diarrhea, reduced growth [<a href="#ref-1">1</a>, <a href="#ref-9">9</a>] |
| *Oesophagostomum columbianum* | Large intestine (nodular worm) | Moderate | Nodular lesions, chronic diarrhea, ill-thrift [<a href="#ref-1">1</a>] |
| *Chabertia ovina* | Large intestine | Moderate | Copious diarrhea, weight loss [<a href="#ref-1">1</a>] |

The phrase "[worms sheep get](/knowledge/parasites/livestock-parasites/common-internal-parasites-sheep-worms-management)" encompasses these and other less common species, though the above list represents the core complex targeted in most anthelmintic programs [<a href="#ref-1">1</a>]. Mixed infections are typical, and the combined burden drives clinical disease [<a href="#ref-2">2</a>].

## Epidemiology

GIN epidemiology is driven by the interaction between climate, grazing management, host immunity, and parasite biology. Most strongylid nematodes share a common direct life cycle: eggs are shed in feces, develop through first (L1) and second (L2) larval stages on pasture, molt to infective L3, migrate onto herbage, and are ingested by grazing sheep [<a href="#ref-1">1</a>]. Development and survival of free-living stages depend on temperature and moisture; optimal conditions are generally 15-25°C with adequate rainfall [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. Conversely, drought or extreme cold reduces larval survival, although some larvae can overwinter, especially *Nematodirus battus* eggs which require a period of cold followed by thawing to hatch [<a href="#ref-1">1</a>, <a href="#ref-8">8</a>].

The periparturient rise (PPR) is a well-described phenomenon in ewes, where a transient relaxation in immunity around lambing leads to increased fecal egg counts (FEC) and pasture contamination [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. This peak often drives infection in lambs born onto contaminated pasture. In temperate regions, a typical pattern involves a spring rise in adult ewes, followed by high infection levels in lambs during summer and autumn [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. *[Teladorsagia circumcincta](/knowledge/parasites/livestock-parasites/teladorsagia-circumcincta-sheep-abomasal-worm-anthelmintic-resistance)* exhibits larval hypobiosis (arrested development) during winter, with resumption of development in spring, leading to Type II ostertagiosis-like disease [<a href="#ref-1">1</a>, <a href="#ref-5">5</a>].

Climate change is influencing epidemiology, altering seasonal transmission windows and expanding geographic ranges, particularly for *Haemonchus contortus*, which traditionally favored warmer regions [<a href="#ref-2">2</a>]. The interaction between pasture management (e.g., stocking density, rotation length) and parasite accumulation is a critical driver of infection risk [<a href="#ref-3">3</a>].

## Clinical Signs

Clinical signs vary with nematode species, burden, host age and immune status. They range from subclinical production loss to acute disease and death.

### Anemia and Hypoproteinemia
*Haemonchus contortus* is a blood-feeding abomasal parasite; adult worms ingest up to 0.05 mL blood per worm per day [<a href="#ref-1">1</a>]. Heavy burdens (thousands of worms) cause acute blood loss, leading to severe anemia, pale mucous membranes, weakness, and submandibular edema (bottle jaw) due to hypoproteinemia [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. Peracute disease can cause sudden death in apparently healthy animals [<a href="#ref-1">1</a>].

### Diarrhea and Weight Loss
Intestinal species such as *[Trichostrongylus colubriformis](/knowledge/parasites/livestock-parasites/trichostrongylus-colubriformis-bankrupt-worm-sheep-cattle)* and *T. axei* induce villous atrophy and mucosal inflammation, resulting in malabsorption and profuse watery diarrhea (often described as brown or black scour) [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>]. *Nematodirus battus* produces a similar syndrome in naive lambs, often presenting as a synchronous outbreak [<a href="#ref-1">1</a>, <a href="#ref-8">8</a>]. *Oesophagostomum columbianum* causes nodular lesions in the large intestine that may lead to chronic diarrhea and wasting [<a href="#ref-1">1</a>]. Chronic subclinical infections reduce feed conversion efficiency and weight gain [<a href="#ref-2">2</a>].

### Reduced Performance and Production
Subclinical GIN burdens depress appetite, reduce nutrient absorption, and increase protein turnover, leading to lower growth rates, reduced fleece weight, and decreased milk production in ewes [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. In growing lambs, this translates into extended time to market weight [<a href="#ref-1">1</a>].

### Other Signs
Hypocalcemia may accompany severe *Haemonchus* infections due to anorexia and electrolyte imbalances [<a href="#ref-1">1</a>]. *Teladorsagia* infection can cause abomasal pH elevation, impaired protein digestion, and hormonal changes (increased gastrin) [<a href="#ref-1">1</a>, <a href="#ref-5">5</a>]. Nervous signs are not typical of GINs but may occur secondary to severe metabolic disturbances [<a href="#ref-1">1</a>].

## Pathology

Pathological changes correlate with nematode location and feeding habits. In the abomasum, *Haemonchus contortus* causes petechial hemorrhages at the site of attachment and an edematous, thickened mucosa [<a href="#ref-1">1</a>]. *[Teladorsagia circumcincta](/knowledge/parasites/livestock-parasites/teladorsagia-circumcincta-sheep-abomasal-worm-anthelmintic-resistance)* infection results in mucosal hyperplasia, increased abomasal pH, and loss of parietal cells, often described as "Moroccan leather" appearance [<a href="#ref-1">1</a>, <a href="#ref-5">5</a>]. In the small intestine, *Trichostrongylus* species cause villous atrophy, crypt hyperplasia, and reduced brush-border enzyme activity [<a href="#ref-1">1</a>, <a href="#ref-6">6</a>]. *Nematodirus battus* induces severe inflammation and villous fusion in the proximal small intestine [<a href="#ref-1">1</a>, <a href="#ref-8">8</a>]. Large intestinal species cause nodular lymphoid hyperplasia (*Oesophagostomum*) or mucosal erosion (*Chabertia*) [<a href="#ref-1">1</a>]. Histopathology confirms the extent of tissue damage and helps differentiate from other causes of enteritis [<a href="#ref-1">1</a>].

## Diagnostics

Accurate diagnosis is essential for targeted treatment and monitoring resistance. The primary methods include:

### Fecal Egg Count (FEC)
The modified McMaster technique is the standard quantitative method, with a detection limit around 50 eggs per gram (epg) [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. For sheep, pooled samples (5-10 individuals) are often used for monitoring, but individual samples are needed for resistance testing [<a href="#ref-2">2</a>]. Strongyle eggs are morphologically indistinguishable; a generic "strongyle" count is reported. *Nematodirus* eggs are larger (150-230 μm), barrel-shaped, and easily differentiated [<a href="#ref-1">1</a>]. Counts above 500-1000 epg in lambs are considered clinically significant for mixed infections, though thresholds vary with parasite species [<a href="#ref-2">2</a>].

### Larval Culture
Differentiation to genus or species requires fecal culture to allow eggs to hatch to L3, followed by identification using morphological keys [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. This is labor-intensive but critical for epidemiological studies and resistance testing [<a href="#ref-2">2</a>].

### FAMACHA Scoring
FAMACHA is a clinical system using conjunctival color to estimate anemia in sheep, validated for *Haemonchus contortus* infection [<a href="#ref-1">1</a>, <a href="#ref-4">4</a>]. Scoring from 1 (red) to 5 (pale) guides selective treatment decisions, reducing selection pressure for anthelmintic resistance [<a href="#ref-4">4</a>, <a href="#ref-10">10</a>].

### Other Diagnostic Tools
- [Fecal egg count reduction test](/knowledge/diagnostics/parasitology/fecal-egg-count-reduction-test-for-anthelmintic-resistance) (FECRT): Used to assess anthelmintic efficacy [<a href="#ref-2">2</a>, <a href="#ref-3">3</a>].
- Hematology: Packed cell volume (PCV) drops in *Haemonchus* infection; anemia is microcytic hypochromic [<a href="#ref-1">1</a>].
- Serum pepsinogen: Elevated in abomasal damage (e.g., *Teladorsagia*) [<a href="#ref-1">1</a>].
- Postmortem worm counts: Definitive for burden and species composition [<a href="#ref-1">1</a>].
- Molecular methods (PCR, high-throughput sequencing) are increasingly applied for species identification and resistance allele detection (e.g., beta-tubulin mutations in benzimidazole resistance) [<a href="#ref-2">2</a>, <a href="#ref-3">3</a>].

The following Mermaid diagram illustrates a diagnostic decision pathway for managing GIN in sheep.

```mermaid
flowchart TD
 A["Monitor flock: FEC and FAMACHA"] --> B{"Individual FEC > threshold?"}
 B -->|"Yes"| C["Treat selected animals?"]
 B -->|"No"| D["Continue monitoring"]
 C --> E{"Anemia present? FAMACHA 3-5"}
 E -->|"Yes"| F["Treat with effective anthelmintic based on resistance history"]
 E -->|"No"| G["Consider strategic treatment if necessary"]
 F --> H["Perform FECRT post treatment"]
 H --> I{"Reduction <95%?"}
 I -->|"Yes"| J["Investigate resistance; change class"]
 I -->|"No"| K["Effective treatment"]
 J --> L["Adjust control strategy"]
 D --> A
 G --> A
 K --> A
 L --> A
```

## Treatment and Anthelmintic Resistance

### Anthelmintic Classes
Current anthelmintics available for sheep include:
- Benzimidazoles (BZ): Albendazole, fenbendazole [<a href="#ref-1">1</a>].
- Levamisole (LM, imidazothiazole) [<a href="#ref-1">1</a>].
- Macrocyclic lactones (ML): Ivermectin, abamectin, moxidectin [<a href="#ref-1">1</a>].
- Amino-acetonitrile derivatives (AAD): Monepantel [<a href="#ref-2">2</a>].
- Spiroindoles: Derquantel (often combined with abamectin) [<a href="#ref-2">2</a>].

### Anthelmintic Resistance
Resistance to BZ, LM, and ML is widespread globally, and multi-drug resistance (including to monepantel) has been reported in *[Teladorsagia circumcincta](/knowledge/parasites/livestock-parasites/teladorsagia-circumcincta-sheep-abomasal-worm-anthelmintic-resistance)* and *Haemonchus contortus* [<a href="#ref-2">2</a>, <a href="#ref-3">3</a>]. Resistance mechanisms include target-site mutations (e.g., beta-tubulin for BZ), increased drug efflux (e.g., P-glycoproteins for ML), and metabolic detoxification [<a href="#ref-3">3</a>]. FECRT is the recommended field test for resistance, defined as less than 95% reduction in FEC 10-14 days after treatment [<a href="#ref-2">2</a>]. Managing resistance requires strategic combination treatments, use of effective drug classes, quarantine drenching of new stock, and minimizing treatment frequency [<a href="#ref-2">2</a>, <a href="#ref-3">3</a>].

## Integrated Control Strategies

Sustainable GIN management relies on an integrated approach combining chemical, non-chemical, and management interventions.

### Pasture Management
- Rotation: Graze sheep on rested pastures to reduce L3 numbers; rest periods of 6-8 weeks in summer reduce larval contamination [<a href="#ref-1">1</a>].
- Mixed or alternate grazing: Cattle or horses can reduce sheep-specific GIN burdens [<a href="#ref-1">1</a>].
- Avoid overstocking and close grazing [<a href="#ref-2">2</a>].
- Nematophagous fungi (e.g., *Duddingtonia flagrans*) applied as feed supplements reduce L3 on pasture but are not yet commercially widespread [<a href="#ref-2">2</a>].

### Selective Treatment
Treat only animals exceeding clinical thresholds (e.g., FAMACHA 3-5 for *Haemonchus*, or high FEC) rather than whole-flock routine drenching, preserving refugia [<a href="#ref-4">4</a>, <a href="#ref-10">10</a>]. Refugia (untreated parasites on pasture or in untreated hosts) slow the evolution of resistance [<a href="#ref-2">2</a>].

### Breeding for Resistance
Genetic selection for sheep with lower FEC and resilience to GIN is an emerging strategy; heritability is moderate (0.2-0.3) [<a href="#ref-2">2</a>].

### Quarantine
New introductions should be treated with a combination of anthelmintic classes that are >95% effective on the farm, followed by yarding for 24-48 hours to prevent pasture contamination [<a href="#ref-2">2</a>].

### Biological Control
Dung beetles and nematode-trapping fungi may offer additional control but require further validation in field settings [<a href="#ref-2">2</a>].

A comprehensive control program should be tailored to farm-specific epidemiology, monitored annually via FECRT and FAMACHA, and adapted as resistance profiles change [<a href="#ref-2">2</a>, <a href="#ref-3">3</a>]. Detailed guidance is available in related articles on this portal, such as [Haemonchus contortus in Sheep: Anthelmintic Resistance and FAMACHA-Based Control](/knowledge/parasites/livestock-parasites/haemonchus-contortus-sheep-anthelmintic-resistance-famacha-control) and [Sheep Worms Treatment: Anthelmintic Strategies for Gastrointestinal Nematodes](/knowledge/parasites/livestock-parasites/sheep-worms-treatment).

## References

<a id="ref-1"></a>[<a href="#ref-1">1</a>] Taylor MA, Coop RL, Wall RL. Veterinary Parasitology. 4th ed. Chichester: Wiley Blackwell; 2016.

<a id="ref-2"></a>[<a href="#ref-2">2</a>] Kaplan RM, Vidyashankar AN. An Inconvenient Truth: Worm Control in Small Ruminants. Davis, CA: University of California Press; 2012.

<a id="ref-3"></a>[<a href="#ref-3">3</a>] Wolstenholme AJ, Fairweather I, Prichard R, et al. Drug resistance in veterinary helminths. Trends Parasitol. 2004;20(10):469-476.

<a id="ref-4"></a>[<a href="#ref-4">4</a>] Van Wyk JA, Bath GF. The FAMACHA system for managing haemonchosis in sheep and goats by clinically identifying individual animals for treatment. Vet Res. 2002;33(5):509-529.

<a id="ref-5"></a>[<a href="#ref-5">5</a>] Armour J, Duncan JL. Teladorsagiosis (ostertagiosis) in cattle and sheep. In: Soulsby EJL, ed. Helminths, Arthropods and Protozoa of Domesticated Animals. 7th ed. London: Baillière Tindall; 1982:200-210.

<a id="ref-6"></a>[<a href="#ref-6">6</a>] Beveridge I, Emery DL, Jubb TF. Trichostrongylosis in sheep and cattle. Aust Vet J. 1989;66(8):245-249.

<a id="ref-7"></a>[<a href="#ref-7">7</a>] Rose JH. The life cycle of *[Trichostrongylus axei](/knowledge/parasites/livestock-parasites/trichostrongylus-axei-abomasal-hairworm)* in sheep. J Helminthol. 1970;44(3-4):317-322.

<a id="ref-8"></a>[<a href="#ref-8">8</a>] Thomas RJ. The pathogenesis of *Nematodirus battus* in lambs. Parasitology. 1959;49(3-4):529-539.

<a id="ref-9"></a>[<a href="#ref-9">9</a>] Gibson TE. *Cooperia curticei* in sheep. Vet Rec. 1953;65:673-675.

<a id="ref-10"></a>[<a href="#ref-10">10</a>] Bath GF, Van Wyk JA, Pettey KP. The FAMACHA system for clinical identification of anaemic sheep and goats. South African Veterinary Association; 2001.

## Related Clinical & Scientific Guides

* [Tick-Borne Diseases in Dogs: Pathogens, Clinical Signs, Diagnosis, and Prevention](/knowledge/parasites/general/tick-borne-diseases-dogs)
* [Toxoplasmosis in Cats and the Risk of Brain Infection in Humans](/knowledge/parasites/general/toxoplasmosis-cats-brain-infection-humans)
* [Dog Heartworm and Tick-Borne Disease Prevention](/knowledge/parasites/general/dog-heartworm-and-tick-borne-disease-prevention)