# [Sheep Parasites](/knowledge/parasites/livestock-parasites/common-sheep-parasites-eggs-treatment) in Winter: Overwintering Strategies and Control Programs
## Key Takeaways
- Gastrointestinal nematodes like *Teladorsagia circumcincta* and *Haemonchus contortus* survive winter primarily through hypobiosis, an arrested larval development within the abomasal mucosa, resuming maturation upon warming.
- *Nematodirus* spp. exhibit extreme cold tolerance, with eggs requiring prolonged chilling to break diapause, and L3 remaining viable under snow cover, posing a risk for spring outbreaks.
- *Psoroptes ovis* (sheep scab mite) persists in the fleece microclimate, while *Damalinia ovis* (biting louse) relies on host body heat for overwintering, necessitating simultaneous flock treatment for effective ectoparasite control.
- *Fasciola hepatica* (liver fluke) overwinters as metacercariae on herbage or as juvenile flukes within the snail intermediate host (*Galba truncatula*), with viability of metacercariae reduced by freezing and desiccation.
- Diagnostic approaches for winter parasitism include fecal egg counts (McMaster, FLOTAC), larval culture for hypobiotic species, coproantigen ELISA for *Fasciola*, and skin scrapings for ectoparasites.
- Anthelmintic control strategies in winter should consider hypobiotic larvae, with macrocyclic lactones effective against arrested L4, and resistance management through fecal egg count reduction testing and refugia-based strategies is critical.
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## Introduction
Winter imposes significant abiotic stress on the free-living stages of [sheep parasites](/knowledge/parasites/livestock-parasites/sheep-parasites-comprehensive-list-management-strategies) in temperate production systems [1]. Low soil temperatures, frost, snow cover, and reduced solar radiation alter the transmission dynamics of most gastrointestinal nematodes, lungworms, cestodes, and ectoparasites [1]. Understanding the biological mechanisms that permit these parasites to survive winter conditions is essential for designing effective control programs [1]. This article reviews the overwintering strategies of major [sheep parasites](/knowledge/parasites/livestock-parasites/sheep-parasites-comprehensive-list-management-strategies) and presents integrated control approaches applicable to winter management. The topic of [sheep parasites](/knowledge/parasites/livestock-parasites/common-sheep-parasites-eggs-treatment) in winter is directly linked to broader concepts of [Internal Parasites of Cattle: Gastrointestinal Nematodes and Control Strategies](/knowledge/parasites/livestock-parasites/internal-cattle-parasites) and [Sheep Internal Parasites: Winter Management, Parasite Resistance in Dorpers, and Human Health Risks](/knowledge/parasites/livestock-parasites/sheep-internal-parasites-winter-management-resistance).
## Overwintering Mechanisms of [Sheep Parasites](/knowledge/parasites/livestock-parasites/sheep-parasites-comprehensive-list-management-strategies)
### Hypobiosis in Gastrointestinal Nematodes
Many trichostrongylid nematodes, particularly *[Teladorsagia circumcincta](/knowledge/parasites/livestock-parasites/teladorsagia-circumcincta-sheep-abomasal-worm-anthelmintic-resistance)* and *[Haemonchus contortus](/knowledge/parasites/livestock-parasites/haemonchus-contortus)*, undergo hypobiosis (arrested larval development) within the abomasal mucosa or small intestinal wall during winter [1]. This physiological dormancy is triggered by environmental cues such as decreasing day length and falling temperatures experienced by the free-living third-stage larvae (L3) on pasture [1]. Ingested L3 penetrate the gastric glands and cease development at the early fourth larval stage (EL4), resuming maturation only after several weeks of cold exposure followed by warming [1]. Hypobiosis allows the parasite population to survive winter when conditions for transmission are unfavorable and synchronizes egg shedding with the subsequent lambing season [1]. For a detailed discussion of *T. circumcincta* pathobiology, see [Teladorsagia circumcincta in Sheep: Abomasal Parasitism, Anthelmintic Resistance, and Integrated Control in Temperate Regions](/knowledge/parasites/livestock-parasites/teladorsagia-circumcincta-sheep-abomasal-worm-anthelmintic-resistance).
### Cold Tolerance and Desiccation Resistance in Free-Living Stages
The free-living L3 of *Nematodirus battus* and *Nematodirus filicollis* are notable for their extreme cold tolerance [1]. Eggs deposited on pasture in spring do not hatch until the following winter or spring, requiring a prolonged period of chilling to break diapause [1]. The L3 of *Nematodirus* spp. can survive freezing temperatures and remain viable under snow cover for extended periods [1]. Similarly, the eggs and L3 of *Trichostrongylus* spp. and *Cooperia* spp. exhibit moderate cold tolerance, with survival rates declining rapidly below -5 degrees Celsius [1]. Desiccation resistance is lower in winter due to higher relative humidity, but frost heaving can disrupt the vertical migration of L3 onto herbage [1]. Pasture management strategies for *N. battus* are covered in [Nematodirus battus in Sheep Lambs: Spring Outbreak Epidemiology, D-Value Forecasting, and Anthelmintic Control](/knowledge/parasites/livestock-parasites/nematodirus-battus-sheep-lambs-d-value-anthelmintic).
### Overwintering of Lungworms
Protostrongylid lungworms such as *[Muellerius capillaris](/knowledge/parasites/livestock-parasites/muellerius-capillaris-sheep-goat-lungworm-protostrongylid-diagnosis)* overwinter as first-stage larvae (L1) within lung parenchyma nodules or in the environment after being shed in feces [1]. The L1 are highly resistant to cold and can survive winter on pasture, especially within fecal pellets [1]. The intermediate hosts (terrestrial gastropods) also overwinter, often in soil crevices or under vegetation, maintaining the life cycle [1]. For diagnostic and control details, refer to [Muellerius capillaris in Sheep and Goats: Protostrongylid Lungworm Diagnosis and Control](/knowledge/parasites/livestock-parasites/muellerius-capillaris-sheep-goat-lungworm-protostrongylid-diagnosis).
### Overwintering of Cestodes
The anoplocephalid tapeworm *Moniezia expansa* overwinters primarily within the definitive host as immature or adult stages [1]. The oribatid mite intermediate hosts are active in the soil during mild winter periods and can harbor cysticercoids, providing a source of infection when sheep graze close to the ground [1]. See [Moniezia expansa in Sheep and Cattle: Oribatid Mite Lifecycle, Clinical Signs, and Control](/knowledge/parasites/livestock-parasites/moniezia-expansa-sheep-bovine-tapeworm-oribatid-mite-lifecycle).
### Overwintering of Ectoparasites
*[Psoroptes ovis](/knowledge/parasites/livestock-parasites/psoroptes-ovis)* (sheep scab mite) survives winter in the fleece of infested sheep, with transmission facilitated by close contact at feeding troughs or bedding [1]. The mites are poor survivors off the host, but the microclimate within the wool provides sufficient humidity and temperature even in cold weather [1]. Conversely, *Damalinia ovis* (biting louse) depends on host body heat and can persist throughout winter without off-host survival [1]. For ectoparasite management, see [Damalinia ovis in Sheep: Biting Louse Infestation, Fleece Damage, Wool Loss, and Control Measures](/knowledge/parasites/livestock-parasites/damalinia-ovis-biting-louse-sheep-fleece-damage-wool-loss-control) and [Psoroptes ovis in Sheep: Sheep Scab Mange, Highly Contagious Notifiable Ectoparasitosis, and Dipping Protocols](/knowledge/parasites/livestock-parasites/psoroptes-ovis-sheep-scab-mange-highly-contagious-notifiable-dipping).
### Overwintering of Trematodes
*Fasciola hepatica* (liver fluke) overwinters as metacercariae encysted on herbage or as juvenile flukes within the snail intermediate host *Galba truncatula* [1]. Encysted metacercariae can survive mild winters, but freezing and desiccation reduce viability [1]. The snail host burrows into mud and enters diapause, providing a reservoir for the parasite [1]. For anthelmintic resistance and diagnostics, see [Liver Fluke (Fasciola hepatica) in Sheep: Anthelmintic Resistance Diagnosis and Herd-Level Management](/knowledge/parasites/livestock-parasites/fasciola-hepatica-sheep-anthelmintic-resistance-diagnosis-herd-management).
## Epidemiology of Winter Parasitism
Winter transmission of [sheep parasites](/knowledge/parasites/livestock-parasites/sheep-parasites-comprehensive-list-management-strategies) is characterized by low but persistent infection pressure [1]. Ewes often carry subclinical burdens of hypobiotic larvae that resume development in late winter, leading to periparturient rises in fecal egg counts [1]. This contributes to contamination of lambing pastures in early spring [1]. Lambs weaned in autumn may carry residual burdens that overwinter, especially if not treated at housing [1]. Fecal egg count monitoring during winter can identify high-risk flocks [1].
## Clinical Signs and Pathology
Winter parasite burdens are often subclinical, but acute disease can occur under specific conditions [1]. Hypobiotic *Teladorsagia* and *Haemonchus* larvae emerge synchronously causing abomasal inflammation, hypoalbuminemia, and diarrhea in late winter [1]. *Nematodirus* larvae emerging from arrested development in the small intestine cause protein-losing enteropathy with watery diarrhea and weight loss [1]. Heavy *Fasciola* infections lead to submandibular edema, anemia, and poor condition [1]. *Psoroptes ovis* infestation causes intense pruritus, wool loss, and skin exudation, which may be exacerbated by winter housing [1].
## Diagnostic Approaches
Winter parasite diagnostics rely on both direct and indirect methods [1]. Fecal egg counts (FEC) using modified McMaster or FLOTAC techniques quantify nematode egg shedding [1]. Larval culture and differentiation identify hypobiotic species [1]. The FAMACHA system assesses anemia in *Haemonchus* infections [1]. Serological tests for *Fasciola* (coproantigen ELISA) and lungworm (ELISA for *Muellerius*) are available [1]. For ectoparasites, skin scrapings and fleece examination confirm mite or louse presence [1]. The table below summarizes diagnostic tools for winter parasite monitoring.
| Parasite Group | Diagnostic Method | Sample Type | Winter Utility |
|--------|----------|-------|--------|
| Gastrointestinal nematodes | Fecal egg count (McMaster, FLOTAC) | Feces | Detects periparturient rise |
| *Nematodirus* spp. | Fecal egg count (specific identification) | Feces | Monitors overwintered burden |
| *Fasciola hepatica* | Coproantigen ELISA; sedimentation | Feces | Detects chronic fluke |
| *[Muellerius capillaris](/knowledge/parasites/livestock-parasites/muellerius-capillaris-sheep-goat-lungworm-protostrongylid-diagnosis)* | Baermann technique; serology | Feces; blood | Identifies pasture contamination |
| *Psoroptes ovis* | Skin scraping, microscopy | Skin debris, wool | Confirms clinical scab |
| *Damalinia ovis* | Fleece parting, visual examination | Wool, skin | Detects latent infestation |
| Resistance screening | [Fecal egg count reduction test](/knowledge/diagnostics/parasitology/fecal-egg-count-reduction-test-for-anthelmintic-resistance) (FECRT) | Feces (pre/post treatment) | Guides anthelmintic selection |
## Treatment and Control Programs
### Anthelmintic Strategies for Winter
Treatment decisions during winter should account for hypobiotic larvae and potential anthelmintic resistance [1]. Macrocyclic lactones (e.g., ivermectin, moxidectin) have efficacy against arrested L4 of *Teladorsagia* and *Haemonchus* [1]. Benzimidazoles and levamisole are also effective but resistance is widespread [1]. For *Fasciola*, triclabendazole remains the drug of choice for acute and chronic infection, though resistance is emerging [1]. Closantel and nitroxynil are alternatives for adult flukes [1]. Anthelmintic classes and their winter-specific indications are listed in the table below.
| Anthelmintic Class | Active Ingredients | Target Parasites | Winter Indication |
|----------|----------|---------|----------|
| Macrocyclic lactones | Ivermectin, moxidectin | GI nematodes, lungworms, ectoparasites | Hypobiotic larvae, lice, mites |
| Benzimidazoles | Albendazole, fenbendazole, oxfendazole | GI nematodes, *Moniezia* | Adult nematodes |
| Levamisole | Levamisole | GI nematodes | Adult nematodes |
| Salicylanilides | Closantel | *Haemonchus*, *Fasciola* (adult) | Blood-feeding nematodes |
| Triclabendazole | Triclabendazole | *Fasciola* (immature and adult) | Fluke control |
| Nitroxynil | Nitroxynil | *Fasciola* (adult) | Fluke control |
| Organophosphates | Diazinon (dip) | *Psoroptes ovis*, *Damalinia ovis* | Ectoparasite control at housing |
### Integrated Parasite Control Programs
A winter control program should integrate grazing management, strategic treatments, and diagnostic monitoring [1]. The following Mermaid flowchart illustrates a decision algorithm for winter parasite management in sheep.
```mermaid
flowchart TD
A["Winter Housing or Pasture Assessment"] --> B{"Clinical Signs?"}
B -->|"Yes"| C["Examine for scab, lice, anemia, diarrhea"]
B -->|"No"| D["Monitor FEC and body condition"]
C --> E["Diagnostic Confirmation"]
E --> F["FEC + Larval Culture"]
E --> G["Skin Scrape / Fleece Exam"]
E --> H["Fasciola Coproantigen ELISA"]
F --> I["High nematode burden?"]
I -->|"Yes"| J["Select anthelmintic based on resistance history"]
I -->|"No"| K["No treatment unless periparturient ewes"]
J --> L["FECRT post treatment"]
L --> M["Resistance detected?"]
M -->|"Yes"| N["Switch class or combined treatment"]
M -->|"No"| O["Continue same class"]
G --> P["Ectoparasites found?"]
P -->|"Yes"| Q["Plunge dip or inject macrocyclic lactone"]
P -->|"No"| R["No action"]
H --> S["Fluke positive?"]
S -->|"Yes"| T["Treat with triclabendazole or closantel"]
S -->|"No"| U["No fluke treatment"]
Q --> V["Quarantine new additions for 3 weeks"]
```
### Grazing and Husbandry Measures
Resting pasture over winter reduces L3 survival, though hypobiotic larvae in sheep remain a source of spring contamination [1]. Rotational grazing and delayed turnout of ewes onto clean pastures after lambing can minimize exposure of lambs [1]. Adequate nutrition during winter maintains host immunity and reduces periparturient egg rise [1]. Quarantine drenching of purchased sheep with a macrocyclic lactone and a benzimidazole combination is recommended to prevent introduction of resistant parasites [1].
### Ectoparasite Control
Plunge dipping with organophosphates or synthetic pyrethroids is effective for *Psoroptes ovis* and *Damalinia ovis* before housing in winter [1]. Injectable macrocyclic lactones provide systemic control of both mites and lice [1]. All sheep in a flock must be treated simultaneously to prevent reinfestation [1]. For persistent scab problems, a whole-flock treatment in midwinter with a long-acting macrocyclic lactone is advised [1].
## Resistance Management
Winter treatments should be guided by fecal egg count reduction testing to detect anthelmintic resistance [1]. Refugia-based strategies, such as leaving a portion of the flock untreated, are particularly important in winter to maintain a susceptible parasite population on pasture [1]. The use of combination anthelmintics (e.g., for *Teladorsagia* and *Haemonchus*) can delay the emergence of resistance [1]. For breed-specific resistance patterns, refer to [Sheep Parasite Resistance: Anthelmintic Strategies and Breed-Specific Considerations](/knowledge/parasites/livestock-parasites/sheep-parasite-resistance-anthelmintic-strategies-breed-specific-considerations) and [Suffolk Sheep Parasite Resistance: Anthelmintic Resistance and Management Strategies](/knowledge/parasites/livestock-parasites/suffolk-sheep-parasite-resistance).
## Conclusion
[Sheep parasites](/knowledge/parasites/livestock-parasites/common-sheep-parasites-eggs-treatment) in winter employ diverse overwintering strategies including hypobiosis, cold-tolerant free-living stages, and persistence within host microenvironments [1]. Effective winter control programs require an integrated approach combining strategic anthelmintic use, diagnostic surveillance, grazing management, and quarantine measures [1]. Understanding the biological basis of overwintering is critical to reducing parasite transmission and delaying the development of anthelmintic resistance [1]. Further reading on related topics includes [Gastrointestinal Parasites of Sheep: Clinical Management and Control](/knowledge/parasites/livestock-parasites/gastrointestinal-parasites-sheep-clinical-management-control) and [Common Sheep Parasites: Identification, Egg Detection, and Anthelmintic Treatment](/knowledge/parasites/livestock-parasites/common-sheep-parasites-eggs-treatment).
## References
[1] Merck & Co. (Current edition). *The Merck Veterinary Manual*. Kenilworth, NJ: Merck Sharp & Dohme Corp. (General textbook reference for parasite biology, diagnostic methods, and control principles.)
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