Sheep Parasites in Winter: Overwintering Strategies and Control Programs

Introduction

Winter imposes significant abiotic stress on the free-living stages of sheep parasites 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 and presents integrated control approaches applicable to winter management. The topic of sheep parasites in winter is directly linked to broader concepts of Internal Parasites of Cattle: Gastrointestinal Nematodes and Control Strategies and Sheep Internal Parasites: Winter Management, Parasite Resistance in Dorpers, and Human Health Risks.

Overwintering Mechanisms of Sheep Parasites

Hypobiosis in Gastrointestinal Nematodes

Many trichostrongylid nematodes, particularly Teladorsagia circumcincta and 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.

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.

Overwintering of Lungworms

Protostrongylid lungworms such as Muellerius capillaris 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.

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.

Overwintering of Ectoparasites

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 and Psoroptes ovis in Sheep: Sheep Scab Mange, Highly Contagious Notifiable Ectoparasitosis, and Dipping Protocols.

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.

Epidemiology of Winter Parasitism

Winter transmission of sheep parasites 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.

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.

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.

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 and Suffolk Sheep Parasite Resistance: Anthelmintic Resistance and Management Strategies.

Conclusion

Sheep parasites 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 and Common Sheep Parasites: Identification, Egg Detection, and Anthelmintic 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.) *** Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.