Sheep Parasite Resistance: Anthelmintic Strategies and Breed-Specific Considerations

Introduction

Gastrointestinal nematode (GIN) infections represent a major constraint to sheep production worldwide, causing reduced weight gain, decreased milk yield, impaired wool quality, and increased mortality, particularly in lambs and periparturient ewes [82, 92]. The most pathogenic species include Haemonchus contortus, Teladorsagia circumcincta, Trichostrongylus spp., Nematodirus battus, and Cooperia spp. [92]. Anthelmintic drugs have been the cornerstone of control for decades, but widespread and increasing resistance now threatens their efficacy [62, 81, 94]. Resistance has been documented against all major anthelmintic classes, including benzimidazoles (BZ), macrocyclic lactones (ML), imidazothiazoles (e.g., levamisole), and the amino-acetonitrile derivative monepantel [63, 71, 85, 97]. This article reviews the mechanisms of anthelmintic resistance, current diagnostic methods, sustainable treatment strategies (including targeted selective treatment, combination therapy, phytochemicals, and biological control), and the role of host genetics and breed-specific resistance in integrated parasite management.

Anthelmintic Resistance Mechanisms

Resistance to benzimidazoles is primarily mediated by single nucleotide polymorphisms (SNPs) in the β-tubulin isotype 1 gene, most commonly at codons 167, 198, and 200, which reduce drug binding to tubulin [33, 78, 89]. These mutations are detected by PCR-based genotyping and are prevalent in H. contortus and T. circumcincta populations globally [1]. Resistance to macrocyclic lactones (e.g., ivermectin, moxidectin) involves multiple mechanisms, including P-glycoprotein efflux pumps, altered glutamate-gated chloride channel subunits, and metabolic detoxification [31, 94]. Metabolomic and proteomic analyses have revealed distinct differences in energy metabolism and detoxification pathways between susceptible and BZ-resistant H. contortus adults, with sex-specific variation [2]. Multidrug resistance is increasingly common, with populations exhibiting resistance to two or more drug classes simultaneously [71, 85, 96]. The genetic diversity of H. contortus isolates, as assessed by microsatellite markers and mitochondrial DNA, is high, facilitating rapid selection for resistance under drug pressure [3]. Resistance in N. battus is also emerging, driven by refugia management and climatic factors [44].

Diagnostic Approaches for Resistance Detection

Phenotypic detection of anthelmintic resistance relies on the fecal egg count reduction test (FECRT), which compares pre- and post-treatment egg counts [94, 97]. Molecular methods, including allele-specific PCR and pyrosequencing for BZ resistance SNPs, provide rapid genotyping of pooled egg or larval samples [33, 78]. Nemabiome metabarcoding using the ITS-2 ribosomal DNA region enables species-level identification and quantification of resistance alleles within mixed-species infections [1]. This approach has revealed differences in species composition between co-grazed sheep and goats [79] and between farms with different management histories [1]. Bioinformatics pipelines for nemabiome analysis require careful validation to avoid biases in species detection [56]. Additionally, the FAMACHA system, which scores conjunctival pallor as an indicator of anemia caused by H. contortus, is a practical field tool for targeted treatment decisions [4]. Process mining of FAMACHA score changes over time can guide optimal monitoring intervals [4]. Immunoglobulin levels (e.g., IgA, IgE) have been investigated as biomarkers for resistance, with higher levels associated with lower fecal egg counts [90].

Anthelmintic Strategies

Targeted Selective Treatment (TST) and Refugia-Based Approaches

Targeted selective treatment (TST) aims to treat only those animals that require intervention, leaving a proportion of the parasite population unexposed to drug (refugia) to slow resistance development [93]. TST criteria include FAMACHA score, body condition score (BCS), fecal egg count (FEC), and performance indicators such as weight gain or dag score [5, 6]. A meta-analysis confirmed that TST reduces anthelmintic use without compromising flock performance compared to suppressive treatment [6]. The "Happy Factor" threshold, based on the proportion of lambs with low FEC, has been shown to be transferable between farms [74]. Supplementation with minerals and vitamins can influence optimal TST thresholds, likely by modulating host immunity [45]. In dairy sheep, BCS-based TST effectively controlled subclinical infections [5]. Periparturient ewes, which experience a peri-parturient rise in FEC, are a key target for TST, and nemabiome composition can guide treatment decisions in this group [38]. Refugia can be maintained by leaving untreated animals, by strategic grazing of low-contamination pastures, or by using slow-release boluses that allow some parasites to survive [46, 93]. Region-specific modeling of refugia dynamics in Nigerian small ruminants highlights the importance of local climate and management factors [7]. Mathematical models such as GI-NemaTracker predict the long-term consequences of different control strategies on parasite populations and resistance evolution [8].

Combination Therapy and Drug Rotation

Combining anthelmintics with different modes of action can improve efficacy against multidrug-resistant populations and delay resistance emergence [62, 76]. A four-year study of combined moxidectin-levamisole treatment showed sustained efficacy against resistant trichostrongylids [76]. However, antagonistic interactions have been reported, for example between spinosad and macrocyclic lactones against Lucilia cuprina larvae, highlighting the need for careful in vitro testing of combinations [32]. Pharmacokinetic studies of albendazole-clorsulon combinations for Fasciola hepatica control demonstrate that drug interactions can affect flukicidal efficacy [9]. Diaryl dichalcogenides blended with ivermectin have shown synergistic activity against GINs in small ruminants [10]. Drug rotation between classes remains a recommended practice, but its effectiveness depends on the rate of reversion to susceptibility, which is often slow [62, 94].

Phytochemical and Plant-Based Anthelmintics

Plant extracts and essential oils offer alternative or complementary anthelmintic options, particularly for organic systems and regions with high resistance [11]. Nepeta racemosa extracts containing rosmarinic acid have demonstrated in vitro activity against sheep GINs, with in silico tubulin-binding studies suggesting a mechanism similar to BZ [30]. Origanum vulgare essential oil shows ovicidal and larvicidal effects [53]. Bark extracts from Nordic trees have activity against T. circumcincta in vitro [70]. Eucalyptus wood vinegar exhibits ovicidal effects on GIN eggs [12]. Hedera helix-derived α-hederin (IVL-11) has both ex vivo and in vivo flukicidal activity against F. hepatica [13]. The combination of albendazole with thymol (a phytochemical) enhances anthelmintic efficacy in vivo and in vitro [91]. Cinnamaldehyde combined with pink grapefruit extract modulates the pharmacokinetics of doramectin in sheep, potentially improving efficacy [14]. Traditional Latvian herbal remedies are also used for parasite control [69]. However, the variability in phytochemical content and the need for standardized extracts remain challenges [11].

Biological Control

Nematophagous fungi, such as Duddingtonia flagrans, reduce larval contamination on pasture by trapping and digesting nematode larvae in feces [15]. Supplementation with heather (Calluna vulgaris) did not reduce the trapping ability of D. flagrans in lambs infected with H. contortus [65]. Bacillus thuringiensis toxins have shown anthelmintic potential against H. contortus in vitro [48]. A systematic review confirmed that nematophagous fungi can significantly reduce GIN larval counts in small ruminants, but field efficacy depends on formulation and environmental conditions [15]. Other biological approaches include the use of copper oxide wire particles and bioactive forages (e.g., sericea lespedeza, chicory) that contain condensed tannins with anthelmintic properties [77, 92].

Vaccination

Vaccination against GINs is an emerging strategy. A glycoengineered recombinant vaccine against H. contortus has shown safety and efficacy in sheep [16]. The Barbervax vaccine (based on gut membrane antigens of H. contortus) has been compared with host genetic resistance under field challenge, with both approaches reducing parasitism [17]. Recombinant vaccines against T. circumcincta have elicited protective immune responses in lambs, with transcriptomic analysis revealing upregulation of Th2-type immune genes [47]. However, vaccine efficacy can vary between breeds, as demonstrated in Canarian sheep breeds [57]. Transcriptomic and multi-omics approaches are being used to identify protective antigens and host responses [18, 19].

Breed-Specific Resistance to Gastrointestinal Nematodes

Genetic variation in resistance to GINs exists both between and within sheep breeds, offering opportunities for selective breeding [18]. Resistance is typically measured as low fecal egg count (FEC) and is moderately heritable (h² ~0.2–0.4) [72]. Breeds adapted to tropical or subtropical environments, such as Red Maasai, West African Dwarf, and Santa Inês, often exhibit higher resistance to H. contortus compared to exotic breeds [20, 18]. In temperate regions, local breeds such as the Canarian sheep breeds show variability in response to T. circumcincta [52, 57]. Katahdin sheep have been studied for genetic markers associated with low FEC at weaning, with variants in genes EDIL3 and ADGRB3 identified [60]. Genome-wide association studies (GWAS) in two local dairy sheep breeds have identified genomic regions associated with estimated breeding values for GIN resistance [29]. Cluster analysis of additive-genetic patterns can classify sheep as resistant, resilient, or susceptible based on FEC and production traits [66]. In Argentinean Corriedale and Pampinta sheep, SNPs in candidate genes (e.g., MHC, IL-4, IFN-γ) are associated with nematode resistance and resilience [80]. Selective breeding for reduced FEC in Dorper lambs altered their response to H. contortus artificial challenge, confirming the genetic basis of resistance [39]. Multi-omics data (transcriptomics, proteomics, metabolomics) are elucidating the host-parasite-microbiota interactions underlying resistance [40]. RNA-sequencing studies have identified key immune genes and pathways, including Th2 cytokines, mast cell proteases, and mucins, that differ between resistant and susceptible animals [18, 21, 19]. The integration of genomic selection into breeding programs, as modeled in Uruguayan Merino sheep, can accelerate genetic gain for resistance while maintaining production traits [72].

Integrated Control Strategies

Sustainable control of GINs requires an integrated approach combining grazing management, TST, biological control, vaccination (where available), and genetic improvement [22, 23]. Grazing strategies such as rotational grazing, mixed-species grazing (e.g., sheep with cattle or goats), and use of low-contamination pastures (e.g., hay aftermath or crops) reduce larval exposure [41, 79, 92]. Biotic interactions in soil and dung, including dung beetles and earthworms, influence larval survival and transmission [41]. Climate and farm management factors drive adaptation in N. battus populations [44]. Habitat suitability modeling using MaxEnt can predict regions at high risk for GIN transmission [84]. Farmer education and perception are critical for adoption of best practices; studies in Norway, Sweden, Scotland, New Zealand, Australia, and the Caribbean highlight gaps between perceived and actual parasite challenges, and barriers to implementing TST and resistance testing [24]. Inappropriate drug usage, such as underdosing and frequent treatments, is common and accelerates resistance [59, 83]. Community-based breeding programs in Ethiopia have demonstrated that strategic anthelmintic treatment combined with improved nutrition and management can reduce parasite burden [49]. In tropical regions, integrated control must account for the year-round transmission of H. contortus and the effects of climate on parasite survival [20, 25, 23]. For liver fluke (Fasciola hepatica), integrated control includes snail habitat management, grazing avoidance, and strategic flukicide use, with triclabendazole resistance now widespread [26, 9]. Detection of liver fluke environmental DNA in water samples can aid in risk mapping [75]. Ectoparasites such as Psoroptes ovis (sheep scab) and Lucilia cuprina (blowfly strike) also require integrated management, with anthelmintic resistance emerging in blowflies [32, 86].

The following decision tree summarizes the key components of an integrated parasite control program for sheep.

flowchart TD
    A[Integrated Parasite Control Program] --> B[Diagnosis & Monitoring]
    A --> C[Anthelmintic Strategies]
    A --> D[Non-Chemical Control]
    A --> E[Breeding for Resistance]

    B --> B1[FECRT / FEC monitoring]
    B --> B2[FAMACHA scoring]
    B --> B3[Nemabiome metabarcoding]
    B --> B4["Resistance genotyping (e.g., BZ SNPs")]

    C --> C1["Targeted Selective Treatment (TST)"]
    C --> C2[Combination therapy]
    C --> C3[Phytochemicals / plant extracts]
    C --> C4["Vaccination (where available")]

    D --> D1[Grazing management / rotation]
    D --> D2["Biological control (nematophagous fungi)"]
    D --> D3[Bioactive forages / tannins]
    D --> D4[Copper oxide wire particles]

    E --> E1[Selection for low FEC]
    E --> E2[GWAS / genomic selection]
    E --> E3[Use of resistant breeds]

Conclusion

Anthelmintic resistance in sheep GINs is a global challenge that demands a multifaceted response. Accurate diagnosis of resistance, implementation of TST and refugia-based strategies, integration of phytochemical and biological control, and exploitation of host genetic resistance through selective breeding are all essential components of sustainable parasite management. Breed-specific differences in resistance offer a valuable tool for reducing reliance on anthelmintics, particularly in production systems where local breeds are well adapted to the parasite challenge. Continued research into the molecular mechanisms of resistance, host immunity, and novel control methods will be critical to preserving the efficacy of current and future anthelmintics.

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