Relative maturity and the development of immunity to gastrointestinal nematodes in sheep: an overlooked paradigm?

Evidence for a constitutive cost of host resistance on body fat growth in ewe lambs from lines selected for resistance or susceptibility to experimental infections with Haemonchus contortus

Although benefits of selection for host resistance to gastro-intestinal nematodes have long been recognized, its costs on production traits remain unclear. A main difficulty when studying those costs is to disentangle genetic effects due to selection from plastic responses induced by infection. Putative costs of host resistance have been extensively investigated in growing sheep. However, while most of those studies have relied on live weight to assess body growth, more comprehensive assessments accounting for body composition are advocated to detect trade-offs. In this study we used 90 female lambs from lines divergently selected on resistance to Haemonchus contortus that we experimentally infected (n = 60) or not (n = 30) under controlled conditions. As those conditions were defined to enable uninfected lambs to fully express their growth potential, we sought to precisely identify the effects of selection for host resistance on health traits and on growth traits. We assessed muscular and fat growth based on repeated measurements with dorsal ultrasonography for all lambs on farm, and with whole-body computed tomography (CT) scans for a subgroup of 18 infected lambs. Lambs achieved a high growth rate, including infected ones despite their high worm burden (confirmed at necropsy in the subgroup). As expected, lambs from the resistant (R) line were less infected than those from the susceptible (S) line. However, the clear pathogenic effects observed on muscular growth and voluntary feed intake were similar between lines. In contrast, a line difference in body fat was supported both by dorsal and volumetric CT measurements. Specifically, lower fats in the R line compared with the S line was observed equally in infected and uninfected groups, thus providing evidence for a constitutive cost of host resistance. Although this cost is not necessarily disadvantageous in nutrient-rich environments exposing animals to excess fat deposition, its consequences in nutrient-scarce environments may be important to promote sustainable breeding strategies for host resistance.

Genetic profile of adaptive immune traits and relationships with parasite resistance and productivity in Scottish Blackface sheep

Gastrointestinal (GI) parasites cause significant production losses in grazing ruminants which can be mitigated by breeding animals resistant to disease. Lymphocyte cytokine production and parasite-specific Immunoglobulin A (IgA) are adaptive immune traits associated with immunity to GI parasites. To explore the utility of these traits for selective breeding purposes, this study estimated the genetic parameters of the immune traits in sheep and assessed their relationship with disease and productivity traits. Whole blood stimulation assays were performed on 1 040 Scottish Blackface lambs at two months of age in 2016-2017. Blood was stimulated with either pokeweed mitogen (PWM), a non-specific activator of lymphocytes, and Teladorsagia circumcincta (T-ci) larval antigen to activate parasite-specific T lymphocytes. The type of adaptive immune response was determined by quantifying production of cytokines interferon-gamma (IFN-γ), interleukin (IL)-4, and IL-10, which relate to T-helper type (Th) 1, Th2 and regulatory T cell responses, respectively. Serum T-ci specific IgA was also quantified. Heritabilities were estimated for each immune trait by univariate analyses. Genetic and phenotypic correlations were estimated between different immune traits, and between immune traits vs. disease and productivity traits that were recorded at three months of age. Disease phenotypes were expressed as faecal egg counts (FEC) of nematode parasites (Strongyles and Nematodirus), faecal oocyst counts (FOC) of coccidian parasites, and faecal soiling score; production was measured as lamb live weight. Significant genetic variation was observed in all immune response traits. Heritabilities of cytokine production varied from low (0.14 ± 0.06) to very high (0.77 ± 0.09) and were always significantly greater than zero (P < 0.05). IgA heritability was found to be moderate (0.41 ± 0.09). Negative associations previously identified between IFN-γ production and FOC, and IL-4 production and strongyle FEC, were not evident in this study, potentially due to the time-lag between immune and parasitology measures. Instead, a positive genetic correlation was found between FOC and PWM-induced IFN-γ production, while a negative genetic correlation was found between FOC and T-ci induced IL-10. Live weight was negatively genetically correlated with IFN-γ responses. Overall, IFN-γ and IL-4 responses were positively correlated, providing little evidence of cross-regulation of Th1 and Th2 immunity within individual sheep. Furthermore, T-ci specific IgA was highly positively correlated with PWM-induced IL-10, indicating a possible role for this cytokine in IgA production. Our results suggest that while genetic selection for adaptive immune response traits is possible and may be beneficial for parasite control, selection of high IFN-γ responsiveness may negatively affect productivity.

Exploring the transcriptomic changes underlying recombinant vaccine efficacy against Teladorsagia circumcincta in 3-month-old lambs

Teladorsagia circumcincta is an abomasal parasitic nematode that can cause serious issues in small ruminant production, which are aggravated by drug resistance. Vaccines have been suggested as a feasible, long-lasting alternative for control since adaptation to the host's immune mechanisms by helminths develops at a much slower pace than anthelmintic resistance. Recently, a T. circumcincta recombinant subunit vaccine yielded over a 60% reduction in egg excretion and worm burden and induced strong humoral and cellular anti-helminth responses in vaccinated 3-month-old Canaria Hair Breed (CHB) lambs, but Canaria Sheep (CS) of a similar age were not protected by the vaccine. Here, we compared the transcriptomic profiles in the abomasal lymph nodes of such 3-month-old CHB and CS vaccinates 40 days after infection with T. circumcincta to understand differences in responsiveness at the molecular level. In the CS, differentially expressed genes (DEG) identified were related to general immunity processes such as antigen presentation or antimicrobial proteins and down-regulation of inflammation and immune response through regulatory T cell-associated genes. However, upregulated genes in CHB vaccinates were associated with type-2 oriented immune responses, i.e., immunoglobulin production, activation of eosinophils, as well as tissue structure and wound repair-related genes and protein metabolism pathways such as DNA and RNA processing. These results highlight potentially more optimal timing and orientation of immune responses in CHB sheep compared to CS associated with vaccine-induced protection. The data obtained in this study thus deepens our understanding of variations in responsiveness to vaccination in young lamb and provides insights for vaccine refinement strategies.

Novel epidemiological model of gastrointestinal nematode infection to assess grazing cattle resilience by integrating host growth, parasite, grass and environmental dynamics

Gastrointestinal nematode (GIN) infections are ubiquitous and often cause morbidity and reduced performance in livestock. Emerging anthelmintic resistance and increasing change in climate patterns require evaluation of alternatives to traditional treatment and management practices. Mathematical models of parasite transmission between hosts and the environment have contributed towards the design of appropriate control strategies in ruminants, but have yet to account for relationships between climate, infection pressure, immunity, resources, and growth. Here, we develop a new epidemiological model of GIN transmission in a herd of grazing cattle, including host tolerance (body weight and feed intake), parasite burden and acquisition of immunity, together with weather-dependent development of parasite free-living stages, and the influence of grass availability on parasite transmission. Dynamic host, parasite and environmental factors drive a variable rate of transmission. Using literature sources, the model was parametrised for Ostertagia ostertagi, the prevailing pathogenic GIN in grazing cattle populations in temperate climates. Model outputs were validated on published empirical studies from first season grazing cattle in northern Europe. These results show satisfactory qualitative and quantitative performance of the model; they also indicate the model may approximate the dynamics of grazing systems under co-infection by O. ostertagi and Cooperia oncophora, a second GIN species common in cattle. In addition, model behaviour was explored under illustrative anthelmintic treatment strategies, considering impacts on parasitological and performance variables. The model has potential for extension to explore altered infection dynamics as a result of management and climate change, and to optimise treatment strategies accordingly. As the first known mechanistic model to combine parasitic and free-living stages of GIN with host feed-intake and growth, it is well suited to predict complex system responses under non-stationary conditions. We discuss the implications, limitations and extensions of the model, and its potential to assist in the development of sustainable parasite control strategies.

Variability in the Response against Teladorsagia circumcincta in Lambs of Two Canarian Sheep Breeds

The increasing resistance to anthelmintics has necessitated the exploration of alternative control strategies of gastrointestinal nematode (GIN) infections. A sustainable option is genetic selection based on differences in susceptibility to GIN infection between and within breeds of sheep. Here, three-month-old Canaria Hair breed (GIN-resistant) and Canaria Sheep breed (GIN-susceptible) showed no significant between-breed differences after trickle infection with Teladorsagia circumcincta, whereas considerable individual variability was found in both breeds. Next, data from lambs of both breeds were used to explore the relationships between parasitological variables and T. circumcincta-specific IgA levels, local immune cell populations, and abomasal lymph node gene expression to understand the possible mechanisms underlying resistance. Mucosal IgA levels as well as numbers of globular leukocytes and MHC-II⁺ cells were associated with protection. Analysis of lymph node gene expression revealed the associations between lower parasite numbers and cumulative fecal egg counts and several immune pathways, such as leukocyte cell adhesion, activation and differentiation of T cells, in particular CD4⁺ and IL-4 production. The data obtained here may inform on the relationship between phenotypic resistance variability and protective responses at the humoral, cellular, and transcriptomic levels, thus contributing to identifying immune responses in young lambs that could be used as markers for selection.

Comparative proteomics reveals genetic mechanisms of body weight in Hu sheep and Dorper sheep

Body weight (BW) is a critical economic trait for meat production in sheep, and it is a complex trait affected by numerous elements. The aim of this study was to investigate the genetic mechanisms of sheep BW by a label-free proteomics approach. The result showed, a total of 27, 14, 61, and 65 differentially abundant proteins (DAPs) were identified in the Hu_HBW vs. Hu_LBW, DP_HBW vs. DP_LBW, Hu_HBW vs. DP_HBW, and Hu_LBW vs. DP_LBW comparisons, respectively. Five proteins (including ILK, AHCYL2, MLIP, CYB5A, and SMTNL1) related to fat synthesis and muscle development were detected in the Hu sheep group. In the Dorper sheep group, the screened DAPs strictly related to muscle development and fat synthesis were significantly enriched in MAP kinase activity (MAPK12), Arachidonic acid metabolism, and Steroid hormone biosynthesis (PGFS, LOC101107119) pathways. Several DAPs related to immune responses (SERPINA1, FGG, SERPINC1, and LOC101108131), fat deposition (APOH, GC, AHSG, SKP1, ACSL1, ACAT1, and ACADS), and muscle development (LMOD3 and LRRC39) were detected in the Hu vs. Dorper sheep comparison. These analyses indicated that the BW of sheep is regulated via a variety of pathways, and these DAPs can be further investigated as candidate markers for predicting the BW of sheep. SIGNIFICANCE: Body weight is one of the key traits in sheep and involves multiple coordinated regulatory mechanisms, but the genetic mechanism of BW is still unclear in sheep. In the current study, the label-free method was used to identify the proteins and pathways related to BW using LT muscle of Hu sheep and Dorper sheep with different BW. These findings will provide new candidate proteins and vital pathways into the molecular mechanisms involved growth traits in sheep.

Variants Within Genes EDIL3 and ADGRB3 are Associated With Divergent Fecal Egg Counts in Katahdin Sheep at Weaning

Gastrointestinal nematodes (GIN) pose a severe threat to sheep production worldwide. Anthelmintic drug resistance coupled with growing concern regarding potential environmental effects of drug use have demonstrated the necessity of implementing other methods of GIN control. The aim of this study was to test for genetic variants associated with resistance or susceptibility to GIN in Katahdin sheep to improve the current understanding of the genetic mechanisms responsible for host response to GIN. Linear regression and case-control genome-wide association studies were conducted with high-density genotype data and cube-root transformed weaning fecal egg counts (tFEC) of 583 Katahdin sheep. The case-control GWAS identified two significant SNPs (P-values 1.49e-08 to 1.01e-08) within introns of the gene adhesion G protein-coupled receptor B3 (ADGRB3) associated with lower fecal egg counts. With linear regression, four significant SNPs (P-values 7.82e-08 to 3.34e-08) were identified within the first intron of the gene EGF-like repeats and discoidin domains 3 (EDIL3). These identified SNPs were in very high linkage disequilibrium (r² of 0.996–1), and animals with alternate homozygous genotypes had significantly higher median weaning tFEC phenotypes compared to all other genotypes. Significant SNPs were queried through public databases to identify putative transcription factor binding site (TFBS) and potential lncRNA differences between reference and alternate alleles. Changes in TFBS were predicted at two SNPs, and one significant SNP was found to be within a predicted lncRNA sequence with greater than 90% similarity to a known lncRNA in the bovine genome. The gene EDIL3 has been described in other species for its roles in the inhibition and resolution of inflammation. Potential changes of EDIL3 expression mediated through lncRNA expression and/or transcription factor binding may impact the overall immune response and reduce the ability of Katahdin sheep to control GIN infection. This study lays the foundation for further research of EDIL3 and ADGRB3 towards understanding genetic mechanisms of susceptibility to GIN, and suggests these SNPs may contribute to genetic strategies for improving parasite resistance traits in sheep.

Metabolizable energy balance in hair sheep lambs artificially infected with Haemonchus contortus

In sheep, infection with Haemonchus contortus may increase the need for energy, and this demand may vary according to the infection level. In this study, the energy intake, digestibility, and energy retention of lambs artificially infected with different levels of H. contortus were estimated. A total of 24 hair sheep lambs reared parasite-free were experimentally infected with H. contortus at one of three infection levels: non-infected (n = 6); infected with 300 infective larvae (L₃) of H. contortus/kg body weight (BW) (n = 9); and infected with 500 H. contortus L₃/kg BW (n = 9). The lambs were fed for an individual weight gain of 100 g/day, and intake of organic matter (OMI) and gross energy (GEI), digestible (DEI) and metabolizable energy (MEI) were measured weekly. The digestibility of organic matter (OMD) and GE (GED) and the metabolizable energy (ME) balance adjusted to zero nitrogen balance (ME_adj) were measured for each lamb during the prepatent and patent periods of infection. From day 21 post-infection (PI), the individual eggs per gram (EPG) of feces and the total number of eggs in feces (TEF) were estimated weekly. After humane slaughter on day 42 PI, the worm burden (WB) was determined. Correlation and regression analyses were performed to estimate the relationships between the parasitological variables (L₃, EPG, TEF and WB) and the response variables (OMI, GEI, DEI, MEI, OMD, GED, ME_adj). During the prepatent period, there were no significant relationships of L₃ with the response variables (OMI, GEI, DEI, MEI, OMD, GED, ME, ME_adj). Similarly, during the patent period, no relationship was evident between infection (EPG, TEF or WB) and OMI, GEI, DEI, GED, OMD, ME or ME_adj. Thus, the gradient of H. contortus infection examined in the present study did not influence energy balance in hair sheep lambs, and infection did not impose any detectable energy cost. Further studies are needed to fully assess the impact of H. contortus infection on energy metabolism in hair sheep.

Artificial Haemonchus contortus infection as a strategy to induce protective immune response to natural infection in Pelibuey lambs

The objective of this study was to evaluate the reduction in nematode faecal egg count (FEC) in Pelibuey lambs segregated as resistant (RES), susceptible (SUS) and intermediate (INT) to gastrointestinal nematodes. Twenty-nine weaned Pelibuey lambs, aged five months old, free of nematode infection, were used. Nine lambs were RES, six were SUS and 14 were INT lambs. The study consisted of two phases: in Phase 1 the lambs were infected experimentally with Haemonchus contortus. In Phase 2, the lambs were naturally infected by grazing. Faecal and blood samples were taken every week. The packed cell volume and total protein were quantified. The FEC value (FECmax) per lamb was recorded together with a natural reduction in FEC in the two phases. The data were analysed with a model of measures repeated over time. During Phase 1, the RES lambs showed the lowest FEC (1061 ± 1053) compared to the other groups (INT: 2385 ± 1794 eggs per gram of faeces (EPG); and SUS: 3958 ± 3037 EPG). However, in Phase 2 no significant differences (p > 0.05) were observed between the groups of lambs (RES: 275 ± 498 EPG; SUS: 504 ± 1036 EPG; and INT: 603 ± 1061 EPG). At the end of Phase 1, the FEC of RES lambs was naturally reduced by 75.5% in respect to FECmax (p < 0.05), and at the end of Phase 2 the reduction in FEC was 90% in respect to FECmax (p > 0.05); the same behaviour was observed in RES and SUS lambs. It is concluded that the artificial infection in the lambs induced a more rapid immune response in RES than SUS lambs, and all lambs developed high acquired resistance by continuous infection.

Immune response related to Pelibuey sheep naturally infected with gastrointestinal nematodes in a tropical region of Mexico

We analysed the immune response involved in sheep naturally infected with gastrointestinal (GI) nematodes. Fifteen Pelibuey lambs were grazed in paddocks contaminated with GI nematodes for 13 weeks. To assess the infection, the number of eggs per gram (epg) and the percentage of packed cell volume (pcv) were evaluated. Blood and abomasal tissue samples were collected at week 8 post-infection to analyse the expression levels of IL-4, IL-5, IL-6, IL-8, IL-13, TGF-β and FCεR1A genes. The nematode Haemonchus contortus was the main species identified. In addition, two groups of lambs were classified based on the x ± SE of epg and pcv values: G-1, with 151 ± 28 and 29 ± 0.33%, respectively, and G-2, with 475 ± 59.5 and 26 ± 0.38%, respectively. For G-1, upregulation of IL-4, IL-8, IL-13, TGF-β and FCεR1A genes from 2.42- to 14.99-fold was observed in blood and abomasal tissue samples (p > .05), and IL-5, IL-8 and TGF-β genes had significant gene expression levels in blood (p < .05). For G-2, moderate gene expression levels, ranging from 1.22- to 3.45-fold, were observed in abomasal tissue (p > .05), and the IL-5 gene presented significant gene expression in blood (p < .05). Strong positively correlated values (r) between pcv and IL-4, IL-8 and TGF-β genes were observed in G-1. In contrast, significant negative correlations between epg and IL-4, IL-5 and FCεR1A genes indicate acute infection for G-2. Our results suggest that IL-4, IL-5, IL-13, TGF-β and FCεR1A genes are important modulators of GI nematode infections of Pelibuey lambs.

Distinct immune response profile during rhipicephalus (boophilus) microplus infestations of guzerat dairy herd according to the maternal lineage ancestry (mitochondrial DNA)

Rhipicephalus (Boophilus) microplus ticks cause major constraints to public and livestock health, and serious economic losses. It is well known that the immune response to infestations with cattle ticks is influenced by the host genetic background leading to distinct immunological profiles between bovine hosts genetically susceptible and resistant. The influence of Bos indicus (Bi) and Bos taurus (Bt) maternal lineage ancestry of mitochondrial DNA in the profile of the immune response of Zebu cattle to ticks remains unknown. The present work evaluated the hematological parameters and the immune response profile in the peripheral blood of a Guzerat dairy herd, further categorized into two maternal lineage ancestry subgroups (Bi-mtDNA and Bt-mtDNA) after experimental infestation with larvae of R. microplus. Our data demonstrated that although hematological and erythrogram analysis showed a similar profile throughout, some cell populations present a distinct profile between the groups. Especially MON, CD335⁺ and CD8⁺ T-cells are predominant in Bi-mtDNA. Moreover, an overall picture of R. microplus infestation demonstrated that Bi-mtDNA presented a more efficient and earlier innate immune response. Bi-mtDNA showed a greater number of connections with R. microplus counts and also with the CD25⁺ activation marker of the immune response. Bi-mtDNA showed greater number of connections, with an important participation of the innate immune while Bt-mtDNA showed a delay in the immune response. Elucidating the mechanisms by which resistant animals prevent heavy tick infestation is a crucial step in the development of predictive biomarkers for tick resistance for use in selective breeding programs, and is also potentially useful for the development of anti-tick vaccines.

Variation in phenotypic resistance to gastrointestinal nematodes in hair sheep in the humid tropics of Mexico

The objective of the study was to evaluate phenotypic resistance against gastrointestinal nematodes in Blackbelly, Pelibuey and Katahdin ewes before pregnancy in the humid tropics of Mexico. Individual faecal and blood samples were taken in 59 Pelibuey, 69 Blackbelly and 73 Katahdin ewes. The egg count per gram of faeces (EPG) of gastrointestinal nematodes (GINs) was determined. The percentage of packed cell volume (PCV) and body condition score (BCS) of each animal were also recorded. The ewes were segregated as susceptible, intermediate or resistant based on the EPG using the quartile method. The data were analysed using the general linear method, and the means between breeds were compared by Tukey's test. The relationships between the EPG, PCV and BCS were evaluated by Spearman correlation. The Katahdin ewes showed the highest EPG counts (3613.6 ± 5649) compared to the Blackbelly and Pelibuey ewes (576.1 ± 1009 and 56.8 ± 187, respectively, P < 0.01). The PCV values between breeds were similar (P > 0.05). The susceptible ewes had the highest EPG counts and the lowest PCV percentage (5069 ± 6404 and 22.8% ± 8.1% respectively) compared to the resistant ewes (P < 0.01). A higher percentage of Katahdin ewes were susceptible compared to the other breeds (P < 0.05). The main GIN species were Haemonchus contortus, Trichostrongylus colubriformis and Cooperia curticei. In conclusion, Katahdin ewes showed susceptibility to GIN compared to Blackbelly and Pelibuey ewes before the pregnancy period in the humid tropics of Mexico.

Comparing the phenotypic susceptibility of Pelibuey and Katahdin female lambs against natural gastrointestinal nematode infections under hot humid tropical conditions

This study compared the phenotypic susceptibility of Pelibuey and Katahdin female lambs against gastrointestinal nematodes (GINs) under grazing conditions in the hot humid tropics of Mexico. The study was performed during the rainy season (August to October, 2014). It included 27 Pelibuey and 12 Katahdin female lambs from 6 months of age and live weight of 21.0 ± 3.7 and 23.3 ± 3.6 kg, respectively. Lambs were reared free of GIN infection before the study. The study lasted 91 days. Animals were weighed and sampled (blood and feces) on days 0 and 28 and every 7 days onwards. Fecal samples were obtained to determine fecal eggs of GIN per gram (EPG), and blood samples were used to determine the packed cell volume (PCV), the peripheral eosinophil counts (PECs), and optical densities (ODs) for IgA. The EPG counts were significantly lower for Pelibuey lambs compared to Katahdin throughout the study (P < 0.001). Similarly, Pelibuey lambs had higher mean PCV (P < 0.01) and PEC (P < 0.05) than Katahdin lambs during the study. The total weight gain and OD for IgA were similar between breeds (P > 0.05). Negative associations (P < 0.05) between EPG and PCV or PEC were moderate to strong for the lambs of both breeds. No association was found between EPG and IgA. In conclusion, Pelibuey lambs showed phenotypic evidence of higher resistance to natural GIN infections compared to Katahdin lambs sharing the same grazing conditions in the hot humid tropics. The most accurate phenotypic markers to identify a difference in susceptibility were EPG and PEC.