Immune development and performance characteristics of Romney sheep selected for either resistance or resilience to gastrointestinal nematodes

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 fat 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.

Immunohistochemistry analyses of the abomasal mucosa show differences in cellular-mediated immune responses to Haemonchus contortus infection in resistant and susceptible young lambs

Haemonchus contortus is known for its high pathogenicity in sheep, and the uncontrolled use of anthelmintics resulted in the emergence of multiple drug-resistant populations. Breeding sheep for gastrointestinal nematode resistance is a sustainable alternative to reduce dependence of anthelmintic drugs, and differences in the degree of resistance between breeds have been reported. Here we compare two sheep breeds (Santa Ines and Ile de France), concerning the differences in innate and adaptive immune response involved in the resistance against H. contortus infection. Immunohistochemical analyses of the abomasum were conducted in naïve Santa Ines (n = 14) and Ile de France (n = 12) lambs randomized into four groups: infected Santa Ines (n = 8), non-infected control Santa Ines (n = 6), infected Ile de France (n = 8), and non-infected control Ile de France (n = 4). The infected lambs were initially infected with H. contortus infective larvae at 14 days of age, and multiple infections were conducted every second day until they reached 66 days of age. There was a significant effect (P < 0.001) of the infection with increase in numbers of CD3+ T; CD79α+ B; GATA3+ Th2/ILC2; POU2F3+ tuft cells; FOXP3+ T reg; and IgE + cells in the fundus of the abomasal mucosa in both Santa Ines and Ile de France lambs. Nevertheless, the infected Santa Ines lambs presented the highest averages for CD79α+ B; GATA3+ Th2/ILC; IgE + cells; and POU2F3+ tuft cells and there was a significant association of the breed and infection status with regards to POU2F3+ tuft cells, with the highest mean in the infected Santa Ines group. The infected Santa Ines group had three lambs with high degree of resistance and five lambs that showed a moderate infection. Our results suggest a mechanism of synergistic coordination between different immune-cell types in promoting resistance of suckling lambs under H. contortus infection.

Bacterial communities in the gastrointestinal tract segments of helminth-resistant and helminth-susceptible sheep

Background

Helminth parasitism is a world-wide problem in livestock industries, with major impacts on health, welfare and productivity. The role of the gut microbiota in host-helminth interactions in ruminants has been extensively examined and the present study added to this body of knowledge by assessing the effects of resistance and susceptibility to helminth infection in the gastro-intestinal tract (GIT). Australian Sheep Breeding Values (ASBVs) for faecal egg count (FEC) were used to select the 10 highly helminth-susceptible (High-FEC) and 10 highly helminth-resistant (Low-FEC) sheep. FEC status was confirmed during the experiment. Using samples from the faeces and the lumen of the rumen, abomasum, duodenum, jejunum, ileum, caecum, and colon, DNA was extracted and used for 16 rRNA gene amplicon sequencing.

Results

The most frequent genera identified along the GIT were Eubacterium, Oscillibacter, and Ruminococcus. Intersectoral-specialization zones were identified along the GIT, with the duodenum displaying major differences between the High-FEC and Low-FEC animals in values for alpha and beta diversity. After taking all samples into account and adjusting for GIT segment, the High-FEC and Low-FEC sheep differed significantly for four genera Butyrivibrio, Mycoplasma, Lachnoclostridium and Succiniclasticum. In the duodenum, the abundances of Aminipila, Lachnoclostridium and Mogibacterium differed significantly between the High-FEC and Low-FEC sheep. In the ileum, on the other hand, the genus Mycoplasma was significantly depleted in the Low-FEC group.

Conclusions

The gastro-intestinal microbial profile varies widely between helminth-resistant and helminth-susceptible sheep. Each GIT section appears to support a particular bacterial composition leading to inter-sectoral differences among the various microbial communities. The microbial populations were most rich and diverse in the duodenum of helminth-resistant sheep, comprising bacterial genera that generally ferment carbohydrates. This observation suggests that helminth-resistant sheep can reorganize the duodenal microbiome taxa which may restrict the development of parasites.

Supplementary Information

The online version contains supplementary material available at 10.1186/s42523-022-00172-1.

The critical importance of planned small ruminant livestock health and production in addressing global challenges surrounding food production and poverty alleviation

Small ruminants are particularly well suited to meet United Nations Sustainable Development Goals surrounding food security, human wellbeing and poverty alleviation in different environmental and climatic settings. However the current efficiency of food production from small ruminants in both developed agricultural regions and in lower and middle income countries is woefully inadequate to meet predicted global needs over the forthcoming decades. Most global research to address this challenge is focussed on the genetics of animal growth, conformation and disease tolerance or resistance traits, albeit the practical consequences of such selection and strategies to use genetically improved animals in the field are uncertain. Any long-term benefits derived from small ruminant genetic selection will only be impactful if steps are first taken to keep animals alive, healthy and productive through iterative planned health management. Parasites are the foremost global infectious disease constraints to efficient small ruminant production. Their genetic adaptability to exploit opportunities afforded by effects of climatic or management changes on free-living stages, or exposure of parasitic stages to drugs, presents specific challenges to their sustainable control. Hence, parasite control provides a relevant means of engagement with livestock keepers and farmers on the topic of planned animal health management. The value of parasitology in this regard is enhanced by the availability of simple to use and accessible diagnostic tools.

Saccharomyces cerevisiae (YT001) supplementation for the control of Haemonchus contortus and modulation of the immune response of sheep

Studies aiming at the development and evaluation of alternative methods to minimise losses caused by the gastrointestinal nematode Haemonchus contortus are extremely important. Such research is essential, given the high morbidity rates among sheep and the significant mortality rates of lambs, allied to the low efficacy of commercial products for the control of this parasite. The purpose of this study was to evaluate the effect of the Saccharomyces cerevisiae (YT001 - YEASTECH) on the control of H. contortus and its modulation of the immune response in experimentally infected sheep. Eighteen sheep were divided into two groups. Group 1, the control group, comprised animals infected with H. contortus and supplemented with distilled water, while Group 2, the treated group, consisted of animals infected and supplemented with S. cerevisiae (400 million cfu/day of suspension for 49 days). The following parasitological parameters were evaluated: number of eggs per gram of faeces, number of infective larvae (L3) recovered per faecal culture, and parasitic load of the abomasum. The following immunological parameters were quantified: immunoglobulin (Ig)A in the mucous secretions and serum IgG; cytokines interleukin (IL)-4, IL-5 and IL-10; number of eosinophils in the abomasal mucosa and groups of cells positive for the markers: MHCII, CD4⁺CD25⁺, CD5⁺CD8⁺, WC4, CD5⁺CD4⁺, CD8⁺CD11b⁺ and CD5⁺WC1 by whole blood flow cytometry. The results revealed a significant decrease (P<0.05) in the number of larvae and significantly higher serum IgG levels (P<0.05) in the group supplemented with S. cerevisiae. The supplemented animals showed significantly larger numbers of eosinophils (P<0.05), as well as more cells positive for MHCII, CD4⁺CD25⁺, CD5⁺CD8⁺ than the control animals. This study confirmed the beneficial action of S. cerevisiae on the host immune response to H. contortus, as evidenced mainly by the smaller number of L3 recovered from the faeces of sheep supplemented with S. cerevisiae.

Temporal changes in circulatory blood cell parameters of sheep genetically different for faecal worm egg count and diarrhoea from late summer to spring in a Mediterranean environment

Differences in haematology traits were investigated in worm-resistant and worm-susceptible Merino sheep that were genetically prone or less prone to developing diarrhoea in a winter rainfall region. The experiment was a 2 × 2 × 2 × 5 design. Male and female sheep aged 8–9 months old identified as having high (H) or low (L) breeding values for dags (breech soiling) and for worm egg counts (WEC) were allocated to one of four genetic groups: HH, HL, LH and LL (the first letter refers to dags and the second to WEC). The animals were sampled five times from autumn through winter to spring, in March, May, June (ewes only), July (rams only), August and in September after exposure to a natural paddock infection dominated by Trichostrongylus, Teladorsagia, Chabertia and Oesophagostomum. Measurements were packed cell volume, haemoglobin, red blood cell count, differential white cell counts, WEC and faecal consistency score. At all sampling times, there were large (P &lt; 0.01) differences between groups within sex for both the faecal consistency score and WEC. The genetic effect for WEC was large (P &lt; 0.01), as worm-susceptible sheep shed 10- (ewes) and 6-fold (rams) as many worm eggs at their WEC peak as the worm-resistant sheep at the end of the experiment. In the high-dag groups, the faecal consistency score was ~0.5 units higher for rams and 0.4 units higher for ewes, throughout the experiment. The optimum time to measure dags and WEC in this environment appears to be 6–8 weeks after the start of the winter rain. No differences were found among any of the treatment groups for any haematology trait. The haematogram changed significantly throughout the experiment, but genetic selection for low-dag score (reduced diarrhoea) or low WEC did not provide an acceptable discriminator in circulatory haematology traits between these genetically distinct groups of Merino sheep. It was concluded that circulatory haematology traits do not offer opportunities to select against diarrhoea or increased worm resistance in sheep.

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.

Comparison of the timing of development of immunity in Romney lambs from resistant and resilient selection lines

Variation in the timing of development of immunity to gastro-intestinal nematode parasites was assessed in resistant and resilient Romney selection lines exposed to mixed natural infection. From weaning, at mean 92 days-of-age, animals (n = 53) were sampled for faecal egg count (FEC) expressed as eggs per gram of faeces (epg), saliva for immunoglobulin (IgG and IgA) determination and fasted live weight (LW) every 10 days until 351 days-of-age. Overall, mean back-transformed FEC were consistently low for resistant animals (<200 epg) whereas resilient counterparts' FEC increased with time to reach a peak of 1400 epg at day 230 for females and 1800 epg for males at day 280 before declining to less than 500 epg by day 300, respectively (P < 0.001). Resistant lambs reached a threshold for Trichostrongylus colubriformis L3-specific IgG which was indicative of the presence of immunity earlier at 220.6 ± 8.8 days-of-age compared with resilient-line animals which reach this threshold 40 days later at 263.4 ± 6.9 days-of-age (P < 0.001). In addition, resistant females reached sexual maturity earlier compared with their resilient counterparts viz. 263.5 ± 3.7 c.f. 274.4 ± 3.4 days-of-age, respectively, (P = 0.048). Mean fasted live weight (LW) showed a selection line by time interaction (P < 0.001) which reflected greater LW in the early phase of the study in resilient males but increasing for all groups until day 280 before declining and being similar for all groups from day 330. In summary, differences appear to exist in the timing of immune development between these Romney lines, with resistant animals developing immunity earlier and these resistant-line animals also appear to be more physiologically mature at the same chronological age than resilient animals. These observations have implications on the timing of identification and selection of resistant animals.

100 Questions in Livestock Helminthology Research

An elicitation exercise was conducted to collect and identify pressing questions concerning the study of helminths in livestock, to help guide research priorities. Questions were invited from the research community in an inclusive way. Of 385 questions submitted, 100 were chosen by online vote, with priority given to open questions in important areas that are specific enough to permit investigation within a focused project or programme of research. The final list of questions was divided into ten themes. We present the questions and set them briefly in the context of the current state of knowledge. Although subjective, the results provide a snapshot of current concerns and perceived priorities in the field of livestock helminthology, and we hope that they will stimulate ongoing or new research efforts.