Major histocompatibility complex DRB1 gene: its role in nematode resistance in Suffolk and Texel sheep breeds

Reducing fecal egg count through selective breeding alters dorper lamb response to Haemonchus contortus in an artificial challenge trial

Infection by gastrointestinal nematodes (GIN), particularly Haemonchus contortus, can be detrimental to sheep health and performance. Genetic susceptibility to GIN varies between breeds, with those lacking high levels of natural resistance often requiring frequent anthelmintic treatment when facing parasitic challenge. Genetic technology can serve as a tool to decrease GIN susceptibility via selection for sheep with reduced fecal egg count (FEC) estimated breeding values (EBVs). However, the physiological changes that result from implementation of this strategy are not well described. Additionally, there is a need for comparison of animals from recent selective breeding against breeds with inherent GIN resistance. In this study we administered a challenge of H. contortus to Dorper x White Dorper (DWD; n = 92) lambs that have been genetically selected for either low (DWD-) or high (DWD+) FEC EBVs and Barbados Blackbelly x Mouflon (BBM; n = 19) lambs from a genetically resistant breed backgrounds. Lamb FEC, packed-cell volume (PCV) and serum IgG were measured at intermittent levels over 5 weeks. At day 21 and day 35, the selectively bred DWD- had a lower mean FEC compared to DWD+, but were higher than BBM. Reductions in both PCV and serum IgG from initial day 0 levels were observed in DWD lambs, but not in BBM. Furthermore, from a subset of lambs (n = 24) harvested at day 21, DWD- only tended (p = 0.056) to have lower mean worm counts than DWD+, with BBM having the lowest mean worm count. Differentially expressed genes (DEGs) identified via RNA-sequencing of abomasal tissue at day 21 indicate a more pronounced Th2 immune response and more rapid worm expulsion occurred in iBBM than iDWD- and iDWD+ lambs. However, gene expression in DWD- suggests an association between reduced FEC EBV and gastric acid secretion and the ability to limit worm fecundity. Ultimately, selection of Dorper sheep for low FEC EBV can reduce susceptibility to GIN, but it will likely require multiple generations with this trait as a breeding priority before presenting a similar resistance level to Caribbean breeds.

Putative SNPs in Ovar-DRB1 and GALNTL6 Genes Conferring Susceptibility to Natural Infection of Haemonchus Contortus in Southern Indian Sheep

AIM

To explore associations between phenotypic traits and polymorphisms in the DRB1 and GALNT6 gene in Nellore, Deccani and Kenguri sheep naturally infected with Haemonchus contortus.

MATERIALS AND METHODS

Blood and faecal samples were collected to evaluate fecal worm egg counts (FEC), packed cell volume (PCV), hemoglobin (Hb), eosinophilia and for DNA isolation.

RESULTS

Animals were grouped into susceptible and resistant groups based on EPG counts. FEC and circulating eosinophilia were higher in a susceptible group. Log FEC was negatively correlated (P < 0.01) with PCV, and Hb estimates. The second exon of DRB1 and intron variant of GALNTL6 genes were amplified from DNA samples of resistant and susceptible sheep. Characterization of Ovar-DRB1 amplicon by RFLP revealed two genotypes ('bb' and 'ab'). The genotype frequencies differed significantly between both groups (P < 0.05). The 'bb' genotypes had higher (P < 0.05) log FEC value than 'ab' genotypes and 'b' allele was linked with susceptibility to haemonchosis in sheep. The mean FEC of Nellore sheep was high indicating susceptibility of the breed and also in which the frequency of 'b' allele was more compared to the other two breeds. OVAR-DRB1 genotypes associated with FEC did not affect PCV and Hb. PCR-RFLP assay developed to determine the genotypes with respect to SNP rs424521894 of GALNTL6 revealed monomorphic nature at the locus in the breeds studied.

CONCLUSION

MHC polymorphism could be used as a genetic marker for the selection of sheep resistant to H. contortus. However, a more intensive study, involving controlled infections and other GALNTL6 SNPs may be enforced to make any decisive assertion.

Exploring genetic diversity and variation of Ovar-DRB1 gene in Sudan Desert Sheep using targeted next-generation sequencing

INTRODUCTION

The Ovar-DRB1 gene, a crucial element of the Major Histocompatibility Complex (MHC) Class II region, initiates adaptive immunity by presenting antigens to T-cells. Genetic diversity in sheep, particularly in MHC Class II genes like Ovar-DRB1, directly influences the specturm of presented antigens impacting immune responses and disease susceptability. Understanding the allelic diversity of Ovar-DRB1 gene in Sudan Desert Sheep (SDS) is essential for uncovering the genetic basis of immune responses and disease resistance, given the the breeds significance in Sudan's unique environment.

METHODS

Utilizing Targeted Next-Generation Sequencing (NGS) we explore allelic diversity in Ovar-DRB1 gene within SDS. Successfully ampliying and and sequencing the second exon of this gene in 288 SDS samples representing six breeds provided a comprehensive allelic profile, enabling a detalied examination of the gene's genetic makeup.

RESULTS

We identifed forty-six alleles, including four previously unreported, enrichness the genetic diversity of SDS breeds. These alleles exhibiting non-uniform distribution, varying frequencies across breeds, indicating a breed-specific genetic landscape. Certain alleles, known and novel, show higher frequencies in specific populations, suggesting potential associations with adaptive immune responses. Identifying these alleles sets the stage for investigating their functional roles and implications for disease resistance. Genetic differentiation among SDS breeds, as indicated by FST values and clustering analyses, highlights a unique genetic makeup shaped by geographic and historical factors. These differentiation patterns among SDS breeds have broader implications for breed conservation and targeted breeding to enhance disease resistance in specific populations.

CONCLUSION

This study unveils Ovar-DRB1 gene allelic diversity in SDS breeds through targeted NGS and genetic analyses, revealing new alleles that underscore the breeds' unique genetic profile. Insights into the genetic factors governing immune responses and disease resistance emerge, promising for optimization of breeding strategies for enhanced livestock health in Sudan's unique environment.

The Immune Response to Nematode Infection

Nematode infection is a major threat to the health of humans, domestic animals and wildlife. Nematodes vary in their effect on the host and in the mechanisms underlying immunity but the general features are becoming clear. There is considerable variation among individuals in resistance to infection and much of this variation is due to genetic variation in the immune response. The major histocompatibility complex has a strong influence on resistance to infection but other genes are collectively more important. Resistant individuals produce more IgA, eosinophils, IgE and mast cells than susceptible individuals and this is a consequence of stronger type 2 (Th2) immune responses. A variety of factors promote Th2 responses including genetic background, diet, molecules produced by the parasite and the location of the infection. A variety of cells and molecules including proteins, glycolipids and RNA act in concert to promote responses and to regulate the response. Nematodes themselves also modulate the host response and over 20 parasite-derived immunomodulatory molecules have been identified. Different species of nematodes modulate the immune response in different ways and probably use multiple molecules. The reasons for this are unclear and the interactions among immunomodulators have still to be investigated.

Investigating the development of diarrhoea through gene expression analysis in sheep genetically resistant to gastrointestinal helminth infection

Gastrointestinal helminths infect livestock causing health problems including severe diarrhoea. To explore the underlying biological mechanisms relating to development and control of diarrhoea, we compared 4 sheep that were susceptible to development of diarrhoea with 4 sheep that were diarrhoea-resistant. Transcriptomes in the tissues where the parasites were located were analyzed using RNASeq. By considering low-diarrhoea sheep as control, we identified 114 genes that were down-regulated and 552 genes that were up-regulated genes in the high-diarrhoea phenotype. Functional analysis of DEGs and PPI sub-network analysis showed that down-regulated genes in the high-diarrhoea phenotype were linked to biological processes and pathways that include suppression of ‘antigen processing and presentation’, ‘immune response’, and a list of biological functional terms related to ‘suppression in immune tolerance’. On the other hand, up-regulated genes in the high-diarrhoea phenotype probably contribute to repair processes associated with tissue damage, including ‘extracellular matrix organization’, ‘collagen fibril organization’, ‘tissue morphogenesis’, ‘circulatory system development’, ‘morphogenesis of an epithelium’, and ‘focal adhesion’. The genes with important roles in the responses to helminth infection could be targeted in breeding programs to prevent diarrhoea.

A novel 2 bp deletion variant in Ovine-DRB1 gene is associated with increased Visna/maedi susceptibility in Turkish sheep

Visna/maedi (VM) is a multisystemic lentivirus infection of sheep that affecting sheep industry across the globe. TMEM154 gene has been identified to be a major VM-associated host gene, nevertheless, a recent study showed that the frequency of the VM-resistant TMEM154 haplotypes was very low or absent in indigenous sheep. Thus, the present study was designed to determine other possible co-receptors associated with VM. For this purpose, DRB1 gene, which is renowned for its role in host immune response against various diseases was targeted. A total number of 151 case–control matched pairs were constructed from 2266 serologically tested sheep. A broad range of DRB1 haplotype diversity was detected by sequence-based genotyping. Moreover, a novel 2 bp deletion (del) in the DRB1 intron 1 was identified. For the final statistic, the sheep carrying VM-resistant TMEM154 diplotypes were removed and a McNemar’s test with a matched pairs experimental design was conducted. Consequently, it was identified for the first time that the 2 bp del variant is a genetic risk factor for VM (p value 0.002; chi-square 8.31; odds ratio 2.9; statistical power 0.90) in the dominant model. Thus, negative selection for 2 bp del variant could decrease VM infection risk in Turkish sheep.

Documentation of extensive genetic diversity in the Ovar-DRB1 gene in native Turkish sheep

Indigenous breeds have a high level of genetic diversity that might contribute to develop animal breeds with desired traits such as disease resistance and high productivity. Major histocompatibility complex (MHC) is a key component of adaptive immune system and consists of highly polymorphic genes that take part in adaptive immune response and disease resistance. Exploring and understanding the effect of polymorphisms in MHC could be beneficial to future animal breeding strategies. In this study, we sequenced the highly polymorphic Exon2 of the ovine DRB1 gene using Sanger sequencing to explore the diversity of this gene in six indigenous Turkish sheep breeds and two crossbreeds. In total, 894 haplotypes from 447 sheep were investigated, and 69 different haplotypes including 27 novel ones were identified. Among the identified haplotypes there were common and breed specific haplotypes. There was a relatively high diversity of the alleles within indigenous breeds. Allelic diversity patterns were mostly associated with geographical differences. The results of this study highlight the genetic variation within indigenous breeds which has important implications for biodiversity and the adaptability of breeds to specific environments. There is value to further studies which include other genomic regions and traits, and these could guide breeding strategies.

Single nucleotide polymorphisms from candidate genes associated with nematode resistance and resilience in Corriedale and Pampinta sheep in Argentina

Selective breeding of genetically resistant animals is considered a promising strategy to face the problem of nematode resistance to anthelmintics and mitigate concerns about the presence of chemical residues in animal food products and the environment. Gastrointestinal nematode resistance is a complex, multifactorial trait related to host immunity. However, the mechanisms underlying host resistance and response to infection remain to be fully elucidated. In this context, the objective of this study was to provide insight into the chromosomal regions determining nematode resistance and resilience in Corriedale and resistance in Pampinta sheep breeds. A total of 170 single nucleotide polymorphisms (SNP) from 76 candidate genes for immune response were studied in 624 Corriedale and 304 Pampinta animals. Lambs underwent artificial or natural challenges with infective larvae mainly from Haemonchus contortus. Fecal egg counts, estimated breeding values for fecal egg counts, and rate of packed cell volume change and FAMACHA© score change over the challenge were used, when available, as indicators of host parasite resistance or resilience. Phenotype-genotype association studies were conducted and significance values obtained were adjusted for multiple testing errors. Eight SNPs, located on OARs 3, 6, 12, and 20, reached significance in Corriedale sheep under artificial challenge. Those SNP represent allelic variants from the MHC-Ovine Lymphocyte Antigen-DRA, two C-type lectin domain families, the Interleukin 2 receptor β, the Toll-like receptor 10, the Mannan binding lectin serine peptidase 2, and the NLR family, CARD domain containing 4 genes. On Pampinta lambs under natural challenge, we found three significant SNPs, located in the TIMP metallopeptidase inhibitor 3, the FBJ murine osteosarcoma viral oncogene homolog, and the Interleukin 20 receptor alpha genes, on OARs 3, 7, and 8, respectively. The results obtained herein confirm genomic regions previously reported as associated with nematode resistance in other sheep breeds, reinforcing their role in host response to parasites. These findings contribute to gain knowledge on parasite resistance and resilience in Corriedale sheep and report for the first time SNPs associated with resistance to gastrointestinal parasite infections in Pampinta breed.

Immunoglobulins as Biomarkers for Gastrointestinal Nematodes Resistance in Small Ruminants: A systematic review

The rise of anthelmintic resistance worldwide has led to the development of alternative control strategies for gastrointestinal nematodes (GIN) infections, which are one of the main constraints on the health of grazing small ruminants. Presently, breeding schemes rely mainly on fecal egg count (FEC) measurements on infected animals which are time-consuming and requires expertise in parasitology. Identifying and understanding the role of immunoglobulins in the mechanisms of resistance could provide a more efficient and sustainable method of identifying nematode-resistant animals for selection. In this study we review the findings on immunoglobulin response to GIN in the literature published to date (june 2019) and discuss the potential to use immunoglobulins as biomarkers. The literature review revealed 41 studies which measured at least one immunoglobulin: 35 focused on lamb immune response (18 used non-naïve lambs) and 7 on yearlings. In this review we propose a conceptual model summarizing the role of immunoglobulins in resistance to GIN. We highlight the need for more carefully designed and documented studies to allow comparisons across different populations on the immunoglobulin response to GIN infection.

Dynamic transcriptomic changes of goat abomasal mucosa in response to Haemonchus contortus infection

Gastrointestinal nematode (GIN) infections are one of the major constraints for grazing sheep and goat production worldwide. Genetic selection for resistant animals is a promising control strategy. Whole-transcriptome analysis via RNA-sequencing (RNA-seq) provides knowledge of the mechanisms responsible for complex traits such as resistance to GIN infections. In this study, we used RNA-seq to monitor the dynamics of the response of the abomasal mucosa of Creole goat kids infected with Haemonchus contortus by comparing resistant and susceptible genotypes. A total of 8 cannulated kids, 4 susceptible and 4 resistant to GIN, were infected twice with 10 000 L3 H. contortus. During the second infection, abomasal mucosal biopsies were collected at 0, 8, 15 and 35 days post-infection (dpi) from all kids for RNA-seq analysis. The resistant animals showed early activation of biological processes related to the immune response. The top 20 canonical pathways of differentially expressed genes for different comparison showed activation of the immune response through many relevant pathways including the Th1 response. Interestingly, our results showed a simultaneous time series activation of Th2 related genes in resistant compared to susceptible kids.

Liver fluke in Irish sheep: prevalence and associations with management practices and co-infection with rumen fluke

BACKGROUND

The present study aimed to identify the national prevalence of Fasciola hepatica in Irish sheep and to conduct a risk analysis assessment based on management and treatment practices in participating flocks. Also, co-infection with rumen fluke was quantified and its association with liver fluke and management practices was assessed.

METHODS

A total of 305 sheep flocks were selected ensuring even national representation of the sheep population. Participating farms were asked to complete a survey questionnaire on farm management practices and submit faecal samples during the winter of 2014-2015. Pooled faecal samples were analysed for the presence of F. hepatica and co-infection with rumen fluke. Apparent and true prevalence were calculated, additionally, the rate of co-infection with rumen fluke was also obtained. Correlation and regression analyses were used for assessing associations between management practices, liver fluke infection and co-infection with rumen fluke.

RESULTS

The national true prevalence of F. hepatica was 50.4% (n = 305). Regional prevalence varied from 41% in the east to 52% in the south. Co-infection with rumen fluke was observed in 40% of the studied population and correlated with increased F. hepatica egg counts (OR = 2.9; P ≤ 0.001). Predominant breeds were Suffolk, Texel and Horned Mountain breeds. Beef cattle were the most frequent type of other livestock present on farms and mixed species grazing was frequently reported (73%). More than half of the flocks reported a mid-to-late lambing period (March-April). Use of mountain land for grazing was of 32%. Flukicides were most commonly used twice over the autumn-winter period. Regression analyses highlighted significant association of F. hepatica status, with the presence of other livestock on farm, frequency of flukicides used during the winter and clinical presentation of liver fluke. A significant increase in eggs per gram of faeces was observed in Charollais sheep in comparison with all other breeds. Co-infection with F. hepatica and Calicophoron daubneyi was also significantly associated with the presence of other livestock on the farm, type of flukicide used and clinical fasciolosis.

CONCLUSIONS

The present study provides up-to-date information on the prevalence of F. hepatica in Irish sheep and adds insight to the epidemiology of the disease. These findings will be useful for designing new holistic control measures for F. hepatica infection.

Identification of the amino acids in the Major Histocompatibility Complex class II region of Scottish Blackface sheep that are associated with resistance to nematode infection

Lambs with the Major Histocompatibility Complex DRB1*1101 allele have been shown to produce fewer nematode eggs following natural and deliberate infection. These sheep also possess fewer adult Teladorsagia circumcincta than sheep with alternative alleles at the DRB1 locus. However, it is unclear if this allele is responsible for the reduced egg counts or merely acts as a marker for a linked gene. This study defined the MHC haplotypes in a population of naturally infected Scottish Blackface sheep by PCR amplification and sequencing, and examined the associations between MHC haplotypes and faecal egg counts by generalised linear mixed modelling. The DRB1*1101 allele occurred predominately on one haplotype and a comparison of haplotypes indicated that the causal mutation or mutations occurred in or around this locus. Additional comparisons with another resistant haplotype indicated that mutations in or around the DQB2*GU191460 allele were also responsible for resistance to nematode infections. Further analyses identified six amino acid substitutions in the antigen binding site of DRB1*1101 that were significantly associated with reductions in the numbers of adult T. circumcincta.

Association of MHC class II haplotypes with reduced faecal nematode egg count and IgA activity in British Texel sheep

Nematode infection is one of the principal diseases suffered by sheep and the class II region of the MHC has been repeatedly associated with differences in susceptibility and resistance to infection. The aim of this study was to examine the association of MHC class II haplotypes in a flock of Texel sheep with faecal egg counts and antibody responsiveness. Two haplotypes carried the DRB1*11:01 allele which has previously been associated with reduced egg counts in Scottish Blackface and Suffolk sheep. One of the two haplotypes was associated with reduced egg counts in the Texel breed, and both haplotypes were associated with reduced IgA activity against an extract from fourth-stage larvae. The reduced IgA activity is probably a consequence of reduced numbers of fourth-stage larvae in sheep carrying the resistance allele. The association of specific MHC alleles with reduced egg counts, reduced worm numbers and decreased IgA activity provides a mechanism for the density-dependent regulation of parasite growth and fecundity.

Effect of Ovar-DRA and Ovar-DRB1 genotype in small ruminants with haemonchosis

The effect of Ovar-DRA and Ovar-DRB1 genotypes on faecal egg count (FEC) was determined in sheep and goats infected with Haemonchus contortus. One hundred and forty-three sheep from 3 different breeds (St. Croix, Katahdin and Dorper) and 150 goats from three different breeds (Spanish, Boer and Kiko) were used. Parasitological (FEC), haematological (packed cell volume) and immunological (IgA, IgG and IgM) parameters were measured. Sheep populations showed a higher FEC and humoural response than goat breeds. Genotypes were determined by high-resolution melting assays and by conventional PCR. For Ovar-DRA, sheep and goats carrying the AA genotype showed significant lower FEC than AG and GG genotypes. The additive effect was found to be 115.35 less eggs per gram of faeces for the A allele for goats. For Ovar-DRB1, only in sheep, the GC genotype was associated with low FEC. The additive effect was 316.48 less eggs per gram of faeces for the G allele, and the dominance effect was 538.70 less eggs per gram of faeces. The results indicate that single nucleotide polymorphisms within Ovar-DRA and Ovar-DRB1 could be potential markers to be used in selection programmes for improving resistance to Haemonchus contortus infection.

Genetic characterization of some Turkish sheep breeds based on the sequencing of the Ovar-DRB1 gene in the major histocompatibility complex (MHC) gene region

In this research, Ovar-DRB1 gene in the major histocompatibility complex (MHC) gene region was surveyed by DNA sequencing in some of the native sheep breeds that are reared in Turkey. A total of 80 samples were collected from eight different Turkish native sheep breeds, and these samples were used for DNA sequencing.

The exon 2 region of Ovar-DRB1 in the MHC gene region was polymerase chain reaction (PCR) amplified and sequenced. A total of 25 new alleles were revealed in the Ovar-DRB1 gene in Turkish native sheep breeds with 24 variable sites; only 13 sites were parsimony informative. The average pairwise genetic distance was 0.029 % for the Ovar-DRB1 gene exon 2 region. The sequence variations at eight different positions (7026, 7036, 7040, 7053, 7059, 7069, 7131 and 7214) are found in all of the studied samples. G → C transversion at position 7081 is only seen in Akkaraman sheep breed, whereas T → C transition at position 7097 is only seen in one sample from the Akkaraman breed. Overall, two main groups were detected among the 25 alleles from Turkish native sheep breeds. All Daǧliç and Kivircik alleles and one allele from Karayaka, Malya and Sakiz are grouped together while all the other breeds are grouped in the other branch.

Exploring the molecular basis of resistance/ susceptibility to mixed natural infection of Haemonchus contortus in tropical Indian goat breed

The present investigation was carried out with the objective to identify putative candidate genes / Quantitative trait loci for resistance / susceptibility towards Haemonchus infestation in tropical goat breed (Rohilkhandi goat) of India. The mean faecal egg count (FEC) and packed cell volume (PCV) of the population were 142.78 ± 22.54 epg (eggs per gram) and 31.73% ± 0.49, respectively. Grouping of animals as per dot ELISA test showed 41.33% (n = 124) positive and 58.66% (n = 176) negative for Haemonchus infestation. The microsatellite loci DYA and ODRB1.2 were significantly associated (P ≤ 0.05) to parasite resistance. The locus DYA showed significant association with log FEC and dot ELISA and the locus ODRB1.2 showed significant association with log FEC, PCV and dot ELISA at P ≤ 0.05. Real time expression profiling revealed that the susceptible group (high FEC group) had 11.1-fold more expression of IFNγ mRNA (Th₁ cytokine) and 0.11-fold lower expression of IL-10 mRNA (Th₂ cytokine), which was found to be statistically significant (P ≤ 0.05).

Immunogenomics of gastrointestinal nematode infection in ruminants - breeding for resistance to produce food sustainably and safely

Gastrointestinal nematode (GIN) infection of ruminants represents a major health and welfare challenge for livestock producers worldwide. The emergence of anthelmintic resistance in important GIN species and the associated animal welfare concerns have stimulated interest in the development of alternative and more sustainable strategies aimed at the effective management of the impact of GINs. These integrative strategies include selective breeding using genetic/genomic tools, grazing management, biological control, nutritional supplementation, vaccination and targeted selective treatment. In this review, the logic of selecting for "resistance" to GIN infection as opposed to "resilience" or "tolerance" is discussed. This is followed by a review of the potential application of immunogenomics to genetic selection for animals that have the capacity to withstand the impact of GIN infection. Advances in relevant genomic technologies are highlighted together with how these tools can be advanced to support the integration of immunogenomic information into ruminant breeding programmes.

The genetic architecture of the MHC class II region in British Texel sheep

Understanding the structure of the major histocompatibility complex, especially the number and frequency of alleles, loci and haplotypes, is crucial for efficient investigation of the way in which the MHC influences susceptibility to disease. Nematode infection is one of the most important diseases suffered by sheep, and the class II region has been repeatedly associated with differences in susceptibility and resistance to infection. Texel sheep are widely used in many different countries and are relatively resistant to infection. This study determined the number and frequency of MHC class II genes in a small flock of Texel sheep. There were 18 alleles at DRB1, 9 alleles at DQA1, 13 alleles at DQB1, 8 alleles at DQA2 and 16 alleles at DQB2. Several haplotypes had no detectable gene products at DQA1, DQB1 or DQB2, and these were defined as null alleles. Despite the large numbers of alleles, there were only 21 distinct haplotypes in the population. The relatively small number of observed haplotypes will simplify finding disease associations because common haplotypes provide more statistical power but complicate the discrimination of causative mutations from linked marker loci.

Breeding for resistance to gastrointestinal nematodes - the potential in low-input/output small ruminant production systems

The control of gastrointestinal nematodes (GIN) is mainly based on the use of drugs, grazing management, use of copper oxide wire particles and bioactive forages. Resistance to anthelmintic drugs in small ruminants is documented worldwide. Host genetic resistance to parasites, has been increasingly used as a complementary control strategy, along with the conventional intervention methods mentioned above. Genetic diversity in resistance to GIN has been well studied in experimental and commercial flocks in temperate climates and more developed economies. However, there are very few report outputs from the more extensive low-input/output smallholder systems in developing and emerging countries. Furthermore, results on quantitative trait loci (QTL) associated with nematode resistance from various studies have not always been consistent, mainly due to the different nematodes studied, different host breeds, ages, climates, natural infections versus artificial challenges, infection level at sampling periods, among others. The increasing use of genetic markers (Single Nucleotide Polymorphisms, SNPs) in GWAS or the use of whole genome sequence data and a plethora of analytic methods offer the potential to identify loci or regions associated nematode resistance. Genomic selection as a genome-wide level method overcomes the need to identify candidate genes. Benefits in genomic selection are now being realised in dairy cattle and sheep under commercial settings in the more advanced countries. However, despite the commercial benefits of using these tools, there are practical problems associated with incorporating the use of marker-assisted selection or genomic selection in low-input/output smallholder farming systems breeding schemes. Unlike anthelmintic resistance, there is no empirical evidence suggesting that nematodes will evolve rapidly in response to resistant hosts. The strategy of nematode control has evolved to a more practical manipulation of host-parasite equilibrium in grazing systems by implementation of various strategies, in which improvement of genetic resistance of small ruminant should be included. Therefore, selection for resistant hosts can be considered as one of the sustainable control strategy, although it will be most effective when used to complement other control strategies such as grazing management and improving efficiency of anthelmintics currently.

Rumen fluke in Irish sheep: prevalence, risk factors and molecular identification of two paramphistome species

BACKGROUND

Rumen flukes are trematode parasites found globally; in tropical and sub-tropical climates, infection can result in paramphistomosis, which can have a deleterious impact on livestock. In Europe, rumen fluke is not regarded as a clinically significant parasite, recently however, the prevalence of rumen fluke has sharply increased and several outbreaks of clinical paramphistomosis have been reported. Gaining a better understanding of rumen fluke transmission and identification of risk factors is crucial to improve the control of this parasitic disease. In this regard, a national prevalence study of rumen fluke infection and an investigation of associated risk factors were conducted in Irish sheep flocks between November 2014 and January 2015. In addition, a molecular identification of the rumen fluke species present in Ireland was carried out using an isolation method of individual eggs from faecal material coupled with a PCR. After the DNA extraction of 54 individual eggs, the nuclear fragment ITS-2 was amplified and sequenced using the same primers.

RESULTS

An apparent herd prevalence of 77.3 % was determined. Several risk factors were identified including type of pasture grazed, regional variation, and sharing of the paddocks with other livestock species. A novel relationship between the Suffolk breed and higher FEC was reported for the first time. The predominant rumen fluke species found was C. daubneyi. Nevertheless, P. leydeni was unexpectedly identified infecting sheep in Ireland for the first time.

CONCLUSIONS

An exceptionally high prevalence of rumen fluke among Irish sheep flocks has been highlighted in this study and a more thorough investigation is necessary to analyse its economic impact. The isolation of individual eggs coupled with the PCR technique used here has proven a reliable tool for discrimination of Paramphistomum spp. This technique may facilitate forthcoming studies of the effects of paramphistomosis on livestock production. The most noteworthy finding was the identification of P. leydeni affecting sheep in Ireland, however further studies are required to clarify its implications. Also, a significant relationship between Suffolk breed and a heavier infection was found, which can be used as a starting point for future research on control strategies of rumen fluke infection.

Genomic Regions Associated with Sheep Resistance to Gastrointestinal Nematodes

Genetic markers for sheep resistance to gastrointestinal parasites have long been sought by the livestock industry as a way to select more resistant individuals and to help farmers reduce parasite transmission by identifying and removing high egg shedders from the flock. Polymorphisms related to the major histocompatibility complex and interferon (IFN)-γ genes have been the most frequently reported markers associated with infection. Recently, a new picture is emerging from genome-wide studies, showing that not only immune mechanisms are important determinants of host resistance but that gastrointestinal mucus production and hemostasis pathways may also play a role.

Major Histocompatibility Complex class IIB polymorphism in an ancient Spanish breed

Genes from the Major Histocompatibility Complex class II region are involved in the presentation of antigens. Therefore, they have the key role in regulating the immune response and in the resistance to infections. We investigated the Major Histocompatibility Complex class IIB genes, DRB and DQB, in Churra sheep, one of the most important indigenous breeds of Spain. These genes are among the most polymorphic in the mammalian genome. Furthermore, often different numbers of class IIB genes per haplotype exist, complicating the genotyping and sequencing of these genes. Especially the DQB region is only partially characterized in sheep and the repertoire of DRB and DQB alleles in Churra sheep, an ancient breed, is unknown. Here, we sequenced the class IIB genes for 15 rams that are the pedigree heads of a selection Nucleus herd. In total, we found 12 DRB and 25 DQB alleles. From these, 3 and 15 were new, respectively. Fourteen haplotypes carrying one or two DQB alleles could be deduced and the evolutionary relationship of these was investigated by phylogenetic trees. Based on the sequences of these most common class II alleles, a more efficient genotyping system for larger numbers of Churra sheep will be developed.

Fecal egg counts for gastrointestinal nematodes are associated with a polymorphism in the MHC-DRB1 gene in the Iranian Ghezel sheep breed

Genetic variation among sheep breeds in resistance to gastrointestinal nematodes (GIN) has been demonstrated in several production environments. Relationships between the ovine major histocompatibility complex and resistance to GIN have been studied, but few studies have systematically examined this issue in less-developed and semi-arid regions. The aim of the current study was to explore associations between fecal worm egg counts (FEC) for several GIN and polymorphisms in the DRB1 gene. One hundred male lambs were selected at 4–6 months of age from weaned animals in five flocks (n = 20 per flock). Body weights were determined, FAMACHA scores based on color of the ocular mucous membranes were assigned as an indicator of anemia, and blood and fecal samples were collected twice to evaluate FEC and blood packed cell volume (PCV) and for DNA isolation. A repeated-measures analysis of variance was used to test effects of genotype on FEC. The model included fixed effects of flock, genotype, time of measurement (1 or 2), and flock × time and genoype × time interactions, and a random (repeated) effect of lamb. Two genotypes (A1A1 and A1A2) were observed following digestion of Region 1 of Ovar-DRB1 with PstI. Genotypic frequencies were 0.73 for A1A1 and 0.27 for A1A2. FEC differed between Ovar_DRB1 genotypes A1A1 and A1A2 for Marshallagia marshalli, Strongyle, and total nematode FEC. Observed FEC were 30–41% lower for genotype A1A1. Differences among genotypes were consistent across measurement times, with no effect of genotype × measurement time interaction for any parasite class (P ≥ 0.34). A significant association was observed between FAMACHA scores and lamb PCV, and the residual correlation between these two variables was -0.51 (P < 0.001). FAMACHA scores can thus be used to detect differences among lambs in PCV, and polymorphic markers of Ovar-DRB1 have potential value as an indicator of parasite resistance in applied animal breeding programs on sheep farms in this region.

An evolutionary perspective on gastrointestinal nematodes of sheep

The purpose of this paper was to discuss from an evolutionary perspective the interaction between domestic sheep (Ovis aries) and their gastrointestinal nematodes. Although evolution is the central theme of biology, there has been little attempt to consider how evolutionary forces have shaped and continue to shape the relationships between domestic animals and their parasite community. Mathematical modelling of the host-parasite relationship indicated that the system is remarkably robust to perturbations in its parameters. This robustness may be a consequence of the long coevolution of host and parasites. Although nematodes can potentially evolve faster than the host, coevolution is not dominated by the parasite and there are several examples where breeds of cattle or sheep have evolved high levels of resistance to disease. Coevolution is a more equal partnership between host and nematode than is commonly assumed. Coevolution between parasites and the host immune system is often described as an arms race where both host immune response genes and parasite proteins evolve rapidly in response to each other. However, initial results indicate that nematode antigens are not evolving rapidly; the arms race between the immune system and nematodes, if it exists, is happening very slowly. Fisher's fundamental theorem of natural selection states that genes with positive effects on fitness will be fixed by natural selection. Consequently, heritable variation in fitness traits is expected to be low. Contrary to this argument, there is considerable genetic variation in resistance to nematode infection. In particular, the heritabilities of nematode-specific IgA and IgE activity are moderate to high. The reasons for this apparent violation of the fundamental theorem of natural selection are not clear but several possible explanations are explored. Faecal nematode egg counts increase at the beginning of the grazing season - a phenomenon known as the periparturient rise. This increase benefits host and parasite and appears to be a consequence of coevolution. In conclusion, an evolutionary perspective can shed light on many aspects of the host-parasite relationship in domestic animals.

MHC-DQB1 Variation and Its Association with Resistance or Susceptibility to Cystic Echinococcosis in Chinese Merino Sheep

Cystic echinococcosis (CE), one of the world’s most geographically widespread diseases, still represents a considerable economic and public health significance, although a variety of methods has been used to control the disease. It has been demonstrated that genetic factors, especially variations in MHC loci, can influence the outcome of CE infection in the human population. The study described here was designed to determine whether variation in MHC-DQB1 was associated with susceptibility or resistance to CE in sheep. If so, it would lay a theoretical foundation for breeding disease resistance sheep in future. This study was carried out on 204 Chinese Merino sheep, including 101 CE sheep and 103 healthy controls. The polymorphism of MHC-DQB1 exon 2 was detected by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP) method, and x² test was used to compare genotype frequencies between CE sheep and healthy controls. A total of 22 alleles and 42 genotypes were identified in DQB1 exon 2 in Chinese Merino sheep. In addition, x² test showed that frequencies of DQB1-TaqIaa and DQB1-HaeIIInn genotypes were significantly higher in the healthy group (82.5% and 57.3%, respectively) than that in the CE group (57.4% and 28.9%, respectively) (both p values = 0, OR = 0.286, 0.303, respectively), suggesting that these genotypes appeared to be associated with resistance to CE. Whereas, frequencies of DQB1-TaqIab and DQB1-HaeIIImn genotypes were significantly higher in the CE group (36.9% and 32.0%, respectively), as compared with the healthy group (16.5% and 11.15%, respectively) (p = 0.001, 0.001 and OR = 2.963, 3.629, respectively), indicating that these genotypes might be associated with susceptibility to CE. It is concluded that the genetic polymorphism within MHC-DQB1 might influence immune responses to pathogens, thus leading to the development of CE or protection against CE in Chinese Merino sheep, which would pave the way for breeding disease resistance sheep in future.

Functional investigation of a QTL affecting resistance to Haemonchus contortus in sheep

This study reports a functional characterization of a limited segment (QTL) of sheep chromosome 12 associated with resistance to the abomasal nematode Haemonchus contortus. The first objective was to validate the identified QTL through the comparison of genetically susceptible (N) and resistant (R) sheep produced from Martinik × Romane back-cross sheep. The R and N genotype groups were then experimentally infected with 10 000 H. contortus larvae and measured for FEC (every three days from 18 to 30 days post-challenge), haematocrit, worm burden and fertility. Significant differences in FEC and haematocrit drop were found between R and N sheep. In addition, the female worms recovered from R sheep were less fecund. The second step of the characterization was to investigate functional mechanisms associated with the QTL, thanks to a gene expression analysis performed on the abomasal mucosa and the abomasal lymph node. The gene expression level of a candidate gene lying within the QTL region (PAPP-A2) was measured. In addition, putative interactions between the chromosome segment under study and the top ten differentially expressed genes between resistant MBB and susceptible RMN sheep highlighted in a previous microarray experiment were investigated. We found an induction of Th-2 related cytokine genes expression in the abomasal mucosa of R sheep. Down-regulation of the PAPP-A2 gene expression was observed between naïve and challenged sheep although no differential expression was recorded between challenged R and N sheep. The genotyping of this limited region should contribute to the ability to predict the intrinsic resistance level of sheep.

MHC-DRB1/DQB1 Gene Polymorphism and Its Association with Resistance/Susceptibility to Cystic Echinococcosis in Chinese Merino Sheep

The aim of this study was to analyze the relationship between polymorphism of the MHC-DRB1/DQB1 gene and its resistance to Cystic Echinococcosis (C.E), as well as to screen out the molecular genetic marker of antiechinococcosis in Chinese Merino sheep. The MHCII-DRB1/DQB1 exon 2 was amplified by polymerase chain reaction (PCR) from DNA samples of healthy and hydatidosis sheep. PCR products were characterized by restriction fragment length polymorphism (RFLP) technique. Five restriction enzymes (Mval, HaeIII, SacI, SacII, and Hin1I) were employed to cut DRB1, while seven restriction enzymes (MroxI, ScaI, SacII, NciI, TaqI, Mval, and HaeIII) were employed to cut DQB1.Results showed that frequencies of patterns Mvalbb (P < 0.01), SacIab in DRB1 exon 2 (P < 0.05), and TaqIaa, HaeIIInn (P < 0.01) in DQB1 exon 2 were significantly higher in the healthy group compared with the C.E individuals, which implied that there was a strong association between these genotypes and hydatidosis resistance or susceptibility. Chi-square test showed that individuals with the genic haplotype DRB1-SacIab/DRB1-Mvalbb/DQB1-TaqIaa/DQB1-HaeIIInn (P < 0.01) were relatively resistant to C.E, while individuals with the genic haplotypes DRB1-Mvalbc/DQB1-Mvalyy/DQB1-TaqIab/DQB1-HaeIIImn (P < 0.01) and DRB1-Mvalbb/DQB1-Mvalcc/DQB1-TaqIab/DQB1-HaeIIImn (P < 0.01) were more susceptible to C.E. In addition, to confirm these results, a fielding experiment was performed with Chinese Merino sheep which were artificially infected with E.g. The result was in accordance with the results of the first study. In conclusion, MHC-DRB1/DQB1 exon 2 plays an important role as resistant to C.E in Chinese Merino sheep. In addition, the molecular genetic marker of antiechinococcosis (DRB1-SacIab/DRB1-Mvalbb/DQB1-TaqIaa/DQB1-HaeIIInn) was screened out in Chinese Merino sheep.

The immunology and genetics of resistance of sheep to Teladorsagia circumcincta

Teladorsagia circumcincta is one of the most economically important gastrointestinal nematode parasites of sheep in cool temperate regions, to which sheep show genetically-varying resistance to infection. This is a very common parasite and viable sheep production requires the extensive use of anthelmintic drugs. However, the emergence of drug-resistant parasites has stimulated the search for alternative control strategies to curb production losses. Lambs become infected soon after weaning and begin to control parasite burden within 8-10 weeks of continual infection. This control is an acquired characteristic mediated by the development of parasite-specific antibodies. This paper describes the immunology associated with resistance and susceptibility, focussing on differential T cell activation that regulates the production of specific effector mechanisms. It continues by summarizing the methods used to identify genes that could be exploited as molecular markers of selection for resistance. In particular it focusses on the link between understanding the molecular immunology of infection and the identification of candidate genes for selection.

Detecting genes for variation in parasite burden and immunological traits in a wild population: testing the candidate gene approach

Identifying the genes underlying phenotypic variation in natural populations can provide novel insight into the evolutionary process. The candidate gene approach has been applied to studies of a number of traits in various species, in an attempt to elucidate their genetic basis. Here, we test the application of the candidate gene approach to identify the loci involved in variation in gastrointestinal parasite burden, a complex trait likely to be controlled by many loci, in a wild population of Soay sheep. A comprehensive literature review, Gene Ontology databases, and comparative genomics resources between cattle and sheep were used to generate a list of candidate genes. In a pilot study, these candidates, along with 50 random genes, were then sequenced in two pools of Soay sheep; one with low gastrointestinal nematode burden and the other high, using a NimbleGen sequence capture experiment. Further candidates were identified from single nucleotide polymorphisms (SNPs) that were highly differentiated between high- and low-resistance sheep breeds. A panel of 192 candidate and control SNPs were then typed in 960 individual Soay sheep to examine whether they individually explained variation in parasite burden, as measured as faecal egg count, as well as two immune measures (Teladorsagia circumcincta-specific antibodies and antinuclear antibodies). The cumulative effect of the candidate and control SNPs were estimated by fitting genetic relationship matrices (GRMs) as random effects in animal models of the three traits. No more significant SNPs were identified in the pilot sequencing experiment and association study than expected by chance. Furthermore, no significant difference was found between the proportions of candidate or control SNPs that were found to be significantly associated with parasite burden/immune measures. No significant effect of the candidate or control gene GRMs was found. There is thus little support for the candidate gene approach to the identification of loci explaining variation in parasitological and immunological traits in this population. However, a number of SNPs explained significant variation in multiple traits and significant correlations were found between the proportions of variance explained by individual SNPs across multiple traits. The significant SNPs identified in this study may still, therefore, merit further investigation.

Conserved haplotype blocks within the sheep MHC and low SNP heterozygosity in the Class IIa subregion

This report describes single-nucleotide polymorphisms (SNPs) in the sheep major histocompatibility complex (MHC) class II and class III regions and provides insights into the internal structure of this important genomic complex. MHC haplotypes were deduced from sheep family trios based on genotypes from 20 novel SNPs representative of the class II region and 10 previously described SNPs spanning the class III region. All 30 SNPs exhibited Hardy-Weinberg proportions in the sheep population studied. Recombination within an extended sire haplotype was observed within the class II region for 4 of 20 sheep chromosomes, thereby supporting the presence of separated IIa and IIb subregions similar to those present in cattle. SNP heterozygosity varied across the class II and III regions. One segment of the class IIa subregion manifested very low heterozygosity for several SNPs spanning approximately 120 Kbp. This feature corresponds to a subregion within the human MHC class II region previously described as a 'SNP desert' because of its paucity of SNPs. Linkage disequilibrium (LD) was reduced at the junction separating the putative class IIb and IIa subregions and also between the class IIa and the class III subregions. The latter observation is consistent with either an unmapped physical separation at this location or more likely a boundary characterized by more frequent recombination between two conserved subregions, each manifesting high within-block LD. These results identify internal blocks of loci in the sheep MHC, within which recombination is relatively rare.

The influence of MHC and immunoglobulins a and e on host resistance to gastrointestinal nematodes in sheep

Gastrointestinal nematode parasites in farmed animals are of particular importance due to their effects on production. In Australia, it is estimated that the direct and indirect effects of parasite infestation cost the animal production industries hundreds of millions of dollars each year. The main factors considered by immunologists when studying gastrointestinal nematode infections are the effects the host's response has on the parasite, which immunological components are responsible for these effects, genetic factors involved in controlling immunological responses, and the interactions between these forming an interconnecting multilevel relationship. In this paper, we describe the roles of immunoglobulins, in particular IgA and IgE, and the major histocompatibility complex in resistance to gastrointestinal parasites in sheep. We also draw evidence from other animal models to support the involvement of these immune components. Finally, we examine how IgA and IgE exert their influence and how methods may be developed to manage susceptible animals.

A differential interplay between the expression of Th1/Th2/Treg related cytokine genes in Teladorsagia circumcincta infected DRB1*1101 carrier lambs

Substantial debate exists on whether the immune response between sheep resistant and susceptible to gastrointestinal nematodes can be differentiated into a Th1 and Th2 phenotype. The present study addresses the hypothesis that variation in resistance to Teladorsagia circumcincta between DRB1*1101 (associated with reduced faecal egg count and worm burden) carriers and non-carriers is due to a differential interplay in the expression of Th1/Th2 and regulatory T (Treg) related cytokine genes. Lambs from each genotype were either slaughtered at day 0 (un-infected control) or infected with 3 × 10⁴ Teladorsagia circumcincta L3 and slaughtered at 3, 7, 21, and 35 days later. Lambs carrying the DRB1*1101 allele had a significantly lower worm burden (P < 0.05) compared to the non-carriers. Abomasal mucosal cytokine gene expression was evaluated by quantitative real-time PCR and comparison made for time and genotype effects. The response generated varied through the course of infection and was affected by genotype. DRB1*1101 carriers had an up-regulated expression of the Th1-related cytokine genes (IL-1β, TNFα, and IFN-γ) at day 3, but this was replaced by an up-regulated expression of Th2-related cytokine genes (IL-10 and IL-13) and Treg-related cytokine genes (IL-2RA-CD25, TGFα, TGFβ, Arg2, MIF and FOXP3) by day 7. Conversely, in the non-carriers these changes in gene expression were delayed until days 7 and 21 post infection (pi), respectively. It is concluded that resistance to Teladorsagia circumcincta in animals carrying the DRB1*1101 allele is influenced by an earlier interplay between Th1, Th2 and T regulatory immune response genes.

The dynamic influence of the DRB1*1101 allele on the resistance of sheep to experimental Teladorsagia circumcincta infection

Suffolk sheep carrying the DRB1*1101 (previously referred to as-DRB1*0203 or G2) allele have been reported to show increased resistance to natural Teladorsagia circumcincta infection compared to non-carriers. The objective of this study was to compare the biochemical and physiological responses of DRB1*1101 carrier and non-carrier twin lambs to an experimental infection with 3 × 10⁴ L3 Teladorsagia circumcincta. The variables studied included worm burden, faecal egg count, abomasal mast cells, IgA, IgE, IgG1 plus IgG2 and haematological parameters at 0, 3, 7, 21 and 35 days post infection (dpi), and duodenal smooth muscle contractility at 0 and 35 dpi. DRB1*1101 carrier lambs had significantly lower worm burden, higher mast cell and plasma platelet counts than the DRB1*1101 non-carriers (P < 0.05). Before infection, the non-carrier lambs exhibited significantly higher mucosal levels of all antibody isotypes measured compared to the carriers; these levels remained relatively stable over the course of infection in the non-carriers while there was a slow build up of these antibodies in the carriers up to day 21 post infection (pi). The DRB1*1101 non-carrier lambs had a significantly higher plasma lymphocyte count, and produced greater duodenal contractile force relative to the carrier lambs (P < 0.05). There was no significant difference between genotypes in the level of plasma eosinophils, monocytes, neutrophils or FEC. This evidence suggests that resistance conferred by DRB1*1101 is acquired rather than innate, depends on worm expulsion rather than fecundity and is dependent on mucosal mast cell proliferation, platelet activation, and IgA and IgE antibody responses.

A single nomenclature and associated database for alleles at the major histocompatibility complex class II DRB1 locus of sheep

The development of standardised nomenclatures with associated databases containing reference sequences for alleles at polymorphic loci within the major histocompatibility complex (MHC) has been facilitated by the development of the immuno polymorphism database (IPD). Recently, included within IPD-MHC is information on allelic diversity within sheep species (IPD-MHC-OLA). Here, we present the first report of progress in populating the sheep IPD-MHC database with alleles at the class II MHC DRB1 locus. The sequence of 63 Ovar-DRB1 alleles within 24 allelic families is now held within the database, each meeting the minimum requirement of a complete second exon. These sequences are derived from a combination of genomic and cDNA-based approaches and represent the most extensive collection of validated alleles at the sheep DRB1 locus yet described. Although these 63 alleles probably represent only a fraction of the DRB1 allelic diversity in sheep species worldwide, we encourage the research community to use the official allelic nomenclature and to contribute allelic sequences to the database via its web-based submission tool. In time, the IPD-MHC-OLA resource will underpin population-based MHC genotyping studies and help to simplify meta-analyses of multi-source data from wild and domestic sheep populations.

Sequence-based genotyping of the sheep MHC class II DRB1 locus

The immunopolymorphism database (IPD) provides a single nomenclature for alleles at the major histocompatibility complex (MHC) loci for a range of different species. The minimum requirements for inclusion of a sheep class II DRB1 sequence is a submission that includes all polymorphic sites within the second exon from at least two independent polymerase chain reactions (PCR). In order to meet these requirements, we have developed a DNA-based genotyping method for the rapid analysis of allelic diversity at the DRB1 locus in domestic sheep, Ovis aries. Using a series of primers located within introns flanking exon 2 and genomic DNA from a cohort of 214 sheep representing 15 different breeds and crossbreeds, the complete exon 2 sequences of 38 Ovar-DRB1 alleles were obtained. This sequence resource allowed the development of a generic set of locus-specific primers which amplify a fragment that includes all polymorphic sites within the second exon. Bidirectional sequence analysis of the PCR product provides a composite sequence where each polymorphic site is represented by the corresponding International Union of Biochemistry nucleotide code. A Basic Local Alignment Search Tool search of alleles held within the IPD or National Center for Biotechnology Information databases allows individual allele sequences to be identified. Low levels of homozygosity (7.48%) within the cohort and verification of previously genotyped samples confirmed the broad allelic specificity of this method. It improves on currently available methods and is broadly applicable to the analysis of MHC diversity in studies investigating linkages with resistance or susceptibility to disease.

IL-10 and TGF-beta1 are associated with variations in fluke burdens following experimental fasciolosis in sheep

Infection with Fasciola hepatica causes an economically important disease in ruminants. Variability in parasite load may indicate innate differences in the host immune system. This study aimed to investigate the immunological mechanisms that are associated with variability in parasite burden following experimental F. hepatica infection in cross-bred sheep. Of a total of 16 animals, four were randomly chosen as uninfected controls, and the remainder infected with 100 viable metacercariae. Uninfected animals were used as the control group for evaluation of cytokine gene expression levels. For comparative analysis, specific animals were selected on the basis of extremes of fluke burdens, and were categorised into light (n = 4) and heavy burdened (n = 3) cohorts. Serum antibody levels, haematological parameters, and expression of IL-4 and IFN-gamma genes in hepatic lymph nodes were equivalent in both groups. However, significant differences in mitogen-specific lymphocyte proliferation in vitro and in expression of TGF-beta1 and IL-10 genes in hepatic lymph nodes were observed at acute and chronic phases of infection, respectively. These results provide useful information in developing further understanding of natural resistance to fasciolosis in sheep.

High heritability for Ascaris and Trichuris infection levels in pigs

Aggregated distributions of macroparasites within their host populations are characteristic of most natural and experimental infections. We designed this study to measure the amount of variation that is attributable to host genetic factors in a pig-helminth system. In total, 195 piglets were produced after artificial insemination of 19 sows (Danish Landrace-Yorkshire crossbreds) with semen selected from 13 individual Duroc boars (1 or 2 sows per boar; mean litter size: 10.3; 5-14 piglets per litter). Starting at 10 weeks of age, piglets were repeatedly infected with the gastrointestinal helminths Trichuris suis and Ascaris suum by administering eggs in the feed for 14 weeks until necropsy. Faecal egg counts (FECs) were estimated regularly and A. suum worm burden was obtained at necropsy. Heritability calculations for log (FEC+1) at weeks 7-10 post-infection (p.i.) showed that 0.32-0.73 of the phenotypic variation for T. suis could be attributed to genetic factors. For A. suum, heritabilities of 0.29-0.31 were estimated for log (FEC+1) at weeks 7-14 p.i., whereas the heritability of log worm counts was 0.45. Strong positive genetic correlations (0.75-0.89) between T. suis and A. suum FECs suggest that resistance to both infections involves regulation by overlapping genes. Our data demonstrate that there is a strong genetic component in resistance to A. suum and T. suis infections in pigs. Identification of responsible genes would enhance our understanding of the host immune response to these common nematodes and for the closely related species (T. trichiura and A. lumbricoides) in man infecting more than a billion people.

[Association of polymorphisms of exon 2 of GOLA-DQB1 gene with immune traits in goats]

PCR-RFLP was used to analyze the polymorphisms of exon 2 of GOLA-DQB1 gene in Laiwu black goats, Lubo goats, and Boer goats. The effects of genotypes of exon 2 of GOLA-DQB1 gene on immune traits were estimated. Seven genotypes (AA, BB, CC, AB, AC, BC, and DD) were detected by analyzing restriction maps. Polymorphic sites were detected at base position 24, 151 of exon 2 of GOLA-DQB1 gene. The results showed that the effects of breed were major effect. W-SCR in Laiwu black goats with genotype BC was significantly higher than that with genotype AC and CC (P<0.05), W-LCR in Laiwu black goats with genotype BC was significantly lower than that with genotype CC (P<0.05); W-LCC in Laiwu black goats with genotype BC was lower than that with genotype AC, CC (P>0.05). W-LCC in Boer goats with genotype BC was lower than that with genotype AA, AB, and BB (P>0.05); W-LCC in Lubo goats with genotype BC and AC was significantly lower than that with genotype AA (P<0.05). It was concluded from the results that GOLA-DQB1 was the gene affecting the immune traits. These results may be applied to marker assisted selection in disease resistance breeding of goats.

Genomic organisation and allelic diversity within coding and non-coding regions of the Ovar-DRB1 locus

In all but the most primitive vertebrates, multiple polymorphic genes associated with lymphocyte-mediated immunological surveillance are linked together within genomic regions termed the major histocompatibility complex (MHC). The extensive diversity at many MHC loci provides a valuable source of genetic markers for examining the complex relationships between host genotype and disease resistance or susceptibility. Such studies in domestic sheep (Ovis aries) have generally focused on exon 2 of the polymorphic class II MHC DRB1 gene and its adjacent sequences. We have determined the complete genomic sequences of two Ovar-DRB1 alleles, which has allowed an analysis of diversity at coding, non-coding and promoter regions of this gene. On the basis of these sequences, oligonucleotide primers were designed for amplification of full-length Ovar-DRB1 transcripts and used to evaluate diversity within an MHC-defined resource flock maintained at the Moredun Institute. We describe nine novel full-length Ovar-DRB1 sequences along with an improved direct-sequencing method for analysis of the entire exon 2 region of the Ovar-DRB1 gene based on previously unknown intronic sequences. We discuss how these data provide evidence on the evolution of MHC DRB diversity in domestic sheep.

The dynamic influence of genetic variation on the susceptibility of sheep to gastrointestinal nematode infection

The interaction between sheep and the nematode Teladorsagia circumcincta is one of the best understood of all host-parasite interactions. Following infection, there is considerable variation among lambs in the number of nematode eggs produced, the number of early fourth-stage larvae and the number of adult worms in the mucosa. These traits have a high variance to mean ratio (i.e. they are overdispersed or aggregated among hosts), they are skewed and approximately negative binomially distributed. The sources of overdispersion are differences among lambs in the ingestion of infective larvae and the immune response. Both forces can produce aggregation but their relative importance is unknown. The key components of variation can be identified by variance analysis. The sum of the average effects of polymorphic genes is known as additive genetic variation and this increases essentially from zero at one month of age to quite high values at six months of age. The major mechanism underlying genetic variation appears to be the differences among individuals in immune responses. Two of the major sources of variation in immune responses are differences in antigen recognition and differences in the type of cytokines produced. Genes that influence both these sources of variation are associated with differences in resistance to nematode infection. Therefore, much of the heterogeneity among animals in parasite transmission appears to be due to genetic variation in immune responsiveness.

Detection of genes with moderate effects on disease resistance using ovine mhc and resistance to nematodes as an example

Detecting some of the genes that influence disease resistance would improve our understanding of the processes that cause disease and also simplify disease control. Genes within the major histocompatibility complex (mhc) are strong candidates for disease resistance and they have been intensely studied for the last 30 years. Recently, several groups working independently have reported the existence of alleles within the mhc that are associated with enhanced resistance to nematode infection. This article uses hindsight to describe some of the potential pitfalls that hinder the search for valid disease resistance genes. The search requires a good understanding of disease biology, molecular genetics, statistical genetics and especially, the design and analysis of experiments. The power to detect mhc effects is quite low and is quite sensitive to the frequency of the putative resistance alleles.