Genetic parameters for resistance to trichostrongylid infection in dairy sheep

Abstract

In sheep, the traditional chemical control of gastrointestinal nematode (GIN) parasites with anthelmintics has led to the widespread development of anthelmintic resistance. The selection of sheep with enhanced resistance to GIN parasites has been suggested as an alternative strategy to develop sustainable control of parasite infections. Most of the estimations of the genetic parameters for sheep resistance to GIN parasites have been obtained from young animals belonging to meat-and/or wool-specialised breeds. We present here the estimated genetic parameters for four parasite resistance traits studied in a commercial population of adult Spanish Churra dairy ewes. These involved two faecal egg counts (FECs) (LFEC 0 and LFEC 1 ) and two serum indicator traits, the anti-Teladorsagia circumcincta fourth stage larvae IgA (IgA) and the pepsinogen (Peps) levels. In addition, this study has allowed us to identify the environmental factors influencing parasite resistance in naturally infected Spanish Churra sheep and to quantify the genetic component of this complex phenotype. The heritabilities estimated for the two FECs analysed (0.12 for LFEC 0 and 0.09 for LFEC 1 ) were lower than those obtained for the examined serum indicators (0.19 for IgA and 0.21 for Peps). The genetic correlations between the traits ranged from 0.43 (Peps2IgA) to 0.82 ( LFEC 0 2LFEC 1 ) and were higher than their phenotypic counterparts, which ranged between 0.07 and 0.10. The heritabilities estimated for the studied traits were lower than previously reported in lambs. This may be due to the differences in the immune mechanisms controlling the infection in young (antibody reactions) and adult (hypersensitivity reactions) animals/sheep. In summary, this study demonstrates the presence of heritable variation in parasite resistance indicator traits in the Churra population studied, which suggests that genetic improvement is feasible for this complex trait in this population. However, further studies in which the experimental variables are controlled as much as possible are needed to identify the best trait that could be measured routinely in adult sheep as an indicator of parasite resistance.

Quantitative trait loci for resistance to trichostrongylid infection in Spanish Churra sheep

“…This agrees with the observations reported by Davies et al [7], who did not find any coincident QTL between parasitic traits and IgA activity. The lack of coincidence between the QTL influencing LFEC 0 and LFEC 1 , although intriguing, agrees with certain differences observed regarding the correlations between these two traits and the serum indicator traits [9]. As suggested in that work, this could be related to the limited sample period between the faecal egg counts, which could indicate that LFEC 1 is a better indicator of the initial immune response triggered by larvae at the beginning of infection.…”

Causes and consequences of intra‐ and inter‐host heterogeneity in defence against nematodes

“…Higher heritability estimates for length of adult Teladorsagia circumcincta , a highly prevalent sheep nematode, than for worm number (0·62 ± 0·20 vs. 0·14 ± 0·10) suggest that variation in lamb FEC is due to variation in regulation of T. circumcincta length (and therefore fecundity) rather than ability to kill worms . This reduction in worm length is associated with a significantly heritable parasite‐specific IgA response . An important outcome of selection experiments is that selection of sheep for resistance to Haemonchus contortus , an abomasal parasite, enhances resistance to Trichostrongylus colubriformis , an intestinal parasite, due to a positive genetic correlation between resistance to them .…”

Mapeo de alta densidad para la identificación de genes relacionados con la resistencia a las infecciones gastrointestinales por nematodos en el ganado ovino de raza churra = High density mapping to identify genes associated to gastrointestinal nematode infections resistance in Spanish Churra sheep

“…The QTL on OAR4 and OAR20 were confirmed by different variants of the RHM method, whereas a QTL on OAR20 positioned in the MHC region was identified as the most significant result. In relation to the result on OAR14, it is worth mentioning that a previous study had reported a significant selection sweep in the same region (40.1-55 Mb according to the OAR v2.0) of OAR14 (Fariello et al, 2013) and the QTL region (42)(43)(44)(45)(46)(47)(48)(49) to the Oar_v3.1) reported by Riggio et al (2014). Based on the multi-locus haplotypes identified in that selection sweep region by Fariello et al (2013), two possible candidate genes had been identified, the IRF3 (interferon regulatory factor 3) gene and the TGF-B1…”

“…In the last decades there have been many studies focused on improving our understanding about the genetic determinism of parasite resistance in sheep, including studies focused on: (i) the estimation and assessment of genetic parameters of indicator traits Morris et al, 2004;Gutiérrez-Gil et al, 2010), (ii) the genetic variability and the role of some candidate genes (Sayers et al, 2005a;Benavides et al, 2002Benavides et al, , 2009, and (iii) the scanning of the genome to identify regions underlying the variation observed in the indicator traits , Gutiérrez-Gil et al, 2009bSallé et al, 2012). We provide below a brief overview of the results and conclusions derived from the three types of studies mentioned.…”

“…Apart the strong effort of this research group to dissect the genetic variation underlying milk production traits, disease resistance traits have also been an important point of interest for this group because of the impact that some diseases such as subclinical mastitis and GIN infections have on the dairy sheep farm´s global economy. In relation to the study of parasite resistance traits, the ULE MEGA research group, through its participation in the Europeanfunded GeneSheepSafety project (5 th Framework Programme), reported the estimation of genetic parameters for FEC and serum levels of IgA and pepsinogen (Gutiérrez-Gil et al, 2010), and the results of a microsatellite-based genome scan for detection of QTL influencing the mentioned indicator traits in a commercial half-sib population of Spanish Churra sheep (Gutiérrez-Gil et al, 2009b).…”