Unravelling the genetic variability of host resilience to endo- and ectoparasites in Nellore commercial herds

Abstract

Background: Host resilience (HR) to parasites can affect the performance of animals. Therefore, the aim of this study was to present a detailed investigation of the genetic mechanisms of HR to ticks (TICK), gastrointestinal nematodes (GIN), and Eimeria spp. (EIM) in Nellore cattle that were raised under natural infestation and a prophylactic parasite control strategy. In our study, HR was defined as the slope coefficient of body weight (BW) when TICK, GIN, and EIM burdens were used as environmental gradients in random regression models. In total, 1712 animals were evaluated at five measurement events (ME) at an average age of 331, 385, 443, 498, and 555 days, which generated 7307 body weight (BW) records. Of the 1712 animals, 1075 genotyped animals were used in genome-wide association studies to identify genomic regions associated with HR.

Results: Posterior means of the heritability estimates for BW ranged from 0.09 to 0.54 across parasites and ME. The single nucleotide polymorphism (SNP)-derived heritability for BW at each ME ranged from a low (0.09 at ME.331) to a moderate value (0.23 at ME.555). Those estimates show that genetic progress can be achieved for BW through selection. Both genetic and genomic associations between BW and HR to TICK, GIN, and EIM confirmed that parasite infestation impacted the performance of animals. Selection for BW under an environment with a controlled parasite burden is an alternative to improve both, BW and HR. There was no impact of age of measurement on the estimates of genetic variance for HR. Five quantitative trait loci (QTL) were associated with HR to EIM but none with HR to TICK and to GIN. These QTL contain genes that were previously shown to be associated with the production of antibody modulators and chemokines that are released in the intestinal epithelium.

Conclusions: Selection for BW under natural infestation and controlled parasite burden, via prophylactic parasite control, contributes to the identification of animals that are resilient to nematodes and Eimeria ssp. Although we verified that sufficient genetic variation existed for HR, we did not find any genes associated with mechanisms that could justify the expression of HR to TICK and GIN.

Fig. 1

Diagram explaining data collection for performance tests of pasture raised cattle on the Mundo Novo farm—Brazil. Body weight (BW), ticks (TICK), eggs of gastrointestinal nematodes (GIN) and oocysts of Eimeria spp. (EIM) counts were collected at each measurement event (ME). “Age” represents the average age of animals at each ME. “nb” is the number of bulls and “nc” is the number of cohorts evaluated at each ME. Red arrow indicates a 70-day interval between evaluations, while blue arrows indicate a 56-day interval

Fig. 2

Additive genetic variances [${\mathrm{\sigma }}_{\text{a}}^2$ (kg²)] and heritability estimates (h²) for body weight (BW) across the trajectories of tick (TICK), nematodes (GIN), or Eimeria ssp. (EIM) burden at five measurement events (ME). ME.331, ME.385, ME.443, ME.498, ME.555 are body weights at each measurement event when the average age of animals was 331, 385, 443, 498 and 555 days, respectively

Fig. 3

Pairwise SNP correlations between body weight (BW), host resilience to ticks (HR.TICK), gastrointestinal nematodes (HR.GIN), and Eimeria spp. (HR.EIM) measured at five measurement events (ME) when the average age of animals was 331, 385, 443, 498, and 555 days. The values above the diagonal are the Pearson correlations between SNP effects (and standard errors of SNP correlations)

Fig. 4

Manhattan plots for the meta-analysis of the genome-wide association studies for HR to ticks (TICK), gastrointestinal nematodes (GIN), and Eimeria spp. (EIM) measured at different measurement events. The dotted line (y = 5.64) indicates the threshold for statistical significance. The dashed line (y = 4.00) indicates the threshold for suggestive evidence of association