Interpretive Summary: Genetic correlations between endo-parasite phenotypes and economically important traits in dairy and beef cattle.
By Anne Zinn
A study recently published in the March 2018 issue of the Journal of Animal Science explores the genetic correlations between endo-parasite phenotypes and traits in dairy and beef cattle. To the research team's knowledge, this study is the first to quantify the genetic correlations between parasitic diseases and economically important traits in cattle, as well as among the antibody responses to parasites.
Animal parasites continue to be a growing concern in the dairy and beef cattle production industry and can be associated with significant economic losses. Milk production, reproductive performance, and meat yield have been compromised in cattle herds infected with Fasciola hepatica (F. hepatica), Ostertagia ostertagi (O. ostertagi), or Neospora caninum (N. caninum. The high prevalence of infection suggests that the current control practices are sub-optimal, but effective control strategies can be very costly. The objective of the current study was to provide the necessary information to facilitate evaluation of the potential to breed for resistance to parasitic diseases in cattle.
Records on individual animal antibody responses to F. hepatica, O. ostertagi, and N. caninum were available from cows in 68 Irish dairy herds and national abattoir data on F. hepatica-damaged livers were made available from dairy and beef cattle. 9,271 dairy cows were in the study herd dataset after data edits and 19,542 dairy and 68,048 young dairy and beef animals had a record for the presence or absence of F. hepatica-damaged liver in the national dataset. Milk, reproductive, and carcass phenotypes were also available for a portion of these animals. Linear mixed models were used to estimate variance components of antibody responses to the 3 parasites and covariance components were estimated between the parasite phenotypes and economically important traits.
Results on this analysis showed that heritability of antibody responses to the different parasites ranged from 0.07 (O. ostertagi) to 0.13 (F. hepatica), and the antibody response to N. caninum was genetically correlated with the antibody response to both F. hepatica (-0.29) and O. ostertagi (-0.67). A moderately positive genetic correlation between the parasite phenotypes and the milk production traits were all close to zero, as were the genetic correlations between F. hepatica-damaged livers and the carcass traits of carcass weight, confirmation, and fat score evaluated in cows. In addition, the genetic correlation between F. hepatica-damaged livers in cows and milk somatic cell score was 0.32. Overall, antibody responses to F. hepatica and O. ostertagi had favorable genetic correlations with fertility traits, but the antibody response to N. caninum and F. hepatica-damaged livers were unfavorably genetically correlated with fertility.
Overall, the results of this study indicate that genetic selection for the investigated endo-parasite phenotypes is possible and could be recommended for use in populations exposed to high parasite load. Even though current cattle breeding programs with high emphasis on fertility traits appear to have reducing liver damage caused by F. hepatica, the introduction of F. hepatica-damaged liver phenotypes into national breeding programs could further reduce the prevalence of damaged livers. However, the research team suggests that the genetic selection for antibody response to only one parasite would be unwise because the antibody response to N. caninum is negatively genetically correlated with the antibody response to both F. hepatica and O. ostertagi. Therefore, the conflicting genetic impact of parasitism on fertility traits and the lack of routine data will hinder the introduction of antibody response to parasites into breeding programs.