The potential of genetic selection to reduce livestock infectious diseases is often considered limited because of low heritability of disease traits. Researchers of the Animal Breeding and Genomics and Quantitative Veterinary Epidemiology groups of Wageningen University & Research have recently shown that this potential is actually much larger, because of indirect genetic effects occurring in the transmission of the infections causing the diseases. The results were published in Genetics last month.
Alternative strategy to combat infectious diseases
To combat infectious diseases in livestock, several intervention strategies, such as vaccination and hygienic measures are applied. For some diseases, such as mastitis and digital dermatitis, however, these strategies have not been successful in fully controlling the disease. Therefore, there is an ongoing need for alternative strategies, one of which could be genetic selection. The prospects of selecting animals for lower disease prevalence, however, are often considered limited, because the heritability of disease resistance traits is generally very low.
In their study, the researchers simulated herds of animals with breeding values for disease susceptibility. Next, they simulated an endemic infectious disease in these herds and recorded the disease status of individual animals at given timepoints. These disease status records were used for heritability estimation. The genetic variance in susceptibility was tuned such that observed heritability corresponded to commonly observed values, and was then used to estimate response to selection.
Indirect genetic effects
Despite the low heritability, the researchers found a large response to selection, even eradication of the disease occurred. “Surprisingly, we needed to simulate considerable genetic variation in susceptibility to reach the low heritabilities”, Dries Hulst, one of the researchers, explains. Next to the large variance, indirect genetic effects contributed considerably to the large response. These indirect effects arise because animals with low susceptibility are not only less likely to get infected themselves, but also infect fewer other animals. “It matters who your herd mates are”, Hulst summarises.
Important insights for the future
The results show that genetic selection is a much more promising strategy to combat infectious diseases in livestock than currently believed. Hulst is careful to create too high expectations: “We did one round of selection and the disease was gone, in practice it will for sure not be that easy.” Several factors might limit the response. Pathogens could evolve resistance to the selected animals and disease traits are often unfavourably correlated with production traits. “We are currently investigating strategies to deal with these problems, but our study certainly provides important new insights that will change current thinking about selection against infectious diseases”, Hulst concludes.