Breeding against infectious diseases is promising

Infectious diseases have profound effects on life, both in nature and agriculture, and zoonoses also threaten the human population. In livestock, breeding can help to make animals less susceptible to infection and also to reduce their tendency to spread the infection to other animals. In this way animal breeding can reduce the prevalence of an infection in the population, and thus contribute to improved animal health and welfare. However, because the heritability of infectious diseases is considered to be small, the current belief is that animal breeding can only have a limited impact.

Much higher heritability than we thought

Researchers from Wageningen University & Research developed a mathematical model for the heritability of infectious diseases. In contrast to existing genetic models, this model accounts for the transmission of the infection between individuals. Results show that the heritability is much greater than currently believed, and that heritability increases as the infection becomes increasingly rare. For example, if on average 20% of the individuals falls victim to the infection, then the heritable variation is 1 divided by 20% = 5-fold greater than currently believed. This is because individuals who are less susceptible to become infected will also infect fewer other individuals, simply because they are less likely to be infected themselves. This positive feedback effect was ignored in previous models for the heritability of infectious diseases. These results imply that breeding against infectious diseases is much more promising than we thought.

Hidden heritability can lead to eradication

The possibility of individuals transmitting the infection to other individuals creates hidden heritability. A traditional genetic analysis captures only the effect of an individual’s genes on its own infection status. However, as an infection becomes increasingly rare, an increasing proportion of the full impact of an individual’s genes is expressed in the infection status of other individuals. In other words, as an infection becomes increasingly rare, the fact that genetically less susceptible individuals also infect fewer other individuals makes up an increasingly large proportion of the total effect of their genes. Computer simulations indicate that animal breeding can even be used to eradicate an infectious disease because of this strong increase of the heritability.

This research was a collaboration of Animal Breeding and Genomics and Quantitative Veterinary Epidemiology of WUR, and was recently published in the scientific journal Genetics.