Genetic variation hidden in transmission of infectious diseases

Infectious diseases form a continuous threat to life. Various interventions have been developed to combat infectious diseases in humans, animals, and plants, for example vaccination, and culling of (infected) animals. Genetic selection as intervention against infections has been quite successfully applied in plants. In animals, genetic selection against infections is also put into practice, but low heritability of resistance traits typically restricts the rate of genetic improvement. “In this approach, the transmission of infections, the fact that animals infect each other, is generally ignored. In my research I have found that this is a factor of major importance in combatting infectious diseases”, explains Dries Hulst, researcher at the Animal Science Group of Wageningen University & Research.

Transmission of infections

In his research, Hulst shows that transmission of infections causes indirect genetic effects, which drastically changes response to selection in the prevalence of the infection, and even makes eradication of an infection through genetic selection possible. “The origin of the indirect genetic effects is very comparable to the mechanism that causes herd immunity in a vaccination program.” If animals are genetically more resistant to infection, this reduces their likelihood to become infected and the average number of infectious individuals in the population. “This in turn reduces the chances of becoming infected for all animals in the population.” Hulst showed that the size of this indirect effect increases with decreasing prevalence of the infection. “The lower the prevalence, the larger the indirect effect, and the larger the response to genetic selection.” Importantly, this rule does not only apply to genetic selection, but to any intervention that reduces the probability of infection.

Eradication by genetic selection

A general problem with infectious disease interventions is that they create selection for adaptations in the pathogen that make the intervention less or not effective. In his research Hulst showed that to prevent such adaptations, the aim of genetic selection should be to eradicate an infection as fast as possible from a local livestock population. In that way, the pathogen gets limited chance to adapt. “Combining genetic selection with other interventions is advised to achieve local eradication”, he says. The research done by Hulst shows that genetic selection of animals against infectious diseases is very promising. “To maximize the effect however, the typical approach in animal breeding needs to change. Instead of working on gradual reduction in prevalence an approach that aims for stepwise local eradication of the infection is likely to be much more effective”, concludes Hulst.