Table 2.

Examples for genetic selection for disease resistance considered in current breeding programmes, and their potential effect on pathogen transmission and herd resilience.

Disease & Species	Resistance phenotype	Characteristics of a resistant animal	Effects on pathogen transmission and herd resilience
Bovine tuberculosis (bTB) in cattle	Binary infection status from in vivo diagnostic skin test applied in herds exposed to bTB	Less likely to have a positive test result when exposed to bTB1	Some uncertainty in whether genetically more resistant cows are less likely to become infected and to transmit bTB2
Marek’s disease (MD) in poultry	Clinical signs (e.g. lameness, lesions) and mortality after exposure to MD virus in challenge trials	Less likely to develop clinical MD and subsequently die3	Chicken with high genetic resistance can still become infected and transmit the MD virus1. Unknown if breeding for MD resistance reduces MD virus transmission
Viral and bacterial infections in Atlantic salmon	Binary survival or time of death after pathogen exposure in challenge trials	Less likely to die when exposed to the pathogen in consideration4	Unknown whether fish considered genetically more resistant have greater disease resistance or tolerance, or both. Unknown if breeding for disease resistance reduces pathogen transmission.
Gastro-intestinal parasite resistance in ruminants	Parasite egg count in faeces (Faecal egg counts, FEC)	Lower FEC may reflect the ability of an animal to limit parasite establishment, growth, fecundity and/or shedding5	Breeding for disease resistance reduces parasite shedding and thus the environmental parasite load, with beneficial effects on herd resilience6
Porcine Reproductive & Respiratory Syndrome (PRRS) in pigs7	Blood viral load of pigs over 21 day infection period after inoculation with the PRRS virus	Pigs that carry the beneficial (GBP5) allele associated with greater natural8 PRRS resistance are still susceptible to PRRS virus infection by inoculation, but tend to have lower virus replication9	Currently unknown if pigs that carry the GBP5 resistance allele are more resistant to infection and less infectious when infected in natural challenge conditions. Hence the effect of genetic selection on PRRS virus transmission is unknown.

¹Banos et al. (2017).

²Bishop and Woolliams (2014).

³Bacon et al. (2001).

⁴Ødegård et al. (2011).

⁵Bisset and Morris (1996).

⁶Stear et al. (2001) and Eady et al. (2003).

⁷Currently included in genetic evaluations, but not explicitly included in the formal selection criteria.

⁸As opposed to resistance through gene editing, which confers full resistance to PRRS virus infection.

⁹Boddicker et al. (2014).