Transmission of digital dermatitis in dairy cattle: population dynamics and host quantitative genetics

Susceptibility, infectivity, the contact rate, and the duration of the infectious period together determine the basic reproduction ratio (R0). The R0 is the average number of secondary cases caused by a typical infectious individual in a fully susceptible population. It determines the ability of an infection to establish itself in a population. The threshold value is one; if R¬0 < 1 a typical infectious individual will infect on average less than one susceptible individual and the disease will die out with certainty. If R0 > 1 a major outbreak is possible, and sometimes such a disease may persist in a population. For endemic diseases in homogeneous populations, the prevalence in the equilibrium follows from R0 as 1-1/R_0 . Breeding strategies that aim to reduce the prevalence of endemic diseases should thus aim to reduce R0. Because R0 depends on both susceptibility and infectivity of the host population, genetic variation in both those traits should be taken into account. This thesis focusses on Digital Dermatitis (DD) in dairy cattle. DD is an endemic infectious claw disease associated with lameness. We collected time-series data on individual disease that might facilitate genetic selection against DD. In this thesis, we investigated transmission dynamics for DD and estimated genetic effects for both host susceptibility and host infectivity. We proposed a generalized linear mixed model to estimate SNP effects on both host susceptibility and host infectivity from time-series data on individual disease status. The model accounted for variation in exposure of susceptible individuals to infectious group mates, and for the infectivity genotypes of those group mates. The power to detect SNP effects was high for susceptibility but lower for infectivity. We applied the model to field data on DD to investigate the contribution of different disease classes to R0. The estimated R0 was 2.36, to which the class with irregular skin contributed 88.5%. Genomic estimated breeding values for R0 ranged from 0.62 to 6.68 with an accuracy of ~0.6. There were 135 SNPs with a suggestive association for several host susceptibility traits, and heritability estimates for these traits ranged from 0.09 to 0.37. These results show that genetic selection against DD is very promising; there is substantial heritable variation and a meaningful accuracy can be obtained from a limited amount of data.