Free Access
Genet. Sel. Evol.
Volume 35, Number Suppl. 1, 2003
Second International Symposium on Candidate Genes for Animal Health
Page(s) S3 - S17
Genet. Sel. Evol. 35 (2003) S3-S17
DOI: 10.1051/gse:2003013

Genetic management strategies for controlling infectious diseases in livestock populations

Stephen C. Bishopa and Katrin M. MacKenziea, b

a  Roslin Institute (Edinburgh), Roslin, Midlothian, EH25 9PS, UK
b  Current address: BioSS, Scottish Crop Research Institute, Invergowrie, Dundee, UK

(Accepted 4 February 2003)

This paper considers the use of disease resistance genes to control the transmission of infection through an animal population. Transmission is summarised by R 0, the basic reproductive ratio of a pathogen. If ${\rm R}_0 > 1.0$ a major epidemic can occur, thus a disease control strategy should aim to reduce R 0 below 1.0, e.g. by mixing resistant with susceptible wild-type animals. Suppose there is a resistance allele, such that transmission of infection through a population homozygous for this allele will be ${\rm R}_{02} < {\rm R}_{01}$, where R 01 describes transmission in the wildtype population. For an otherwise homogeneous population comprising animals of these two groups, R 0 is the weighted average of the two sub-populations: R 0 = R 01 $\rho + R_{02}$ ( $1-\rho$), where $\rho$ is the proportion of wildtype animals. If R 01 > 1 and R 02 < 1, the proportions of the two genotypes should be such that ${\rm R}_0 \leq 1$, i.e. $\rho \leq
({\rm R}_0 - {\rm R}_{02})/({\rm R}_{01} - {\rm R}_{02})$ . If R 02 = 0, the proportion of resistant animals must be at least $1-1/{\rm R}_{01}$. For an n genotype model the requirement is still to have ${\rm R}_0 \leq 1.0$. Probabilities of epidemics in genetically mixed populations conditional upon the presence of a single infected animal were derived. The probability of no epidemic is always $1/({\rm R}_0 + 1)$. When ${\rm R}_0 \leq 1$ the probability of a minor epidemic, which dies out without intervention, is ${\rm R}_0/({\rm R}_0 + 1)$. When ${\rm R}_0 > 1$ the probability of a minor and major epidemics are $1/({\rm R}_0 + 1)$ and ${\rm R}_0 - 1)/({\rm R}_0 + 1)$. Wherever possible a combination of genotypes should be used to minimise the invasion possibilities of pathogens that have mutated to overcome the effects of specific resistance alleles.

Key words: genetics / epidemiology / disease resistance / livestock / R 0

Correspondence and reprints: S.C. Bishop

© INRA, EDP Sciences 2003