Genetic management strategies for controlling infectious diseases in livestock populations

Stephen C. Bishop; Katrin M. MacKenzie

doi:doi:10.1051/gse:2003013

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Volume 35 / No Suppl. 1 (2003)

Genet. Sel. Evol., 35 Suppl. 1 (2003) S3-S17

Abstract

Free Access

Issue		Genet. Sel. Evol. Volume 35, Number Suppl. 1, 2003 Second International Symposium on Candidate Genes for Animal Health


Page(s)		S3 - S17
DOI		https://doi.org/10.1051/gse:2003013

Genet. Sel. Evol. 35 (2003) S3-S17
DOI: 10.1051/gse:2003013

Genetic management strategies for controlling infectious diseases in livestock populations

Stephen C. Bishop^a and Katrin M. MacKenzie^{a, 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)

Abstract
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
e-mail: Stephen.Bishop@bbsrc.ac.uk

© INRA, EDP Sciences 2003