Bishop S.C., Mc Kenzie K..
2002. Strategies for utilising genes conferring enhanced disease resistance.
In : Second international symposium on Candidate Genes for Animal Health (C.G.A.H), Montpellier, France, August 16-18th 2002 : abstracts. CIRAD, INRA
Note générale : Session 1 : Present and future strategies and tools of the candidate gene approach
Résumé : Much effort directed towards finding disease-resistance genes, somewhat less toward the optimal use of such genes in breeding programmes. For disease control, two issues are of importance, how may resistance genes be used to control the transmission of infection and what are the impacts of such strategies upon risks of pathogen evolution? These questions are addressed in this paper, combining animal breeding and epidemiology theory. Disease transmission through a population may be summarised by R0, the basic reproductive ratio - the number of secondary infections caused by the introduction of an infected animal. If R0 > 1.0 it is expected that a major epidemic will occur but if R0=1.0 either there will be no epidemic or minor epidemics which die out without intervention. A disease control strategy should aim to reduce R0 to below 1.0, e.g. by mixing genetically resistant animals with susceptible wild-type animals. Thus, optimal genetic management strategies require knowledge (or conservative estimates) of R0 for the pathogen. Assume that for a given disease, transmission in a host population comprising wildtype animals is R01. Now suppose there is a resistance allele, such that a population of animals homozygous for this allele will have altered disease transmission R02<R01. A general result is that for an otherwise homogeneous population comprising animals of these two groups, R0 is the weighted average of that in the two sub-populations, i.e. R0=R01p+R02(1-p), where p is the proportion of the population that is wild-type. This can be extended to n genotypes. When there are two resistance categories, there are three situations. If R01<1, there is no disease risk. If R02>1, epidemic risks are minimised by having all animals homozygous for the resistance allele. If R01>1 and R02<1, the proportions of the two genotypes should be such that R0=1, i.e. p=(R0-R02)/(R01-R02). In the case where resistant animals have R02=0, then the proportion of resistant animals must be at least 1-1/R01. If the population is divided into discrete units, then these proportions may be applied to each individual unit. For the n genotype model, nonunique solutions exist, but the requirement is still to have the weighted average R0 below 1.0. For any combination of genotypes, potential outcomes of pathogen invasion are, no epidemic, a minor epidemic or a major epidemic. Generally, for the n genotype model, the probability of no epidemic can be shown to be 1/(R0+1). When R0=1 the probability of a minor epidemic is R0/(R0+1) and the probability of a major epidemic is zero. Pathogen evolution risks depend upon the heterogeneity of the host population resistance, the epidemic outcome and the relationship between the targeted host population and the meta-population. Generally, mutations cause pathogens to be less fit than wild-type pathogens, thus less able to compete, so pathogen evolution will mainly be a risk when the mutated pathogen has a competitive advantage over the wild-type. Thus, evolution is a minimal risk in wild-type populations and in populations where no animals become infected. If the genetic makeup of the population reduces the probability of a major epidemic to zero, then the risk of evolution is proportional to the risk of a minor epidemic. Additionally, where disease control can be achieved through a combination of genes and/or alleles, this should be attempted as it will minimise the invasion possibilities of the mutated pathogen. (Texte intégral)
Mots-clés Agrovoc : génétique, résistance aux maladies
Classification Agris : L10 - Génétique et amélioration des animaux
Auteurs et affiliations
- Bishop S.C., Roslin Institute (GBR)
- Mc Kenzie K., Roslin Institute (GBR)
Autres liens de la publication
Source : Cirad - Agritrop (https://agritrop.cirad.fr/512004/)
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