title: Genetic management strategies for controlling infectious diseases in livestock populations.
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 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 R(02) <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 R(0)< or =1, i.e. rho < or =(R(0)-R(02))/(R(01)-R(02)). If R(02)= 0, the proportion of resistant animals must be at least 1-1/R(01). For an n genotype model the requirement is still to have R(0) < or =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/(R(0) + 1). When R(0)< or =1 the probability of a minor epidemic, which dies out without intervention, is R(0)/(R(0) + 1). When R(0) >1 the probability of a minor and major epidemics are 1/(R(0) + 1) and (R(0) -1)/(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.
authors.affiliation: Roslin Institute (Edinburgh), Roslin, Midlothian, EH25 9PS, Scotland, UK. Stephen.Bishop@bbsrc.ac.uk
authors.forename: Stephen C
authors.initials: SC
authors.lastname: Bishop
authors.forename: Katrin M
authors.initials: KM
authors.lastname: MacKenzie
mesh_terms: D000483:Alleles; D000818:Animals; D000829:Animals, Domestic / Q000235:genetics*; D005075:Biological Evolution; D004196:Disease Outbreaks / Q000662:veterinary; D018562:Disease Transmission, Infectious / Q000517:prevention & control* / Q000662:veterinary*; D005828:Genetics, Population; D007113:Immunity, Innate; D008957:Models, Genetic; D015233:Models, Statistical