SIMULATION ANALYSIS OF THE EFFECT OF HERD-IMMUNITY AND AGE STRUCTURE ON INFECTION OF A CATTLE HERD WITH BLUETONGUE VIRUSES IN QUEENSLAND, AUSTRALIA

A state-transition model based on Leslie matrix formulation was used t o investigate the effects of herd immunity and age structure on the in fection of a simulated cattle herd with bluetongue viruses under Austr alian climatic conditions. Increasing duration of immunity decreased t he prevalence of infection. A duration of immunity of 33 months was co nsistent with prevalence estimates made from previous serological stud ies of bluetongue virus. Herd prevalence displayed slowly dampening cy clical variation over time (most pronounced when a short duration of i mmunity was simulated). Increasing calving and mortality risk rates in the simulated herd increased prevalence, whereas increasing age at fi rst calving decreased prevalence. Manipulation of calving rates had th e greatest effect on the predicted prevalence of infection in the herd . Simulation of a number of herd-management scenarios suggested that m anagement systems in which cattle are bred early and where high calvin g rates are achieved are likely to contribute to high levels of infect ion with bluetongue viruses. Results confirm the importance of managem ent factors in influencing the prevalence of infectious diseases in an imal populations. (C) 1997 Elsevier Science B.V. All rights reserved.