ESTIMATE OF BASIC REPRODUCTION NUMBER (R0) OF LOW PATHOGENICITYAVIAN INFLUENZA OUTBREAKS USING A BAYESIAN APPROACH

摘要

Low pathogenicity avian influenza (LPAI) is a mild disease in poultry with low mortality. Certain virus subtypes may evolve into the highly pathogenic forms, with devastating consequences. Characterising the epidemiological determinants of LPAI is critical to facilitate effective control measures. The basic reproduction number (Ro, the average number of new cases generated by one infectious individual in a fully susceptible population) is an important epidemiological parameter which summarises the transmission potential of a disease. Ideally, Ro is estimated from the complete course of an outbreak, taking into account the infection times of each bird and the length of the infectious period. Unfortunately, onset and duration of LPAI infections are difficult to establish in the field because of the mainly subclinical course of the disease and the high number of birds per flock. Using serosurveillance data, a Bayesian hierarchical model was implemented to estimate Ro of LPAI outbreaks in meat turkey flocks from the final size p (the proportion of a population that had been infected by the end of an outbreak) through the equationp = 1 - exp(-p Ro). Two different probability distributions for R0 (gamma and lognormal) were compared and test sensitivity(as a fixed value or a distribution) was further included into the model. A lower sensitivity increased the estimates of Ro, but better allowed for the fact that the data came from a serosurveillance programme, the results of which depend on both the true infectious status of the flock and the accuracy of the diagnostic assays. Based on the deviance information criterion, the best model had a gamma-distributed Ro with mean 5.7 (95% CI: 3.3-20.5), and a posterior median sensitivity of 97.6%. The estimated variance of 12.6 (95% CI: 1.8-382.8) indicates that Ro in the population of infected flock is possibly highly variable, an argument in strong favour of the use of field data to estimate transmission parameters.