Estimation of the reproduction number of salmon pancreas disease virus subtype 3 in homogeneously mixed populations of Norwegian farmed Atlantic salmon

The reproduction number (R) of salmon pancreas disease (PD) was estimated within homogeneously mixing populations (within-cage) of Norwegian farmed Atlantic salmon (Salmo salar L.) based on data collected during PD epidemics from 10 cages at 2 farming sites. Two approaches were used: (a) estimation of an overall reproduction number (R-(cmd)) and a time-dependent reproduction number (R-(t)) using mortality records during PD epidemics, and (b) estimating the reproduction number during the early stage of infection (R-(sd)) based on data from a surveillance program for SPDV subtype 3. The R-(cmd) estimates based on the mortality data ranged from 1.02 to 1.45, and the R-(sd) estimates ranged from 1.0 to 2.9. Plots of the R-(t) estimates covering the whole epidemic period yielded an increasing slope prior to SPDV3 detection. This study presents a framework for the quantitative measurement of a PD epidemic that could be useful for the evaluation of prevention methods. The time-dependent R-(t) estimate can provide an early warning of PD outbreaks. (c) 2013 Elsevier B.V. All rights reserved.