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.