Salmonella Dublin infection in young dairy calves: transmission parameters estimated from field data and an SIR-model.

In this study we used field data collected from October 2001 to January 2002 to estimate number of days of faecal excretion of Salmonella Dublin bacteria and time to seroconversion in infected calves below the age of 180 days. Based on these estimates all calves in four endemically infected dairy herds were grouped into the following infection states: susceptible (S), infectious (I) and resistant/recovered (R). Resistant calves had either acquired maternal antibodies through colostrum or they have recovered from previous infection and had a high level of antibodies directed against Salmonella Dublin possibly protecting them from becoming infected again until the level of antibodies had decreased to sufficiently low levels. Using the antibody measurements and faecal excretion periods, it was possible to assign the most likely infection state to each calf per week of the study period. Estimates of transmission parameter, beta, were obtained from a generalised linear model relating the number of new infections to the proportion of susceptible and infectious calves per week. From beta, the reproduction ratio R at steady state and the basic reproduction ratio R(0) were estimated for each herd and across herds. The R(0) denotes the average number of new infections caused by one infectious individual that is introduced to a fully susceptible population. The point estimates for R(0) ranged from 1.1 to 2.7 in the study herds. However, the confidence intervals were wide. Data were too limited to show possible significant differences in the parameters between the study herds. However, the tendency in the data suggested that there may be important differences. Across herds the R(0) was close to two suggesting that on average one infectious calf will produce two new infectious calves when introduced into a fully susceptible population under typical Danish dairy production systems. Further, the analyses indicated that environmental contamination from infectious calves plays an important role in transmitting Salmonella Dublin between calves.