A modelling approach to estimate the sensitivity of pooled faecal samples for isolation of Salmonella in pigs

Abstract

The objective of this study was to develop and parametrize a mathematical model of the sensitivity of pooled sampling of faeces to detect Salmonella infection in pigs. A mathematical model was developed to represent the effect of pooling on the probability of Salmonella isolation. Parameters for the model were estimated using data obtained by collecting 50 faecal samples from each of two pig farms. Each sample was tested for Salmonella at individual sample weights of 0.1, 0.5, 1, 10 and 25 g and pools of 5, 10 and 20 samples were created from the individual samples. The highest test sensitivity for individual samples was found at 10 g (90% sensitivity), with the 25 g test sensitivity equal to 83%. For samples of less than 10 g, sensitivity was found to reduce with sample weight. Incubation for 48 h was found to produce a more sensitive test than incubation for 24 h. Model results found increasing sensitivity with more samples in the pool, with the pools of 5, 10 and 20 being more sensitive than individual sampling, and the pools of 20 being the most sensitive of those considered.

Figure 1

Flow diagram of the experiments to estimate the sensitivity of pooling of pig faeces for the detection of Salmonella.

Figure 2

The predicted sensitivity of the pooled pen sample from the estimated parameters (solid line) and the proportion of positives from the pooling study (asterisks) for (a) 5×5 g pools, (b) 10×2.5 g pools and (c) 20×1.25 g pools.

Figure 3

The predicted mean test sensitivity of a 25 g pooled pen sample from a pen of 50 pigs when the number of faecal samples in the pool was varied between 1 and 25. Results are given for 7 and 41 pigs infected in the pen, and for the best guess of the national prevalence.