ModellingSalmonellaspread within a farrow-to-finish pig herd

Abstract

-Delivery of infected pigs to the slaughterhouse is a major source of pork meat contamination by bacterial hazards to humans. We propose a model of Salmonella spread within a farrow-to-finish pig herd, assuming the prevalence in infected delivered pigs depends on the whole pig life-time and growing process. This stochastic discrete-time model represents both the population dynamics in a farrow-to-finish pig herd using batch management, and Salmonella spread. Four mutually exclusive individual health states were considered: Salmonella-free, seronegative shedder, seropositive shedder and seropositive not shedding carrier, making the distinction between seropositive animals and shedders. Since indirect transmission is the main route of transmission, the probability of infection depends on the quantity of Salmonella in the pigs' environment (Q). A dose effect function is used with two thresholds, assuming saturation in exposure for high Q vs. a minimum exposure for low Q. Salmonella is introduced in an initially Salmonella-free 150-sow herd. Prevalence of shedders and seroprevalence are calculated over time in batches of sows and pigs, and in groups of delivered pigs, composed of pigs from different batches. The model shows very variable seroprevalence over time within a herd among delivered groups, as well as among replications. The mean seroprevalence and the mean shedding prevalence are 19.3% and 13.8% respectively. A sensitivity analysis shows that the Salmonella quantity shed and the maternal protective factor are the most influential parameters on Salmonella prevalence in delivered pigs. swine / Salmonella / epidemiological model / population dynamics / environment

Mathematical Modeling of the Transmission and Control of Foodborne Pathogens and Antimicrobial Resistance at Preharvest

“…For bacteria that behave as commensals (e.g., Campylobacter jejuni in poultry), the susceptible and infectious states are often the only epidemiological states considered in the model. Differences in infectiousness because of different levels of pathogen shed- (Lurette et al, 2008). In addition to their epidemiological status, animals can be classified in age or group classes if there are differences in risk of acquiring and spreading the foodborne pathogen (discussed in the next section).…”

“…Farm structure has been included in models for foodborne pathogens for dairy farms (Turner et al, 2003) and pig farms (Lurette et al, 2008). To model Salmonella spread in a farrow-to-finish pig farm, Lurette et al (2008) explicitly modeled the batch farrowing management system and the mix of batches that can take place at the finishing stage.…”

Understanding the effects of intermittent shedding on the transmission of infectious diseases: example of salmonellosis in pigs

“…[4,5,18,20,21,29,42]). For each of these models, the pig population is divided into different compartments depending on their infection status.…”

“…Both of the models developed in [21,42] assume direct (host-to-host) transmission of infection which has been argued to be unrealistic since, for pigs, salmonellosis is transmitted primarily through the environment [18]. The model developed by Lurette et al [29] included indirect (environment-to-host) transmission by modelling the concentration of free-living Salmonella within the environment. Similarly, the models developed by Berriman et al [4,5] and Hill et al [20] consider multiple sources of indirect transmission (i.e.…”

“…Some of the previously developed models for salmonellosis [4,5,18,21,29] assume that infected pigs will shed Salmonella into the environment intermittently. That is, infected pigs will cycle through shedding or non-shedding (carrier) states of infection.…”