Evaluation of Control Strategies for Porcine Reproductive and Respiratory Syndrome (PRRS) in Swine Breeding Herds Using a Discrete Event Agent-Based Model

Abstract

The objective of this study was to develop a discrete event agent-based stochastic model to explore the likelihood of the occurrence of porcine reproductive and respiratory syndrome (PRRS) outbreaks in swine herds with different PRRS control measures in place. The control measures evaluated included vaccination with a modified-live attenuated vaccine and live-virus inoculation of gilts, and both were compared to a baseline scenario where no control measures were in place. A typical North American 1,000-sow farrow-to-wean swine herd was used as a model, with production and disease parameters estimated from the literature and expert opinion. The model constructed herein was not only able to capture individual animal heterogeneity in immunity to and shedding of the PRRS virus, but also the dynamic animal flow and contact structure typical in such herds under field conditions. The model outcomes included maximum number of females infected per simulation, and time at which that happened and the incidence of infected weaned piglets during the first year of challenge-virus introduction. Results showed that the baseline scenario produced a larger percentage of simulations resulting in outbreaks compared to the control scenarios, and interestingly some of the outbreaks occurred over long periods after virus introduction. The live-virus inoculation scenario showed promising results, with fewer simulations resulting in outbreaks than the other scenarios, but the negative impacts of maintaining a PRRS-positive population should be considered. Finally, under the assumptions of the current model, neither of the control strategies prevented the infection from spreading to the piglet population, which highlights the importance of maintaining internal biosecurity practices at the farrowing room level.

Predicting vaccine effectiveness in livestock populations: a theoretical framework applied to PRRS virus infections in pigs

“…Other model limitations include that specific herd or farm structure, known to influence within-herd transmission dynamics of pathogens (see e.g. [48,49,51]), were not explicitly considered in our model. Furthermore, we only considered horizontal infection transmission in this study, thus ignoring the potential impact of vaccination on vertical transmission from infected sows or boars to piglets from the time of insemination during gestation until after birth [25,64].…”

“…The model presented in equations (1)-(2) above can be applied to model PRRSv transmission dynamics in pig herds under different vaccination scenarios [48,49,51]. Values of the model parameters were obtained from the literature (see references listed in Table 2) and represent different PRRSv strains (e.g.…”

Role of animal movement and indirect contact among farms in transmission of porcine epidemic diarrhea virus

“…In addition to direct contact among farms via animal movements, indirect contact may occur between farms due to windborne propagation of aerosols and dissemination of fomites by personnel, contaminated vehicles, and feed (Alonso et al, 2014;Alvarez et al, 2016;Beam et al, 2015;Dee et al, 2014;Kim et al, 2017;Lowe et al, 2014;O'Dea et al, 2015;Pasick et al, 2014). Although the potential importance of such mechanisms in creating transmission opportunities between swine premises has been shown in experimental studies and outbreak investigations (Alonso et al, 2014;Bowman et al, 2015;Lowe et al, 2014;Pasick et al, 2014), indirect contact is less often accounted for in epidemiological models (Arruda et al, 2016;Martinez-Lopez et al, 2011;Thakur et al, 2015;Yadav et al, 2016). Models of pathogen spread in livestock populations focus primarily on animal movement and, in some cases, local spatial spread based on proximity between premises (Brooks-Pollock et al, 2015).…”

Modeling the spread of porcine reproductive and respiratory syndrome virus (PRRSV) in a swine population: transmission dynamics, immunity information, and optimal control strategies

“…Mathematical modeling has been used as an important tool to help understand the dynamics of infectious diseases among farm animals. However, mathematical models for describing the spread of PRRSV are not numerous [9][10][11][12][13]. Evans et al [9] developed a stochastic model of a farrow-finish heard and found that the persistence of PRRSV increases according to increased herd size, increased contact between different age groups, and increased reintroduction of infectious swines.…”