Measuring Progress on the Control of Porcine Reproductive and Respiratory Syndrome (PRRS) at a Regional Level: The Minnesota N212 Regional Control Project (Rcp) as a Working Example

Abstract

Due to the highly transmissible nature of porcine reproductive and respiratory syndrome (PRRS), implementation of regional programs to control the disease may be critical. Because PRRS is not reported in the US, numerous voluntary regional control projects (RCPs) have been established. However, the effect of RCPs on PRRS control has not been assessed yet. This study aims to quantify the extent to which RCPs contribute to PRRS control by proposing a methodological framework to evaluate the progress of RCPs. Information collected between July 2012 and June 2015 from the Minnesota Voluntary Regional PRRS Elimination Project (RCP-N212) was used. Demography of premises (e.g. composition of farms with sows = SS and without sows = NSS) was assessed by a repeated analysis of variance. By using general linear mixed-effects models, active participation of farms enrolled in the RCP-N212, defined as the decision to share (or not to share) PRRS status, was evaluated and used as a predictor, along with other variables, to assess the PRRS trend over time. Additionally, spatial and temporal patterns of farmers' participation and the disease dynamics were investigated. The number of farms enrolled in RCP-N212 and its geographical coverage increased, but the proportion of SS and NSS did not vary significantly over time. A significant increasing (p<0.001) trend in farmers' decision to share PRRS status was observed, but with NSS producers less willing to report and a large variability between counties. The incidence of PRRS significantly (p<0.001) decreased, showing a negative correlation between degree of participation and occurrence of PRRS (p<0.001) and a positive correlation with farm density at the county level (p = 0.02). Despite a noted decrease in PRRS, significant spatio-temporal patterns of incidence of the disease over 3-weeks and 3-kms during the entire study period were identified. This study established a systematic approach to quantify the effect of RCPs on PRRS control. Despite an increase in number of farms enrolled in the RCP-N212, active participation is not ensured. By evaluating the effect of participation on the occurrence of PRRS, the value of sharing information among producers may be demonstrated, in turn justifying the existence of RCPs.

Fig 1. Observed (grey lines) and predicted (black lines) probably of sharing PRRS-status from July 2012 to June 2014.

Continuous and dotted lines correspond to sharing PRRS-status for SS and NSS enrolled in the RCP-N212 respectively. Months from July 2012 to June 2014 are represented from 1 to 24.

Fig 2. Time-space clusters of high (A, continuous line) and low (B, dashed line) predicted probability of sharing PRRS status from July 2012 to June 2014 in the RCP-N212.

Counties delineated at the edges have premises enrolled in the RCP-N212 and shade scale depicts the total number of premises per county (Census 2102. USDA, 2014).

Fig 3. Observed PRRS incidence (grey line) and estimated linear trend (black line) of PRRS incidence between July 2012 and June 2014 in in the RCP- N212.

Months from July 2012 to June 2014 are represented from 1 to 24.

Fig 4. Temporal (left) and spatial (right) density of PRRS incident cases from July 2012 to June 2014 in the RCP-N212.

Fig 5. Perspective plot of space-time pair correlation function (g^(u,v)) for PRRS incident cases from July 2012 to June 2014 in the RCP-N212.

Time (in weeks, from 0-wk to 104-wks) is represented by axis v; while space (in km, from 0-km to 6-km) is represented by axis u. Vertical axis represents the space and time correlation.