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According to the results of the thesis presented to The Royal Veterinary College, University of London: "The Epidemiology of Porcine Reproductive and Respiratory Syndrome
(PRRS) in England: and evaluation of surveillance using scenario tree analysis", current control measures are not effective and that surveillance needs enhancement. Moreover, biosecurity measures and vaccination need to be evaluated to provide effective control and prevention of the PRRS infection. Vaccination may minimise the immediate impact seen on farms, but does not prevent virus circulation and may lead to complacency among farmers, respectively PRRS seems to be ruled immediately if unspecific clinical signs occur, and therefore these farms will not be detected with the current surveillance system.
Data from 147 farrow-to-finish farms was collected during April 2008 – April 2009 through a cross-sectional survey carried out in England. Infection status on farms was determined using PRRS ELISA on non-vaccinating farms, and detection of virus via PCR for vaccinating farms. Risk factors for active PRRS infection were identified using multivariable logistic regression analysis. A stochastic scenario tree analysis model was developed to evaluate how efficient the current passive surveillance system is in detecting infected farms.
The results indicated a prevalence of active PRRS infection of 30.61% (95%CI: 23.07-38.15) in the English pig population with a higher proportion of infected farms located in areas with high pig density. In total, 63 (43.45%) farms reported using vaccination against PRRSV. A higher proportion of farms used live vaccine rather than killed vaccine (82.54% vs.17.46%). Farms were more likely to show evidence of circulating virus if they used live virus vaccine (OR=3.65, 95%CI: 1.24-10.73), were located in high pig density areas (OR=4.64, 95%CI: 2.18-9.84) and had dead pigs collected (OR=9.00, 95%CI: 2.40-33.67). However, farms that weaned pigs at the age of 28 days or later had lower odds of being PRRS positive (OR=0.22, 95%CI: 0.07-0.65). An increased frequency of breeding stock brought on the farm, identified in univariable analysis, was no longer significant (OR=2.87, 95%CI: 0.74-11.25) after controlling for the effect of herd size and type of farm (indoor/outdoor), but was retained in the analysis as it improved the model fit.
The probability of an infected farm being detected through passive surveillance was very low (mode=0.0118, 5th and 95th percentiles: 0.0074; 0.0203 respectively). Further, farms which used live virus vaccine had lower probabilities for detection compared to those which did not.
November 2011/ BPEX/ United Kingdom.
http://www.bpex.org.uk
