Elimination of PRRS virus in five pig farms using a test and removal procedure in the breeding herd

Abstract - Since 1993, controlling the disease of Porcine Reproductive and Respiratory Syndrome (PRRS) has been a frustrating experience. Characteristics of the PRRS virus including the ability to mutate, recombine, initiate persistent infection and prolonged viremia have limited the efficacy of strategies such as Nursery Depopulation and Gilt Development. Due to the ability of PRRSV to induce transplacental infection, or to be shed during lactation through saliva, milk or colostrum, production of non-infected piglets using Segregated Early Weaning and Multiple Site Production has been inconsistent as well. Furthermore, the use of commercially available vaccines, both of a modified-live and killed nature has brought about a number of safety and efficacy issues that remain unanswered today. Therefore, a study to evaluate the feasibility of PRRS eradication using Test and Removal was initiated. Preliminary data indicate that the process of Test and Removal (T&R) is an effective strategy for the detection and removal of persistently infected breeding swine, preventing vertical transmission and the production of naïve offspring. T&R has been applied for the elimination of Aujeszky's disease virus and Actinobacillus pleuropneumoniae . The principe of T&R is to collect sera from all breeding females, and evaluate the samples using ELISA for the detection of PRRSV antibodies, and Polymerase Chain Reaction (PCR) for the detection of viral nucleic acid. Animals that are positive by ELISA and serum PCR are immediately removed, as are those determined to be ELISA (+) / PCR (-) or ELISA (-) / PCR (+). Prior to testing, all boars are removed as well. The study in progress consists of 12 breeding stock farms in the USA. Breeding herd inventories within study farms range from 200 to 1500 sows. Application of the diagnostic protocol has resulted in the removal of 6 to 11% of the breeding herd; however, only a small number, i.e. 2 to 4/800 have been serum PCR positive at the time of sampling. Re-infection has not been detected despite monthly monitoring programs within the breeding (sample size at 95/5) and finishing populations (95/10) for up to 10 to 24 months following completion of the protocol. Diagnostic costs range from $8 to 10 US dollars per sow. Removed sows have been primarily older animals (6+ parities), and appear to cluster within groups of 3 to 4 (foci of infection), or reside as single entities randomly placed throughout the gestating population. ELISA data within infected foci tend to be normally distributed, suggesting that virus transmission initiates from an individual animal and occurs only over short distances. In conclusion, while such strategies still require further evaluation, other studies are currently underway to determine stimuli for the initiation of shedding, the true prevalence of persistently infected breeding swine and the assessment of ante-mortem diagnostic tests to enhance their detection.