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Tilmicosin
Is tilmicosin useful in reducing viremia and the clinical impact of Porcine Reproductive and Respiratory Syndrome (PRRS)?
Robert Friendship1, Ron Johnson2, Susy Carman3, Josepha Delay3, Zvonimir Poljak1, Tim Blackwell4, Terri O’Sullivan1 Department of Population Medicine, University of Guelph. Guelph, Ontario1, Department of Biomedical Science, University of Guelph, Guelph, Ontario2, Ontario Ministry of Agriculture Food and Rural Affairs, Fergus, Ontario3, Animal Health Laboratory, Laboratory Services Division, University of Guelph. Guelph, Ontario4
Porcine reproductive and respiratory syndrome (PRRS) is the most important swine disease in Canada today. A common strategy for controlling PRRS in a breeding herd is to inoculate pregnant sows with serum from a viremic pig in order to create uniform exposure and hopefully develop herd immunity rapidly. The procedure generally results in expression of clinical disease in some of the inoculated sows. It has also become common practice to medicate the sow herd with antibiotics during this controlled exposure to the disease with the original intention of controlling secondary bacterial infection. There have been anecdotal reports that tilmicosin has become the preferred option and this might be based on a belief that the antibiotic has antiviral properties. In-vitro studies have demonstrated some evidence of anti-viral activity that might account for the apparent beneficial effect beyond the fact that tilmicosin is an effective antibiotic for the control of bacterial respiratory diseases. However controlled field trials have not been performed to confirm an in-vivo antiviral effect.
The primary objective of this study is to determine if feed medication with tilmicosin reduces viremia in pigs exposed to PRRS. The secondary objective is to determine the effect of tilmicosin on lung pathology and macrophage activity.
Two hundred pigs (approximately 20 kg) will be randomly assigned to one of 4 treatment groups: 1a. Negative control - receiving feed containing 400ppm of tilmicosin but not infected with PRRS virus, 1b. Negative-negative control – receiving non-medicated feed and not infected with PRRS virus, 2. Positive control – infected with PRRS virus but not receiving tilmicosin, 3. Infected with PRRS virus while receiving feed containing 200 ppm of tilmicosin, 4. Infected with PRRS virus while receiving feed containing 400 ppm of tilmicosin. Infection will be achieved via an intramuscular injection of a commercially available PRRS MLV.
Pigs will be monitored on a daily basis and behavior, clinical signs and body temperature recorded. Quantitative PCR will be performed on sera to assess viremia and tilmicosin serum concentrations will be determined using HPLC. Pulmonary alveolar macrophages will be isolated from the broncheoalveolar lavage fluid of 20 pigs, 2 and 14 days following infection, to evaluate the effects of tilmicosin on macrophage activity across the different groups. On Day 14 a postmortem will be performed on 10 pigs including histology and immunohistochemistry on lungs for PRRS. The lung levels of tilmicosin will be determined from these pigs from samples collected from several different lobes using HPLC.
The cost of the tilmicosin treatment has been estimated at about $25 per sow which is relatively expensive. The benefit of this study to the Canadian swine industry is that if tilmicosin only has activity against secondary bacteria there may be more cost effective alternatives. On the other hand if tilmicosin does indeed reduce viremia it might be a useful tool in controlling PRRS outbreaks and minimizing spread. Canadian swine veterinarians will also benefit because this study will provide them with information which will help them in planning therapeutic strategies.