An experimental study with a vaccine strain of porcine reproductive and respiratory syndrome virus to determine effects on viremia assessed by reverse transcriptase-polymerase chain reaction in pigs fed rations medicated with tilmicosin or non-medicated

Abstract

Objectives: To determine if feed medicated with tilmicosin affects viremia (assessed using reverse transcriptase-polymerase chain reaction [RT-PCR]) in pigs exposed to a vaccine strain of porcine reproductive and respiratory syndrome virus (PRRSV), clinical signs associated with vaccination (body temperature), and average daily gain. Materials and methods: Purebred Yorkshire pigs (N = 192) were each assigned to one of five treatment groups. Groups 1a and 1b remained PRRSV-negative (controls), while Groups 2, 3, and 4 were injected with a modified-live (MLV) PRRSV vaccine. Groups 1b and 2 were fed non-medicated feed. Rations contained tilmicosin at 400 mg per kg for Group 1a and Group 4 and 200 mg per kg for Group 3. Blood samples were collected to measure serum tilmicosin concentrations and assess PRRSV viremia. Bronchoalveolar lavage was performed and macrophages assessed for PRRSV viremia and tilmicosin concentrations. Results: Groups 1a and 1b remained PRRSV-negative. Number of PRRSV copies per mL in serum was highest in inoculated pigs at 10 days post inoculation, but did not differ among the three inoculated groups. Average daily gain (ADG) was higher in groups fed rations containing 400 mg per kg tilmicosin than in groups on non-medicated rations. Clinical signs of disease were absent in all pigs. Implications: Viremia associated with an MLV vaccine strain of PRRSV does not differ between pigs fed rations containing 200 or 400 mg per kg of tilmicosin. In the absence of clinical disease, pigs consuming tilmicosin-medicated feed have higher ADG than pigs consuming non-medicated feed.