Examination of viraemia and clinical signs after challenge with a heterologous PRRSV strain in PRRS Type 2 MLV vaccinated pigs: A challenge-dose study

Abstract

Vaccination with porcine reproductive and respiratory syndrome (PRRS) Type 2 modified-live vaccines (MLVs) has been shown to improve clinical signs and survival rates in PRRS virus (PRRSV)-challenged pigs. This study evaluated the dose of PRRSV challenge needed to cause and maintain viraemia in PRRS Type 2 MLV-vaccinated pigs and assessed clinical responses to various doses of virulent challenge. This controlled, randomised, blinded vaccination-challenge study involved 95 pigs who were either vaccinated with 2 mL of a PRRS Type 2 MLV on Day 0 or left unvaccinated. On Day 28, pigs were challenged intranasally with virulent PRRSV isolate (dose range <1.5 to 4 log 10 50% tissue culture infectious dose/ mL). Five pigs were left unchallenged and served as environmental controls. Viraemia levels, pyrexia, average daily weight gain and clinical signs were assessed. At all challenge doses, vaccinated groups had reduced viraemia levels and clinical signs, and higher average daily weight gain compared with non-vaccinated groups. Vaccinated groups challenged with 2 log had similar viraemia levels and clinical performance (days pyrexic and average daily weight gain) as the non-challenged group. Vaccinated groups had significantly reduced pyrexic days compared with non-vaccinated groups across all challenge doses (P <3 log had significantly improved average daily weight gain (P