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 log10 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 <.05). Vaccinated pigs challenged with <3 log had significantly improved average daily weight gain (P <.05). In vaccinated groups, challenge dose correlated positively with viraemia levels and number of days pyrexic, and negatively with average daily weight gain. This is the first study to use a challenge-dose model to evaluate the efficacy of vaccination against PRRSV. PRRS Type 2 MLV was shown to mitigate the consequences of PRRSV infection at all evaluated PRRSV challenge doses. Lower levels of challenge had minimal impact on health and performance of vaccinated pigs, supporting the benefit of vaccinating swine with PRRS Type 2 MLV.
Highlights
Porcine reproductive and respiratory syndrome (PRRS) is responsible for substantial animal and economic losses to the swine industry [1]
The objective of this study was to evaluate the dose of virulent PRRS virus (PRRSV) challenge needed to cause and maintain viraemia in PRRS Type 2 modifiedlive vaccines (MLVs)-vaccinated pigs
All pigs except six were found to have seroconverted following vaccination. These six pigs were omitted from the analysis: two vaccinated pigs challenged with 4 log PRRSV (Group 4) and four vaccinated pigs challenged with 3 log PRRSV (Group 3)
Summary
Porcine reproductive and respiratory syndrome (PRRS) is responsible for substantial animal and economic losses to the swine industry [1]. The PRRS virus (PRRSV) causes viraemia, which typically leads to pyrexia, pneumonia with abnormal respiratory behaviour, and reduced average daily weight gain (ADWG) [2]. Previous studies have shown that pigs can be infected by low doses of the virus [3,4,5]. PRRSV is highly infectious and spreads via intranasal, intramuscular, haematogenous and aerosol routes. As pigs are often raised in areas of high density, the spread of infection is difficult to control [3,4,5]. Preventing infection and transmission of the virus is essential in reducing its negative impact on individual farms, and on the entire swine production industry
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