Effects of a modified-live virus vaccine against porcine reproductive and respiratory syndrome in boars

Abstract

Objectives. To determine whether vaccine virus is found in serum and semen of vaccinated boars, whether vaccination prevents subsequent shedding of wild-type virus after challenge exposure, and whether semen and blood variables are altered after vaccination or challenge exposure with wild-type virus, or both. Design. Throughout the 50-day postvaccination period, serum and semen from exposed boars were evaluated for the presence of porcine reproductive and respiratory syndrome virus (PRRSV). All boars were then challenge-exposed with PRRSV isolate VR-2332 and evaluated for an additional 27 days. Semen quality variables, serostatus, and blood variables were monitored. Animals. 7 PRRSV-seronegative adult boars. Procedure. Semen was collected 3 times weekly and evaluated by use of a nested reverse-transcriptase polymerase chain reaction for detection of PRRSV RNA. Serum was obtained weekly and evaluated by nested reverse-transcriptase polymerase chain reaction, virus isolation, and PRRSV ELISA. Semen quality variables were evaluated 3 times weekly, and CBC was performed weekly. Results. Vaccine virus was shed in the semen of all vaccinated boars, but shedding was of shorter duration in 4 of 5 vaccinated boars than that generally observed after exposure to wild-type virus. After challenge exposure, shedding of wild-type virus in semen was shortened or eliminated in 4 of 5 vaccinated boars. Percentage of forward movement and normal spermatozoal morphology and motility were significantly reduced in vaccinated boars after challenge exposure. Conclusions. Vaccine virus was shed in semen of vaccinated boars, but vaccination generally reduced or eliminated shedding of wild-type PRRSV after challenge exposure. Semen quality appeared to be less than optimal, particularly after vaccination and subsequent challenge exposure with wild-type virus. Clinical Relevance. Extra-label use of the PRRSV vaccine in boars remains controversial because some boars may still shed wild-type virus in semen after challenge exposure at postvaccination day 50. Semen quality also appeared to be altered after vaccination and subsequent challenge exposure.