Once a sow vaccination program has been initiated on a farm that was previously negative for PRRS but has recently had an outbreak, when should piglet vaccination start?
When a negative farm becomes infected with PRRS virus it will continue to produce piglets that have been infected during gestation and are therefore viremic for at least a 4-month period.
When these piglets are vaccinated, they tend not to handle the vaccine virus well as their immune response will not be entirely adequate, having lived with the virus while their immune system was developing during late gestation.
Vaccination of piglets will only make sense when there is a low percentage of piglets born viremic. The percentage of piglets born viremic will be very high during the first 2 months after infection of the farm and will progressively decrease over time. Ideally, vaccination of piglets should be initiated once all piglets are negative, but reaching this point may take many months, depending on the farm, so vaccination is generally initiated when there is a reduced percentage of viremic piglets.
Since not all piglets can be tested individually, this can be done by quantifying the virus present in testicular fluids by following the Ct value of the PCR.
The higher the value, the lower the presence of virus. So, vaccination could be initiated when the Ct values are higher than 29, indicating that a significant percentage of piglets are born negative and, therefore, by vaccinating the batch it is expected that there will be an acceptable vaccination response that will reduce problems in the post-weaning phase.
If you want to know more about piglet vaccination read out chapter:
1. Angulo J, et al. (2020) Implementation of PRRS surveillance using processing fluids (PF) RT-PCR Ct value as predictor for nursery mortality. 26th International Pig Veterinary Society Congress, 2020.
2. Lopez WA, et al. Porcine Respiratory and Reproductive Syndrome monitoring in breeding herds using processing fluids. J. Swine Health and Prod. 2018; 26(3):146150
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