Practice tip

May and June, 1999

Determining whether replacement stock is "free" from PRRSV

To be sure that the multiplier from which you plan to purchase stock is indeed free from porcine reproductive and respiratory syndrome virus (PRRSV), get the company to allow you to conduct an "external audit" of the multiplier.

Individual animal serology is not sufficient. We obtain 40 blood samples from only the late nursery pigs because we assume that group is the most likely to be viremic. We test it with the PRRSV ELISA and PRRSV polymerase chain reaction (PCR). To save money on the PCR, we pool three blood samples at a time. If only 1% of pigs are viremic, then we have about a 70% probability of not detecting a positive. However, if 10% are viremic, then the probability of not detecting a positive falls to about 5%. We are comfortable with this because we expect at least 10% of nursery pigs to be viremic in a positive herd. The potential for animals to go negative over time, or to develop low positive ELISA results that are in the "grey zone" between 0.4-0.7 (i.e., potential false positives) is real. Individual animal serology can also miss long-term viremic pigs that may test seronegative but still have virus in their system.

Screening the adult and growing herds at least quarterly should allow you to detect virus circulation in the source herd. Test incoming animals from "negative herds" at 14-16 days after arrival to assess exposure during transport to the destination farm.

--submitted by Drs. Carlos Pijoan and John Kolb

This practice tip is not refereed