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
|
