Pork Checkoff Report
Newsletter Article 12/13
Submitted by Dave Wright, D.V.M.
How Many Pigs Do I Need to Test?
The goal of any disease surveillance program is to accurately identify the status of a
herd—positive or negative—with the fewest number of samples. Most of us think of the
magic number “30”, but this might make us overconfident if the prevalence of disease in
our herd is low.
Assuming that we use a test with perfect sensitivity (no false negatives) and perfect
specificity (no false positives), if we test 30 pigs in a group we can be 95% confident that
we will find at least one positive pig in a herd that has a prevalence of at least 10%.
Unfortunately, in herds that are attempting to eliminate PRRS virus the number of
infected pigs becomes lower than 10% but still poses a significant threat to re-infecting
the herd.
We can improve our chances of detecting a positive pig by increasing the number of pigs
tested, by increasing the frequency of testing, by pooling samples, by selecting sicklooking pigs that may be more likely to be infected, and by introducing negative or naïve
pigs that are tested after they have been exposed to the herd. Talk to your veterinarian to
determine the best surveillance protocol for your herd. And remember to include your
premises ID on all lab submissions to accurately identify the site.