Surveillance of Iowa swine herds for influenza-like illness: combined serologic and virus isolation method.

Five Iowa swine herds (involving about 7,000 swine) were placed under surveillance for signs of influenza-like illness. Blood samples for hemagglutination-inhibition (HI) tests of serums and nasal secretions on swabs for viral isolation were collected from 20 feeder swine in each herd at the outset of surveillance. On the basis of results of HI tests, 6 swine in each herd tested were chosen to be resampled 6 weeks after the first blood sample was collected if swine influenza virus (SIV) was not isolated, but 3 weeks after the first blood sample was collected if SIV was isolated at the outset of surveillance. The swine chosen for resampling were considered sentinels in a herd for the duration of surveillance. Swine influenza virus was isolated from 20 of 20 swine in each of 2 herds that had signs of influenza-like illness. The initial HI titer of each of the 20 swine in the 2 herds was less than 10. However, serum samples prepared from blood collected from sentinel swine in the 2 herds 3 weeks after isolation of SIV had HI geometric mean titers (GMT) of 23 and 34. One herd had an initial HI GMT of 21. A SIV was not isolated from this herd, and serum samples obtained from 3 of the 6 sentinel seine 6 weeks after the first blood sample was collected still had demonstrable HI antibody.