A pre-pandemic outbreak of triple-reassortant swine influenza virus infection among university students, South Dakota, 2008.

Background: After identifying a student with triple-reassortant swine influenza virus (SIV) infection and pig exposure at a livestock event, we investigated whether others were infected and if human-to-human transmission occurred.

Methods: We conducted a cohort study and serosurvey among persons exposed to (1) event pigs, (2) other pigs, (3) the index case, and (4) persons without pig or index case exposure. Confirmed cases had respiratory specimens positive for SIV within 2 weeks of the index case's illness. Probable and suspected cases had illness and (1) exposure to any pig or (2) contact with a confirmed case preceding illness. Probable cases were seropositive. Suspected cases did not give serum samples.

Results: Of 99 event pig-exposed students, 72 (73%) participated in the investigation, and 42 (42%) provided serum samples, of whom 17 (40%) were seropositive and 5 (12%) met case criteria. Of 9 students exposed to other pigs, 2 (22%) were seropositive. Of 8 index case-exposed persons and 10 without exposures, none were seropositive. Pig-exposed persons were more likely to be seropositive than persons without pig exposure (37% vs 0%, P < .01).

Conclusions: We identified an outbreak of human SIV infection likely associated with a livestock event; there was no evidence of human-to-human transmission.