Risk factors for detecting influenza A virus in growing pigs

Objective: To investigate the association between certain farm-level risk factors and the presence of influenza A virus (IAV) in growing-pig farms. Materials and methods: Twenty-six pig farms participated in the study. Thirty nasal swabs from growing pigs were collected per month from each farm for 12 or 24 consecutive months between 2009 and 2011. Nasal swabs were tested for IAV by real-time reverse transcriptase polymerase chain reaction. Weather stations located at every participating farm monitored temperature, relative humidity, light intensity, and wind speed and gusts. Farm-level data was obtained through a questionnaire to assess the relationship between the presence of IAV and farm-level characteristics. Results: Of the 15,630 nasal swabs col-lected from growing pigs, 730 (4.6%) tested positive for IAV. Of the 522 groups of growing pigs from which nasal swabs were collected, 110 groups (20.8%) had at least one positive nasal swab. Positive nasal swabs originated from 23 of the 26 participating farms. Farm-level characteristics associated with the presence of IAV included farm type (farrow-to-finish odds ratio [OR] 3.05; nursery OR 16.69), pig flow (all-in, all out OR 0.31 by barn; OR 0.35 by site), gilt source (born at breeding site, raised off-site. and later returned OR 0.17; off-site multiplier OR 0.25), environmental temperature, and wind speed. Implications: Population dynamics, eg, nursery and farrow-to-finish farms and continuous-flow management, play important roles in the epidemiology of IAV. Possible modifications to farm type and pig flow should be considered when constructing IAV control and prevention strategies.