Effect of strain-specific maternally-derived antibodies on influenza A virus infection dynamics in nursery pigs.

Reducing the number of influenza A virus (IAV) infected pigs at weaning is critical to minimize IAV spread to other farms. Sow vaccination is a common measure to reduce influenza levels at weaning. However, the impact of maternally-derived antibodies on IAV infection dynamics in growing pigs is poorly understood. We evaluated the effect of maternally-derived antibodies at weaning on IAV prevalence at weaning, time of influenza infection, number of weeks that pigs tested IAV positive, and estimated quantity of IAV in nursery pigs. We evaluated 301 pigs within 10 cohorts for their influenza serological (seroprevalence estimated by hemagglutination inhibition (HI) test) and virological (prevalence) status. Nasal swabs were collected weekly and pigs were bled 3 times throughout the nursery period. There was significant variability in influenza seroprevalence, HI titers and influenza prevalence after weaning. Increase in influenza seroprevalence at weaning was associated with low influenza prevalence at weaning and delayed time to IAV infection throughout the nursery. Piglets with IAV HI titers of 40 or higher at weaning were also less likely to test IAV positive at weaning, took longer to become infected, tested IAV RT-PCR positive for fewer weeks, and had higher IAV RT-PCR cycle threshold values compared to piglets with HI titers less than 40. Our findings suggest that sow vaccination or infection status that results in high levels of IAV strain-specific maternally-derived antibodies may help to reduce IAV circulation in both suckling and nursery pigs

Farm management practices associated with influenza A virus contamination of people working in Midwestern United States swine farms

Abstract Indirect transmission of influenza A virus (IAV) contributes to virus spread in pigs. To identify farm management activities with the ability to contaminate farmworkers’ hands and clothing that then could be a source of virus spread to other pigs, we conducted a within-farm, prospective IAV surveillance study. Hands and clothes from farmworkers performing the activities of piglet processing, vaccination, or weaning were sampled before and after the activities were performed. Samples were tested by IAV rRT-PCR and virus viability was assessed by cell culture. A multivariate generalized linear model was used to detect associations of the activities with IAV contamination. Of the samples collected for IAV rRT-PCR testing, there were 16% (12/76) collected immediately after processing, 96% (45/48) collected after vaccination, and 94% (29/31) collected after weaning that tested positive. Samples collected immediately after vaccination and weaning, i.e., activities that took place during the peri-weaning period when pigs were about 3 weeks of age, had almost 6 times higher risk of IAV detection and had more samples IAV positive (p-value < 0.0001) than samples collected after processing, i.e., an activity that took place in the first few days of life. Both, hands and clothes had similar contamination rates (46% and 55% respectively, p-value = 0.42) and viable virus was isolated from both. Our results indicate that activities that involve the handling of infected piglets close to weaning age represent a significant risk for IAV dissemination due to the high level of IAV contamination found in farmworkers’ hands and coveralls involved in the activities. Biosecurity protocols that include hand sanitation and changing clothing after performing activities with a high-risk of influenza contamination should be recommended to farmworkers to control and limit the mechanical spread of IAV between pigs

Impact of nurse sows on influenza A virus transmission in pigs under field conditions

Piglets prior to weaning play a central role in maintaining influenza infections in breeding herds and the use of nurse sows is a common practice to adopt piglets that fall behind and that otherwise would die. Transmission of influenza A virus (IAV) from nurse sows to adopted pigs has been reported experimentally, however, the importance of this route of transmission under field conditions has not yet been elucidated. A cohort study to assess the IAV status in nurse and control sows and their respective litters was carried out in three influenza positive breed-to-wean farms. A total of 94 control and 90 nurse sows were sampled by collecting udder skin wipes and oral swabs at enrollment (similar to 5-7 days after farrowing) and at weaning. Six piglets per litter were sampled randomly at enrollment, 2 days post-enrollment (DPE), 4 DPE, at day 14 of lactation (14DL) and at weaning. At enrollment, 76 % (69/91) of udder wipes and 3 % (3/89) of oral swabs from nurse sows were positive by rRT-PCR compared with 23 % (21/92) of udder wipes and 0 % (0/85) of oral swabs from control sows. Of the 94 control litters sampled, 11.7 %, 14.9 %, 22.9 %, 46.8 % and 63.9 % tested rRT-PCR IAV positive at enrollment, 2DPE, 4DPE, 14 DL and weaning, respectively. Corresponding prevalence for nurse sow litters were 12.2 %, 30.2 %, 37.0 %, 59.4 % and 56.4 %. The odds of IAV positivity were significantly higher (p < 0.05) for litters from nurse sows 2 DPE (odd ratio (OR) = 6.13, 95 % CI = 1.8-21.2), 4 DPE (OR = 5.5, 95 % CI = 1.7-17.8) and 14 DL (OR = 3.7, 95 % CI = 1.1-12.3). However, there were no differences in the proportion of positive samples at weaning. Moreover, approximately 18 % of the control sows and 11 % of nurse sows that tested IAV negative in oral swabs at enrollment, tested IAV positive at weaning. This study indicates that nurse sows can contribute to the transmission and perpetuation of IAV infections in pigs prior to weaning, particularly during the first week after adoption