Risk factors for first and recurrent influenza virus shedding in nursery pigs

Swine influenza outbreaks are usually recognized by the sudden appearance of respiratory signs and also by quick recovery of sick animals. However, influenza A virus (IAV) can endemically circulate without causing such typical clinical outbreaks. In addition, the complexity of influenza circulation in large multi-site and multi-source herds has not been well described. The objectives of this study were to describe the dynamics of IAV circulation in multisource nursery herds and identify risk factors for recurrent infection.

The study was conducted at a ~2000-head nursery operated on an all-in/all-out basis. Pigs from 5 different sow-herds, each with a different health status, were mixed in 4 rooms (each with 24 pens). In the first 2 hours of arrival 400 pigs were selected for the initial virological testing. Additionally, 81 and 75 pigs were included for ongoing weekly testing for influenza virus for Study 1 and 2, respectively. Virus isolation and propagation were done in Madin-Darby canine kidney (MDCK) cells. Serology was performed by hemagglutination inhibition (HI) using 8 different influenza viruses. Risk factor analysis for virological positivity and likelihood of recurrent infection was conducted using logistic regression and survival analysis.

In Study 1, at ~30 days post-weaning, 100% of pigs were positive and shedding viruses, with 35 (43.2%) pigs being positive recurrently. A different pattern was observed in Study 2 with 36 (48%) pigs being positive only once and 8 (10.7%) pigs being positive recurrently. Results indicated that IAV can circulate during the nursery phase in a cyclical pattern and the likelihood of recurrent infections was higher for pigs with higher level of heterologous (withinsubtype) maternal immunity, but only for the H3N2 strain, which could explain ongoing issues in the nursery. However, the presence of high heterologous immunity is not likely to explain all recurrent infections because pigs with low heterologous infections were also noticed to be recurrently infected. High degree of within-pen clustering was also observed, suggesting that transmission within a pen played an important role.

Prolonged or recurrent IAV infections could be very important when trying to control IAV infection in nursery barns. These findings could be useful in developing control strategies.

Disclosure of Interest: None Declared.