Sources of influenza A virus infection on the farm

Influenza A virus in swine (IAV-S) can circulate continuously within a herd. IAV-S often appears on a farm with the introduction of new but infected animals. Once this occurs, the virus can transmit to susceptible animals like newborn piglets or new groups of replacement animals, allowing it to persist. This can lead to the virus becoming endemic in the herd.1

Farm Reservoir Diagram
Weaning —
Young piglets are a source of IAV-S for other pigs if they are infected at the time of weaning.2,3
Growing —
As pigs grow, they can be infected early in the nursery by virus shedding from weaned pigs as well as laterally from neighbor animals.
Gilt development unit (GDU) —
Replacement gilts can be sources of new viruses and reservoirs for endemic viruses in the breeding herd. Replacement gilts newly moved to a farm have a higher likelihood of testing positive for IAV-S than gilts who have been present on the farm for more than 4 weeks.4
Sow —
One study found a positive correlation between sow level serostatus and finishing pigs testing positive.5

References: 1. Brown I. The epidemiology and evolution of influenza viruses in pigs. Vet Microb. 2000;74:29–46. 2. Diaz A, Torremorell M. Dynamics of flu infection in sow farms. In: Proceedings of the Allen D. Leman Swine Conference; September 15–18, 2012; St. Paul, MN. 3. Reynolds JJH, Torremorell M, Craft ME. Mathematical modeling of influenza A virus dynamics within swine farms and the effects of vaccination. PLoS One. 2014;9(8):e106177. doi:10.1371/journal.pone.0106177. 4. Diaz A, Perez A, Sreevatsan S, et al. Association between influenza A virus infection and pigs subpopulations in endemically infected breeding herds. PLoS One. 2015;10(6):e0129213. 5. Poljak Z, Dewey CE, Martin SW, et al. Prevalence of and risk factors for influenza in southern Ontario swine herds in 2001 and 2003. Can J Vet Res. 2008;72:7–17. doi:10.1371/journal.pone.0129213.