Influenza A viruses (IAVs) are recognized as one of the most common respiratory disease agents in pigs. There was a time prior to the early 1980’s when IAV was not present in Ontario. The classic H1N1 IAV strain made its way to Ontario and for a long time it was the only strain of IAV infecting pigs in Ontario. Most clinical disease was restricted to sudden epidemic outbreaks in fall winter and for the most part these infections were spontaneously eliminated from the affected herd. Farrow to finish operations were predominant back then. The breeding herd and growing pigs all became infected at the same time and then the virus burned out due to widespread “herd immunity”. There was no more fresh fuel to keep the fire burning. In 2005, an H3N2 virus made its way from the USA to Canada via turkeys. With the arrival of H3N2, IAV became a year round disease. The viral picture has continued to change and today the H1N2 virus has become the predominant IAV found in pigs in Ontario. As an RNA virus IAV has the tendency to change genetically through genetic drift as well as recombination events. Needless to say, the prevention and control of IAV continues to be complicated.
A recent research project organized through the Ontario Veterinary College looked at characterizing the circulation of IAVs between weaning and market age. This was determined by looking at both
the development of antibodies in response to exposure as well as molecular epidemiology of detected viruses (PCR). Two batches of weaned pigs were followed in the nursery and finisher barns with
a sample size of 81 and 75 pigs in study 1 and 2 respectively. Nasal swabs and blood samples were collected from individual pigs for virological and serological analyses.
The researchers found the following:
- In Study 1, H3N2 subtype virus, of cluster IV, was detected and this virus had a maximum of 97.9% identity to HA gene of viruses previously isolated in Ontario. Pigs with higher numbers of
IAV detection had lower serological titers for the same virus that was confirmed to circulate in the nursery (P < 0.01) (Antibodies that the piglet receives via colostrum can reduce the clinical
impact of disease and reduce shedding but do not necessarily prevent infection).
- In Study 2, H1N1 subtype virus, of 2009 H1N1 pandemic lineage was detected and this virus had a maximum of 97.8% identity to HA gene of viruses previously isolated in Ontario.
- The existence of antibody titers for IAV strains other than the strain of IAV that was isolated helped to confirm that more than one IAV subtype can circulate in the same population.
Take Home Messages
- A thorough knowledge of all viral strains that are present in a particular herd is fundamental for development of infection and disease control.
- The types of IAV that you may find on a first attempt at screening a herd may not represent all of the IAV strains that are truly present. Some IAV strains in a given pig population may be of
lower prevalence and are therefore overshadowed during sampling by the more predominant strains.
- IAV sampling and testing strategies need to be robust in order to detect all of the IAV strains that are present and sampling should be repeated over time as the picture can and likely will change.
Submitted by Greg Wideman, DVM
Ref: Juliana Bonin Ferreira, Zvonimir Poljak, Robert Friendship, Éva Nagy, Greg Wideman, Helena Grgić Assessment of exposure to influenza A viruses in pigs between weaning and market age Vet Res . 2021 Apr 21;52(1):60. doi: 10.1186/s13567-021-00927-9.