Pig Chat – Finding All The IAV Strains In A Herd Requires Robust Sampling, By Greg Wideman from South West Ontario Veterinary Services


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.