Greg Wideman from South West Ontario Veterinary Services, Modelling Influenza Transmission Rates: Dose Matters

Influenza is a major cause of mortality and morbidity in both pigs and people on a worldwide basis. Swine Influenza Virus (SwIV) are swine host adapted viruses that can infect both pigs and people. These SwIV are commonly found in commercial pig populations in Europe and North America. The dynamics of SwIV infection and transmission in individual hosts has not been well defined. Researchers at the University of Edinburgh and the Pirbright Institute wanted to create an experimental model that demonstrated transmission of SwIV by using the H1N1pdm09 pandemic influenza A virus in a transmission model for pigs. The experimental design allowed the researchers to observe individual transmission events occurring within sequential singular 18 hour windows of contact between a donor pig and a potential recipient pig.

Interestingly the researchers used a seeder pig model to create the donor pigs. The seeder pigs were intranasally inoculated with reconstituted Influenza virus and then exposed in a one on one situation with a naive pig to create a virus positive Donor pig. This extra step was used to create the Donor pig because intranasal inoculation resulted in a very quick onset of viral shedding and an unusually high virus titer during shedding which would not have been representative of a real world influenza outbreak and transmission rates in a commercial setting.

The researchers found the following:

  • The “shed virus titre” was above the limit of detection in Donor pigs for 67/88 D-R contact periods.
  • Among these 67 windows of contact where the Donor pig was actually shedding virus, transmission to Recipient pigs occurred on only 42/67 (60%) occasions. ( Lower than expected. )
  • Only one Recipient pig was infected after being in contact with a Donor pigs that tested negative for virus on a nasal swab.
  • On average the first transmission event was observed at 2.27 ± 0.33 days post contact (DPC)(range: 1–4 dpc). This led to the true infectious period (mean 3.9 days) being slightly shorter than that predicted by detection of virus (mean 4.5 days).
  • The generation time of infection (which determines the rate of epidemic spread) was estimated for the first time in pigs at a mean of 4.6 days.
  • The latent period of the contact pig was longer when they had been exposed to smaller amount of shed virus.

Take Home Messages:

  • It is interesting that one recipient pig was infected even when no virus was detected when swabbing the contact donor pig.  Nasal swabs are certainly not foolproof in establishing who is or is not an infection risk and the use of a disease specific quarantine period is always a safer approach.
  • As is the case in many other viral and bacterial diseases this study demonstrates that the infectious dose has a profound effect on shortening the time from exposure to the onset of viral shedding as well as increasing the amount of viral shedding as seen in the seeder pig inoculation part of the study.

Submitted by Dr. Greg Wideman

Ref: Laetitia Canini , Barbara Holzer , Sophie Morgan, Johanneke Dinie Hemmink , Becky Clark , sLoLa Dynamics Consortium; Mark E J Woolhouse , Elma Tchilian , Bryan Charleston   Timelines of infection and transmission dynamics of H1N1pdm09 in swine  PLoS Pathog . 2020 Jul 24;16(7):e1008628. doi: 10.1371/journal.ppat.1008628. Online ahead of print.