There are a number of zoonotic infectious diseases that have the ability to cross from animals to humans and humans to animals. “Zoonotic spillover” describes the transmission of a pathogen
from a vertebrate animal to a human. Examples of Zoonotic spillover include pathogens such as Ebola virus, Influenza A virus (H1N1) pandemic 09, SARS- CoV-2 and Middle East respiratory
syndrome coronavirus (MERS-CoV). For a spillover event to occur there must be some genetic mutation in the infectious agent that would increase compatibility of the animal infection with
human infection. The organism must be shed in sufficient amounts to reach an infection threshold, survive in the environment long enough to be disseminated and finally must get to a susceptible human host that is close enough to the virus to be infected. The transmission of traditionally swine host adapted Influenza A to humans normally requires all of these moving parts to come into alignment. When traditionally swine host adapted Influenza viruses infect people, we describe them as influenza “variants”. The truth is that IAV are quite happy to survive and replicate in any host when the opportunity arises.
It is easy to see why these IAV variants have been commonly detected in association with 4H shows or fall fairs. When pigs that happen to get sick at the time of the show are housed in tight quarters with children working closely with their show animal the odds of a spillover event increases. These events can also happen with people working closely with pigs on a commercial farm. This is one of the reasons that we continue to recommend that everyone should be getting their seasonal influenza vaccine.
It is also easy to see why pig production has the potential to play a role in an Influenza pandemic. Surveillance of specific sentinel animal populations prior to a “spillover” event into the human population is a key part of pandemic prevention and planning. More often than not many of these early signals turn out to be a false alarm. In June, 2021, the Province of Manitoba reported one case of a variant human influenza A (H3N2)v. This was only the second confirmed case of an H3N2variant in Canada since it was first reported in a human case in 2016 in Ontario. This sort of spillover event is going to happen every once in a while. Fortunately, this virus did not spread any further in the human population. It hit a dead end if you will and this is usually the most common outcome. It is always possible, however, that a spillover event could gather some momentum and become increasingly problematic in the human population by spreading from human to human and country to country.
What does a public health response to one of these events look like. In October 2020, a multidisciplinary group detected a variant influenza A subtype H1N2 of swine origin in a person in Alberta, Canada. This kicked off a very collaborative follow up that involved a public health, veterinary, and laboratory investigation to identify the source of the infection and determine whether it had spread any further in the human population. The group identified the probable source of infection as a local pig farm where a household contact of the index (first) patient worked. Phylogenetic (genetics) analysis revealed that the
isolate closely resembled IAV strains that had been found at that farm in 2017. Retrospective and prospective surveillance using molecular (PCR) testing did not identify any secondary cases among
1,532 persons tested in the surrounding area. This is an example of one of the earliest potential signals of an emerging disease or pandemic that can take place at the intersection between animal and human populations.
Take Home Messages
- “Zoonotic spillover” is the transmission of a pathogen from a vertebrate animal to a human and on very rare occasions these spillover events have the capability of kicking off a pandemic.
- Quick collaboration between human and veterinary public health practitioners can enable a rapid response to a potential outbreak and the focus in the initial stages will be at the animal human
interface a well as contact tracing.
- It is not a surprise that public health authorities are extremely interested when these events occur and vigorous follow up can be expected.
Submitted by George Charbonneau DVM
Ref: Jamil N Kanji, Kanti Pabbaraju, Matthew Croxen, Susan Detmer, Nathalie Bastien, Yan Li, Anna Majer, Hussein Keshwani, Nathan Zelyas, Ifeoma Achebe, Corinne Jones, Maureen Rutz, Angela Jacobs, Keith Lehman, Deena Hinshaw, Graham Tipples Characterization of Swine Influenza A(H1N2) Variant, Alberta, Canada, 2020 Emerg Infect Dis. 2021;27(12):3045-3051. doi: 10.3201/eid2712.210298.
