• Jon is a contributing correspondent for Science.

The Continuing Mystery of How Canadian Pigs Caught the Novel Flu Virus

11 June 2009 4:20 pm

On 2 May, a pig farm in Alberta, Canada, made international news when officials revealed that the animals there carried that novel H1N1 virus causing the swine flu outbreak in humans—the first and still the only pigs known to be infected. The Canadian Food Inspection Agency (CFIA) said it was "highly probable"that a Canadian who recently had traveled to Mexico and returned with flu-like symptoms had infected the pigs. At press conferences on 2 May and 7 May, Canadian officials explained that because the farmer did not buy pigs from other farms, the “contractor” who had gone to Mexico was the most likely source of the virus. That scenario ruled out the possibility that the pigs were infected before humans and may have held clues to the origin of the outbreak.

It turns out that the contractor, Adrian Blaak, was a carpenter who had worked on the farm for one day, 14 April, swapping out vents on a pig barn. Although Blaak was feeling ill that day, he had minimal direct contact with the pigs. The farmer first noticed illness in his pig herd on 24 April. Officials quickly suspected that Blaak was the source of the pig infection, but his symptoms had resolved by then, at which point it's typically difficult to find the virus. Nasopharyngeal swabs taken from him, as expected, were negative for the novel H1N1. At a 7 May press conference, Frank Plummer, who heads the National Microbiology Laboratory in Winnipeg, a branch of the Public Health Agency of Canada (PHAC), said that they were going to test Blaak’s blood samples for antibodies to the novel H1N1 virus, which could confirm that he had been infected.  Plummer also discussed other workers at the farm who had flu-like symptoms and were being tested for evidence of infection with the swine flu virus.

ScienceInsider recently asked Plummer by email about the results from the tests of the carpenter’s sera. Plummer did not respond but his spokesperson on 3 June wrote that PHAC was “unable to answer questions related to specific cases.” The spokesperson suggested that the Province of Alberta might have more information, but the chief medical officer there also cited privacy reasons for not discussing the case.

ScienceInsider then contacted the carpenter, Adrian Blaak, who from the outset publicly questioned the assertion that he even had the H1N1 virus. Although Blaak acknowledges he wasn’t feeling well the day he worked on the farm, he told ScienceInsider, “I still don’t believe I infected the pigs.” Blaak was surprised that officials had yet to inform him about the results from his blood tests, and he was planning to ask for them.

Veterinarian Jim Clark, CFIA’s lead spokesperson about the novel H1N1 infection of the Alberta pigs, discussed the case with ScienceInsider on 9 June. Clark revealed that CFIA has not received information from PHAC about the blood tests of Blaak and others on the farm who had flu-like symptoms. Although CFIA remains convinced a human infected the pigs, the agency has downgraded  Blaak’s role from “highly probable” status, Clark revealed. He also said he did not know of any confirmed human cases of the novel H1N1 virus in the vicinity of the fairly remote Alberta farm.

This is a condensed transcript of the interview, edited for clarity.

Q: I wanted to follow up with the questions about the further testing on the sera of the contractor and others at the farm to see if there is any evidence that they were infected with the novel H1N1. We discussed this earlier, and Frank Plummer and others also discussed it at the press conference on 7 May after we spoke. Are there any results from the testing of those sera?

Jim Clark: I don’t know. The public health authorities in Manitoba are holding that information, and they have not made that available to CFIA.

Q: Really? It’s been over a month.

J.C.: I’m sure they have results, as I suggested, but it’s not been made available to anyone outside the human health community.

Q: Why are you sure they have results?

J.C.: Well, negative or positive, they would have results by this point in time.

Q: Is it frustrating that you don’t have those results?

J.C.: It reflects an information sharing issue that has been highlighted previously. The concern prior to this has been with the people who would be responsible for maintaining animal health, sharing their test results with the public health community. I think we have the opposite situation in this particular case, where the public health authorities may have information that may be of use and assistance to the animal health community in making some determination about how the disease progressed or the epidemiology of the disease. Information sharing does become something of concern on an ongoing basis in most disease outbreak investigations, and it’s around privacy concerns of the people involved. I think we can address those concerns, it’s just not being done at this point in time.

Q: There’s an odd wrinkle to the privacy concern angle. I spoke with Adrian Blaak, who is the carpenter suspected of  infecting the pigs. It’s very surprising to him that he hasn’t been given the results of the tests.

J.C.: Ah. I didn’t know that either.

Q: Do you have any different thoughts about the original assertion that it was “highly probable” that the worker who went to Mexico infected the pigs?

J.C.: That’s an excellent question. Given what we know in both our investigation of the probable opportunities for the virus to be introduced to this particular herd, we have come to the conclusion that since there doesn’t seem to be any animal possibility of introducing the virus from pigs, we have to conclude that the most reasonable source has to have been a person. Whether the person was that carpenter or some other individual who had the opportunity to interact with swine, that’s a question at this point in time. And until we get better information about what was going in the individuals on the human side who had an opportunity to interact with the pigs, we’re not really going to be able to say any more definitively.

We’re reasonably confident that there is no opportunity for this virus to have been introduced from another swine herd.

Q: It sounds like your thinking might have changed, that it may have been another human who infected the pigs.

J.C.: I think we have to back off a little bit and say while we still think it was a human introduction to the swine herd, we’re not quite as definitive about whether it was this individual who went to Mexico and returned as we were previous to now.

Q: ScienceExpress last month published a paper co-authored by 59 researchers that said there might have been another species as the intermediate, suggesting the virus might have gone from swine to another species and then to humans. Have you looked at other species that potentially could have been the vector?

J.C.: No. When you look at other species, which species do you pick on?

Q: I could list a few.

J.C.: Ok. Name them.

Q: Equine.

J.C.: We’re not seeing any evidence in the equine population or any other species that would indicate that that virus has moved into that particular species and is propagating. I guess in my own way of thinking of it, since we appear to have a relatively human adapted virus at this point in time, my most probable thought would be that we have an individual who somehow was exposed to swine viruses that had both Eurasian and North American lineages, and that recombinant activity probably occurred in a human rather than another species of an animal.

Q: What surveillance did CFIA do for swine influenza prior to this outbreak, and has it changed?

J.C.: CFIA didn’t do any ongoing surveillance for swine influenza viruses or influenza A viruses other than the avian influenza viruses that would have been H5 and H7 subtypes. Those particular subtypes in the avian influenzas represent the possibility of the pathogenecity change from low to high and obviously have a fairly significant impact on both the animal health and the human health situation—H5N1, an Asian strain, being the virus of most concern.

Influenza A viruses in the swine community were pretty much determined to be ubiquitous, production-limiting diseases that were recognized to be there on an ongoing basis, and had some opportunity for transmission from swine to people and people to swine, well documented within the literature, and didn’t represent any particular threat to either the swine or the human communities.

Q: Following the outbreak, has there been surveillance for the novel H1N1?

J.C.: We’re certainly approaching it from the same position as the United States and Mexico. We want to become aware of any situation where there has been a person who has been diagnosed as having the novel H1N1 and has an opportunity for association with a swine herd so we can go in and investigate to determine whether there has been any transmission. And we’re certainly looking for any novel isolates that may come out of the passive surveillance system. So if we have a provincial or a university lab looking at an influenza A virus from a swine herd and they don’t recognize it as one of the typical North American strains that was previously circulating, then we’re looking for those isolates to be sent to our National Centre for Foreign Animal Diseases in Winnipeg for characterization. But we’re not going into active surveillance in the swine population looking for this particular virus unless there is some direct association with an infected pig or infected person.

Q: Are there any investigations underway right now because of infected human cases being in the proximity of swine?

J.C.: No.  We’ve received no reports from the public health community of people who’ve been confirmed to be infected who have associated with swine. That certainly was of concern to us in the initial part of the investigation, and we did ask them to insert questions within their epidemiological investigation that would highlight that particular opportunity for association. They’ve done that, and we’ve not been advised of any circumstances where they have concerns.

Q: Do you know if there are any confirmed cases in the vicinity of the pig farm?

J.C.: I’m unaware of any. Alberta has confirmed H1N1 in individuals, but whether they have any association with the pig farm we’ve had under quarantine I’m not aware of it.

Q: The pig farm is fairly remote, isn’t it?

J.C.: It’s not in one of the more densely populated areas. Edmonton would be the closest urban area that would have a significant population. Other than that, it’s pretty indicative of the population density of most of the Midwestern states or our prairie provinces. It’s pretty scattered.

Posted In: