TORONTO - There is no smoking gun in the case of the H1N1 infected pigs - and authorities investigating the first known infections of pigs with this new swine flu virus may not be able to unearth one, a senior Canadian Food Inspection Agency official admits.
Testing of people on the Alberta farm - some of which was done too late, some of which may not have used the best technique to get an answer - has turned up no solid proof people brought the virus to the pigs. And it remains to be seen whether blood testing will be able to fill the evidence gap.
The agency still strongly believes the pigs were infected by people, but all the evidence is circumstantial, says the CFIA's Dr. Jim Clark.
"That's absolutely it. Without a laboratory result that clearly says one of those people that had exposure to the pigs was infected with this novel H1N1, it's simply taking probabilities and looking at what is the most likely source of the infection," says Clark, the national manager for disease control for CFIA's animal health division.
Clark, who is also serving temporarily as planning chief of the national emergency response team, says he thinks the chances of finding an answer in blood samples taken from people on the farm is "fairly remote" - though the director of Canada's National Microbiology Laboratory is more optimistic.
"We think we probably will be able to make a call based on the antibody levels in this individual," Dr. Frank Plummer said during a recent news conference. Plummer was speaking of the so-called index case, the man believed to have introduced the virus into the pig farm.
He said there isn't a lot of cross reaction between antibodies created by exposure to human H1N1 viruses and those provoked by infection with the new swine H1N1, so it might be possible to tease out an answer using a blood test - once one is designed.
If that doesn't work, Plummer said, there are other techniques to probe the immune system's memory that might show that the man had been infected.
The news this novel H1N1 virus had been found in pigs made international headlines when it broke on May 2. Though genetic analysis of the new virus shows it is the product of reassortment (gene swapping) between two swine flu viruses, the virus had not been found in pigs to that point.
Making the story more fascinating still was the fact that the infection seemed to have passed from person to pig, not the other way around, though there was some suspicion the pigs might have returned the favour and infected some members of the farm family.
It's believed the virus was introduced to the farm - a closed operation with no recent introduction of pigs - by a carpenter who had just returned from Mexico. The man showed up to work on April 14, but left after only half a day because he was ill, Clark says.
CFIA was only notified of a problem with the pigs on April 28. But looking back, the farm's owner believed pigs started falling sick within a few days of the carpenter's brief appearance, Clark says.
"The owner thinks anecdotally that he noticed some difference in the general demeanour of the pigs and the amount of coughing that was going on about three to four days after the carpenter was there," he says, adding that a chronic respiratory problem that had plagued the herd may have masked the first signs.
Several members of the farm family were ill, though whether it was with the new flu virus or something else remains a mystery. Clark says several family members were sick over the Easter weekend, a few days before the sick carpenter turned up on the farm.
By the time investigators arrived at the farm and testing began on pigs and people, easy answers were no longer within reach for at least some of the people. It was too late to expect to find flu viruses in the upper respiratory tract of the carpenter, who had recovered.
But the farmer was still suffering flu-like symptoms.
Provincial health investigators used a nasal swab, not a nasal pharyngeal swab to test the farmer, and it came back negative for the new flu virus, Clark says.
A nasal pharyngeal test, which scrapes cells deeper into the respiratory tract, would have been more likely to produce a positive result if there is influenza virus present, infectious diseases experts say.
"In terms of what was infecting the farmer, we don't know if he was infected with exposure to his family that had been experiencing influenza-like illness, to the carpenter - that's a really long ... latency period - to the pigs, or to something external to the farm entirely," Clark admits.
Still, genetic analysis of virus retrieved from the pigs shows it is a match for the new H1N1 flu virus. And the evidence available on how the pigs caught it all points to the scenario the CFIA has sketched out, Clark says.
"The only history we have of anything entering the herd or having exposure to the herd is on the human side."
Follow Canadian Press Medical Writer Helen Branswell's flu updates on Twitter at CP-Branswell