And in another development that underscores how dynamic the intersection is between pigs, people and influenza viruses, health authorities in Minnesota announced Friday they have found three people infected with a different swine flu virus, an H1N2 variant virus.
An unidentified Ohio woman became the first to succumb to infection with H3N2v, which was first seen in people in the U.S. last summer. The virus has triggered an explosion of cases — 288 at last count — in 10 states this summer.
The woman, 61, was from Madison County near the centre of the state. She reportedly had chronic health problems before she came down with H3N2v after visiting the pig exhibit at a county fair.
The U.S. Centers for Disease Control expressed sympathies, but used the opportunity to warn that people who have serious health problems should avoid swine at the current time.
"These people should absolutely not have contact with pigs or visit pig arenas at fairs this summer," said Dr. Lyn Finelli, chief of surveillance and outbreak response in the CDC's influenza division.
People with a range of chronic illnesses — heart disease, asthma, chronic lung disease — or who have weakened immune systems are at higher risk of having severe illness if they become infected with influenza.
Most people who have been infected with H3N2v have had only mild illness, though 15 people have been hospitalized so far.
The term variant signals these are viruses of animal origin, not human flu strains.
There is a human H3N2 — it's a distant cousin of the pig virus. There are no human H1N2 viruses, though swine viruses of this subtype were spotted sporadically in the late 1990s and early 2000s. And Minnesota reported one case last fall.
The surprising surge of swine flu cases this summer has been driven by transmissions at fairs in the U.S. The vast majority of the cases have been in children under the age of 18 who have been exposed to the virus at fairs.
Finelli said several hundred thousand children in the U.S. exhibit pigs at fairs every year. The pigs are typically sold at the fairs and taken to slaughter after the events. During the fair, it is common for children to virtually camp out in the pig barns to look after their animals.
To date no other country has reported seeing H3N2v, either in pigs or in people.
The Public Health Agency of Canada reiterated Friday that there have been no cases detected in this country.
But Dr. Danuta Skowronski, a flu expert with the B.C. Centre for Disease Control, said there may not be enough testing going on in Canada to find cases at this point.
"We may say it's not detected in Canada, but I think most (people) recognize that it's only a matter of time until it is," she said from Vancouver.
"It ... may relate to surveillance sensitivity rather than a true absence of virus."
In the U.S., influenza surveillance systems that normally start up in October have been activated early to look for cases, Finelli said.
She said the CDC is concerned that with the resumption of school and the impending change in the weather this virus could become more transmissible among people. So far only a few instances of person-to-person spread have been seen with H3N2v.
But Finelli noted that late last year there was some human-to-human transmission in Iowa and West Virginia. While the numbers last year weren't high, she said the CDC is worried that pattern might be repeated this year.
"We learned from that situation that we have to be ultra-vigilant as weather changes from hot weather to cold weather," Finelli said.
There is evidence that suggests flu transmission is more efficient when temperatures are cooler and absolute humidity drops.
As well, young children are highly susceptible to this virus, having no immunity to it. So if an infected child went to school, there is a possibility the virus could take off among his or her classmates.
Finelli said the CDC is asking state and local authorities to aggressively investigate reports of influenza-like illness or respiratory infections in schools, urging them to collect specimens from sick kids that can be tested for H3N2v.
"We're very concerned about this virus. And we want to do everything we can to have as quick a notification of person-to-person spread as possible," she said.