Influenza A(H1N1): In The Beginning

May 6, 2009

The World Health Organization today reported that the number of confirmed cases worldwide has reached 1,893. Twenty-three countries have confirmed cases; Mexico still leads the list with 942 cases, including 29 deaths. The United States is next with 642 cases and two deaths, followed by Canada (165 cases) and Spain (73 cases).

". . . in the US swine industry, transmission of influenza viruses between swine and humans is fairly common and is bidirectional."

E. Thacker & B. Janke, writing in Journal of Infectious Diseases

In 1988, a pregnant woman was admitted to a community hospital in Wisconsin. She was complaining of fever, headaches, muscle aches, dry cough and shortness of breath. She delivered a healthy baby four days after entering hospital, and died as a result of respiratory failure four days later. After testing clinical samples submitted to it by the state, CDC concluded that the woman had suffered a case of swine influenza. Specifically, she was infected with a strain of Influenza A(H1N1).

The victim had visited a pig barn display at the local county fair four days before falling ill. According to the CDC report, veterinarians at the county fair confirmed that some of the pigs had displayed symptoms of swine influenza. No other human cases were reported in the surrounding communities.

Fast-forward to 2005. A 17-year old boy receives his flu shot on November 11th. On December 4th, he holds the front end of a freshly-slaughtered pig while his brother eviscerates the carcass. Three days later, he begins to complain of headaches, low back pain, a cough and a runny nose. He has no fever. The following day, he visits a local out-patient clinic, where his nasal passage is swabbed. In a couple of days, the youth recovers fully. His clinical specimen yields an influenza virus that is ultimately identified by CDC as an unusual strain of Influenza A(H1N1) – a triple reassortment virus that contains components of swine, human and avian influenza virus genetic material. 

Transmission of influenza virus from infected pigs to humans, while not common, has been documented a number of times. In most cases, the infection is an occupational hazard. Pig farmers, lab workers, and meat-packing workers are at greatest risk, as are visitors to pig farms and livestock shows. Occasionally, an individual becomes infected without having been exposed directly to swine.

This month, Canadians learned that a farm worker in Alberta apparently had passed the current Influenza A(H1N1) strain to a herd of swine. One-tenth of the herd – 220 out of 2,200 pigs – were infected. The herd was quarantined, and the affected pigs are recovering. So far, none of the other workers on the farm have tested positive for the virus, although some of them have complained of symptoms.

Canada's National Microbiological Laboratory has determined that the influenza virus that is responsible for the illnesses in Mexico is identical to the virus that has been isolated from flu-stricken Canadians. It has not yet mutated. Unfortunately, this doesn't provide us with any insight into how the virus strain arose, whether or not it is related to the 2005 virus, or how long it was circulating in Mexico before the country's health authorities became aware of it. In time, some of these questions will be answered.

Meanwhile, there are other questions, including whether pork – especially Canadian pork – is safe to eat. We'll explore that question next.

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