WASHINGTON (Reuters) - How widespread is this new swine flu that has killed as many as 81 people in Mexico? How can a swine flu be infecting people? Why has it killed some people and caused only mild symptoms in others?
The new strain of H1N1 influenza is behaving just as public health experts expect it to — that is, unpredictably.
“It’s very hard to predict exactly,” Dr. Anne Schuchat of the U.S. Centers for Disease Control and Prevention told reporters in a conference call. “We all need to all be prepared for change.”
Within hours on Sunday, officials learned of new cases across the United States and in Canada, for a total of 26, in addition to more than 1,300 suspected cases in Mexico.
Influenza can spread quietly, and people who are infected can spread the virus before they even start showing symptoms. One of the biggest problems with flu is that it causes flu-like symptoms — just like dozens of other viruses and bacteria do.
Fever, headache, muscle aches, dry cough, extreme tiredness — all these can be caused by numerous other infections from the rhinoviruses that cause the common cold to the less-known but also common adenoviruses.
“The syndrome that we are hearing about in Mexico is relatively non-specific,” Schuchat noted. “There are many different causes of respiratory illnesses.”
And people die of these common infections every day. Even during the height of the flu season, only 10 percent or less of deaths from “influenza-like illness” are ever confirmed to be influenza.
So tracking it is difficult.
Health officials all over the world are now testing people with influenza-like symptoms for the new strain of swine flu. It requires DNA tests because it is an influenza A virus — like two of the strains now causing very common seasonal flu. It is also an H1N1 virus — one of the seasonal strains.
Not until the DNA is sequenced does it become clear whether a person with influenza has this new and unusual strain of swine flu.
“It could be all over the place and they just haven’t tested for it,” said Mike Osterholm, a former Minnesota state health official who is now director of the Minnesota-based Center for Infectious Disease Research and Policy.
It already has turned up in Mexico, Canada and the United States, and among people who have nothing else apparent in common. That suggests the connection is an as-yet unseen chain of human infections.
“It is clear that this is widespread. And that is why we have let you know that we cannot contain the spread of this virus,” Schuchat said.
This new strain is just what flu experts have worried about. It has DNA from four different strains of flu. While genetically it most closely resembles a swine H1N1 virus, it contains avian flu and human flu sequences as well.
While most cases in the United States, Canada and Mexico have been mild, it has killed up to 81 people in Mexico, including some young adults. This is disturbing because flu usually kills the very young, the very old and those with other underlying conditions.
“It may look different because we don’t have good enough information. It may look different because the virus is different,” Schuchat said.
Acting CDC director Richard Besser said this virus could itself disappear and reappear. “It’s very hard to say,” he told a White House briefing on Sunday. “We are nearing the end of the season in which flu viruses tend to transmit easily.”
Besser said he would expect the number of cases to decline in the northern hemisphere’s summer. Other pandemics have come in waves months apart and Besser said this virus could do that.
Pandemics hit in 1918 — killing anywhere between 40 million and 100 million people globally — and in 1957 and 1968. The 1968 pandemic was relatively mild, with 1 million deaths — perhaps because the case, an H3N2 virus, was similar to the H3N2 that caused the 1957 pandemic and people had some immunity.
H1N1 has been around since the 1918 pandemic, in various forms but no one knows if this strain is different enough to cause widespread and severe disease. Health experts say the world is overdue for another pandemic but have stressed that no one can predict which strain would cause the next one.
Editing by Philip Barbara