August 30, 2012

Flu Virus Is Still Transmittable Before Symptoms Even Start

Lee Rannals for redOrbit.com — Your Universe Online

You may have contracted the flu, and already passed it on to your neighbors, before you ever started showing symptoms, according to a new study.

Researchers found while examining influenza transmission in ferrets that the virus can actually be passed on to someone else before the symptoms occur.

Knowing whether someone is infectious before symptoms occur is important to try and help authorities stop an epidemic. Previous studies estimated that most flu transmission occurs after the onset of symptoms, but some happen earlier.

The new study, published in the journal PLOS ONE, is the first to investigate the question experimentally in an animal model.

Ferrets are most commonly used for flu studies because the animals are susceptible to the same virus strains as humans, and also show similar symptoms.

During the study, the animals that contracted the virus were put in contact with uninfected ferrets for short periods at different stages after infection.

The scientists found that transmission of the virus took place before the first symptom, fever, appeared when both ferrets were in the same cage.

"This result has important implications for pandemic planning strategies. It means that the spread of flu is very difficult to control, even with self-diagnosis and measures such as temperature screens at airports," professor Wendy Barclay, the study's lead author from the Department of Medicine at Imperial College London, said in a statement.

"It also means that doctors and nurses who don't get the flu jab are putting their patients at risk because they might pass on an infection when they don't know they're infected."

The authors used the flu strain from the 2009 swine flu pandemic, which killed nearly 300,000 people around the world.

They found that ferrets were passing on the 2009 H1N1 influenza virus to others within just 24 hours after becoming infected. The animals did not start showing symptoms until 45 hours after infection.

The results are consistent with previous studies that found sneezing is not necessary to transmit the flu.

After five to six days of being infected, the flu was transmuted much less frequently, suggesting that people can return to work or school soon after symptoms subside.

"Ferrets are the best model available for studying flu transmission, but we have to be cautious about interpreting the results in humans," the first author, Dr. Kim Roberts, who is based at Trinity College Dublin, said in a statement. "We only used a small number of animals in the study, so we can't say what proportion of transmission happens before symptoms occur. It probably varies depending on the flu strain."