Flu can be transmitted before symptoms appear, researchers from Imperial College London reported in the journal PLoS ONE after carrying out experiments on ferrets. The scientists say that if their animal experiments apply to humans, people infected with the flu virus may be passing it on to others unwittingly, making it extremely hard to prevent epidemics.

When health authorities plan for epidemics, they expect to know whether people are infectious before symptoms appear. However, this has been hard to establish from data gathered when epidemics occur. Previous studies have indicated that the majority of cases of human-to-human transmission took place after symptoms developed, however, some happen before.

This animal study is the first to try to establish whether transmission occurs before the onset of symptoms, and if so, how often. Scientists use ferrets in flu research because they have very similar symptoms to humans and are susceptible to the same virus strains.

Professor Wendy Barclay and team placed infected ferrets in contact with uninfected ones for short periods at different stages after infection. They found that the infected animals transmitted the virus to the previously uninfected ones before symptoms appeared. Ferret-to-ferret transmission occurred among ferrets placed in the same cage as the infected ones, as well as those placed in adjacent cages.

Professor Wendy Barclay said:

"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. 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 researchers used the 2009 swine flu pandemic influenza strain - H1N1 - which had nearly 300,000 reported fatalities. Initial estimates placed the death toll at 18,500, but it was later found to be much higher.

Sneezing not necessary for virus droplets to be expelled

Within 24 hours of becoming infected, the ferrets passed on the virus to others, the authors reported. However, they did not develop any fever or flu-like symptoms until 45 hours after becoming infected; sneezing did not occur until after 48 hours.

The authors said that their findings back up previous studies which indicated that the transmission of flu does not necessarily depend on people sneezing. Normal breathing can expel droplets of the virus into the air.

Flu is much less transmissible during the late stages of infection - after five or six days. This means that most people can probably go back to school or work as soon as symptoms subside, as the risk of passing on the infection is minimal.

First author, Dr Kim Roberts, said:

"Ferrets are the best model available for studying flu transmission, but we have to be cautious about interpreting the results in humans. 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."

Written by Christian Nordqvist