Flu can spread before symptoms show, according to study in ferrets
Virus can be transmitted into the air through normal breathing
Ferrets are the best animal model for studying human flu



08:14 GMT, 30 August 2012

If you are reaching for your handkerchief it might be too late to stop the spread of flu, research suggests.

Flu and cold viruses are known to be carried in mucus droplets that spray out when a person coughs or sneezes.

But the latest research indicates that flu can be transmitted before any symptoms show.

Close-up of ferret

woman with flu

Viral flu particles can be expelled through normal breathing meaning it spreads before symptoms show, according to a study of ferrets. Ferrets were used as they have a similar response to influenza

The findings, from a study of ferrets, support earlier research suggesting that viral particles can be expelled into the air through normal breathing.

Lead researcher Professor Wendy Barclay,
from Imperial College London, 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.'

Ferrets are often used in flu research
because they are susceptible to the same virus strains as humans, and
show similar symptoms.


Influenza researchers agree that ferrets are the best animal model for studying human flu.

Ferrets and humans have a similar physiological response to flu and show the same symptoms including sneezing, fever, and nasal discharge. However, humans tend to cough while ferrets sneeze more.

They also have a similar early immune system response to various strains, expressing particular proteins, called cytokines, that regulate the body's inflammatory response.

Dr Anice Lowen from Emory University in Atlanta, added: 'The virulence and transmissibility of a wide range of influenza viruses are found to be similar between ferrets and humans.'

Dr Lowen pointed to studies that found common human seasonal strains of flu also cause mild disease that spreads easily between ferrets. Similarly the H5N1 strain of bird flu causes severe illness but doesn't spread easily in humans or ferrets.

So to date ferrets have provided the most reliable flu model. The domesticated animals are part of the weasel family and live up to 10 years.

The new study, reported in the online journal PLoS ONE, is the first to investigate non-symptomatic flu transmission in an animal.

Ferrets with flu were placed close to healthy animals for a short period of time at different stages after infection. Transmission occurred before the flu carriers displayed their first symptom: fever.

The virus passed between animals which were kept both in the same cage and in adjacent cages.

The strain used in the research was the same one that caused the 2009 swine flu pandemic which killed almost 300,000 people worldwide.

Ferrets were able to pass flu onto their neighbours just 24 hours after being infected themselves, the scientists found.

Animals did not show signs of fever until 45 hours after infection and began sneezing after 48 hours.

In the late stages of infection, after five or six days, the virus was transmitted much less frequently. The researchers believe this suggests people could return to work or school soon after symptoms subside with little risk of infecting others.

Co-author Dr Kim Roberts, now based at Trinity College Dublin, 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.'