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
Flu can be spread long before symptoms appear, a study has found - leading to calls for widespread vaccination.
Doctors and nurses could unwittingly be infecting patients after being exposed to the bug, even if they feel perfectly healthy.
And office workers could be passing flu to colleagues before they have so much as coughed or sneezed.
Professor John Oxford of the Queen Mary School of Medicine in London, said the findings back the case for more healthcare workers to be given the flu jab - and even for a universal vaccination programme.
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
Research at Imperial College London showed that ferrets were capable of passing on flu 24 hours after being infected with the virus – even though they did not show symptoms for almost another full day.
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.
WHY FERRETS CAN HELP CURE FLU
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.'