Yes, you can spread the flu before you have symptoms

Once in a while, the conventional wisdom about an illness turns out to be correct. Remember hearing that you’re most contagious before you start showing symptoms? Well, if that illness is the flu and you’re a ferret, that’s absolutely true.

Doctors aren’t blind to the possibility that pre or asymptomatic people could be spreading disease. In fact, epidemiologists often assume that up to a third of disease transmissions originate in people with no obvious signs of illness. Kim Roberts and her colleagues from Imperial College London used ferrets to test whether this was so. Apparently, ferrets make good flu-patient models. They become feverish and sneezy just like we do. 

First, the researchers determined how quickly symptoms appeared during a normal bout of flu. They infected some unfortunate ferrets (donors) and monitored their temperatures and how much virus they shed in their nasal secretions over the next ten days. The peak for both virus and fever was on day two, with a secondary spike of viral secretions on day five. Ferrets stopped secreting virus on day seven. Fever was the earliest sign of illness and appeared between 38 and 45 hours post infection.

Next, the scientists conducted a series of experiments where they exposed healthy ferrets (sentinels) to the sick ones at various times. Sentinels that were housed with the sick donors from 16 to 20 hours post infection did not become infected, but ferrets that were placed with sick roommates from 24 to 28 hours post infection did become ill. Remember, this was at least ten hours before the earliest sign of disease appeared in the donors.

What about aerosol transmissions? The authors infected another set of ferrets and placed new sentinels in adjacent cages from either day 1 to 2 or day 5 to 6 post infection. Only the former group of sentinels became infected despite the fact that far more sneezing and coughing was going on during the latter time period.

Clearly, the sentinel ferrets were becoming infected before any outward sign of flu appeared in their fellows. If this translates to human epidemiology, it could be bad news for trying to contain a future pandemic. However, I want to point out a couple of caveats. For one thing, this was an extremely small study. Each phase included no more than four animals. And second, although the donors may have shown no demonstrable sign of flu (fever, coughing, etc), during the early infectious period, it’s impossible to know whether they felt completely well. If humans feel a bit ‘under the weather’ before any overt flu symptoms appear, they might take greater pains to avoid infecting other people.

Kim Roberts, Holly Shelton, Peter Stilwell, & Wendy Barclay (2012). Transmission of a 2009 H1N1 Pandemic Influenza Virus Occurs before Fever Is Detected, in the Ferret Model PLoS ONE DOI: 10.1371/journal.pone.0043303