The swine flu pandemic is well under way. With the WHO citing almost 60,000 laboratory-confirmed cases at the time of writing, the race is truly on to understand more about the virus. Now, two new studies have painted a fresh but partly contradictory picture about two of the virus’s most important aspects – its infectivity (its ability to spread from host to host) and its virulence (its ability to cause disease in a host). These two traits will largely determine the threat that the virus poses, especially in relation to more familiar garden varieties of seasonal flu.
Both groups, one based in the US and the other in the Netherlands, tested the virus’s behaviour in ferrets. These animals are affected by flu viruses in much the same way as humans, mimicking both the severity of our infections and ease of our viral transmission.
Both studies found that the new swine flu virus takes a slightly greater toll on its host’s health than the usual strains of seasonal flu. These strains limit their infections to a ferret’s nasal passages but the new swine flu virus makes its way into the lungs too. The Americans, led by Taronna Maines at the CDC, even found traces of the virus in the ferrets’ gut.
This helps to explain the unusual profile of symptoms associated with swine flu. Most patients experience typical mild flu symptoms but an unusually large proportion (around 40% or so) have also suffered from unusual symptoms like vomiting and diarrhoea. Some have also been hospitalised due to severe pneumonia and respiratory failure, occasionally with fatal consequences. A flu virus that is unusually good at infecting the lungs and gut certainly make sense of these cases.
However, when it came to the virus’s ability to spread, the two research teams disagree. The Americans found that swine flu is less easily transmitted from ferret to ferret than other seasonal viruses of the same H1N1 subtype. However, the Dutch team, led by Vincent Munster from the Erasmus Medical Center, found that the new virus transmits just as easily as its seasonal counterparts.