Swine flu is worse than seasonal flu, research suggests
Swine flu is more severe than seasonal strains of the influenza virus, according to new research which suggests that the H1N1 pandemic should not be regarded as mild.
Scientists have discovered that the H1N1 inflicts significantly more lung damage in animals than seasonal flu strains, in similar fashion to the highly virulent H5N1 bird flu virus.
The findings, from a team led by Yoshihiro Kawaoka, of the University of Wisconsin-Madison in the United States, may explain why the swine flu pandemic has caused serious illness and deaths among people with no underlying health problems.
Though swine flu has often been described as mild, and most infected patients have had only minor symptoms, thousands of people have been hospitalised after falling ill and hundreds have died.
Two deaths reported today brought the total number of British fatalities to 17. Last week, a patient from Basildon in Essex became the first otherwise healthy person to die from swine flu in Britain.
The research, published in the journal Nature, offers firm evidence that swine flu is not always a mild disease, and that its effects can be much worse than those of seasonal flu.
Ian Jones, Professor of Virology at the University of Reading, said: “This complete analysis of the current H1N1 is what we’ve been waiting for. It shows that the new virus is about five times more pathogenic than seasonal H1N1 but that, nonetheless, the major outcome to infection is recovery. For the few cases of severe infection the data will help in clinical management of hospitalised patients.”
Other scientists cautioned, however, that the study had exposed animals to very high doses of swine flu, much greater than those involved when people typically become infected. This could explain why most people experience mild symptoms, despite the virus’s potential to cause severe illness.
The virus was also compared with H1N1 strains of seasonal flu, which has not caused serious disease for several years, and not with the H3N2 seasonal strain that has been responsible for the majority of recent severe cases.
Wendy Barclay, Professor of Virology at Imperial College, London, said: “This is very solid work, and it shows clearly that the effects can be severe in three animal models. However, it’s important to be clear this isn’t the worst virus that could be out there. It may not be terribly different from early isolates of the H3N2 virus that caused the 1968 pandemic [which killed about a million people worldwide].”
In the study, scientists infected mice, ferrets and cynomolgous macaque monkeys either with a swine flu strain isolated from a Californian patient or with a seasonal H1N1 strain recently in circulation known as A/Kawasaki/UTK-4/09 (KUTK—4).
In all three animals, the swine flu virus caused a greater inflammatory response in the lungs, and the virus replicated more easily than seasonal flu in the lungs. While seasonal H1N1 readily infects the nasal passages, it does not generally infect or damage the lungs, or replicate there.
In the monkeys, the swine flu virus caused a higher fever than the seasonal strain, and more severe lung lesions. The scientists likened the lung damage to that caused by highly pathogenic influenza viruses, such as H5N1 avian flu.
In ferrets, which are regarded as one of the main animal models for flu in humans, the swine flu virus was again more damaging. The ferrets infected with it showed more severe pneumonia, and damage to the alveoli — the cavities deep in the lung where oxygen enters the blood and carbon dioxide leaves it.
The swine flu virus was vulnerable to both the main antiviral drugs, Tamiflu and Relenza, and to two experimental antiviral agents. The scientists warned, however, that resistance could yet emerge and that the virus could mutate to become more virulent.
The findings broadly replicate those of two studies published two weeks ago in Science, which also showed that swine flu causes more severe infections than seasonal flu in ferrets.
The Wisconsin study also found that pigs can be infected with swine flu, but do not show symptoms. This could explain why the virus was not identified in pigs before it spread to humans, even though molecular studies have shown it is of swine origin.
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