H5N1 Bird flu virus linked to inflamation of the brain

As if fever, aching muscles and a sore throat were not enough, researchers have found that flu may also lead to chronic neurodegenerative diseases like Parkinson's and Alzheimer's.

Most influenza infections affect the respiratory system, but there have long been suspicions that they are also linked to neurological disorders. The H1N1 flu pandemic in 1918 was followed by an outbreak of encephalitis and later by an unusually high number of cases of Parkinson's disease.

Hard evidence of such a link has been hard to find, however. So Richard Smeyne of the St Jude Children's Research Hospital in Memphis, Tennessee, sprayed a solution containing a highly pathogenic subtype of H5N1 avian flu into the noses of 225 mice. The team found that the virus infected nerves in the gut, then entered the brain stem and finally reached the brain. In the brain, it led to chronic activation of the immune system, even long after the viral infection had been cleared.

This immune system activity later led to protein aggregation and neuron loss in the brain, and to symptoms like tremor and loss of coordination – the hallmarks of Parkinson's and Alzheimer's.

Brain strains

"Infection with influenza virus might leave the brain vulnerable to damage from future infections with new influenza strains," says Smeyne, adding that this is more likely to happen in young children or during an flu pandemic.

Smeyne suspects that all flu viruses, including the current H1N1 swine flu pandemic, could cause symptoms of encephalitis, or inflammation of the brain. But he says that there is currently no proof that flu viruses other than the H5N1 he worked with can enter the central nervous system.

Reports of extra encephalitis during outbreaks of seasonal flu in Japan and the US, and most recently of swine flu-infected children hospitalised with neurological symptoms in Dallas, Texas, hint at a related mechanism, however.

"The link between flu infection and neurological disorders has been the elephant in the room [ever since 1918]," says virologist John Oxford, a virologist at Barts and The London School of Medicine and Dentistry, Queen Mary, University of London.

Journal reference: Proceedings of the National Academy of Sciences, DOI: 10.1073/pnas.0900096106