Avian influenza and the brain--comments on the occasion of resurrection of the Spanish flu virus.

Recent incidences of direct passage of highly pathogenic avian influenza A virus strains of the H5N1 and H7N7 subtypes from birds to man have become a major public concern. Although presence of virus in the human brain has not yet been reported in deceased patients, these avian influenza subtypes have the propensity to invade the brain along cranial nerves to target brainstem and diencephalic nuclei following intranasal instillation in mice and ferrets. The associations between influenza and psychiatric disturbances in past epidemics are here commented upon, and the potentials of influenza to cause nervous system dysfunction in experimental infections with a mouse-neuroadapted WSN/33 strain of the virus are reviewed. This virus strain is closely related to the Spanish flu virus, which is characterized as a uniquely high-virulence strain of the H1N1 subtype. The Spanish flu virus has recently been reconstructed in the laboratory and it passed once, most likely, directly from birds to humans to cause the severe 1918-1919 pandemic.