Was the 1918 Influenza Pandemic Caused by an Avian Flu Virus?

Recently, the virus responsible for the 1918 influenza pandemic was re-synthesized by a group at the Center for Disease Control and Prevention (CDC). The virus was characterized in mice and the genomic sequence was aligned with other known influenza viruses, yielding very interesting results.

The 1918 influenza virus is thought to be responsible for an estimated 50 million deaths, but the genetic differences between it and current influenza viruses are poorly understood. Trumpey and colleagues at the Atlanta CDC re-created the virus from RNA isolated from a 1918 flu casualty buried in permafrost in Alaska. The 1918 virus was found to produce 39,000 times more viral particles in mouse lungs of infected mice than a current influenza virus. The 1918 virus was also lethal after 5 days in mice, while mice readily overcame infection with current virus.

Histological analysis of mouse lungs infected with the 1918 virus revealed servere damage with an increased infiltration of inflammatory cells in comparison to current virus. Surprisingly, the average 1918 flu virus casualty was between 15 and 34 years old. This however may be explained by the damaging inflammatory response exuded by the virus, as persons in this age range would be expected to mount the most complete immune response.

Interestingly, comparing the genome of the 1918 virus with all known influenza viruses indicated it may have been avian in origin. The 1918 virus was also found to be lethal in fertilized chicken eggs, a feature of avian influenza viruses, while a current mammalian influenza virus was not. This further demonstrates that the 1918 virus may have been passed from birds to humans. While this data is alarming and punctuates the problem avian influenza may pose, precautionary measures, health care and overall knowledge of the virus are exponentially greater than 90 years ago.