Fatal Immunity and the 1918 Flu Virus

The flu virus that killed roughly 50 million people worldwide in 1918 is alive and still very deadly. New research sheds light on how the 1918 Spanish flu virus might have killed so many people so quickly -- and opens new horizons for researchers who hope to avoid a flu pandemic today.

Scientists regenerated the 1918 virus Jurassic-Park-like from a frozen corpse two years ago. Now scientists have discovered that the regenerated virus can kill monkeys much as it killed humans in 1918, by kicking the immune system into dangerous overdrive, which ultimately kills the infected host.

A group of researchers infected one group of monkeys with the 1918 virus, and one group of monkeys with a conventional human flu virus -- called the K173 virus. Within 24 hours, each of the 1918-virus-infected animals became visibly ill, according to the study report.

The report is published in today's issue of the journal Nature.

The monkeys were depressed, the study authors report, and didn't want to eat or drink. They coughed, sniffled and were euthanized within eight days of infection because their symptoms were so severe.

Monkeys infected with the K173 virus didn't show serious signs of a flu infection. Those animals were also euthanized within eight days of infection.

A Fast-Moving and Powerful Virus

When doing autopsies of the 1918 flu and K173 infected monkeys, scientists could see to what extent the 1918 flu virus had ravaged the monkeys it infected.

Scientists easily recovered the 1918 flu virus from the throat and nasal cavities of the infected animals, and found in some monkeys that the virus had spread to the heart and spleen. Their lungs were covered in lesions, and the infected lung tissue -- which covered 60 or even 90 percent of the lung within six to eight days after infection -- was filled with watery and bloody liquid.

The lungs of monkeys infected with the K173 virus had actually begun to heal themselves within six to eight days after infection. The autopsy proves that the 1918-virus-infected monkeys were very sick when they were put down.

The 1918 Spanish flu was the deadliest human plague of the 20th century. The pandemic was unusually severe, causing upward of 40 million deaths worldwide -- including 675,000 Americans. Most of the victims were healthy people in the prime of life.

So what makes the 1918 flu virus so much more deadly than, for example, the K173?

Scientists discovered that the 1918 flu virus is so aggressive -- it multiplies and spreads quickly throughout the body -- that it causes the immune system to overreact.

"The [1918 flu] virus causes more destructive damage to the body and does so early in infection because it changes the body's immune response," said Dr. William Schaffner, professor and chairman of the department of preventive medicine at the Vanderbilt University School of Medicine.

In the face of the 1918 flu virus, the immune system becomes "less effective," Scahffner said.

"Some parts of the immune system are depressed and work less hard, but other parts become unregulated and work harder."

In other words, the 1918 flu virus somehow corrupts our only defense against it.

Answers to a Medical Mystery

The finding finally answers the question of what made the 1918 virus so deadly.

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