How Can the Flu Kill You?

May 7, 2009 1:05pm

ABC News’ Matt Davis and Michelle Schlief report: We spent some time talking with pathologists, medical examiners and infectious disease experts to find out exactly how the flu can kill a person.

Swine flu, like any flu, is transmitted when the virus comes into contact with the eyes, nose or mouth. The virus attaches to the cells on the inside of the airways or lungs. It then invades these cells, making many copies of itself and causing the cells to burst open, releasing more new copies of itself. This process then repeats, with new viruses attacking neighboring cells and releasing more copies during each cycle.

Once this process gets going, the body responds through a complicated process involving the immune system. Immune cells release proteins called cytokines that act as signals to each other, communicating that the body is under attack. These cytokines are the cause of symptoms like fever and fatigue. They work to focus the immune system on fighting the virus by actually turning off other parts of the immune system.

Consequently, the body is unable to attack other types of infections, such as those caused by bacteria.
One of the most devastating things that can happen is that a bacterial infection can take hold in the lungs, leading to pneumonia. Under normal circumstances a patient would be able to fight off the infection, or their defenses would have caused it to have been a much less severe illness. But now, with the defenses disabled, the infection has the potential to become life-threatening.

Bacterial pneumonia leads to a variety of complications in the airways and lungs, like bleeding and swelling, that ultimately can choke off breathing and kill the victim.

It is in fact these pneumonias that are responsible, historically, for 75 to 90 percent of influenza deaths during pandemics, said Dr. Dennis Metzger of Albany Medical College.

Another concern is that for flu patients who are relatively defenseless against infections, exposure to superbugs like MRSA will be particularly devastating – since MRSA is resistant to many traditional antibiotics.
Certain segments of the population – particularly the very young, the very old and those with other medical problems like lung and heart disease – are most susceptible to these infections as a result of the flu. Those with underlying lung disease also have little ability to tolerate an additional strain on their lungs.

While secondary infections cause most influenza deaths during pandemics, damage from the virus replication process itself can be toxic, said Dr. Frederick Hayden of the University of Virginia.

Another theory, the “cytokine storm,” tries to explain why so many young people died in the 1918-1919 flu pandemic. According to this theory, when cytokines – signals from the immune system – are manufactured in high levels in healthy young people, they may cause the immune system to attack healthy cells. This is particularly a problem when cells in the airways come under attack, causing acute respiratory distress, said Dr. Gregory Poland of the Mayo Vaccine Research Group.

But this theory is falling out of favor, said Dr. Hayden. He told us that cytokine response levels have been found to correlate with the level of virus growth in the lungs. So the damage seen in the lungs is most directly attributable to viral replication in the lungs.

Rarely, a flu virus can affect the heart or brain by directly causing inflammation, said Dr. William Schaffner of Vanderbilt University. These complications can be severe enough to result in death.

Once a patient dies and swine flu is suspected, the cause of death is confirmed, surprisingly, through the same nasal swab and diagnostic procedure that would be done on a live patient. But the critical question remains, How did swine flu cause the death? To determine that, tissue samples are sent to a lab where they are given high priority. Results are usually turned around in 7 to 10 days. The samples are probed for evidence of a secondary infection that may have contributed to the death.

One medical examiner told us that once swine flu is established as a cause of death in a geographical region there is no need to do additional autopsies unless more questions arise.

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