How Bird Flu Works

Photo courtesy CDC Influenza virus

If you've kept up with the news lately, you've probably heard dire warnings about avian flu, or bird flu. It's a highly infectious disease that has swept through bird populations in many parts of the world. In October of 2005, the disease reached Eastern Europe, most likely through migratory birds. It continued to spread through Europe, and in February of 2006 it moved into Africa. Although the virus does not infect humans easily, more than half of the people who have contracted it have died.

But what exactly is bird flu? How is it different from the seasonal flu that people experience every year? How does it threaten people? What are governments doing to stop its spread?

In this article, we'll review the basics of how viruses and influenza work, and we'll learn the answers to these and other questions about avian flu, including whether it is likely to cause a global flu epidemic.

Viruses and Influenza: An Overview
A virus particle -- or virion -- is a microscopic packet that contains genetic material wrapped in a layer of protein. Some viruses also have a lipid membrane around the protein coat. Unlike bacteria, they cannot reproduce on their own -- they have to invade host cells. This process destroys cells and makes people sick.

Viruses usually enter the bodies of animals and people through their mouths, mucus membranes or breaks in the skin. Then, they infect specific cells. For example, common cold viruses attack cells in the respiratory system. As they reproduce, they destroy their host cells, releasing copies of the virus to attack other cells. Some viruses are more stable than others, but in general they mutate frequently, sometimes making it difficult for doctors to treat them.

 

Influenza is a specific type of virus that attacks the respiratory system. It can cause fevers, sore throats and congestion. If it attacks muscle cells, it can also cause muscle aches.

There are three types of influenza virus -- types A, B and C. Multiple subtypes exist within those types, and multiple strains exist within each subtype. Like many viruses, influenza can mutate through antigenic drift (small changes that occur as it reproduces) or antigenic shift (major changes that create a new subtype of the virus).

The influenza virus has eight gene segments. When two different types of influenza encounter each other, they can swap segments of their DNA. This can lead to new strains of the virus, some of which can be especially deadly. In fact, scientists believe that the two most recent flu pandemics occurred after human strains of influenza acquired genes from an avian flu virus.

Influenza A SubtypesSubtypes of influenza A are named for surface proteins called hemagglutinin (HA) and neuraminidase (NA). There are 16 HA subtypes and 9 NA subtypes. That's why influenza A viruses have names like:   H1N1: The "Spanish" flu that killed 50 million people worldwide from 1918 to 1919 (scientists identified its proteins long after the pandemic was over) H2N2: The "Asian" flu that caused a pandemic from 1957 to 1958 H3N2: The "Hong Kong" flu, which caused the world's most recent pandemic in 1968

Next, we'll look the strains of influenza that birds carry.