Bird Flu Alarm

Understanding Avian Influenza



Influenza is a category of viruses associated with acute (short), usually self-limited infections, whose symptoms are most commonly fever, muscle pain or aches, and cough. However, illness can be more severe based upon the properties of the virus, the patient's age, pre-existing immunity status, or pre-existing medical conditions.

The influenza virus is described by a three part naming system that includes the virus type, subtype, and strain. There are three major types (A, B, C) and a number of subtypes which are classified based upon the surface coatings of the virus. These surface coatings determine whether the virus will affect humans, pigs, horses or birds, or more than one type of animal.2 Within a specific type and subtype of influenza, there are also important differences in the particular strain of virus. For example, the strain of influenza A (H5N1) that has affected birds and humans in much of Asia is not the same strain that is affecting birds in the U.S. or Pakistan.

Influenza viruses also change or mutate over time. "Scientists know that the avian and human influenza viruses can exchange genes when a person is simultaneously infected with viruses from both the common human influenza virus and the avian type. This process of gene swapping inside the human body can give rise to a completely new subtype of the influenza virus to which few, if any, humans would have any natural immunity..If the new virus contains sufficient human flu virus genes, transmission directly from one person to another (instead of from birds to humans only) can occur."3 Some previous outbreak investigations documented limited human-to-human transmission of avian influenza. It is believed that most cases of avian influenza in humans have resulted from contact with infected poultry or contaminated surfaces.

In particular, influenza A (H5N1) has a documented tendency to acquire genes from viruses infecting other animals.4 There is particular cause for concern because this strain of influenza A (H5N1) is now spreading from birds (e.g., chickens, ducks, turkeys) to humans, and scientists are trying to determine if the virus is also spreading from human to human.5 Since this strain of influenza virus does not commonly infect humans, the general population may not have natural immunity to the virus. The current strain of influenza A (H5N1) that is transmitted from birds to humans is considered to be "highly pathogenic."