Physicians around the country are being bombarded with questions about avian flu. This brief review of the current status of the avian flu outbreak and its treatment and prevention provides the information you will need to answer the most pressing patient questions.
What is avian flu—and what's all the fuss?
Avian flu, or "bird flu," is a predominantly respiratory illness caused by influenza viruses normally found in birds. These viruses are genetically distinct from influenza viruses that infect humans. The avian influenza A viral strain of H5N1 is the focus of media attention for a number of reasons:
•H5N1—1 of 15 avian virus subtypes—mutates rapidly.
•It has the propensity to acquire genes from influenza viruses that affect other species.
•It can cause severe disease in humans.
•Infected birds can spread it through saliva and feces.
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The first cases of known human infection occurred in 1997 in Hong Kong.1 Since then, more reports have surfaced. Between January 2004 and April 2005, avian flu killed more than 50 people in Vietnam, Cambodia, and Thailand.2
Wild, migratory birds are a natural reservoir for H5N1. Infection of domestic poultry in Southeast Asia, western China, Turkey (most recently), and possibly, Romania, has heightened the attention given to this infection. The avian H5N1 virus is spread easily from bird to bird; reports of the spread of this virus from birds to humans are increasing. Human-to-human spread—which is very rare—probably occurred in a cluster of cases in a family from Thailand.3 So far, however, human-to-human transmission has not been observed beyond a single person and therefore is not sustained. As the number of cases of human infection increases, however, the opportunities for the virus to evolve into a strain that is more easily transmissible from human to human will increase.
The influenza virus genome can change frequently; it has a high mutation rate and can segment into 8 separate RNA molecules.2 This segmentation allows genetic exchange (reassortment) in hosts who are coinfected with 2 different influenza viruses. Reassortment could result in a human-avian hybrid that has the ability to spread easily from person to person.
To date, there is no evidence that the 2004-2005 H5N1 isolates have acquired nonavian influenza genes by reassortment. However, the recent isolate circulating in Vietnam appears to be much more virulent than the 1997 isolate.4
Human influenza pandemics have occurred at irregular intervals (in 1918, 1957, and 1968), and it is unknown when the next one will occur. There is worry that the world is overdue for another influenza pandemic and that the potentially life-threatening avian flu virus, to which humans have no past immunity, may become more easily transmissible from person to person. This will be particularly true as the number of cases of bird-to-human transmission increases and another human influenza pandemic may be just around the corner.
Who gets the avian flu—and how do you get it?
At the time of this publication, no human cases of avian flu have occurred in the United States. However, persons of all ages can become infected. Age greater than 13 years may be a risk factor for more severe disease.1
Infected birds shed virus in saliva, nasal secretions, and feces. Exposure to these birds in an area with high infection rates is the greatest risk factor for contracting avian flu. Humans may become infected through contact with the diseased bird in a live market or during preparation for its consumption. Defeathering causes aerosolization of virus particles, which may land on exposed mucosal surfaces of the mouth, eyes, and nose and subsequently be inhaled into the lungs. Bridges and colleagues5 found that the butchering of infected birds is associated with the development of anti-H5 antibodies.
Although ingestion of contaminated poultry is an unlikely mode of transmission (it was not associated with infection in the earliest cases), fecal-oral spread by direct contact of a person's mucous membranes with the contaminated poultry is possible.6 Isolation of H5N1 from fecal specimens of a 4-year-old Vietnamese boy who presented with severe diarrhea and no respiratory symptoms was reported.6
As noted, human-to-human spread is rare but is more likely when one has been in close contact with the index case.3 Contaminated environmental surfaces may also pose a substantial risk.6
