Bird flu: hoard tuna?!? Whelan responds on CNN
What is avian influenza?
Avian influenza, commonly called “bird flu,” refers to a large group of influenza viruses that typically infect and are spread among birds, just as human influenza viruses are spread among people. While some avian flu strains cause only mild or asymptomatic forms of disease in infected birds, others cause illness severe enough to decimate flocks. To date, all avian influenza strains that have been severely disease-causing in birds have involved H5 and H7 subtypes. Worldwide, wild birds—particularly migrating aquatic birds like ducks or geese—often carry mild bird flu viruses and never become ill. However, if passed to domesticated birds—like chickens and turkeys—mild bird flu strains can spread rapidly through flocks where they sometimes change, or “mutate,” to a more pathogenic form. If this occurs, entire flocks can be wiped out.
Why are health experts so concerned about H5N1 if it is a bird flu, not a human flu?
The current strain of bird flu infecting poultry flocks in Asia—H5N1—is highly contagious among birds and has resulted in the death or destruction of over 150 million birds. The outbreaks, which began in 1997, are the most severe on record and the most widespread; infected birds often die within two days and, as of early 2006, birds in at least 30 countries have been affected.
Historically speaking, most bird flus rarely infect humans. While bird flus are generally highly “species specific”—meaning they only infect birds—occasionally they cross the species barrier and infect mammals, such as pigs or humans. Research indicates that, due to natural changes, the current H5N1 is becoming more capable of causing disease in mammals than past bird flus. In fact, there have been reported cases of H5N1 in pigs and some feline species (wild and domestic), the latter of which had previously been considered resistant to natural (in non-laboratory settings) influenza infection. Though none have become ill, some canine species in Thailand have also tested positive for H5N1 antibodies, indicating infection by the virus. Currently, researchers believe the risk posed to humans by these infected animals is very low. Because such viruses rarely infect people, however, scientists fear we may have little or no immunity to them, making us particularly susceptible to illness should H5N1 undergo the changes necessary to spread among humans.
Since 2003, there have been over 170 cases of human infection with H5N1 among people working in close contact with infected domesticated birds in Asia. Direct bird-to-human transmission of any H5 avian influenza virus resulting in illness was unheard of before then. According to the World Health Organization, in the first 173 cases of bird-to-human transmission of H5N1, there have been 93 deaths. In other words, more than half of those infected have died. This mortality rate of roughly 54 percent is one reason public health authorities worldwide are so concerned: it is over a hundred times that of common seasonal flus, and about twenty-fold higher than that of the great flu pandemic of 1918, which had a mortality rate of roughly 2.5 percent. It should be noted, however, that the high mortality rate associated with avian influenza in humans is likely to be an overestimation. This is a common phenomenon when a new disease emerges as new cases are typically found by observing those who are severely ill, many of whom will die. Those who are not as ill often go unobserved. It is important to note, though, that even if the mortality rate currently associated with avian influenza were to fall to 1 percent, upwards of one million deaths could be expected in the first pandemic wave, if it is not rapidly controlled.
Thus far, it seems difficult for humans to acquire the virus from birds and even more difficult for the virus to spread among people. In fact, no suspected cases of person-to-person transmission have been definitively confirmed thus far. This indicates that the species barrier is still fairly strong. However, given how devastating H5N1 has been both for birds and the small number of humans it has infected, health experts are concerned that further mutations of H5N1 could change the virus into a form easily transmitted from person to person, resulting in a worldwide outbreak of the disease, or “pandemic.” The good news is that, if H5N1 does acquire the ability to routinely infect humans, it may have a far lower mortality rate than it currently does. Some researchers have even theorized that H5N1 infection in humans is far more common than reports indicate, but that these infections result only in mild illness and may go unreported as a result. This theory, however, is yet to be supported by serological (blood) data.
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