Influenza viruses in waterfowl are genetically stable; they don't change much over time. However, when the viruses infect
domestic birds like turkeys, chickens and domestic ducks, they undergo rapid genetic changes. In wild waterfowl, influenza
typically doesn't cause any symptoms at all. When transmitted to poultry, influenza virus may undergo mutations and cause
illness, sometimes severe. These viruses can be transmitted from poultry to humans, but typically LPAI doesn't cause clinical
symptoms in people and isn't spread from person to person.
There are two separate issues that concern us regarding avian influenza. 1) Direct infection causing severe illness, such
as is occurring with H5N1, and 2) the potential for genetic changes in the avian virus that give it the ability to become
easily transmitted from human to human.
When an animal or human is simultaneously infected with different strains of influenza, the two viruses can exchange genetic
information. This is called reassortment, and it results in a novel progeny virus to which humans are immunologically naïve.
If the resulting virus acquires the ability to be easily transmitted between humans, a pandemic may result.
Essentially, an avian influenza virus is one that is adapted to birds, and a human influenza virus is one that is adapted
to humans. The technical distinction between an avian virus and a human virus is genetic, with an estimated 52 genetic changes
distinguishing avian influenza strains from those that spread easily among people. A critical difference between avian and
human influenza is that avian influenza viruses bind alpha 2-3 sialic acid receptors while human influenza HA bind alpha 2-6
sialic acid receptors. The alpha 2-3 links are found in the intestinal tract of birds, while the human respiratory tract has
predominantly alpha 2-6 linkages.
This receptor specificity is a key factor in host-range restriction. If an avian virus acquires the ability to bind to human
receptors, it is likely to become adapted to humans and transform into a human virus.
Currently, only H1N1 and H3N2 viruses are circulating widely among humans. Although they originated from avian strains when
they first emerged in the human population, they now are considered human strains. H1N1 emerged suddenly and explosively in
1918 and circulated until 1957, when it was replaced by an H2N2 virus. H2N2 circulated until 1968, when H3N2 emerged and H2N2
disappeared. H1N1 re-emerged in 1977, possibly by escaping from a laboratory, although its origin is uncertain. In each case,
the newly emergent virus had avian origins. The viruses quickly mutated and became adapted to humans, so they were no longer
considered avian viruses. Each time the Hemagglutinin subtype changed, a pandemic occurred.
DVM: Is an H5N1 outbreak in the United States inevitable? How much vaccine would be available?
MYERS: Many do feel that the spread of H5N1 into birds in this country is inevitable. The geographic range of the virus is rapidly
expanding and efforts to contain the virus have been unsuccessful.
The larger question is whether a human pandemic is inevitable. The consensus among the public-health community is that another
influenza pandemic is inevitable. Pandemics occur, on average, about every 30 years, and it has been 39 years since the last
As mentioned earlier, a pandemic occurs when a new subtype enters the human population. The viruses causing the last three
pandemics originated in avian species. Many believe that it is just a matter of time before H5N1 adapts to humans and causes
the next pandemic. Others suggest that if H5N1 were going to cause a pandemic, it would have happened already, and that the
virus may never acquire the ability to transmit efficiently among humans. It is impossible to predict how H5N1 will evolve
or which strain will cause the next pandemic.
The first human vaccine against H5N1 was approved by the FDA in April 2007. A limited amount of it is being stockpiled and
will be considered for early use in the event of an H5N1 pandemic. About 13 million doses, enough for 6.5 million people,
have been ordered by the federal government. The HHS pandemic plan lists priority groups for vaccine distribution. Agricultural
workers and others with exposure to birds are not included in priority lists.
DVM: Were there other findings in the study of particular interest to veterinarians?