Many researchers and health officials are concerned about the potential for an influenza pandemic caused by avian influenza
virus. Outbreaks of a highly pathogenic strain of influenza have occurred recently in domestic and wild-bird populations in
several Asian countries. Incidents of transmission of avian influenza to people have been associated with these outbreaks.
In July, the disease arrived in Russia and Central Asia, leaving health officials concerned about the spread of avian influenza
to western Europe.1 Avian influenza also has been transmitted to a few domestic and nondomestic cats in Thailand, generally through contact with
and ingestion of carcasses of infected poultry.
Margaret C. Barr, DVM, PhD
THE ORIGINS OF A POSSIBLE PANDEMIC
The three major human influenza pandemics of the 20th century have been associated with avian influenza cross-overs into the
human population.2 The Spanish flu of 1918 killed between 20 and 50 million people worldwide. The virus responsible for the pandemic appears
to have been an avian influenza virus that mutated to become highly pathogenic and easily transmitted from person to person.
The 1957 Asian flu outbreak was caused by a virus carrying both avian and human influenza genes. The Hong Kong flu pandemic
of 1968 was similarly caused by a virus with a combination of avian and human influenza genes.2
Since 2003, a strain of virus known as H5N1 has been responsible for outbreaks of avian influenza in domesticated poultry and wild ducks in southeastern and central
Asia, resulting in high morbidity and mortality. An estimated 100 million birds have been affected in these outbreaks, and
standard control procedures have not effectively controlled the spread of disease.3 A particular cause for alarm is that the H5N1 virus has infected several people who had contact with sick birds, including
a disproportionate number of children. Of 89 confirmed cases in people in 2004 and the first three months of 2005, 52 resulted
in death.4 The extent of milder infection and disease in the human population is unknown, but current evidence suggests that this influenza
strain is highly pathogenic in people. A recent report also suggests that person-to-person spread of the virus has occurred
in at least one instance.5 The virus appears to be resistant to two major anti-influenza therapeutic drugs, amantadine and rimantadine.4
If the current circulating avian influenza virus undergoes mutations that allow it to infect people easily or if it recombines
with human influenza virus, the likelihood of another influenza pandemic is great. People have no immunologic memory of this
influenza virus strain, so the entire world population would be at risk in an outbreak. This is also true for other mammalian
species susceptible to influenza virus infections, including pigs, horses, some marine mammals, and cats.
THE VIROLOGY OF INFLUENZA
Influenza viruses belong to the Orthomyxoviridae family. An understanding of their genetic composition is critical for understanding
how pandemics—or panzootics—occur.
Structure and classification
Influenza viruses are enveloped, single-stranded RNA viruses. Their genome is composed of eight segments, with each segment
encoding one or occasionally two proteins. The segments that get the most attention are the hemagglutinin (HA) and neuraminidase
(NA) genes that encode proteins found on the viral envelope.2,6 The HA protein is responsible for cell receptor binding and determines the type of cells that can be infected, while the
NA protein is involved in the release of nascent virus from infected cells. Fifteen different HA antigens, designated H1 through
H15, and nine different NA antigens, designated N1 through N9, have been characterized.
Influenza virus strains are usually identified by their HA and NA subtypes. For example, the Spanish flu virus of 1918 was
an H1N1 virus. Different species of animals tend to harbor different influenza virus strains. Equine influenza viruses are
usually H3N8 or H7N7 derivatives, human and swine influenza viruses are often H3N2 subtypes, and H2N2 is a common avian influenza
Antigenic drift and shift
Influenza viruses are famous for their ability to change quickly and evade the collective immunity of a population of animals
or people. This rapid evolution is accomplished in two main ways: antigenic drift and antigenic shift.