ABSTRACT
Influenza A virus (IAV) infection and carrier states are well
established and continuous in aquatic birds, the natural host of
the virus. Influenza subtypes are also well recognized as the cause
of both seasonal epidemics of disease and, much more rarely,
pandemics. The latter are stochastic in their appearance and are
generally associated with dramatic antigenic changes in one or both
of the major virion function surface proteins, hemagglutinin (HA)
and neuraminidase (NA). Over the past decade, a new subtype of
influenza, H5N1, has caused an epizootic among domestic poultry
(highly pathogenic avian influenza [HPAI]) and some waterfowl,
including migratory birds. Several hundred documented cases of
H5N1 in humans, most clearly originating as a result of direct
transmission from a bird to a human host in most cases, have
also occurred. The specter has been raised of a pandemic, and
the expenditure of substantial resources in pandemic preparedness
mostly focused on this particular virus subtype. The following are
examined in this chapter: the historical course of H5N1-induced
disease in humans and birds, mechanisms for human-to-human
transmission, viral recombination in humans and animals, and
extant immunity to disease in humans. These factors should be
carefully considered in assessing the risk of a new pandemic based
on some variant of the currently circulating clades of HPAI H5N1.
An H5N1 pandemic appears unlikely, but systematic near real-time
surveillance for severe respiratory or gastrointestinal symptoms
in humans and animals is warranted, to identify disease-causing
organisms that have the potential to cause pandemics or panzootics.
Influenza A, B, and C virus types are members of the family
Orthomyxoviridae, all negative-sense RNA viruses possessing a segmented genome divided into either eight or nine strands. One
genus of Orthomyxoviridae contains influenza A and B, and a second contains influenza C. While these influenza types obviously share
structural properties, their host ranges are dramatically different,
with types B and C being almost exclusively human pathogens (on
occasion found in pigs and seals); IAVs have awide host range-wild
birds, pigs, horses, seals and whales, and poultry. Influenza viruses
routinely undergo mutational changes during host cell transcription
of viral RNA. Such mutations may or may not result in a viable virus,
but if they do, a virus with a changed host range or pathogenic
potential may emerge.
