Avian influenza in the 1990s

Avian influenza (Al) viruses comprise the vast majority of the type A Ortho myxoviridae. Evolution has produced an enormous array of viral antigenic su btypes and variants based upon the structure of the two surface glycoprotei ns, the hemagglutinin (HA) and the neuraminidase (NA). These viruses appear to be perpetuated in nature in a select few wild avian species, but some s trains are capable of sporadic and unpredictable entry into other animal po pulations, including humans. The fate of these occasional entries is likewi se unpredictable, and investigators are left only with retrospective analys is. It is clear, however, that Al viruses (or some of their genes) have fix ed themselves into circulating lineages in some mammalian hosts. In birds, particularly commercial poultry, Al can undergo a dramatic shift and take t he unique form of a highly lethal and systemic disease. This has happened a t least eight times in this decade on four different continents. In this re view we explore these outbreaks and what we have learned from them regardin g virulence acquisition and interspecies transmission. We further attempt t o explore the implications of these outbreaks for the future of both avian and non-avian species and discuss current methods of diagnosis and control of Al.