eral, when well-adapted to a host, is among the most highly transmissible viruses known. If a highly transmissible strain is also highly virulent (i.e., highly likely to cause serious disease), the mortality rate for those infected can be significant. Most influenza strains are transmissible only between members of the same species. Other strains may pass from one species to another. For a pandemic to occur in humans, a human-transmissible strain is required. It is only when a strain becomes transmissible from an animal species to humans and then gains the ability to travel easily among humans that a human pandemic may result.

Differences in transmissibility are due to a strain’s genetic make-up. The influenza viruses that most concern scientists are strains of influenza A. The genetic material of influenza A viruses is a single strand of RNA with eight gene segments. (See Figure 3-1.) The genes encoding hemagglutinin and neuraminidase are carried on different segments; therefore, the particular combination of HA and NA type is easily changed, both in nature and in the laboratory. If a single host is infected with two different strains of influenza, a new viral strain may emerge, in a process known as antigenic shift, through a process of gene segment reassortment that results in an influenza virus with a new combination of the hemagglutinin and neuraminidase proteins. Variation is also often introduced during viral reproduction. The replication process in viruses is imprecise, and errors are frequently introduced into viral RNA sequences. This imprecision often benefits the virus, since different variations may confer survival advantages in different environments. New genetic signatures can result in altered traits which may improve the strain’s ability to evade the host’s immunological defenses. While, in many cases, the new traits do not adversely affect the human host, they may, in some cases, cause serious harm. Regardless, the influenza virus’s rapid evolution means that the influenza vaccine must be reformulated each year.

Influenza A Viruses and Their Many Hosts

Influenza A viruses infect a wide variety of animals that include—in addition to humans—whales, seals, pigs, horses, domesticated poultry, and wild birds. (See Figure 3-2.) Wild birds are the principal hosts, and throughout the world there is a large reservoir of influenza viruses in hundreds of avian species. In general, influenza-infected birds are asymptomatic. When an infected bird is symptomatic, mild gastrointestinal symptoms are most common.

An influenza strain that crosses a species barrier is cause for greater concern. The pathogenicity of such a strain is often much higher on the other side of the species barrier, as has been the case with H5 and H7 influ-



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