TABLE 2-3 Evolution of New Viruses: Constraints and Opportunities



Extreme viral alterations are lethal


There may be requirements for co-evolution of viral cellular proteins


Virus survival requires a critical level of virulence


Propagation in alien hosts tends to be attenuating


Adaptation to ecological niches is exquisitely specific


Penetration of human immunologic barrier usually requires major antigenic change


Infection with nonhuman (zoonotic) viruses is sometimes but not always contagious



High viral mutation rates


Interviral genetic interaction


Ecological change increasing opportunity for contact of man with vectors or viruses


Changes in human behavior (e.g., sexual)


Altered behavior of viruses in immunocompromised hosts.

Adapted from Kilbourne, 1991. Used with permission.

(E. Domingo et al., 1978). A virus, then, is identified as a consensus that reflects the predominating mutant(s) in a mixed population. Because predominating mutants seldom change, only unusual selective pressure by the host's immune response or other factors, such as host adaptation, will allow new mutants to gain ascendancy, resulting (rarely) in the emergence of distinguishably new viruses.

Influenza A Virus

Although influenza viruses mutate at a rate similar to other RNA viruses, they are unique in that they also evolve (undergo meaningful changes) relatively rapidly in nature. This is due to selective pressure on the virus from the large population of partially immune people, who have antibodies to the virus as a result of previous infections. To survive, the virus must undergo some degree of genetic mutation (or "antigenic drift"). This process is continuous and results in regional epidemics of influenza.

Much less frequently, the surface proteins—called hemagglutinin (H) and neuraminidase (N)—of the influenza virus undergo a radical change (an "antigenic shift") that creates a virus so different that no person possesses immunity to it. A pandemic of life-threatening disease results. Interestingly, the radical changes that have produced pandemic influenza viruses are rooted in the virus's acquisition of genetic material from animal influenza A viruses. Scientists have hypothesized that agricultural practices in Southeast

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