Figure 23.22 Influenza Virus: Antigenic Drift and Antigenic Shift With drift, repeated mutations cause a gradual change in the antigens composing the hemagglutinin, so that antibody against the original virus becomes progressively less effective. With shift, there is an abrupt, major change in the hemagglutinin antigens because the virus acquires a new genome segment, which in this case codes for hemagglutinin. Changes in neuraminidase could occur by the same mechanism.
Nester-Anderson-Roberts: I IV. Infectious Diseases I 23. Respiratory System I I © The McGraw-Hill
Microbiology, A Human Infections Companies, 2003
Perspective, Fourth Edition
588 Chapter 23 Respiratory System Infections time. As can be demonstrated in the laboratory, dual infections result in an exchange of large segments of the viral genomes, so that totally different viruses result. Animal strains of influenza virus can occasionally infect humans and cause dual infections with human strains. If genetic mixing results in a new virus that is infectious, virulent, and possesses a hemagglutinin for which a population had no immunity, it could cause widespread disease. Ecological studies show that all the known influenza A virus types exist in aquatic birds, generally causing chronic intestinal infections. These bird influenza viruses readily infect domestic fowl, and from them can infect other domestic animals and humans. A well-studied episode in Hong Kong in 1997 involved an H5N1 virus from chickens that caused fatal infections in humans, but fortunately it did not spread easily from person to person. These ecological considerations indicate that operations in which high densities of domestic fowl or other animals are raised should be strictly isolated from contact with wild fowl. Influenza epidemics have devastated human populations in the past, and the emergence of virulent new strains of influenza virus continues to be an extremely serious threat to humankind.
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