However, if the new influenza A subtype does not easily pass from person to person, the disease outbreak will be limited.Īntigenic shift occurs in two ways. Antigenic shift may lead to global disease spread, or pandemic, because humans will have few or no antibodies to block infection. The virus that results has a new HA or NA subtype. Antigenic ShiftĪntigenic shift is a process by which two or more types of influenza A combine to form a virus radically different from the ancestor strains. Other times, the prediction misses the mark, and the vaccine won’t prevent disease. Sometimes the prediction is accurate, and the flu vaccine is effective. They create a vaccine designed to fight the predicted virus. Scientists try to predict which changes are likely to occur to currently circulating flu viruses. For example, an influenza type A H1N1 virus caused the 2009 influenza pandemic.) As mutations accumulate in future generations of the virus, the virus “drifts” away from its ancestor strain.Īntigenic drift is one reason that new flu vaccines often need to be created for each flu season. (Hemagglutinin and neuraminidase lend their first initials to flu subtypes. In such a case, antibodies produced by previous infection with the ancestor strain cannot effectively fight the mutated virus, and disease results. These mutations may cause the virus’s outer surface to appear different to a host previously infected with the ancestor strain of the virus. Influenza viruses can evolve gradually through mutations in the genes that relate to the viral surface proteins hemagglutinin and neuraminidase (HA and NA in shorthand). Mutations in viral RNA and recombinations of RNA from different sources lead to viral evolution. They consist of no more than seven or eight RNA segments enclosed within an envelope of proteins. Influenza viruses are simple entities belonging to one of three types: A, B, or C. RNA virus mutations are frequent and can have important consequences for their hosts. Mistakes in copying RNA happen frequently, and the host cell does not correct these mistakes. RNA, however, is an unstable molecule, and RNA viruses don’t have a built-in proofreading step in their replication. DNA viruses therefore do not change, or mutate, much. If the virus makes a mistake in copying the DNA, the host cell can often correct the mistake. They manage to use the host cell to verify viral DNA replication. DNA is a more stable molecule than RNA, and DNA viruses have a proofreading check as part of their reproductive process. Both of these viruses are RNA viruses, meaning their genetic material is encoded in RNA, not DNA. Many viral adaptations involve changes to the virus’s outer surface.īelow we look at two special cases in viral evolution: how evolution occurs in influenza viruses and in the human immunodeficiency virus (HIV, the virus that causes AIDS). A virus that appears different from other viruses that have infected the host has an advantage: the host has no pre-existing immunity, in the form of antibodies, to that virus. Antibodies lock onto the outer surface proteins of a virus and prevent it from entering host cells. One way hosts protect themselves from a virus is to develop antibodies to it. If it kills its host before the host infects others, that mutation will disappear. The virus needs a new, healthy host for its descendants to survive. Take, for example, a virus with a mutation that makes it particularly deadly to its human host and kills the host within a few hours of infection. Characteristics that make it difficult for the virus to spread to another host tend to be lost. Characteristics that help a virus do its job tend to be kept from generation to generation. The pathogen’s job is to evade the immune system, create more copies of itself, and spread to other hosts. The human immune system uses many tactics to fight pathogens. Though viruses aren’t technically living – they need a host organism to reproduce – they are subject to evolutionary pressures. Just as natural selection has shaped the evolution of humans, plants, and all living things on the planet, natural selection shapes viruses too.
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