Interactions of Animal Viruses with Their Hosts

For bacterial viruses, the host organism is a single cell, and so other kinds of cells do not affect the course of the infection. In the case of animals, however, the outcome of viral infection depends on many factors that are independent of the infected cell. Of special importance are the defense mechanisms of the host, such as the presence of protective antibodies that can confer immunity against a virus ordinarily lethal to an individual without such immunity. Devastating epidemics of measles and smallpox, which decimated the indigenous native population following the arrival of Europeans to the Americas, are good examples of the consequences of the lack of immunity to particular viruses.

Sudden epidemics causing widespread deaths are the most dramatic events of virus interactions with humans. The death of the host, however, also means that the virus can no longer multiply. Obviously, this is not in the best interests of the virus. Just as with the vast majority of bacterial viruses, animal viruses may develop a relationship with their normal hosts in which they cause no obvious harm or disease. Thus, the virus infects and persists within the host, in a state of balanced pathogenicity, in which neither the virus nor the host is in serious danger. Indeed, most healthy animals, including humans, carry a number of viruses as well as antibodies against these viruses without suffering any ill effects. If, however, a virus is transmitted to an animal that has no immunity against it, disease may result.

Many viruses that are carried by one group of organisms without causing disease may cause serious disease when transferred to another group. For example, Lassa fever virus does not cause disease in rodents, in which it is normally found, but when transferred to humans, it kills a large percentage of the infected population.

The relationship between disease-causing viruses and their hosts can be divided into two major categories based on the disease and the state of the virion in the host. These are acute and persistent. Acute infections are usually self-limited diseases in which the virus often remains localized (figure 14.7). In persistent infections the virus establishes infections that persist for years or even life, often without any disease symptoms.

Acute Infections

Acute infections are usually of relatively short duration, and the host organism may develop long-lasting immunity. Viruses that cause acute infections result in productive infections. The infected cells die and may or may not lyse with the release of virions. Viruses that cause lysis of host cells are usually naked, whereas those that do not cause lysis are frequently enveloped. Although infected cells die, however, this does not mean that the host dies. Disease symptoms result from localized or widespread tissue damage following lysis of cells and spreading and infection of new cells. With recovery, the defense mechanisms of the host gradually eliminate the virus over a period of days to months. Examples of acute infections are mumps, measles, influenza, and poliomyelitis. ■ mumps, p. 608 ■ measles, p. 549 ■ influenza, p. 586 ■ polio, p. 677

The reproductive cycle of an animal virus that results in an acute infection with cell lysis can be compared with the productive infection of a bacterium with a virulent phage. Basically the steps are the same except that infection of more complex eukaryotic cells in a multicellular host requires several additional steps. The essential steps include:

■ Attachment

■ Entry into susceptible cells following attachment

Figure 14.7 Time Course of Appearance of Symptoms of Measles and the Measles Virions

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  • grimalda
    How do animal viruses interact with their hosts?
    2 years ago

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