Info

Invertebrates

Ascoviridae

Ascovirus

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Spodoptera frugiperda ascovirus

Invertebrates

aQuotes are used to denote taxa without internationally approved names; kbp, kilobase pairs.

bViruses in families in boldface type have vertebrate hosts, which include human hosts except for the genera Asfivirus, Ranavirus, and Lymphocystivirus.

aQuotes are used to denote taxa without internationally approved names; kbp, kilobase pairs.

bViruses in families in boldface type have vertebrate hosts, which include human hosts except for the genera Asfivirus, Ranavirus, and Lymphocystivirus.

There are also five families of DNA viruses that contain single-stranded DNA (ssDNA) as their genome, as listed in Table 6.2. These viruses infect bacteria, mycoplasma, spiro-plasma, plants, invertebrates, and vertebrates. Members of only one family, the Parvoviridae, infect humans and cause disease, and only this family is considered further. The par-voviruses have a small genome (4-6 kb), whence they receive their name (Latin parvus = small). Note that in the classification used here, viruses with a DNA genome that replicate through an RNA intermediate, such as the hepad-naviruses and the caulimoviruses, are not referred to as conventional DNA viruses and were considered in Chapter 5.

During infection, most vertebrate DNA viruses stimulate host-cell DNA replication, or at least the early stages of DNA replication, in order to prepare a suitable environment for their own DNA replication. Such a favorable environment includes the presence of cellular factors required for DNA replication as well as an increase in the amount of substrates required for making DNA. Further, some viruses cause cells to proliferate, at least early in the infection cycle. For this reason, most DNA viruses are at least potentially transforming. If an incomplete replication cycle occurs and the early genes that stimulate DNA replication continue to be expressed, a cell may become transformed. Many DNA viruses are known to cause tumors in humans or in other animals.

In contrast, the parvoviruses do not encode proteins that stimulate cellular DNA replication. For this reason, they can only replicate in cells that are actively dividing and their target tissues in the host are organs that undergo continual renewal. Members of one genus of the Parvoviridae, the Dependoviruses, require a helper virus for replication, and replication will occur only in cells coinfected with the helper.

The larger DNA viruses interfere with the defenses of the vertebrate host against viruses. Some inhibit the interferon system, some inhibit immune effector functions, some interfere with the complement system, and most large DNA viruses interfere with more than one host defense pathway. In part because of this ability to interfere with host defense systems, infection with some DNA viruses results in a latent or a persistent infection that can endure for the life of the infected individual. Some aspects of these interference pathways are described in this chapter, where needed to understand the replication cycle and epidemiology of the virus, but a more detailed description of these defense mechanisms is presented in Chapter 8.

Some DNA viruses, such as the poxviruses and the aden-oviruses, cause epidemics of symptomatic disease in vertebrates from which recovery is complete (if the infection is not fatal) and immunity is established. Other vertebrate DNA viruses, such as the herpesviruses and the polyomaviruses, establish long-term infections that persist despite a vigorous immune response. For such a strategy of long-term persistence to be successful, infection in the majority of hosts must be inapparent or cause only moderate symptoms that are not unduly deleterious. Spread may be epidemic and accompanied by symptoms during primary infection, but for some herpesviruses, vertical transmission to infant progeny occurs

TABLE 6.2 Single-Stranded DNA Viruses

Family

Genera

Genome size (kb)

Type species

Host"

Inoviridae

Inovirus

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