Groupings Based on Routes of Transmission
1. Viruses that cause disease can be grouped according to their routes of transmission—the enteric viruses, respiratory viruses, zoonotic viruses (transferred from animals to other animals), and sexually transmitted viruses. (Table 14.4)
14.2 Methods Used to Study Viruses
Cultivation of Host Cells
1. Some viruses can only be cultivated in living animals; others can be grown in tissue culture. (Figures 14.2, 14.3)
368 Chapter 14 Viruses, Prions, and Viroids: Infectious Agents of Animals and Plants Quantitation
1. The plaque assay is commonly used to determine the number of infective virions in a sample. (Figure 14.4)
2. Virions can be counted with an electron microscope. (Figure 14.5)
3. Quantal assays determine the infective or lethal dose in a viral preparation.
4. Some viruses can clump red blood cells and their concentration can be measured by determining the dilution of virus able to clump the cells, hemagglutination. (Figure 14.6)
14.3 Interactions of Animal Viruses with Their Hosts
1. Most viruses infect and persist within their hosts in a state of balanced pathogenicity in which the host is not killed.
2. Viruses cause diseases that can be classified as acute or persistent.
1. Acute infections are self-limited; the virus often remains localized, and diseases are of short duration and lead to lasting immunity. (Figure 14.7)
2. The replication cycle of virulent animal virus that causes an acute infection is similar to the lytic cycle shown by phage T4. (Table 14.6, Figure 13.5)
3. The steps in the infection process include attachment to specific receptors; entry of the virion; targeting to the site of viral replication; uncoating of the virion (Figure 14.8); replication of virus nucleic acid and protein; (Figure 14.9); maturation of the virion (Figure 14.10); release of the virion so that it infects other cells in the vicinity (Figure 14.11); shedding of the virions and transmission to other hosts (Table 14.6).
1. In persistent infections, the virions are continually present in the body and are released from cells by budding. (Figure 14.11)
2. Persistent infections can be (1) late complications following an acute infection, (2) latent infections (Table 14.7)
(3) chronic infections (Table 14.8), and (4) slow infections. These categories are distinguished from one another largely by whether a virus can be detected in the body during the period of persistence. (Figure 14.12)
3. Infections with characteristics of more than one category are complex infections. An example is HIV infection which has characteristics of a chronic, latent, and slow viral infection.
4. In HIV infection, the viral RNA is converted to double-stranded DNA by the viral enzyme reverse transcriptase. The DNA is then integrated into chromosomes of host cells concerned with the immune response, where it can remain latent or give rise to intact virions. (Figure 14.14)
5. Unlike DNA polymerase, the enzyme reverse transcriptase has no proofreading ability, and so virions undergo numerous uncorrected mutations.
14.4 Virus-Induced Tumors—General Aspects
1. One class of host cell regulatory genes commonly involved in tumor formation is proto-oncogenes.
2. Mutations in this gene class predispose the host to tumor formation.
3. Proto-oncogenes are transcriptional activators.
4. Viruses can alter the activity of proto-oncogenes.
14.5 Viruses and Animal Tumors
1. Retroviruses are the most important tumor-causing viruses in animals, but not in humans.
1. Oncogenes of tumor viruses can modify the properties of cells growing in tissue culture (Table 14.9). These genes are mutants of the host cell's proto-oncogenes. (Table 14.10)
1. Retroviruses transform cells by inserting oncogenes into the genome of the host cell, where they interfere with the normal intracellular control functions of the cell's proto-oncogenes. (Figure 14.15)
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