A small infectious agent that is unable to replicate outside a living animal cell. Unlike other intracellular obligatory parasites (for example, chlamydiae and rickettsiae), animal viruses contain only one kind of nucleic acid, either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA), but not both. They do not replicate by binary fission. Instead, they divert the host cell's metabolism into synthesizing viral building blocks, which then self-assemble into new virus particles that are released into the environment. During the process of this synthesis, viruses utilize cellular metabolic energy, many cellular enzymes, and organelles which they themselves are unable to produce. For this reason, they are incapable of sustaining an independent synthesis of their own components. Animal viruses are not susceptible to the action of antibiotics. The extracellular virus particle is called a virion, while the name virus is reserved for various phases of the intracellular development. See also: Deoxyribonucleic acid (DNA); Ribonucleic acid (RNA)
Virions are small, 20–300 nanometers in diameter, and pass through filters which retain most bacteria. Due to this property, viruses were called filterable agents of disease. However, large virions (for example, vaccinia, which is 300 nm in diameter) exceed in size some of the smaller bacteria.
The major structural components of the virion are proteins and nucleic acid, but some virions also possess a lipid-containing membranous envelope. The protein molecules are arranged in a symmetrical shell, the capsid, around the DNA or RNA. The shell and the nucleic acid constitute the nucleocapsid.
In electron micrographs of low resolution, virions appear to possess two basic shapes: spherical and cylindrical. High-resolution electron microscopy and x-ray diffraction studies of crystallized virions reveal that the “spherical” viruses are in fact polyhedral in their morphology, while the “cylindrical” virions display helical symmetry. The polyhedron most commonly encountered in virion structures is the icosahedron, in which the protein molecules are arranged on the surface of 20 equilateral triangles. Based on these morphological features, viruses are classified as helical or icosahedral (Fig. 1). Certain groups of viruses do not exhibit any discernible features of symmetry and are classified as complex virions (Fig. 2). Further distinction is made between virions containing RNA or DNA as their genomes and between those with naked or enveloped nucleocapsids. These assignments are shown in the table, in which the major groups of animal viruses are listed.
Viral nucleic acid
The outer protein shell of the virion furnishes protection to the most important component, the viral genome, shielding it from destructive enzymes (ribonucleases or deoxyribonucleases). The viral genome carries information which specifies all viral structural and functional components required for the initiation and establishment of the infectious cycle and for the generation of new virions. This information is expressed in the alphabet of the genetic code (sequence of nucleotides) and may be contained in a double-stranded or single-stranded (parvoviruses) DNA, and double-stranded (reoviruses) or single-stranded RNA. The viral DNA may be linear or circular, and the viral RNA may be a single long chain or a number of shorter chains (fragmented genomes), each of which contains different genetic information. Furthermore, some RNA viruses have the genetic information expressed as a complementary nucleotide sequence. These are classified as negative-strand RNA viruses. Finally, the RNA tumor viruses have an intracellular DNA phase, during which the genetic information contained in the virion RNA is transcribed into a DNA and integrated into the host cell's genome. The discovery of this process came as a surprise, since it was believed that the flow of genetic information was unidirectional from DNA to RNA to protein and could not take place in the opposite direction, from RNA to DNA. The transcription of RNA to DNA was termed reverse transcription, and the RNA tumor viruses are sometimes referred to as retroviruses. A classification based on the relationship between viral genomes and their messenger RNA (mRNA) is shown in Fig. 3. See also: Genetic code; Tumor viruses
When introduced into a susceptible cell by either chemical or mechanical means, the naked viral nucleic acid is in most cases itself infectious. Two exceptions are the negative-strand RNA viruses and the RNA tumor viruses. In these cases, the RNA has to be first transcribed and reverse-transcribed, respectively, into the proper form of genetic information before the infectious process can take place. This task is carried out by means of an enzyme which is contained in the protein shell of the virion nucleocapsid. The whole nucleocapsid is therefore required for infectivity.
Advances in virus research are closely linked with the utilization of new laboratory techniques. The introduction of tissue cultures eliminated the necessity to use experimental animals and revolutionized animal virus research. The application of plaque assays, first developed in studies of bacterial viruses, was of similar importance. In this assay, a monolayer of cells growing on solid support (usually in a petri dish) is infected with a dilute solution of virions and overlaid with a soft nutrient agar layer. The high viscosity of the agar layer prevents newly released virions from diffusing farther than the cells adjacent to the ones originally infected. After a sufficiently large number of surrounding cells are killed by newly released virions (after 1 day to 3 weeks, depending on the virus), a clear plaque becomes visible. Since each plaque was initiated by a single virus particle, the method allows a quantitative determination of the number of viable virions and offers a means of virus purification (the virus population isolated from a single plaque is relatively homogeneous). Plaque assays are frequently used in diagnosis and in the clinical isolation of viruses. Other assays are based on the ability of some viruses to hemagglutinate (aggregate) red blood cells. The highest dilution of a virus-containing extract which is still capable of hemagglutinating (end-point dilution) is used as a measure of virion concentration. Titrations with specific antibodies are similarly applied in the characterization and quantitation of viruses. See also: Tissue culture
Viral infection is composed of several steps: adsorption, penetration, uncoating and eclipse, and maturation and release.
Adsorption takes place on specific receptors in the membrane of an animal cell. The presence or absence of these receptors determines the tissue or species susceptibility to infection by a virus. Enveloped viruses exhibit surface spikes which are involved in adsorption; however, most animal viruses do not possess obvious attachment structures.
Penetration takes place through invagination and ingestion of the virion by the cell membrane (phagocytosis or viropexis). Enveloped viruses often also enter the cell by a process of fusion of the virion and cell membranes. Penetration is followed by uncoating of the nucleic acid, or in some cases by uncoating of the nucleocapsid. At this stage, the identity of the virion has disappeared, and viral infectivity cannot be recovered from disrupted cells (special methods are required to utilize the infectious nucleic acid). See also: Phagocytosis
The absence of infectious particles in cell extracts is characteristic of the eclipse period. During the eclipse, the biochemical processes of the cell are manipulated to synthesize viral proteins and nucleic acids. The survival of viruses depends on this subversive ability. In the process of evolution, they have developed an extraordinary efficiency and a remarkable repertoire of strategies. The eclipse period in infections with DNA viruses starts with the transcription of the genetic information in the nucleus of the cell (except poxviruses), processing into mRNAs, and their translation into proteins (in the cytoplasm). This process is divided into early and late transcription. In the absence of newly synthesized viral components, the immediate early events must be entirely catalyzed by cellular enzymes. The early proteins, however, are virus-encoded functional proteins which will participate in the synthesis of viral DNA and of intermediate and late viral proteins, as well as in the shutoff of various cellular functions which might be detrimental to viral synthesis. The event that distinguishes early from late mRNA transcription and translation is the onset of viral DNA synthesis. The major late products are the structural proteins of the nucleocapsid. Almost as soon as these proteins are synthesized, they assemble with newly synthesized DNA molecules into virion nucleocapsids. The appearance of these particles signals the end of the eclipse period.
The events of the eclipse period in infections with RNA viruses are similar, except that they take place in the cytoplasm (influenza virus excepted), and a division into early and late transcription cannot be made. In the case of positive-strand RNA viruses, the viral RNA is itself the mRNA. In infections with negative-strand RNA viruses, the virion RNA in the nucleocapsid is first transcribed into positive mRNAs. Intracellular nucleocapsids are present throughout the entire infectious cycle, and the eclipse period cannot be defined in the classical sense. RNA tumor viruses reverse-transcribe their RNA into DNA, which enters the cell nucleus and becomes integrated into the cellular DNA. All viral mRNAs and genomic RNAs are generated by transcription of the integrated DNA (Fig. 3).
Maturation and release
The event characteristic of the maturation step is virion assembly and release. In many cases, the protein shell is assembled first (procapsid) and the nucleic acid is inserted into it. During this insertion, processing of some shell proteins by cleavage takes place and is accompanied by a modification of the structure to accommodate the nucleic acid. Unenveloped viruses which mature in the cytoplasm (for example, poliovirus) often exit the cell rapidly by a reverse-phagocytosis process, even before the breakdown of the cell. In some cases, however, a large number of virus particles may accumulate inside the cell in crystalline arrays called inclusion bodies (Fig. 4). Viruses that mature in the nucleus are usually released slowly, and the damage to the cell is extensive. Enveloped viruses exit the cell by a process of budding. Viral envelope proteins (glycoproteins) become inserted at various sites into the cell membrane, where they also interact with matrix proteins and with nucleocapsids. The cellular membrane then curves around the complex and forms a bud which detaches from the rest of the cell. Figure 5 shows virus particles budding from the cell surface of a mouse cell infected with murine leukemia virus.
Effect of viral infections
Two extreme types of effects are identified: lytic infections, which cause cell death by a variety of mechanisms, with cell lysis as the most common outcome; and persistent infections, accompanied either by no apparent change in the host cell or by some interference with normal growth control, as in transformation of normal to cancer cells. The degenerative phenomena in tissue cultures during a lytic infection are called cytopathic or cytolytic effects. In animals, extensive destruction of tissue may accompany an infection by a lytic virus. See also: Lytic infection
As a defense to certain conditions of infection, animal cells generate a group of substances called interferons which, by a complex mechanism, inhibit replication of viruses. They are specific to the cell species from which they were derived but not to the virus which elicited their generation. (Mouse interferon will protect mouse but not human cells from any viral infection.)
Like other microorganisms, viruses mutate either spontaneously or following exposure to mutagenic agents. With the introduction of cell tissue cultures and with the development of plaque purification procedures, isolation and characterization of well-defined mutants became possible. In the past, the most useful mutants in genetic studies of animal viruses were temperature-sensitive (ts) and host-range (hr) mutants. The ts mutants grew efficiently at lower than normal but not at normal temperatures (permissive and nonpermissive temperatures, respectively). The mutation caused a change of an amino acid in a viral protein which resulted in a biologically inactive configuration at the higher temperatures. Studies of the impaired proteins and their respective genes helped to elucidate the molecular mechanisms of various events in the infectious cycle. The hr mutants, which grew normally in certain host cells but not in others, were useful in studies of virus–host interactions. This approach to viral genetics was dependent on the isolation of the proper mutants, which was often fortuitous. With the advent of modern DNA technology of restricting, cloning, and sequencing, it became possible to generate site-specific mutants. Rather than depending on an accidental change of a nucleotide by a mutagen, the investigator could chemically alter a nucleotide at any selected site. In this manner, mutants were generated to answer very specific questions about viral gene expression and virus–host interactions. Since reverse transcription can be carried out in the test tube, these techniques could, in principle, be applied also to RNA viruses as long as their RNAs are infectious. DNA cloning, which permits the preparation of relatively large quantities of viral genomes, is making it possible to study viruses which were inaccessible to investigation either because of low yields obtained in living organisms or because of the lack of suitable tissue cultures. See also: Genetic engineering; Mutation; Recombination (genetics); Restriction enzyme
This modern approach to animal virus genetics also rapidly advanced understanding of the complex molecular biology of the animal cell. Many structural and regulatory features of animal virus genes are similar to those of the animal cell, yet are easier to study because of the smaller size of viral genomes. See also: Genetics; Molecular biology
Virus infections spread in several ways: through aerosols and dust, by direct contact with carriers or their excretions, and by bites or stings of animal and insect vectors. At the point of entry, infected cells undergo viremia. From there, the virus becomes disseminated by secretions. It is carried through the lymphatic system and bloodstream to other target organs, where secondary viremias occur (except in localized infections like warts). In most cases, viral infections are of short duration and great severity. However, persistent infections are not uncommon (herpes, adeno, various paramyxoviruses like measles). See also: Virus infection, latent, persistent, slow
The afflicted organism mounts a variety of defenses, the most important of which is the immune response. Circulating antibodies against viral proteins are generated. Those interacting with virion surface proteins neutralize the infectious potential of the virus. Although the antibodies are specific against the virus which has elicited them, they will cross-react with closely related virus strains. The specificity of neutralizing antibodies obtained from experimentally injected animals is utilized for diagnostic purposes or in quantitative assays. In addition to the circulating antibodies, cell-mediated immune responses also take place. The most important of these is the production of cytotoxic thymus-derived lymphocytes, found in the lymph nodes, spleen, and blood. They destroy all cells which harbor in their membrane viral glycoproteins, regardless of whether these cells are actively involved in virus synthesis or acquired the viral proteins by membrane fusion with inactive virions or cell debris. The cell-mediated immunity has been demonstrated to be more important to the process of recovery than circulating antibodies. In spite of their beneficial role, immune responses often seriously contribute to the pathology of the disease. Circulating antigen–antibody complexes can lodge in organs and cause inflammation; cell-mediated responses have been known to produce severe shock syndromes in patients with a history of previous exposures to the virus. See also: Antibody; Autoimmunity; Cellular immunology; Immunity; Virus interference
Viruses are resistant to the antibiotics commonly used against bacterial infections. The use of chemotherapeutic agents with antiviral activity is plagued by their toxicity to the animal host. However, the application of vaccines has been successful in the control of several viruses. The vaccines elicit immune responses and provide sometimes life‐long protection. Two types of vaccines have been applied: inactivated virus and live attenuated virus. Various inactivation procedures are available. For example, in preparations of polio vaccines (Salk), a combination of formalin and heat was utilized. Protection by attenuated live virus vaccines dates back as early as 1776, when Edward Jenner used infections with harmless cowpox to achieve protection from the devastating effects of smallpox. In modern times, an attenuated laboratory strain of smallpox has been applied so successfully that the disease is considered to be eradicated. A small probability of back mutations of the attenuated virus to a virulent strain makes applications of live vaccines somewhat riskier. On the other hand, protection is longer-lasting and, by virtue of spread to nonvaccinated individuals, more beneficial to the population group (herd effect). See also: Chemotherapy and other antineoplastic drugs; Poliomyelitis; Vaccination; Virus chemoprophylaxis
In order to achieve full protection, it is important that the vaccine contain all the distinct antigenic types of the virus. Development of monoclonal antibodies led to a better characterization of these types in naturally occurring viruses. This information will undoubtedly lead to better vaccines. Moreover, monoclonal antibodies have aided investigations into the molecular structure of viral antigenic groups and brightened future prospects for synthetic vaccines. See also: Monoclonal antibodies; Virus; Virus classification