Virus

Virus

Virus, the smallest form of life (20-300 nanometres), is structurally and functionally unique. Their size is such that they do not contain enough genetic material to code for the proteins they require for reproduction, nor do they have ribosomes needed to synthesize these proteins. Therefore, being obliged to enter permissive host cells that assist them to reproduce, they are obligate intracellular parasites implicated in diseases of humans, other animals and plants.

Specialists in microbiology, molecular biology, biochemistry, immunology and other biological sciences are engaged in interdisciplinary research to identify, describe and control disease-causing viruses. In Canada, research into human viral disease is carried out primarily in the universities and hospitals, and is funded largely by federal and provincial grants and frequently by targeted philanthropic agencies. Agriculture Canada and certain universities have extensive programs on viral diseases of animals. Work on plant viruses is done in Agriculture Canada Research Stations and in plant pathology laboratories connected to university departments of botany or biology.

Structure and Function

The mature virus organism (virion) consists of viral genes contained in a protective protein shell (a capsid). Viral genes may be deoxyribonucleic acid (DNA) or ribonucleic acid (RNA); genetic RNA is unique to viruses. RNA viruses include enteroviruses (eg, causing poliomyelitis), rhinoviruses (common cold), rhabdoviruses (rabies), paramyxoviruses (measles), orthomyxoviruses (influenza) and almost all plant viruses.

DNA viruses include papavoviruses (warts), adenoviruses (acute respiratory disease), herpesviruses (cold sore, infectious mononucleosis, chicken pox), poxviruses (smallpox, cowpox), hepatitis B, and many viruses that infect bacteria (bacteriophages) and insects. Virions have very limited functions and serve mainly to protect the viral genetic material (genome) from adverse environmental factors outside a host cell and to facilitate the entry of the genome into compatible host cells. Specific types of viruses take advantage of the resources of particular host cells. Bacteriophages (bacteria eaters), named by Canadian virologist Felix d'Herelle in 1917, prey only on bacteria and have specific preferences for particular bacterial species.

Similarly, plant-specific viruses (discovered by Russian botanist Dimitri Ivanovsky in 1892) and animal-specific viruses (discovered by Friedrich Loeffler and Paul Frosch in 1898) have preferred host cells. Such preferences may stem from the origins of viruses, which are thought by some theorists to be degenerate, subcellular, self-replicating particles.

Viruses infect cells by fusion, endocytosis or injection. In viruses of a certain structure, the viral envelope fuses with the cell membrane of a host cell and the virus genome it contains is introduced into the cell. In endocytosis the host cell engulfs the virus, engulfment being triggered by contact with a viral particle, and the nucleic material is released from the capsid. Injection, a technique copied in laboratory genetic engineering, is used only when bacteriophages infect bacteria. The bacteriophages (virus) attaches itself to the bacterial cell wall and injects its nucleic acid, leaving the capsid outside the wall. Once inside the host cell, the viral nucleic acid participates in replicative activity.

Once inside a permissive host cell the viral DNA or RNA may integrate itself into the genome of the host cell or may immediately initiate replication through a cytolytic (eg, cell-degenerating) infection, depending on the type of virus. In the case of integration, the host cell undergoes changes during which it acquires many of the properties of cancer cells.

The virus genome, being integrated, is conserved and replicated along with the host genome. The virus or cell may, however, initiate a "lytic" infection replicating the mature virion which is then capable of infecting another host cell. The association between viruses and some cancers in humans is well documented, and viruses are known to cause many cancers in other vertebrate species.

Cells undergoing a lytic virus infection frequently have their own synthetic processes inhibited but produce hundreds of identical copies of the infecting virion. The replicative strategies employed by RNA viruses are diverse and complex, depending upon whether the virus genome can function directly as messenger RNA, which instructs host cell ribosomes (cell organelles through which protein synthesis occurs) to make viral enzymes and proteins, or whether messenger RNA must be synthesized on the viral genetic template, before protein and enzyme synthesis can proceed.

Most DNA viruses replicate in the nucleus of the host cell, using host-cell enzymes to synthesize messenger RNA on the virus's DNA template. Assembly of the virus is a complex and still poorly understood process. As examples, RNA viruses may assume a simple spherical form, complicated forms with enveloping membranes, and other special structures. DNA viruses may assemble in "factories" in the host-cell cytoplasm or be "budded off" from the host-cell membranes. Viruses that acquire outer membranes from the membrane of the cell's nucleus or cytoplasm incorporate virus-coded glycoproteins into these membranes, before the final assembly of the mature virion.

Viruses cause major diseases in plants and animals, but there are few effective methods for cure. Plant breeding may yield strains resistant to viruses, but this avenue of approach is unavailable to humans. Recently, some antiviral chemicals have shown promise as antibiotics; however, this control technique is still limited to a few virus diseases such as the use of acyclovir for herpesvirus infections.

More successful efforts have been directed towards the development of vaccines for preventing viral disease. Properly administered, such vaccines have resulted in the worldwide eradication of smallpox and the virtual disappearance of poliomyelitis, measles and rubella from specific populations. In all Canadian provinces, Public Health authorities maintain vaccination programs directed especially at small children. Some viruses, notably Human Immunodeficiency Virus (HIV), the cause of Acquired Immune Deficiency Syndrome or AIDS, have been, to date, refractory to either approach and innovative therapeutic methods are actively being pursued.

Finally virologists today are deeply concerned about the appearance of new and the resurgence of known pathogenic viruses such as HIV and the ability of the Public Health system to treat and control these organisms. See also Biological Products Industry; Pharmacy; Veterinary Medicine.