Health Significance Of Viruses To Man: Multiplication, Spread And Common Routes Of Entry
An Influenza Virus Molecule
Microbiological Implications of Viral Multiplications
Most of the viruses are not visible under light microscope. They vary in size from 300 nm to 20 nm. Each virus contains a nucleic acid molecule with either RNA or DNA as its genome. A complete viral particle which contains the genetic material (genome) and the surrounding protective coat which serves as a vehicle for its transmission from one cell to another is called a virion. Viruses lack enzymes required for the synthesis of their components and for energy metabolism. They are therefore, dependent on the metabolic pathways and components of their host cells. They are obligate intracellular parasites. They cause lesions in several living organisms such as animals, insects, plants and bacteria. Many viruses may live in their host cells without producing disease but some are pathogenic. The lesions may be inflammatory or oncogenic (including tumours).
Multiplication of viruses is controlled by the genetic information contained in the viral nucleic acid. Viral multiplication involves six sequential steps. These are:
- Absorption to the host cell
- Entry into the cell
- Release of viral nucleic acid into the cell contents
- Biosyntehsis of viral components
- Maturation of the virus particles, and
- Release of numerous virus particles from the host cell.
Viral infection causes demonstrable changes in the host cell such as cytolysis and destruction of the cell. Sometimes, the virus may become latent and persist within the cell to be reactivated later. Incorporation of viral nucleic acid into the genome of the host cell may result in malignant transformation. Several cells may fuse together to form giant cells. In the central nervous system demyelination may occur. Many tissues infected by virus show inclusion bodies within the cytoplasm or the nucleus. These are made up of aggregates of virions as the site of synthesis or these may merely represent products of degeneration caused by virus infection.
The mechanism of transmission and portal of entry of viruses vary. Common routes of entry are as follows:
- Respiratory tract, eg, Coryza, influenza, measles and smallpox
- Alimentary trac, eg, enteroviruses and infectious hepatitis
- Abrasions or wounds on the skin which may be due to animal bites (rabies), arthropod vectors (arboviruses), injection (serum hepatitis) or abrasions of skin (papilloma virus).
- Intact conjunctiva (adenovirus and measles).
- From mother to fetus through the placenta (rubella and cytomegalovirus)
- From the genital tract, during birth (Herpes simplex virus type II).
The outcome of infection by a virus depends on its virulence and the resistance of the host. The effects on the host may be divided into specific and nonspecific. Specific response is the production of immunity, while the nonspecific response include fever and production of interferons. Both humoral and cellular immune responses play their roles in viral infections.
Proteins of the viral capsid stimulate the formation of humoral antibodies which neutralize the virus. They also cause complement mediated lysis of the infected host cells. Although, in many cases the antibodies are protective, sometimes, these are harmful, causing immune complex type of tissue injury to the renal glomeruli, synovial of joints or the skin. IgA antibodies secreted into the respiratory tract protect against infection by other respiratory viruses. Humoral viral infections, eg, enterovirus infections. Some viruses like herpes, rubeola and pox stimulate cells mediated immunity which playes the major role in defense.
© 2014 Funom Theophilus Makama