Detection of Animal Viruses
The presence of viruses in an animal or plant is recognised by the manifestation of some abnormality in whole organisms or host cells. In whole organisms the manifestation of the presence of a virus varies from inapparent infections which may only be detected by the formation of antibodies or by the development of local lesions and mild disease to progressive very severe disease.
In individual cells the symptoms of viral infection may vary from changes in morphology and growth patterns to cytological changes (cytopathic effects) such as breakdown of cells, organelles, development of inclusion bodies etc.
Measurement of animal viruses
Viruses may be measured or enumerated as infectious units, they may also be measured or enumerated as virus Particles irrespective of whether they are infectious or not.
A. Infectious Units
The measurement of the number of virus inféctious units per unit. volume of virus Suspension is known as titration. Several methods of determining the titre of a virus Suspension are available all of which involve infecting host or target cells in such a way that each virus particle capable of causing infection elicits a recognisable response.
The Methods include plaque formation, focus formation, pock formation or assays.
Pock Assay
Embryonated eggs are used to assay for the infectivity of viruses. Herpes simplex virus causes evident pocks to form on embryonated eggs.
Pocks contain many inflammatory cells in addition to cells that are infected.
Plaque formation
Plaque formation is often the most desirable method of titrating viruses. However not all viruses can be measured by this method especially if there are no cells that develop the desired cytopathic effects.
Focus Formation
Assay of viruses by focus Formation involves counting the number of focusing units.
Serial Dilution End Point Method
Some viruses destroy the cells they infect without producing visible cytopathic effects. These viruses can be titrated by the serial dilution end point method.
Enumeration of the total number of virus particles Hemagglutination assay
Many animal viruses adsorb to the red blood cells of various animal species. Viral haemagglutination results from a linking together of the red cells by the viral particles or from the action of soluble substances such as lipoproteins produced by the organisms (e.g pox viruses).
Myxoviruses agglutinate, fowl, guinea pig and human red cells, under favourable conditions. Visible clumping occurs within a few Seconds as the virus particles become attached to mucoprotein receptors on the red cells and form bridges between the cells.
Note: The haemagglutination assay is performed by determining the virus dilution that will agglutinate a standard number of red cells. The number of virus particle necessary for agglutination is readily calculated, hemagglutination serves as a highly accurate and rapid method of quantifying virus particles.
Detection And Enumeration Of Plant Viruses
The effects of viruses on plants are numerous and may be grouped according to their effects on overall growth, colour, water content, tissue life span and shape of organs.
Until viruses themselves are studied in detail they could only be detected indirectly by studying their pathogenic effects. Such viruses that escape isolation and detection can only be studied through indirect detection of their effect in plants that are susceptible.
Infection leads to a series of events which become visible if the plant is sensitive. Often infection fails to yield clear symptoms in spite of intensive multiplication and internal spread of virus. Such plants are insensitive or tolerant and infection is latent, inapparent or symptomless. Many viruses do not provoke reaction in their main hosts and are referred to as latent viruses.
A careful ind close look at the seed may occasionally give an indication of the presence of virus in certain seeds. Growing on tests, infectivity tests, serological, biochemical tests and electron microscopy are some of the techniques used in detecting plant viruses.
There are some simple large scale or routine methods of virus detection designed for particular «crops and for a few common viruses in the expected absence of others. In the production and final certification of virus free or virus-tested plant propagation material such methods are called routine indexing. Several methods rely on or start with visual examination for symptoms in field grown plants or their parts e g. bulbs, seeds, tubers and other vegetative propagation material either in stock or in samples taken to the laboratory.
Visual inspection for external symptoms must be supported by search for histological or anatomical deviations such as inclusion bodies or vascular necrosis. The most typical sign of virus infection is the presence of virus particles in abundant amounts.
Detection in the field or in the laboratory should be supported or replaced by tests for the viruses themselves. Infectivity tests for viruses are usually highly sensitive and employ selected test plants or indicator plants. The ELISA technique has given new impetus to large scale direct and rapid detection (indexing) of a wide range of plant species and their seeds.
Electron microscopy provides the worker with a view of a wide range of plant viruses in a few minutes. Diagnosis of viriod diseases relies on mechanical inoculation to test plants though this is giving way to physico chemical properties of the viruses themselves e.g isolation by polyacrylamide gel electrophoresis and staining of the nucleic acid bands. The cDNA hybridisation technique also is well suited to viroids.
Enumeration Of Plant Viruses
The choice of an appropriate propagation host and of appropriate conditions and chemicals to be used during isolation requires a reliable assay of virus concentration.
Virus concentration is also useful and required to monitor the effect of the different steps in the isolation and purification of viruses, several methods are available for the enumeration of plant viruses. These include biological tests, ultraviolet spectrometry, electron microscopy and serological methods.