Viral particle,

The plaque assay is desirable because it is very sensitive and only detects infectious viral particles. However, there are viral agents which cannot be supported by cell lines. In these cases other methods must be used. The polymerase chain reaction (PGR), which amplifies DNA or RNA from viral agents, can be used to detect the presence and quantity of viral agents. The amount of RNA or DNA target in the initial sample can be determined by competitive PGR where the quantity of amplified product is compared to a control PGR product where the initial amount of target is known. Quantification is also possible by an end-point dilution method similar to that used to determine a tissue culture infections dose. PGR methods can be very sensitive however. 

Mlcrofiltra.tlon, Various membrane filters have been used to remove viral agents from fluids. In some cases, membranes which have pores larger than the viral particle can be used if the filtration is conducted under conditions which allow for the adsorption of the viral particle to the membrane matrix. These are typically single-pass systems having pore sizes of 0.10—0.22 lm. Under situations which allow optimum adsorption, between 10—10 particles of poHovims (28—30 nm) were removed (34—36). The formation of a cake layer enhanced removal (35). The titer reduction when using 0.10—0.22 p.m membrane filters declined under conditions which minimized adsorption. By removal standards, these filters remove vimses at a rate on the low end of the desired titer reduction and the removal efficiency varies with differences in fluid chemistry and surface chemistry of viral agents (26). 

Smaller pore size microfilters in single-pass systems which have pore sizes small enough to remove some vimses by size exclusion have been examined (26,37,38). Minimum levels of vims removal can be estabhshed for these systems if fluid and process conditions are employed which minimize removal of viral particles by mechanisms other than size selection. 

Table 1. Removal of Viral Particles from Fluids by Ultrafiltration...

Improvements ia membrane technology, vahdation of membrane iategrity, and methods to extend filter usage should further improve the performance of membrane filters ia removal of viral particles. Methods to improve or extead filter life and iacrease flow rates by creating more complex flow patterns could possibly be the focus of the next generation of membrane filters designed to remove viral particles. 

The size of this viral particle is of course larger than that of a virus with only 60 subunits. The diameter of tomato bushy stunt virus is 330 A compared with 180 A for satellite tobacco necrosis virus. The increase in volume of the capsid means that a roughly four times larger RNA molecule can be accommodated. 

In contrast to retroviruses, proteolysis is an early event in the replication cycle of (+)-strand RNA viruses and both protease and polymerase inhibitors can be expected to halt the propagation of infectious viral particles from already infected cells. 

Apart from offering a new and highly specific approach to the inhibition of herpesviruses, this new mechanism of action could potentially also have beneficial immunological consequences. During treatment with BAY 38-4766, viral protein synthesis continues, but due to the lack of monomeric genomic length DNA, only empty particles (dense bodies) can be formed. It is conceivable that these non-infections viral particles could aid the establishment of an antiviral immune response, leading to better control of the virus by the host. This mechanism appears... 

Genetic informahon for viral reproduchon resides in its nucleic acid (DNA or RNA see Chapter 3). The viral particle (virion) does not possess enzymes necessary for its own replication after entry into the host cell, the vims uses the enzymes already present or induces the formahon of new ones. Vimses replicate by synthesis of their separate components followed by assembly. 

Consequences of virus infection in animal cells Viruses can have varied effects on cells. Lytic infection results in the destruction of the host cell. However, there are several other possible effects following viral infection of animal cells. In the case of enveloped viruses, release of the viral particles, which occurs by a kind of budding process, may be slow and the host cell may not be lysed. The cell may remain alive and continue to produce vims over a long period of time. Such infections are referred to as persistent infections. 

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