1. PHYSICAL AND CHEMICAL INFLUENCES ON VIRAL INFECTIVITY
Environmental factors
Action in vitro

2. Introduction
Generally viruses are more sensitive to inactivation by physical and chemical agents than bacteria and fungi
Knowledge of their sensitivity to environmental conditions is important for:
Ensuring the preservation of infectivity of the viruses as reference reagents
Preservation of viruses in clinical specimens collected for diagnosis
For the deliberate inactivation for sterilization, disinfection and production of inactivated vaccines

3. Physical and chemical factors
Temperature
Too high a temperature is the principal environmental condition that may adversely virus infectivity in clinical specimens pH
High or low pH also affect infectivity
Lipid solvents and detergents
Mostly affect enveloped viruses

4. Heat stability Viruses vary considerably in heat stability
Surface proteins are denatured within a few minutes at 55° - 60°C, hence the virion is no longer capable of normal cellular attachment and/or uncoating
Infectivity is preserved by storing viral preparations at low temperature
4°C (wet ice or a refrigerator) for a day
Longer term preservation requires much lower temperatures, -70°C (dry ice [CO2] or freezers)
[t½ of viral infectivity 60°C in sec, 37°C in min, 20°C in hrs, 4°C in days, -70°C in years]

5. Heat stability
Enveloped viruses are more heat-labile than the naked viruses
Repeated freezing and thawing also affect infectivity probably due to disruption of the virion by ice crystals
Freezing also pose a problem in the collection and transportation of clinical specimens.
Thus specimens should be delivered as rapidly as practicable, packed without freezing on ice-cold gel packs.

6. Thermostability of Viruses
Most viruses are stable at room temp especially when suspended in complex media
Thermostability differ in the range of 50 60C where there is rapid protein denaturation
When present in high concentrations, monovalent (i.e. Na) and divalent (i.e. Mg & Ca) cations stabilize certain viruses (enteroviruses) to heat-inactivation
Other viruses are not stabilised and are thus quickly inactivated (Arboviruses)

7. Laboratory Preservation of viruses
Rapid freezing of small aliquots of viruses suspended in a medium containing protective proteins and/or dimethylsulfoxide, followed by storage at -70°C or -196°C
Freeze-drying/lyophilisation, that is, dehydration of a frozen viral suspension under reduced pressure, followed by storage of the resultant powder at 4°C or -20°C
Freeze-drying prolongs viability even at ambient temperature and is universally used in manufacture of live-virus vaccines

8. Ionic environment and pH
Viruses are best preserved in an isotonic environment at physiological pH
Some viruses tolerate a wide ionic and pH range
Most enveloped viruses are inactivated at pH 5 – 6, some survive acidic pH of the stomach e.g. rotaviruses and many picornaviruses
Stability to low pH is determined by exposing the virus samples to pH 3.0 (or any pH of interest) for 60 min.
A decrease of 1-log or more in infectivity is indicative of sensitivity to low pH, while a retention of activity or of any decrease < 1-log is indicative of stability

9. Lipid solvents and detergents
Infectivity of enveloped virus is readily destroyed by lipid solvents e.g. ether, chloroform or detergents like sodium desoxycholate
Lipid solvents must be avoided where procedures concerned with maintaining viability of viruses are involved
In the lab, detergents are commonly used to solubilize viral envelopes and liberate proteins for use as vaccines and for chemical analysis
Lipid-solvent containing disinfectants are useful in clearing an area from viral contamination
A given virus is considered sensitive if exposure to the lipid solvent results in a 1-log decrease in infectivity as compared to the unexposed controls.

10. Irradiation UV-light
Irradiations often affect the genome, DNA and proteins
Use of sunlight in disinfection