Survival of Viruses in Water.

Water, a frequent vehicle for the transmission of viruses, may permit their survival, but many environmental factors will have an adverse effect on the viral population. Risk evaluation requires identification of these factors and assessment of the inactivation rate of infectious viruses. A higher temperature means a faster reduction of the viral population, as do increased sunlight, higher antimicrobial concentration, or higher oxygen levels. Another documented impact is linked to the presence of indigenous microbial populations: virus survival is higher in sterile water. Environmental factors inactivate viruses through direct or indirect action on one part of the viral structure: genome, capsid, or envelope if present. Viral populations also have resistance mechanisms, generally involving physical shielding from adverse effects; such protective behaviors include aggregation, adhesion, or internalization inside living structures. Because of these phenomena, inactivation kinetics may deviate from traditional log-linear shapes. It is therefore important to account for all factors that may impact on survival, to carefully design experiments to ensure sufficient data, and to select the right modelling approach. Comparison between studies is difficult. It is suggested that laboratory studies include standard conditions of water, and analyze the impact of different factors as precisely as possible. Larger studies in natural environments, though more difficult, are also much needed.