Pathogenic viruses in drinking-water biofilms: a public health risk?

Biofilms in drinking-water distribution systems may accumulate human pathogenic viruses. Viruses that attach to biofilm are removed from the water phase improving the water quality. However, if released in slough, it may still present a risk of infection. This review describes the available data on the presence of pathogenic viruses in drinking-water biofilms. First, biofilms of distribution systems potentially contribute to viral contamination of tap water only if infectious viruses are present initially in the water, which has been shown in several recent studies. However, only one out of three field studies showed the presence of infectious enteroviruses in natural biofilms from drinking-water networks. The presence of pathogenic viruses in biofilms points to the ability of these viruses to attach to biofilms. This has also been shown in pilot-scale studies in which bacteriophages and vaccine poliovirus strains were spiked into water and could be eluted from artificial biofilms. Reported attachment rates vary greatly and may depend on many factors such as the biofilm characteristics, the virus strain and the efficiency of viral recovery from biofilms. One study reported biphasic viral inactivation in biofilms with rapid initial inactivation followed by slow inactivation, implying that some of the attached viruses are able to remain infectious for a longer time. In several laboratory experiments, virus attachment to biofilms has been reported under various conditions; however, although detachment of sloughs in distribution systems has been observed, the presence of viruses in these sloughs was not studied. Here, we discuss the possible presence of infectious pathogenic viruses in sloughs; the extent to which these will pose a health risk remains to be investigated.