REVIEW ARTICLE Pathogenic viruses in drinking-water biofilms: a public health risk? S. Skraber 1 , J. Schijven 1 , C. Gantzer 2 and A. M. de Roda Husman 1 * ABSTRACT 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. * Corresponding author: Dr A. M. de Roda Husman National Institute for Public Health and the Environment Microbiological Laboratory for Health Protection Health Related Water Microbiology PO Box 1 3720 BA Bilthoven The Netherlands T 31 30 2744325 F 31 30 2744434 E AM.de.Roda.Husman@rivm.nl 1 National Institute for Public Health and the Environment, Microbiological Laboratory for Health Protection, Health Related Water Microbiology, PO Box 1, 3720 BA Bilthoven, The Netherlands 2 Laboratoire Chimie Physique et Microbiologie pour l’Environnement (LCPME), UMR 7564, CNRS/ Universit ´ e Henri Poincar ´ e Nancy I, Facult ´ e de Pharmacie, 5 rue Albert Lebrun, 54000 Nancy, France INTRODUCTION The European Council Drinking Water Directive of 1980 prescribes that consumption of drinking water should not pose a public health threat (European Economic Council Directive 80/778/EEC). Human pathogenic viruses contribute largely to the waterborne disease burden causing gastroenteritis, but they also give rise to more serious diseases such as meningitis and hepatitis (Bosch, 1998; Lee & Kim, 2002; Leclerc et al., 2002). Although waterborne outbreaks have been described in many countries worldwide, virus outbreaks attributed to the consumption of drinking water may be greatly underestimated because they are scarcely distinguishable from disease originating from other sources. In addition, viruses, which can be infectious at a very low dose (Schiff et al., 1984; Graham et al., 1987), can be present in drinking water outside of the range of current detection techniques. An alternative to the direct detection of pathogenic viruses in drinking water is to estimate their concentrations in the drinking water from the concentrations in the source waters used for drinking- water production, in combination with the efficiencies of drinking-water treatment processes. The concentration of pathogenic viruses in the drinking water can be used to assess the health risk associated with the consumption of tap water, as has been done for adenovirus, rotavirus and coxsackievirus (Regli et al., 1991; Haas et al., 1993; Gerba et al., 1996; Crabtree et al., 1997; Mena et al., 2003). However, these studies did not take into account the possible role of biofilms colonizing the distribution system. Since it has been shown that viruses can attach to biofilms (Quignon et al., 1997a,b; Storey & Ashbolt, 2001, 2003a,b; Hock & Botzenhart, 2002, 2003), there is a possibility that drinking-water biofilms accumulate pathogenic viruses present in the water entering the 105