1. Occurrence
Viruses excreted in the feces or urine of animals may pollute water, but those in human feces are of particular importance to human health. These include viruses of the genera Norovirus, Rotavirus, Enterovirus, Hepatovirus, Hepevirus, Mamastrovirus, and Mastadenovirus. Infections from these viruses may result in a wide range of illnesses (depending on virus type), including acute gastroenteritis, conjunctivitis, myocarditis, and infectious hepatitis. With the possible exception of Hepatovirus, each genus consists of several species, which may be further subdivided into distinct genotypic or serological groups; altogether,1 there are more than 200 human enteric viruses.2–6 These viruses most frequently transfer from person to person via the fecal-oral route. Because these viruses are excreted with feces in relatively large numbers, they are present in domestic sewage and, therefore, may be present in sewage-contaminated surface and ground waters7 that are used as sources of drinking water.
In temperate climates, some viruses demonstrate seasonality (e.g., enterovirus concentrations in sewage peak during late summer and early fall). One notable exception may be rotaviruses. The incidence of rotavirus disease increases in colder months, but quantitative information on rotaviruses’ seasonal patterns of occurrence in water and wastewater is lacking because these viruses cannot be assayed readily with conventional cell culture techniques. Nevertheless, several studies using modern molecular techniques show that domestic sewage contains many types of viruses year-round.8,9
Culture-based methods may be the gold standard for detecting and characterizing viruses, but many remain recalcitrant to culture. For example, human noroviruses have not been cultivated in any cell cultures, although immunochemical assay methods have been developed to detect them as antigens.10–12 Human rotaviruses and hepatitis A virus have been cultivated recently in cell cultures, but the techniques are difficult and require concomitant use of polymerase chain reaction (PCR) and immunoassays (e.g., immunofluorescence) to detect virus growth or gene probes.13–17
Rotaviruses mostly infect infants and young children, while noroviruses infect adults. Infection rates vary considerably from area to area, depending on sanitary and socioeconomic conditions. Viral pathogens are not normal flora in the intestinal tract; they are excreted only by infected individuals. The concentration excreted, however, is usually several orders of magnitude lower than that of coliform bacteria. Enteric viruses multiply only within living, susceptible cells; their numbers cannot increase in sewage. Sewage treatment, dilution, natural inactivation, and water treatment further reduce viral numbers. So, while massive sewage contamination of a water supply may cause large outbreaks of waterborne viral disease,17 the incidence of waterborne viral infections in technologically advanced nations depends on whether minimal quantities of viruses can produce infections.
Researchers have demonstrated that infection can be produced experimentally by a very few virus units,18,19 although the risk of infection increases with increasing ingested doses.20 The risk of infection for an individual living in a community where the water system contains very few virus units has not been determined.21 However, risk analysis has suggested that low numbers of enteric viruses in a drinking water system could result in significant infection risk.22 The percentage of individuals who develop clinical illness may be as little as 1% for poliovirus and as much as 97% for hepatitis A.22
In the United States, most recognized waterborne virus disease outbreaks have been caused by cross-connections and intrusions in distribution systems, and sewage contamination of untreated or inadequately treated private and semi-public water supplies. Virus disease outbreaks associated with community water supply systems usually are caused by contamination via the distribution system.23–26