By Kelly A. Reynolds, MSPH, Ph.D.
From 1971 to 2000, seven percent of the 751 documented waterborne outbreaks in the U.S. were due to human viruses. In about half of these outbreaks, no causative agent was identified; however, the event characteristics were consistent with viral etiologies. Therefore, viruses are thought to be the primary agent of waterborne disease outbreaks. Yet today, monitoring our water supplies for human enteric viruses is not practical, as current detection methods are difficult, expensive and time consuming. A variety of indicators, including physical, chemical and biological markers, have been used to monitor water quality and treatment efficacy but have not always correlated well with human virus presence. A recent study adds to the mounting evidence that more reliable indicator systems are needed to protect human health. The recommended use of a group of bacterial viruses (coliphage) continues to gain momentum.
More than 140 different types of viruses are known to infect the human intestinal tract and are subsequently excreted in feces. Enteric viruses associated with polluted waters include the enterovirus group (poliovirus, coxsackievirus and echovirus), hepatitis A virus, rotavirus, adenovirus and norovirus. These viruses are responsible for a wide range of illnesses including meningitis, paralysis, myocarditis, hepatitis, encephalitis, diabetes, respiratory illness and diarrhea.
Viruses and other pathogens find their way into the environment via many routes: municipal waste disposal, septic tank seepage, storm water runoff, wastewater reclamation practices and recreational bathers, just to name a few. Transmitted by the fecal-oral route into drinking water, low numbers are able to initiate infection in humans. In fact, the infectious dose may be as low as one culture-able organism. Therefore, virus monitoring tools must be able to detect low pathogen concentrations in large water volumes.
Viable human viruses have been detected in surface waters, ground waters and even treated drinking waters. A recent nationwide survey, using molecular detection methods, found that one third of all drinking water wells used by utilities tested positive for human enteric viruses.1 Although the molecular methods used do not determine if the viruses were infectious, their presence indicates a potentially increased level of exposure risk. Although other pathogens are also present in wastewater, viruses cause the greatest concern due to their small size and long-term survival. They have been documented to travel through the subsurface for up to 100 meters (328 feet), much further than fecal bacteria or other pathogens.
Post-treatment contamination of drinking water is also a concern where intrusion can occur, allowing viruses to enter the distribution system after which there are no additional barriers prior to consumption. During such events, residual chlorine levels in the distribution water are known to be ineffective at inactivating the introduced viruses and current monitoring methods often do not correlate with other health-related contaminants.
Reliability of indicators
Typical indicators used in water quality monitoring include nitrogen and phosphorous (markers for the presence of nutrients); chlorophyll-a (a marker for algal blooms); suspended solids and turbidity (water clarity indicator); dissolved oxygen (an indicator of the oxygen available to aquatic organisms); pH (a measure of the acidity or alkalinity of the water); conductivity (a measure of the salinity of water); and coliform bacteria, among others. Monitoring the physical, chemical and biological markers of a particular water source provides a means to determine the overall quality of the source water without directly monitoring the infinite number of potential toxicants that may be present.
Internationally, bacterial indicators, such as total and fecal coliforms, are used to monitor and predict drinking water quality related to microbial contaminants. Guidelines and standards have been developed by the U.S. Environmental Protection Agency (EPA), the European Union (EU) and others. Coliform bacteria are used by drinking water laboratories for microbial analysis of potable water, but are also used to evaluate food, pharmaceuticals, distribution lines, treated effluents, bottled water, groundwater, marine water and other environmental samples. In June 1989, the U.S. EPA published the Total Coliform Rule (TCR), requiring all public water systems to monitor for the presence of coliforms in their distribution systems. Total coliforms are regulated by U.S. EPA standards where zero is considered the maximum contaminant level (MCL) goal. The enforced MCL is that no more than five percent of samples can be coliform-positive in a month. Every sample that’s positive for total coliforms must be analyzed for fecal coliforms. No fecal coliforms are permitted in any sample.
Bacterial indicators have been highly effective for indicating presence of disease-causing bacteria, such as those associated with typhoid, dysentery and cholera. The use of bacteria to ensure the safety of drinking water is questioned, however, in regard to predicting the presence of viral and protozoan pathogens (see On Tap, September 2003). This may be due to a number of factors including the different survival, transport and growth characteristics of viruses, bacteria and protozoa.
Promising phage indicators
Coliphages are viruses that infect E. coli bacteria (yes, bacteria get viruses, too). They tend to persist wherever their requisite host is found, thus coliphage, like E. coli (a classic fecal bacterial indicator organism) are consistently present in the guts of warm-blooded animals and excreted in feces. The bonus is that they are viruses and more closely mimic the characteristics of fate, transport and survival of human viruses in the environment, making them an attractive option for an indicator of such contamination in water. Even better, coliphage are much easier to detect in environmental samples than human viruses.
There are two types of coliphage: somatic phage, infecting via the bacterial cell wall and male-specific phage (also known as F+ coliphage), infecting via the bacterial pili (a tail-like extension). Identification of specific pollution sources is possible with F+ coliphage monitoring since predominant groups, differentiated genetically, are linked to either human or animal wastes.2
A recent study found that F+ coliphage were suitable for monitoring distribution system integrity, a major concern considering that 19 percent (n=120) or the 751 documented outbreaks from 1971-2000 were due to contamination in the distribution system. In the study, F+ phage were detected in 5.6 percent of the 2,471 samples collected during an intensive study of a single water company, compared to 0.01 percent positive for total coliforms and no samples positive for E. coli.4 Also of interest is that the vast majority of the coliphage positive results corresponded to main breakages documented within 72 hours prior. Viruses and other pathogens, potentially contaminate drinking water following negative pressure transients that occur frequently in distribution systems.3 Current monitoring requirements do not routinely include human viruses or coliphage. The authors express confidence that routine survey of coliphage indicators in the future will increase, considering that the U.S. EPA is in the process of approving standard methods for their detection in source waters.
Letting go of the past
No indicator has proven perfect, but routine use of alternative indicators, in particular specific coliphage, is long overdue. While coliforms will continue to be useful monitoring tools, the total coliform group lacks specificity with regard to health risk; fecal coliforms do not necessarily indicate recent fecal contamination or correlate with major of enteric pathogens. Although more specific and more indicative of fecal contamination, E. coli is a poor indicator of viruses and protozoa able to survive for much longer periods of time in the environment. Many scientists are calling for more reliable monitoring methods for the future.
- Abbaszadegan, M. 2002. Viruses in Drinking Water and Ground Water. Encyclopedia of Environmental Microbiology. John Wiley & Sons, New York, NY.
- Cole et al., 2003. Evaluation of F+ RNA and DNA coliphages as source-specific indicators of fecal contamination in surface waters.
- Federal Register. Guidelines Establishing Test Procedures for the Analysis of Pollutants; Analytical Methods for Biological Pollutants in Ambient Water. 68: 139. July 21, 2003: www.epa.gov/fedrgstr/EPA-WATER/2003/July/Day-21/w18155.htm
- LeChevallier et al., 2006. Colliphage as a potential indicator of distribution system integrity. Journal of the American Water Works Association. 98:7:87
- LeChevallier et al., 2003. Potential for health risks from intrusion of contaminants into distribution systems from pressure transients. Journal Water and Health. 1:1:3.
- Reynolds, et al., 1996. Detection of infectious enteroviruses by an integrated cell culture-PCR procedure. Applied Environmental Microbiology 62:1424.
About the author
Dr. Kelly A. Reynolds is an associate professor at the University of Arizona College of Public Health. She holds a master of science degree in public health (MSPH) from the University of South Florida and a doctorate in microbiology from the University of Arizona. Reynolds has been a member of the WC&P Technical Review Committee since 1997. She can be reached via email at [email protected]