Appendix 1C1: Indicator Organisms
This appendix provides further background information about indicator organisms and EPA's review of epidemiological studies. For a more complete discussion, refer to EPA's Implementation Guidance for Ambient Water Quality Criteria for Bacteria (USEPA, 2000).
1. Organisms That Can Indicate Fecal Contamination
Because many pathogens are not easily detected, indicator organisms are a fundamental monitoring tool used to measure both changes in environmental (water) quality or conditions and the potential presence of hard-to-detect target pathogenic organisms. An indicator organism provides evidence of the presence or absence of a pathogenic organism surviving under similar physical, chemical, and nutrient conditions. For fecal contamination, indicator organisms should (Sloat and Ziel, 1992; Thomann and Mueller, 1987):
- Be easily detected using simple laboratory tests.
- Generally not be present in unpolluted waters.
- Appear in concentrations that can be correlated with the extent of contamination.
- Have a die-off rate that is not faster than the die-off rate of the pathogens of concern.
Indicator bacteria are usually harmless, more plentiful, and easier to detect than pathogens (Wilhelm and Maluk, 1999). Methods are not currently available to culture or enumerate all the disease-causing organisms that might be present in natural waters. For example, viruses and protozoans are generally not used as indicators because of difficulties associated with isolating them and detecting their presence in environmental samples. The bacteria species chosen as indicators are indigenous to the intestines of warm-blooded animals and indicate the potential presence of dangerous pathogens that can cause human illnesses.
Use and reliability are two factors that states and tribes should consider when selecting a pathogen indicator. The lack of correlation between certain indicators and pathogen-caused diseases in humans, as well as the uncertain relationship between indicators and different sources of pathogens, is a limitation of bacterial indicators. A positive result for the indicator organism means that the indicator is present in the waterbody, not necessarily that waterborne pathogens are also present. The presence of an indicator might not indicate whether those pathogens (if present) are viable or capable of causing disease and whether the source of the contamination is humans or other animals.
Indicators vary in their ability to reliably predict potential risks to human health. Some indicators have been shown to have a greater statistical relationship to disease than others. Also, currentdisease from other pathogens that can cause skin, upper respiratory tract, eye, ear, nose, and throat disease (USEPA, 1999). More research on the use of other bacteria and viruses as indicators is being conducted at the federal, state, and local
levels. Despite variability in the ability of indicators to reliably predict potential risks to human health, EPA studies indicate that enterococci and E. coli are the most effective available primary indicators for predicting the presence of gastrointestinal illness-causing pathogens, and for marine waters, enterococci is most appropriate.
One area of current scientific debate is whether indicator bacteria react differently under various climatic and environmental conditions. Preliminary evidence suggests that E. coli and enterococci can be detected at tropical locales such as Puerto Rico, Hawaii, and Guam in waters where there is no apparent source of contamination from warm-blooded animals (USEPA, 1999). EPA and others are evaluating whether the current indicator bacteria grow and persist in natural tropical environments. If E. coli and enterococci are determined to propagate naturally in tropical conditions, EPA will conduct additional research to identify alternative indicators for tropical areas.
2. EPA's Review of Recent Epidemiological Studies
Since the publication of EPA's 1986 criteria, a number of studies related to bacterial indicators have been completed. Therefore, EPA reviewed relevant recent studies to determine whether the studies continue to support EPA's recommendation to use E. coli and enterococci as bacterial water quality indicators. EPA's review focused on the epidemiological studies that related swimming-associated health effects to marine and freshwater bacterial water quality using studies performed after 1984. (For a complete discussion of these studies, see EPA's Implementation Guidance on Water Quality Criteria for Bacteria, USEPA, 2000).
EPA's Office of Research and Development (ORD) concluded:
The epidemiological studies conducted since 1984, which examined the relationships between water quality and swimming-associated health effects, have not established any new or unique principles that might significantly affect the current guidance EPA recommends for maintaining the microbiological safety of marine and freshwater bathing beaches. Many of the studies have, in fact, confirmed and validated the findings of the U.S. EPA studies. There would appear to be no good reason for modifying the Agency's current guidance for recreational waters at this time (Dufour, 1999).
The new studies added an additional body of evidence that supports EPA's 1986 criteria. As a result of this examination, EPA determined that its 1986 water quality criteria for bacteria continue to represent the best available science and serve as a defensible foundation for protecting public health from gastroenteritis in recreational waters. EPA found no reason to undertake a revision of the criteria at that time (USEPA, 2000).
The following table includes the relevant findings of the research EPA reviewed that has been conducted on indicator organisms since 1986.
Table 1. Summary of Research Conducted Since 1986