Are Existing Bacterial Indicators Adequate for Determining Recreational Water Illness in Waters Impacted by Nonpoint Pollution?

Abstract

Tens of millions of persons swim in coastal waters in the United States each year. In many instances, these waterways are contaminated with fecal pollution that can contribute to recreational water illness.' Unfortunately, monitoring recreational waters for disease-causing microorganisms is not as straightforward as it would seem. In addition to the well-known pathogenic bacteria such as Escherichia coli, Salmonella, and Shigella, protozoa including Cryptosporidium and Giardia and viruses, in particular noroviruses and adenoviruses, are also major contributors to recreational water illness. Protozoa and viruses persist in the environment for much longer periods of time than do bacteria, and virus transport through surface and subsurface water is both faster and farther than that of bacteria. The great diversity in pathogenic microorganisms that can be transmitted by water and the difficulty in developing and implementing detection strategies for all possible microbes has resulted in the selection of a few indicator organisms to determine the presence and magnitude in a waterway of microorganisms causing recreational water illness. These indicators include total coliforms, fecal coliforms, E. coli, and enterococci, many of which have been used as indicators for over 100 years. Several studies have shown a strong correlation between conventional bacterial indicators and human illness resulting from recreation in water contaminated by human sewage outfalls.2 However, nonpoint pollution is also a major source of contamination of marine and fresh water recreation areas, and other studies have concluded that there is a poor correlation between levels of classic bacterial indicators and the presence of sources of nonpoint pollution. Compared with point source pollution, nonpoint pollution sources are diffuse and heterogeneous, and they typically include agricultural runoff, malfunc tioning or poorly maintained septic tanks, leaking sewer infrastructure, as well as waste from wildlife such as deer and birds. In many instances, animals shed high numbers of bacteria, many of which are not of human health concem, resulting in false-positive interpretation of the magnitude of health risk when relying on classic bacterial indicators. Importantly, nonpoint pollution can also contribute to high levels of unmeasured poten tially pathogenic protozoa and viruses that can result in false-negative interpretations of the risk to human health when only bacteria are used to evaluate microbial water quality. Thus, a major challenge in assessing the risk to swimmers posed by nonpoint pollution is selecting appropriate indicators. One microbial group that shows considerable promise as a potential viral indicator are bacteriophage. Bacteriophage are viruses that infect bacteria. Although they do not cause illness in humans, many bacteriophages are similar in size, shape, and environmental persistence as human enteric viruses. In particular, male-specific coliphage show considerable promise as a more reliable indicator of viral pollution. Climatic events also are a major factor in assessing the level of risk to bathers of recreational water illness. After as little as 1 to 2 cm of rainfall, the levels of microor ganisms in water can increase by several orders of magnitude.3 Conventional sample analysis of water for classic indicators can take 24 to 48 hours. This delay in the