Too Dangerous to Dip?

Marine Pollution Makes Swimmers Sick

Reports of illness after exposure to marine water appear to be increasing, and there is evidence that the rate of infection is proportional to both the amount of time swimmers are exposed and the levels of pollution in the waters where they swim. In this issue, researchers from Harvard University led by Sarah E. Henrickson conclude that human activities are contributing to illnesses associated with marine recreation [EHP 109:645-650]. The team also suggests ways to reduce the incidence of illness by both monitoring pathogens to prevent human exposure and correcting environmental conditions that boost pathogen concentrations in marine waters.

Marine pollution is increasing: up to 90% of untreated sewage from urban areas in developing nations is dumped into oceans and streams, and high nitrogen loads from agricultural runoff and other types of pollution may upset the ecological balances that normally keep pathogens in check. Warming oceans and extreme weather events also contribute to the growth of algal blooms that can harbor pathogens.

Research reviewed by the Harvard team suggests that bacterial pathogens can survive in salt water, especially in waters with low salt concentrations (such as those found in estuaries) or high nitrogen loads. For example, marine species of Vibrio bacteria have been linked to disease in swimmers, including cases of gangrene and endometritis. Outbreaks of noninfectious disease are also of concern. These include various types of "swimmer's itch" caused by marine larvae, and neurologic symptoms from exposure to algal toxins. Swimmers can be exposed to pathogens through broken and unbroken skin. Exposure to algal toxins can also occur through the lungs when the toxins are aerosolized by heavy surf.

Although public health authorities most often test for bacterial contamination, the authors' review of 131 articles suggests that the primary health hazard in coastal waters may not, in fact, be bacterial exposure, but viral instead. The authors examine, for example, one randomized controlled trial of swimming exposure conducted in the United Kingdom in which swimmers became ill even on days when concentrations of fecal coliform bacteria were at acceptable levels. Viruses are extremely abundant in marine waters, where they often survive longer than bacteria. They are also more resistant to sewage treatment. The authors speculate that viruses may be a chief cause of swimming-associated gastrointestinal infections.

The Harvard team concludes that bacterial indicators are inadequate surrogates for overall pathogen levels and that surveillance methods that focus only on these indicators are no longer adequate to protect human health. They recommend the development of surveillance methods using polymerase chain reaction, fluorescent antibody, or monoclonal antibody techniques. They also recommend monitoring algal populations. To reduce the number of pathogens in marine waters, they recommend reducing nutrient loads through maintenance of wetlands that filter runoff, an increase in the efficiency of sewage treatment and other urban waste disposal systems, and reductions in the use of fertilizers. They cite the example of Sydney, Australia, which greatly reduced health hazards at its beaches by extending sewage outfalls to the edge of the continental shelf. The authors also state that reducing emissions from fossil fuel combustion would decrease the amount of nitrogen deposited in marine ecosystems and help prevent long-term warming of the world's oceans.