Considering that 30% of the US population visits a coastal area beach each year, 42% use recreational water sites and these locales account for 85% of US tourist revenue, you'd think we'd know a lot more about the hazards of recreational water than we do. When a beach is closed, most of the time it is because the waters there have exceeded the state allowed levels of fecal or enteric coliforms (a group of organisms whose presence is thought to signal potential contamination by the feces of warm blooded animals. These organisms are not the pathogens, however, only indicators. Pathogens could be other bacteria, viruses or protozoal parasites. The two papers from the Hopkins group undertook to see if there was a relationship between pathogenic parasites and the number of people at the beach. The answer is not obvious. If the source of pathogens is animals (e.g., aquatic birds or run-off from surrounding land) then there might be little relationship. However even if there is a relationship this does not automatically mean humans are the source. They might just be stirring up the sediment where the bugs are hanging out. The more people, the more the sediment gets stirred up. Wastewater discharges (e.g., combined sewer overflows) is another other possible source.
The parasites in these two papers were of two categories, the known pathogens Cryptosporidium parvum ("Crypto") and Giardia lamblia ("Giardia"); and the more recently discovered microsporidia, Enterocytozoon bieneusi and Encephalitozoon intestinalis. The latter two have been known to cause human disease, especially (but not exclusively) in immunocompromised individuals, but they are hard to test for and identify so we know little about their epidemiology, including where those infected encountered them. Crypto and Giardia, on the other hand, have caused numerous waterborne disease outbreaks (including the largest in US history in Milwaukee in 1993, from Crypto) and both are highly resistant to chlorination. Their cysts can remain viable in a hostile environment as long as a year.
These papers are taken up with technical details relating how the samples were taken, handled and analyzed. These are important because much still needs to be worked out and variations in how these things are done is necessary to compare one study with another. But the bottom line is relatively simple: For either class of parasite there was a significant relationship between levels and the humber of bathers. Samples were taken on busy weekends and compared with lightly populated weekdays. Almost half the samples had microsporidia and levels of Crypto ranged from zero to 42 cysts/liter and Giardia from zero to 33 cysts/liter. How much of this stuff does it take to make you sick. For the microsporidia we aren't sure. We'll need human volunteer feeding studies to nail this down. For Crypto and Giardia it depends on the strain. In some cases the answer is less than 10 cysts.
What is the source? Again, we still aren't sure. These studies show clearly that the turbidity or cloudiness of the water is directly related to bather density. So stirring up sediments is happening and this could be the source. But bathers are also sources. It has been estimated that anal fecal residue shed to water amounts to 0.14 g and may be as high as 10 grams (this is from a paper by Chuck Gerba, Quantitative Microbiology 2:55-68, 2000). Despite this estimates of Crypto and Giardia risk from swimming in lakes is estimated to be pretty low (less than one in ten thousand).
But one of the things these studies show is that there are pathogens in recreational waters even when they meet standards deemed acceptable by state and federal bacterial standards. These beaches were legally open to the public. Thus the bacterial indicator standards are of doubtful protection. It further underlines the commonsense notion that public drinking water sources should be separated from recreational bathing locales.
And there are other possible interventions. Limit the number of bathers, prohibit children in diapers and advise anyone symptomatic to stay away
.
The parasites in these two papers were of two categories, the known pathogens Cryptosporidium parvum ("Crypto") and Giardia lamblia ("Giardia"); and the more recently discovered microsporidia, Enterocytozoon bieneusi and Encephalitozoon intestinalis. The latter two have been known to cause human disease, especially (but not exclusively) in immunocompromised individuals, but they are hard to test for and identify so we know little about their epidemiology, including where those infected encountered them. Crypto and Giardia, on the other hand, have caused numerous waterborne disease outbreaks (including the largest in US history in Milwaukee in 1993, from Crypto) and both are highly resistant to chlorination. Their cysts can remain viable in a hostile environment as long as a year.
These papers are taken up with technical details relating how the samples were taken, handled and analyzed. These are important because much still needs to be worked out and variations in how these things are done is necessary to compare one study with another. But the bottom line is relatively simple: For either class of parasite there was a significant relationship between levels and the humber of bathers. Samples were taken on busy weekends and compared with lightly populated weekdays. Almost half the samples had microsporidia and levels of Crypto ranged from zero to 42 cysts/liter and Giardia from zero to 33 cysts/liter. How much of this stuff does it take to make you sick. For the microsporidia we aren't sure. We'll need human volunteer feeding studies to nail this down. For Crypto and Giardia it depends on the strain. In some cases the answer is less than 10 cysts.
What is the source? Again, we still aren't sure. These studies show clearly that the turbidity or cloudiness of the water is directly related to bather density. So stirring up sediments is happening and this could be the source. But bathers are also sources. It has been estimated that anal fecal residue shed to water amounts to 0.14 g and may be as high as 10 grams (this is from a paper by Chuck Gerba, Quantitative Microbiology 2:55-68, 2000). Despite this estimates of Crypto and Giardia risk from swimming in lakes is estimated to be pretty low (less than one in ten thousand).
But one of the things these studies show is that there are pathogens in recreational waters even when they meet standards deemed acceptable by state and federal bacterial standards. These beaches were legally open to the public. Thus the bacterial indicator standards are of doubtful protection. It further underlines the commonsense notion that public drinking water sources should be separated from recreational bathing locales.
And there are other possible interventions. Limit the number of bathers, prohibit children in diapers and advise anyone symptomatic to stay away
.
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