RWIs: Framing a Healthy Perspective
By Jeff Williams and Nathan Kinney
The recent report from the Centers for Disease Control and Prevention (CDC) on "Microbes in Pool Filter Backwash" (May 17, 2013, Morbidity and Mortality Reports, 62 (#19): 385-388) certainly stirred up the mass media airwaves, and put something of a cloud over public perceptions of swimming pool safety, just as this year's season was entering full swing. Reminding bathers about personal hygiene as a part of public pool use is always a good thing, but this report's results readily became sensationalized by content-hungry news channels. A rather unsurprising set of microbiological findings on a limited range of pool filter samples got converted from what should have been a news story in the "dog bites man" category, into lurid coverage in the "poop in pools everywhere" style. It probably ended up doing little to focus public attention on the behavioral changes that CDC strives so hard to bring about.
And that's unfortunate because making such a beneficial and wholesome activity as swimming even more attractive, and encouraging participation in it, is critical to national efforts to get Americans off the couch, and doing something healthful, yet delightful. Raising fears about risks of what have come to be called "recreational water infections" (RWI) can be a setback to those efforts. That is what makes it important to put the MMWR data in a healthier perspective.
Interpreting 'High-Tech' Findings
CDC researchers collected 161 backwashed pool filter samples from Atlanta-area pools, and tested them for biological contaminants using DNA probes. In this way, filtered particulates ended up being highly concentrated in a small amount of backwash water. If you then use the powerful tool of DNA "fingerprinting," you will detect whatever is trapped on the filter medium with an extraordinary sensitivity. That turned out to be the case because many of the filters, doing their proper job, were sure enough found to have trapped bacteria, to the extent that more than half of the samples showed evidence of the presence of E.coli (commonly used as an indicator of fecal contamination). About the same proportion contained fingerprints of Pseudomonas aeruginosa, a microbe that can be human-associated, sometimes causing disease, but which can occupy and multiply in a wide range of environmental niches.
What these findings signify has to be framed by the limitations of the technology; DNA probes provide for quick and sensitive ID analysis of samples for specific microbes, but the meaning of results is limited without using other kinds of microbiological tools to arrive at an assessment of RWI infection risks. For example, the DNA technology will give a positive signal in the backwash water sample, whether the microbe is alive, and potentially infectious, or dead and harmless—killed in this instance most likely by the pool's chlorine sanitizer, doing its regular job, taking care of the inevitable microbial contamination that comes from the bather load in the pool. It would have taken a lot more microbiological procedures to get a handle on whether the E.coli or the Pseudomonas "fingerprints" truly represented a RWI risk and signified that the sanitation/filtration system was faulty or not properly maintained—the kinds of signals that properly should raise alarms and are often discovered in the aftermath of disease outbreaks. But there were no such outbreaks in the Atlanta area in the summer of 2012, the findings that garnered so much public attention were not indicators of wholesale failures of proper sanitation and hygiene, and the bacteria detected were not in the primary disease agent category (such as E.coli O157:H7). So, all the anxieties raised by the headlines were not necessarily in a good cause. Safe pool management practices were clearly working.
It's a fair proposition to make that adopting good personal hygiene practices could beneficially affect the amount of contamination that the sanitation/filtration system has to deal with. But it remains to be determined whether showering ahead of pool bathing would significantly reduce the rate at which ultra-sensitive DNA probing turns up positive signals of human-related microbial material in filter backwashes. It is certainly a reasonable notion, but it will be a challenge to test it out experimentally.
Significant, but almost lost in the high visibility of the "pool poop" story, was the detection in some samples of parasite cyst fingerprints—significant because in this instance even the best chlorine sanitizing system will not inactivate Cryptosporidium oocysts. They are most likely to have a human feces origin in pool water, and almost certainly would still be alive and infectious in the filter. Of course, filtering them out, and regularly backwashing them into the septic outflow is another component of ongoing effective pool management. But here we encounter a higher risk to bathers, and much less certainty about the capacity of the filter medium to capture and eventually remove oocysts. Close enough to bacteria in size to be able to pass through the pore tunnels of commonly used filter media beds, these microbes are truly an RWI hazard, and one with explosive potential due to their high infectivity to humans; one serious contamination episode can seed a large pool with enough chlorine-resistant cysts to ensure that a mouthful of water contains an infectious dose. Almost all cases of Cryptosporidiosis in the United States each year—and there are usually thousands—are acquired from pool exposure, so there's good reason to take its presence in backwash samples, even at a low rate (2 percent), as a serious indicator of potential problems.
This risk is not readily diminished by adopting sanitizing alternatives to chlorine. For all its downsides, including objectionable smell, taste, eye and skin irritation, and disinfection byproduct generation, amongst others, chlorine serves up an unmatched array of advantages in maintaining pool safety; it deserves its centerpiece role in recreational water sanitation. That it does not cope with the toughness of Crypto oocysts, added to the other shortcomings, has led to clamor for more attractive and powerful replacements. That hasn't proven easy, and while practical alternatives are available they bring their own weaknesses to the challenge. Chemical agents with oocyst-inactivating power, such as chlorine dioxide and ozone, pose more operational demands on users than chlorine, and cannot compare in their inability to sustain biocidal activity in the main body of pool water that is so readily accomplished with chlorine residuals. Ultraviolet irradiation can destroy the infectivity of oocysts, but besides furnishing no residual protection to the recirculating water, it must operate by line-of-sight exposure to the inactivating rays. Any obstruction in the form of turbidity in heavily bather- loaded pools can undermine the utility of UV. And all the inactivating power of UV has to wait upon the arrival of germs over the course of the recycling turn of all the pool water; in the meantime they're free to cause trouble.
Clearer Is Safer
This brings us to the final piece of the framework for our perspective on the CDC report—water clarity as a contributor to pool water safety, regardless of the method of disinfection being deployed. Turbidity is the enemy of effective pool safety management, and efficient filtration is the way to counter it. Turbidity interferes with UV, consumes disinfectants (it's a "demand" for halogen, meaning less is available for sanitation), and it contributes to the risk of pool drowning accidents by obscuring the view of bathers who get into trouble and sink out of sight. Filter backwashes should contain evidence for what the medium is taking out—just as the CDC researchers found—but helping that process by flocculating suspended particles is a worthy goal, and it can be achieved as a part of regular pool management. There are plenty of products available to make that happen.
Encouraging a multipronged approach to pool sanitation, with proper attention to disinfection, clarification and filtration, as well as emphasizing the role of personal hygiene measures for all bathers, is likely to lead to more widely enjoyed and enjoyable recreational water experiences. Properly implemented, these kinds of pool management practices do work. That would be a nice message to put out at the start of the summer bathing season!
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