We're all familiar with how vaccines work: you get a small dose of a nasty virus or other pathogen, your body works out how to fight it, and then you're pretty much protected against coming down with the illness that the virus causes.
This semester, as part of the Water Resources Program at UNM, I am studying water reuse with Dr. Bruce Thomson. He assigned us an article by his colleague, Floyd J. Frost. Dr. Frost hypothesized that exposure to small doses of some pathogens in drinking water can actually provide a protective immunity against that particular disease.
Cryptosporidium is a persistent water borne protozoan that can cause serious gastrointestinal illness, even death. There have been a sufficient number of outbreaks of cryptosporidiosis in the U.S. in the last 15 years, that the EPA has published new rules under the Safe Drinking Water Act requiring water utilities to specifically implement treatment methods to remove this organism.
In response to this concern, Floyd developed a study to test whether people who are exposed to small doses of Cryptosporidium oocysts on a regular basis might be less susceptible to the gastrointestinal distress caused by a Cryptosporidium infection.
He studied people who got their drinking water from a good quality surface source. He tracked episodes of gastrointestinal illness among the sample populations and correlated those with levels of two antigens believed to be produced by the human body in response to Cryptosporidium. He found that those exposed to subclinical doses of Crypto as evidenced by moderately high levels of the antigens experienced significantly less outbreaks of gastrointestinal distress episodes.
More research is needed to fully understand what is happening. The study suggests that "a moderately strong serological response to a Cryptosporidium antigen group is related to a lower risk of enteric illness." (pp 812-813) The implication is that "[i]f future improvements in water treatment reduce serological responses for users of surface water, then the risk of cryptosporidiosis will likely increase. Thus, reducing low-dose waterborne exposures may increase rather than reduce the risks of diarrheal and gastrointestinal illnesses." (p813)
Obviously, this raises some difficult questions for those responsible for developing policy on water treatment standards. Can your drinking water be too clean?
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Special thanks to Dr. Bruce Thomson for his review and comments on this post. LRK.
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Floyd J. Frost, Melissa Roberts,Twila R. Kunde, Gunther Craun, Kristine Tollestrup, Lucy Harter, and Tim Muller. How Clean Must Our Drinking Water Be: The Importance of Protective Immunity. The Journal of Infectious Diseases. 2005. 191:809-14.
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This recalls the old saying, "What doesn't kill you makes you stronger." The same argument can be applied to our excessive use of anti-bacterial soaps, cleansers, etc.
About 15 years ago, Dennis Goldman, then of the National Ground Water Association, wrote a paper that he and his co-author (whose name I cannot remember) titled "Engineered Immune Deficiency Syndrome" about the very subject you mention. As far as I know the paper was never published - complaints over the title, for one thing.
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