By Susan B. Rivera, Ph.D., CPO

Cryptosporidiosis is a diarrheal disease caused by microscopic parasites of the genus Cryptosporidium. Once an animal or person is infected, the parasite lives in the intestine and passes as an oocyst in the stool.

The oocysts are protected by an outer shell that allows the parasite to remain infective outside the body for long periods of time. The same outer shell – the oocyst wall – makes the parasite resistant to chlorination.

The most common symptom of cryptosporidiosis is watery diarrhea. Other symptoms include dehydration, weight loss, stomach cramps or pain, fever, nausea and vomiting. Both the disease and the parasite are commonly known as crypto.

How does transmission occur in a pool?
Unlike bacterial pathogens, Cryptosporidium oocysts are resistant to chlorine disinfection and can survive for days in treated recreational water venues such as public and residential swimming pools and community and commercial water parks despite maintenance of 1-3 ppm[1] chlorine residual.[2] Millions of oocysts can be released in a bowel movement from an infected human or animal.

Studies indicate that diarrheal fecal releases constitute the largest risk for oocyst contamination.[3] Formed stools, while unpleasant, are simpler to handle from a disinfection management standpoint.[4]

If a person swallows oocyst-contaminated water, they may become infected. Studies show that swimmers inadvertently swallow between 16 and 37 ml (over an ounce) of water when they swim. Children tend to swallow twice as much as adults.[5]

Game changing management practices in public swimming pools
In February 2008, the Centers for Disease Control and Prevention (CDC) revised the recommendations for responding to fecal accidents in disinfected swimming venues. Ct[6] values for Cryptosporidium oocyst inactivation were changed from 9,600 mg-min/L to 15,300 mg-min/L.

This means that pool operators following CDC’s guidelines, must adjust either chlorine dosages (and residual concentrations) or time in order to meet the guidelines. For an operator accustomed to superchlorinating to 10 ppm and waiting for 960 minutes (16 hours), he or she now must wait for 1,530 minutes (25.5 hours) to meet the guideline.

As incidents of cryptosporidiosis outbreaks are increasingly reported,[7] industry professionals are looking for methods to prevent infectious oocysts from entering pools. The CDC and the National Swimming Pool Foundation (NSPF) have established a cryptosporidiosis outbreak alert system. When either of these organizations becomes aware of an outbreak, a regional alert is broadcast to professionals and aquatics groups.[8]

Education regarding inadvertent contaminations by all patrons, but especially by younger patrons, has increased. Research indicates that children and babies are more likely to contaminate recreational water as a result of incontinence or greater perianal fecal contamination.[9]

However, all persons ill with diarrhea can contaminate water and should not swim, preferably for at least two weeks after diarrhea symptoms have ceased. People infected with cryptosporidiosis will excrete oocysts for several weeks after recovery from diarrhea.[10]

Because it is difficult to identify persons with diarrhea and even more difficult to enforce guidelines new management practices are being reported. In the summer of 2007, during a hot spell, a large outbreak of cryptosporidiosis severely affected Utah pool management practices.

In this case, children under five were banned at some locations while superchlorination was completed.[11] Other cities have closed multiple pools as a strategy to minimize the risk of transmission as infected patrons move to other venues. For example, in Phoenix, Arizona, 29 city pool facilities were closed for one week and superchlorinated at double the levels recommended by the CDC.[12]

On-site generation of mixed oxidant solution
A proven technology, on-site generation of mixed oxidant solution may pose the perfect solution for lowering the risk of contracting cryptosporidiosis at recreational venues. This allows for the use of current pool test kit methodologies for measuring free available and total chlorine levels (FAC and TC, respectively).

Chlorine-based mixed oxidants were originally developed for use in the municipal water market over 10 years ago. Both hypochlorite and mixed oxidant solution generators are available on the market. These oxidants are produced on site from salt, water, and electricity, and stored so that proper dosing may be achieved based on bather load and resulting free available chlorine (FAC).

This technology is not a salt-water chlorinator so the pool remains low in salt. Numerous studies show that mixed oxidants inactivate a wide range of microorganisms faster than traditional chlorination technologies at the same FAC residuals.[13] Included in this list of microorganisms is Cryptosporidium parvum oocysts. The improved inactivation rate is due to the way the oxidant is generated in the electrolytic cell. See reference 11 and references therein for more details on the technology.

The work of five independent laboratories has shown that when tests are performed correctly, mixed oxidants are consistently able to inactivate C. parvum oocysts over 10 times faster than chlorine alone.[14],[15],[16],[17],[18] Table 1 takes representative data from these studies to compare the mixed oxidant inactivation rate to that of conventional chlorine. At a reasonable residual of 5 mg/L as measured FAC, chlorine (as bleach) takes at least 10 times longer to achieve the same inactivation as does mixed oxidant solution. More work is needed to validate with independent laboratories.

Weighing the options and adopting a multi-barrier approach
As in the drinking water industry, a multi-barrier approach will provide bathers with the greatest level of protection. Several factors must be considered when choosing or designing a complete disinfection system. These include local, state and national codes and guidelines, chemical safety, and operational and maintenance costs.

Filtration is a physical means of removing oocysts. In general, sand filters are not effective, while diatomaceous earth (DE) filters have a good track record. Membranes may be an emerging technology that can be used for oocyst removal.

UV systems are effective at inactivating oocysts as long as the systems are maintained properly and the circulation systems are optimal. UV is not effective in the pool. The water must be circulated from the pool to the UV system before disinfection can occur. Even in a properly circulating pool, oocysts may not be exposed to the UV system for 2 days.

Outbreaks of cryptosporidiosis have occurred in pools using UV, further illustrating the need for on-demand disinfection in the pool.[20] Mixed oxidant disinfectants provide better protection from cryptosporidiosis than liquid sodium hypochlorite, stabilized forms of chlorine, and calcium hypochlorite.

Finally, mixed oxidant systems are inherently safer than many other technologies. No handling of concentrated chlorine is required, pool personnel do not have to wear protective gear, and there are no storage compatibility issues. An environmentally ‘green’ feedstock of simple food-grade sodium chloride salt converted on site via electrolysis is all that is needed. Because no chlorine dioxide or ozone is formed during the electrolytic process, chlorate and bromate are not formed as disinfection byproducts.

On-site generated mixed oxidant disinfectants may also be an acceptable solution for many aquatic facility managers who wish to address the emerging risk of recreational water illness, but who cannot afford to implement and maintain three stages of protection (filtration, chlorination and UV). With the rising costs of calcium hypochlorite (average is $2.65 per pound of FAC) and bulk sodium hypochlorite ($2.25 per pound of FAC), on-site generation of disinfectant is more cost effective at $0.65 per pound of FAC. Depending upon the system, anywhere between 3.5 and 5.2 kW are needed to generate one pound of chlorine. The annual maintenance costs for the system itself are minimal.


  1. The unit ppm (parts per million) is also represented as milligrams per liter (mg/L).
  2. Korich D. G., Mead, . R., Madore, M.S., Sinclair, N.A., and Sterling, C.R. 1990. “Effects of Ozone, Chlorine Dioxide, Chlorine, and Monochloramine on Cryptosporidium parvum oocysts viability.” Applied and Environmental Microbiology 56: 1423-128.
  3. CDC. “Prevalence of Parasites in Fecal Material from Chlorinated Swimming Pools — United States, 1999”. MMWR 2001;50(20):410–2.
  4. CDC. “Fecal Incident Response Recommendations for Pool Staff.” updated August 1, 2008
  5. Dufour, A.P., Evans. O.M., and Dehymer, T. D. (2006). “Water Ingestion During Swimming Activities in a Pool: A Pilot Study.” Journal of Water and Health 4(4): 425-430.
  6. The Ct value is the concentration (C) in mg/L multiplied by the contact time (t) in minutes necessary to inactivate a given percentage of the microorganisms present in the water. The Ct value tends to increase linearly with the percentage of microorganisms inactivated.
  7. Boone, R., “Cryptosporidium Outbreak Hits the West.” Associated Press, Sept. 21, 2007.
  8., downloaded from the NSPF website August 15, 2008.
  9. Gerba, C.P., 2000. “Assessment of Enteric Pathogen Shedding by Bathers during Recreational Activity and its Impact on Water Quality.” Quantitative Microbiology. 2:55-68.
  10. Cryptosporidium and Water: A Public Health Handbook. 1997 Atlanta, Georgia:Working Group on Waterborne Cryptosporidiosis.; accessed September 3, 2008
  11. Rizzo, R., “Parasitic epidemic: Kids 5-and-under Banned from N. Utah Pools.” The Salt Lake Tribune, August, 29, 2007.
  12. Ferraresi, M., and Smokey, SJ. “Pool may have Sickened 35; Facilities Closed Indefinitely”. The Arizona Republic, July 15, 2008.
  13. A summary of these studies can be found in the document entitled “Master Features Summary.pdf” located at in the library of documents
  14. Venczel, L.V., M. Arrowood, M. Hurd, and M.D. Sobsey, 1997, “Inactivation of Cryptosporidium parvum Oocysts and Clostridium perfringens Spores by a Mixed-Oxidant Disinfectant and by Free Chlorine”, Applied and Environmental Microbiology, 63(4):1598-1601.
  15. Biovir Laboratories Inc.,Reports, 2004. A summary of the microorganism inactivation studies conducted at BioVir Laboratories, Inc. can be found in the “BioVir Summary Report” located at in the library of documents
  16. Sasahara, T., Aoki, M., Sekiguchi, T., Takahashi, A., Satoh, Y., Kitasato, H., and Inque, M. 2003. “Effect of the Mixed-Oxidant Solution on Infectivity of Cryptosporidium parvum Oocysts in a Neonatal Mouse Model”, The Journal of the Japanese Association for Infectious Diseases, 77(2) 75-82.
  17. Yozwiak, M.L., W.L. Bradford, F.A. Baker, S.D. Olfers, M.M. Marshall and C.R. Sterling, 1994, “Effect of a Mixed-Oxidant Solution on Cryptosporidium parvum (oocyst) viability”, Poster Session, Proceedings of the 47th Annual Meeting of the Society of Protozoologists, June 24-29, 1994, Cleveland, OH.
  18. Bajszar, G., 2006, “Report: Analysis of Disinfection Efficacy of Mixed Oxidant Solutions Produced in MIOX Cap Units with Different Electronic Control Boards for DC Output”, M3C Systems, Albuquerque, NM, November 2, 2006.
  19. A summary Cryptosporidium ssp. inactivation studies using mixed oxidant solution can be found in the library of documents at The document is entitled “Crypto Inactivation.pdf”
  20. CDC MMWR Weekly Report. 2007. “Cryptosporidiosis Outbreaks Associated with Recreational Water Use—Five States, 2006” 56(29); 729-732.

About the author
Dr. Susan Rivera is the Manager of Research and Development at MIOX Corporation. She holds a Ph.D. in biochemistry from the University of Utah and a Certified Pool Operator (CPO) certification from the National Swimming Pool Foundation®. Rivera assists customers with pool disinfection management and has traveled overseas to consult on pool disinfection issues. She has been a member of the WC&P Technical Review Committee since September 2007.

About MIOX Corporation
MIOX Corporation, based in Albuquerque, NM, is focused on solving one of the world’s most pressing issues: the need for affordable, safe and healthy water. MIOX’s patented water disinfection technology replaces the need to purchase, transport and store dangerous chemicals. The company has over 1,500 installations, with equipment used in over 30 countries. MIOX equipment is used in recreational water venues, in hundreds of communities across the US for public drinking water systems, water reuse projects and a variety of commercial and industrial applications. More information is available at <>.



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