Contaminant of the Month: Cryptosporidium and Giardia

March 4, 2015

There are several species of Cryptosporidium and Giardia, and some can infect humans.

What they are:

  • Cryptosporidium and Giardia are the most common protozoa of concern in U.S. drinking water.
  • They are fecal, oral intestinal parasites of many warm-blooded animals.
  • There are several species of each, and some can infect humans.
  • Cryptosporidium has a more complex reproductive cycle, although both can survive in water or food environments, and they reproduce and become infectious in the host’s intestines.
  • Dimensions of Giardia lamblia (or G. intestinalis) cysts are about 8 µm by 14 µm with several whip-like flagella that help them move in fluids. Cryptosporidium parvum or Cryptosporidium hominis oocysts are about 3 µm by 6 µm.
  • Protozoa are considerably larger than bacteria that range from about 0.2 µm to 2 µm in width or diameter.
  • Exposure and infections occur from contact with contaminated recreational water or from food, and several outbreaks from drinking water have been reported.

Health effects:

  • CDC reported five giardiasis outbreaks with 67 cases, and two with 44 cases of cryptosporidiosis in 2009 -2010 from drinking waters or recreational waters.
  • Cryptosporidium oocysts and Giardia cysts cause gastrointestinal infections by colonizing the lower GI tract where they attach to the intestinal wall and reproduce.
  • About 20 percent of detected organisms in water are infectious. Some are not viable and certain viable organisms are not infectious.
  • Infection requires ingestion of about one to 10 organisms. Some infections are asymptomatic so some people are not aware they are infected.
  • Symptoms can include diarrhea and sometimes nausea, vomiting and fever.
  • The infections are usually self-limiting, lasting several days for healthy people; but they can be chronic or fatal for less healthy or immunocompromised people.

Water treatment:

  • Both survive well in cold water, but Cryptosporidium cysts are very sensitive to water temperature. A 2 log inactivation occurs in 70 days at 20o C, 35 days at 25o C and seven days at 30o C.
  • They are more resistant than bacteria to disinfectants; Cryptosporidium is completely resistant to chlorination.
  • Conventional drinking water treatment (coagulation, flocculation, sedimentation, filtration, etc.) is effective.
  • Chlorine, hypochlorite, chlorine dioxide, ozone and ultraviolet (UV) are effective for Giardia; chlorine dioxide, ozone and UV are effective for Cryptosporidium.
  • Microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO) are very effective for removing both microorganisms.
  • Conservative regulators give limited credit for even RO because of concerns about potential membrane leakage.
  • Appropriate POU and POE filters, and UV can remove both microorganisms.
  • Boiling of water, as in distillation, isanother effective means to kill microorganisms, including Cryptosporidium and Giardia lamblia.


  • Analyses for both microorganisms are available but are expensive and complex because 10 to 100 liters of water must be filtered and the organisms must be identified.


  • Drinking water regulations have reduced the waterborne disease risks.
  • Surface water treatment rules require filtration of almost all surface water supplies.
  • The LT2 rule requires surface classification of water supplies into "bins" based upon Cryptosporidium concentrations in raw water. Bin numbers determine required log removals.
  • The LT2 turbidity requirement is less than 0.3 ntu in 95 percent of filtered water samples per month and never to exceed 1 ntu. This is determined by continuous monitoring of each filter in the treatment plant. Virtually all surface water systems were in bin one and seven percent were in bin two, none were in three or four.

Dr. Cotruvo is president of Joseph Cotruvo and Associates, LLC, Water, Environment and Public Health Consultants. He is a former director of the U.S. EPA Drinking Water Standards Division.

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