Giardia is one of the most common causes of diarrhoea in North America . Like Cryptosporidium, it is also a waterborne protozoan cyst. Unlike Cryptosporidium, Giardia, which is about 5-6 microns in diameter, can be killed by being exposed to long contact with chlorine disinfection. Filtration is also an effective method for trapping and removing this microorganism from the water supply. As with Cryptosporidium, a combination of treatments works best in keeping this parasite away from our drinking water.
Ingestion of water contaminated by Giardia in healthy individuals can cause such symptoms as severe diarrhoea, nausea, anorexia, and fever. These symptoms can last from days to weeks. For those with weakened immune systems, antibiotics are necessary in order to rid the body of this parasite.
The good news is that tap water coming from your faucets at home are probably not contaminated by Cryptosporidium and Giardia, since it has been through a lot of filtering and disinfection before the water actually gets to you. Ozone and filtration, when used together remove at least 99.9 % of the Giardia cysts that may be present.
The bad news is that, although rare, there have been occasions where, despite treatment, microorganisms like Cryptosporidium or Giardia make their way into the water supply. Examples of this are contamination caused by activity of animals in the watershed area of the water supply or the introduction of sewage into the water supply.
Treated water stored in open reservoirs is also subject to contamination due to storm runoff.