Giardia is slowly inactivated, but it is inactivated even in normally chlorinated water. With the FC at around 10% of the CYA level, a 3-log reduction (99.9%) kill takes around 232 minutes so nearly 4 hours. Basically, half are inactivated every 23 minutes. So in a chlorinated pool, this probably wouldn't prevent person-to-person transmission of this pathogen if there were an incident and you were nearby and drank some pool water, but a full-fledged ongoing multi-day outbreak would very likely be prevented.ptbrunet said:What about someone with Giardia?
In a commercial/public pool, personnel are supposed to be on the lookout for formed stools or diarrhea incidents and to close the pool when they occur. Of course, this isn't always possible to see and is why one gets outbreaks. In practice, if you look at the Cryptosporidiosis Surveillance compared to the Giardiasis Surveillance and you look at Surveillance for Waterborne Disease, you will find the following:
From the graphs in Figure 4 in the Surveillance for Waterborne Disease report, it shows that in treated recreational water in 2007-2008, 82.9% of the gastrointestinal illness outbreaks were due to Crypto alone while 4.3% were Giardia and 1.4% were both. Table 7 shows there were 60 Crypto outbreaks with 12,492 cases while there were 3 Giardia outbreaks with 19 cases and 1 combination outbreak of Crypto and Giardia with 19 cases.Cryptosporidium has become the leading cause of reported treated recreational water–associated outbreaks of gastroenteritis.
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During 1999–2008, Giardia was identified as a causal agent of eight (3.5%) of 228 reported recreational water-associated gastroenteritis outbreaks.
This paper shows that 6.2% of pools without outbreaks had Giardia while 1.2% had Crypto giving further evidence that Crypto is more likely to cause an outbreak when it is present. This may be partly due to its greater infectivity, but is also due to its greater chlorine resistance. If an incident remains undetected, it may take weeks before a correlation of later gastrointestinal illness is associated with swimming in a particular pool. That's why you see so many more Crypto outbreaks and that they affect so many more people.