Chlorine level to kill ringworm fungus?

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PoolESQ

PoolESQ

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Dec 3, 2007
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Albuquerque, NM
Trying to set aside my complete anger at someone letting their kid with ringworm come to swim in our pool. An adult happened to ask the parent, "what are those marks on your daughter's back?" "Oh, it's just a fungus - ringworm." !!!!!! So here we go. Parents immediately removed kids from pool. Have thrown away the towel she used to dry off.

To what level, if any, do I need to raise the CL to make sure this fungus is dead? How long will it take before it's safe to swim again? Is our whole weekend now ruined from being able to use the pool??
 
From what I know, ringworm requires direct contact to spread. Unfortunately, this does not just mean person to person contact. It will spread to animals as well, and then they can further spread it. In a pool though... with chlorine... I am pretty certain you are in the clear.
 
Normal maintenance levels should be sufficient to kill the fungus very quickly in the pool. Would not hurt to bring it to shock level and give the pool a good scrubbing, especially ladders, railings and diving board if you have one.
 
The ringworm fungus itself is not so difficult to kill with chlorine. As noted in this old 1933 paper, 0.4 ppm FC in swimming pool water (that would be without CYA in those days) effectively did a complete kill in around 2 hours. Our pools with the minimum FC/CYA ratio have an equivalent FC with no CYA of at least 0.06 ppm in non-SWCG pools so that would be a complete kill in around 13 hours. At SLAM levels that are equivalent to around 0.3 ppm FC with no CYA, the complete kill would be in under 3 hours. So a SLAM is the reasonable thing to do.

However, it's the spores that are more difficult and these are more like other spores or oocysts that are more resistant to chlorine. The test described above used scrapings of live active fungus from a patient and while that is the most likely mode of transmission in a swimming pool, it is also possible for spores to be transmitted and the experimental procedures they used for culturing may not have activated spores (i.e. it was more for testing the viability of live fungus). However, unlike the protozoan oocyst Cryptosporidium parvum where the infectious dose is around 10 oocysts (see this link), exposure to dermatophyte spores does not always lead to infection (see this fact sheet). The infectious dose for some foot fungus is on the order of 280 for half the people to get infected (see this link). So water dilution will likely prevent transmission of this disease if only one infected individual were using the pool.

If one wanted to ensure getting rid of spores or oocysts from a pool, one could use chlorine dioxide, but that's not a normal procedure and should really be something reserved for emergency handling of a diarrheal release thought to contain Cryptosporidium parvum.

This chapter in a book goes into a lot of detail on skin fungus infections including ringworm. The pictures with extreme cases are pretty awful.
 
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chem geek - I've read (not so much understood completely) the reference materials you linked to. What is the difference between the fungus itself and the spores? And why are spores more difficult to kill with chlorine?
 
There are several different types of microorganisms that have (at least) two primary states: an active state where they reproduce, grow, metabolize, etc. basically being what you'd normally attribute to being alive but where they are more susceptible to being killed, and an inactive state where there is no growth and the organism is like being in hibernation and is also surrounded by a thick shell that protects the organism from the environment. Some protozoa have these states and the inactive one has the organisms called oocysts. Some fungi and algae have these states and the inactive one has the organisms called spores.

The primary purpose of oocysts or of spores is to be able to be spread into the environment that would otherwise be harsh and could kill these microorganisms. Technically, plants also use this mechanism but they group their spores into bunches that we know as pollen or seeds. Oocysts and spores are harder to kill from chlorine because they have thicker protective cell walls and are not metabolizing so have slower transport of chemicals (even water) from outside to inside the oocyst or spore. The reason chlorine dioxide does better is that it is a dissolved gas and is less reactive to the cell wall material so penetrates more deeply whereas chlorine tends to get hung up reacting with the cell wall material.

For ringworm, the fungus on the skin will be mostly active fungus but it will also produce spores that shed and can get on bedding and other materials so that it can be transmitted more easily from person-to-person without having direct contact with the infected area. In a swimming pool, there may be shedding of both live fungus (along with a person's surface skin cells) and spores, but the spore numbers are likely to get so diluted that they would not be an issue for transmission from one infected person in a residential pool to others. This paper showed that for two types of fungal spores, they were not killed in over a week in a chlorinated swimming pool while two other spores only survived for 24 and 48 hours, respectively. The good news is that the spores associated with human infection were more susceptible than those from other species.
 

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Very interesting! I had no idea there were two ways a fungus could be transmitted. But it makes complete sense when you think of it in the terms you've described. The journal in your link says that for two species of fungus, a single spore landing in an abrasion or hair follicle can set up an infection! I'm not going to ask - but am highly impressed - you found a relevant study from a Thai medical journal. Talk about a research superhero..haha. Thank you again!
 
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