How To Safely Manage Legionella Control And Aquatic Life

The audits at FCS (including those for Legionella pneumophila management) are risk-based.  If and when there is scientific evidence to the contrary we have and will amend our recommendations.

In this article we are going to discuss any natural-type freshwater feature that has a means to routinely generate aerosols. Here the water feature will be any artificial pool, pond, lake, moat or fountain that contains fish and/or water-plants, algae, water snails and any other similar organisms that cannot be dosed with chlorine or other typical chemical sanitisers used to control the number of Legionella pneumophila bacteria without causing harm to the organisms therein.

The second factor is the ability to generate aerosols, such us by spraying water through a nozzle, allowing a stream of water to impact the water surface from some height or a means to blow air bubbles through the water for aeration.

We believe that the main issue is indeed whether the water feature produces an aerosol or not.

Whether the fountain is outdoors or inside and the temperature of the water both have a bearing on the issue but they are secondary to this key fact.

How Does Legionella Disease Infections Happen?

As a quick re-cap,  the following chain of events need to take place for an outbreak of Legionellosis:

1. Legionella pneumophila (usually Sero-group 1) bacteria must be present in the water.

These bacteria are naturally occurring in the environment and they cannot always be detected by any of the standard testing methods since they can hide in amoebae cells. For which there are no readily-available testing methods at any commercial lab to our knowledge). They also tend to frequent bio-films and are usually at low concentrations in nature.

A negative Legionella pneumophila test result for any pond/fountain is therefore no proof that they are not there.

Indeed we had a client in Cape Town some years ago who experienced an actual outbreak and yet their water samples had been consistently negative for the months leading up to the outbreak, using an accredited laboratory. As the philosophers state, “absence of evidence is not evidence of absence”. We cannot assume a pond or fountain does not have them.

2. The bacteria need to multiply to an infectious dose.

For this they need food, the right temperatures; and time. Since Legionella can grow on many substrates such as slime, scale, dirt and the like, a natural-type water-body will have plenty of suitable substrates for them.

This leaves the temperature. It is indeed true that Legionella pneumophila are not happy to grow below 20 ºC, though they can survive happily at cooler temperatures.

However, I have my doubts whether most water-features in South Africa can remain below 20 ºC through the long summers when most of the country has temperatures that fluctuate from lows not much below 16ºC to highs of 30ºC or more for months on end.

Since the much larger body of water in a swimming pool can easily reach 24ºC under these conditions I am not convinced any fountain or similar feature will stay below 20 ºC all the time.

3. The Bacteria Need to Infiltrate the Lungs.

This leaves the last step: the organisms must be present in the required numbers in an aerosol spray that people can breathe in.

Prevention is Better than Cure

Taking these in reverse, we note that it is very rare (indeed, almost unheard-of) to get legionellosis from a traditional fish-pond, lake or dam. But this is primarily because of two factors: there is almost no aerosol generation (at least none near any humans) and the volume of water in most cases is so large that the Legionella pneumophila cells almost certainly present are diluted down to very low doses.Neither factor applies to artificially-constructed but natural-type water features described above that are near large numbers of people. The volume of water is relatively small and aerosols are being generated.

Thus the standard FCS recommendation to ALL clients in a similar situation is:

  1. Eliminate the aerosols (i.e. remove any parts that can generate spray)
  2. Sanitise the water to prevent large numbers of Legionella pneumophila ever developing. Of course traditional sanitisers like chlorine are not compatible with the natural organisms, which then need to be removed.

On that last point, there are apparently alternative sanitisers that are compatible with the fish and plants: however, we are not too familiar with these and cannot say how effective they would be against Legionella. All the standard guidelines tend to focus on chlorine and other strong oxidisers. So to sum up in one phrase: If it has fish it can’t be a fountain and if it’s a fountain it can’t have fish.

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