:: Legionella Control::

Legionella is an example of what is termed an “ environmental disease” which has basically been created by man and the industrialised world we occupy. There are numerous other such diseases that are constantly being brought onto the world stage.
We highlight the work done on legionella as an example of the power of chlorine dioxide to combat these “ environmental diseases”.

INTRODUCTION TO LEGIONELLA
The history of Legionnaires’ Disease is well documented in that the 58th  Annual convention of the American Legion’s Pennsylvania Chapter held their meeting in Philadelphia in July 21-24 1976 and 221 people got ill with 34 deaths. The bacterium, which caused this disease outbreak and deaths is now called legionella.

There are numerous references outlining the medical aspects of the disease, its medical diagnosis, conditions supportive of legionella growth, sampling and testing requirements even guidelines for acceptable and actionable limits for legionella in various systems including cooling towers, hot and cold water systems. Various organisations like OSHA, Association of Water Technologies and the WHO have all put out somewhat different guidelines as to acceptable or actionable legionella counts.

Research has clearly shown that controlling Legionella in the bulk water is not adequate to prevent a system from becoming a site of infection. Hence, any biocide that can only kill legionella in its planktonic ( free floating form) will not be effective to prevent a system from becoming a site of infection. Most biocides that have so-called efficacy against legionella have been tested against planktonic cells only.

CHEMICAL CONTROL OF LEGIONELLA
From a strictly chemical control perspective, any biocide ( we will use the term biocide to mean biocide, microbiocide, disinfectant, sanitiser, anti-microbial agent ) that is chosen for the control of legionella must be able to do five things:

1. Inactivate legionella and other bacteria in the bulk water
2. Control / prevent / remove biofilm and inactivate associated biofilm bacteria
3. Control / prevent / remove algae associated with biofilm
4. Inactivate protozoa
5. Inactivate cysts.

We will examine each of these points in more details hereunder.

1 Inactivate Legionella and Other Bacteria in the Bulk Water
Despite the diversity of opinions as to whether Legionella can grow in the bulk water or not, the fact remains that Legionella can be present in the bulk water. It is therefore important to be able to control Legionella that are present in the bulk water.
Most tests have been done on laboratory grown legionella and testing biocide efficacy in the laboratory. Few biocides cannot control legionella under these ideal circumstances. Research has shown that laboratory cultured bacteria have significant differences in their resistance to disinfection compared with those found in nature.

2 Inactivate Legionella in Biofilms
Biofilms have been shown to be a major source of legionella, a major source for other pathogenic bacteria, it can include suspended solids, corrosion products, fungi and alagae. Biofilms have been shown to be the source of corrosion, product wastage, a major problem in the health care industry and reduction of productivity in industry. It has been estimated that in the USA alone 0.03% of its GDP is lost because of biofilm formation.
The bacteria in biofilm have been shown to have significantly greater resistance to disinfection than those same bacteria found in the bulk water. In general, oxidising biocides are more effective than non-oxidising biocides in biofilm control and removal. However, it has been shown that chlorine, bromine and ozone have been found to be relatively ineffective in the control and removal of biofilms because they are considered to reactive. A general observation is that the weaker the oxidant, the more likely it is to be able to penetrate a biofilm, inactivate the bacteria within it and removal of the biomass. Chlorine dioxide has been shown to be one of the most effective biofilm control and removal products. As little as 0.18 mg / l Chlorine Dioxide residual is required to prevent the formation of biofilm. ( A biofilm and biofouling manual summarising the latest research is to be found in our web page at www.btcproducts.co.za in the “ downloads’ section).

BSRIA in their Technical Note TN 2/98 showed that chlorine dioxide was very effective in the removal of legionella from biofilm matrixes. There are numerous technical papers which have monitored the impact of chlorine dioxide treatment in hospitals over many years ---Dr Hood of Glasgow Infirmary study was over 6 years---and showed that upon the onset of chlorine dioxide treatment the incidence of legionella in the bulk water was reduced and over this period of time no outbreaks of legionella were recorded.

3 Inactivate Legionella in Algal biofilms
Research has shown the following : a) algae can be found in bacterial biofilms b) certain algae are known to provide suitable nutrients for the proliferation of legionella and c) legionella can grow more readily in the presence of algae thaninits absence.
Many biocides have very good effect on algae and chlorine dioxide is toxic to some algae at levels lower than 1 mg / l ClO2 residual.

4 Inactivate Protozoa
The interesting relationship between legionella and certain protozoa has been one of the more thoroughly and ongoing areas of research. It has been suggested that the survival, growth and amplification of legionella in nature will require the presences of protozoa. Protozoa cab be thought of as “ microscopic cows” that graze on the biofilm, ingesting bacteria as a food source. This process of ingesting bacteria is known as “ phagocytosis “. Unlike most bacteria, whaih are used as a food source by protozoa, the legionella can under specific conditions, after phagocytosis, multiply inside the protozoa. The legionella can cause the amoeba to lyse and thereby release large quantities of legionella into the bulk water system.
Few non-oxidising biocides have been tested and reporting amoebicidal properties. QAC products, particularly based on didecyl dimethyl ammonium chloride have been shown to be highly effective against protozoa. Most the work has been done on oxidising agents used in the drinking water arena and these oxidants like chlorine, chloramines, chlorine dioxide and ozone have been found to be particularly effective against protozoa.

5 Inactivate Cysts
A motile, feeding and replicating trophozoite form characterises all free-living amoeba and with many species being able to produce a cyst stage. Work has been shown that encysted protozoa are considerably more difficult to destroy than the trophozoite form.
The oocysts found in drinking water have resulted in considerable research where it has been shown that only ozone and chlorine dioxide are able to inactivate the cryptosporidium parvum cysts ans that even at 1 ppm chlorine dioxide can inactivate the giardia cysts. Chlorine is need atunacceptably high concentrations or contact times to inactivate giardia cysts.

LEGIONELLA CONTROL STRATEGIES
All the studies conducted to date provide us with valuable data about the legionella puzzle and when viewed together it will give us a clearer picture of the necessary strategies needed to adequately control legionella in various industrial processes. Legionella is not only confined to be found in cooling towers it has been found in potable water storage facilities ( both hot and cold water systems), emergency water systems eg safety showers, eye wash stations and fire sprinkler systems, spas / Jacuzzi / hydrotherapy pools, fountains / water fall systems, evaporative air coolers / misters / air washers/ sprayers and humidifiers, metal working systems. Basically, any industrial water holding or distribution system that can form biofilm, and or produce aerosol droplets for the convection of the disease can become an area for concern or an area to be treated in order to avoid the presence of legionella.

As can be seen above, only a few chemical control options for control of legionella are effective across the broad conditions favourable for the promotion of legionella. One of these control technologies is chlorine dioxide, which our company has in excess of 10  years of technological application data. Our partners in the UK have close to 20 years experience in the supplying of chlorine dioxide technology for legionella control in both the healthcare and industrial markets. Chlorine dioxide has proven to be very versatile in its utilisation as a control chemistry against legionella in that it can be used to a) disinfect potable water systems b) used to “ soak and disinfect” systems off-line c) can treat small and large water systems from hospital hot /cold water systems to industrial systems of varying types.Chlorine dioxide as shown above is approved for use in drinking water.
Here in South Africa people have tended to pay lip-service to the problems of legionella until an outbreak causes the deaths of one or more people.

However, the most effective control strategies are likely to be combination of two or more control chemistries depending upon the system and its application needs. E see the combinations of ozone / UV light together with a chemical like chlorine dioxide to provide a measurable residual in the water; combinations of wetting agents and biocides; non-oxidising and oxidising biocide combinations.

CONCLUSIONS ON CHEMICAL CONTROL FOR LEGIONELLA CONTROL
More and more research into biofilms and their effect on industrial processes are pointing towards the use of chlorine dioxide as one of the most powerful biocides available to solving these “ new frontier” problems ( presence of biofilms). The other major problem associated with biofilms and therefore legionella control is how does one measure their presence. As has been described herein the analysis of water samples does not give the whole story. Indeed, looking at only planktonic analysis can generate wrong conclusions.
If one cannot measure the problem how do we know if we have solved the problem other than through the passage of time. This passage of time could result in deaths of plant personnel / visitors or someone from the community, reduction in productivity, destruction of products and corrosion of plants. 
Our company together with the University of Pretoria believes that we will be in a position to offer industry a biofouling monitor which will monitor biofilm formation in real time, 24 hours per day, in-line and non-destructively. This unit will be cost effective as well as being able to measure the impact of the existing chemical control strategies for bioflm removal or legionella control