The Naegleria fowleri (N. fowleri) microbe, a pathogen known to be fatal to humans, isn't one you want swimming in your water supply. It's a pathogen that's been known about and treated for since the 1960s but a recent study conducted by CSIRO has provoked a rethink on whether current water chlorination guidelines deliver a sufficiently deadly dose to these killer amoeba.
In 1965, Australian physicians Malcolm Fowler and Rodney Carter linked1 an unknown amoeba with a fatal case of meningitis in Adelaide’s Children’s Hospital. Following that, the disease came to be distinguished as primary amoebic meningoencephalitis (PAM), an illness that’s since claimed the lives of 350 people worldwide, including 19 Australians.
So far, N. fowleri has been found in drinking water distribution systems in Australia, the United States and Pakistan. The good news is that you cannot become infected by drinking a glass of water containing the amoeba. Infection is the result of contaminated water coming into contact with the olfactory nerves in the nasal cavity, rather than through drinking or bathing. From there the amoeba creeps its way into the central nervous system, where it survives on body tissues.
With a shorter distance between their nasal membranes and brain tissue, not to mention the increased chance of water getting into their nose in the first place, children are at a greater risk of infection. A recent tragedy, in which a 12 month old boy in north-west Queensland died from PAM after playing with a garden hose, was covered in the documentary Out of the Water by the ABC's Australian Story, which aired in November 2015.
Symptoms can take up to a week or so to develop. When they do, however, they can be quickly fatal, within as little time as a day. While the disease seems to be relatively uncommon, a fatality rate of 95 percent makes the presence of N. fowleri in our water supplies a serious concern.
Since the early 20th century, the main strategy for dealing with any disease-causing microbes in our water has been to hit them with a dose of chlorine. Depending on the state, Australia’s water utilities suggest a constant level of between 0.2 and 0.5 mg/L of free chlorine in our drinking water distribution system2.
For many microorganisms, this dosage of chlorine is a veritable death sentence. Yet N. fowleri has a couple of tricks up its sleeve that allow it to hold out against such a concentration of disinfectant.
A tough nut to crack
When the going gets tough, species of amoeba in the Naegleria genus can enter a hardy cyst stage that offers them significant protection by way of a tough wall.
N. fowleri is no different. However, it has the added protection of being able to bury itself in another form of protective shell: a biofilm.
Many microbes create sticky webs called biofilms that help them adhere to surfaces and offer protection from the environment. It’s the prime reason we brush our teeth – physically disrupting the matrix of protein and polysaccharide frees the bacteria lurking within it.
Yet the biofilm coating created by bacteria inside water pipes offers not only protection for N. fowleri; it provides the amoeba with a ready food source in the bacteria themselves.
Geoffrey Puzon from CSIRO’s Environmental Contaminant and Mitigation Technologies Program has studied what it takes to protect us from this deadly amoeba. Puzon is confident that without their defensive talents, in water free of organic material to soak up the chlorine, N. fowleri is susceptible to the disinfectant’s toxic bite.
Concentrations of 0.5 to 0.74 mg/L for at least 30 minutes is enough to take care of the amoeba while it’s in its unprotected free-living form. Even when it’s a cyst, slightly higher chlorine levels of between 0.5 and 1.5 mg/L for an hour can finish it off.
If the real world was as clean as a laboratory, we’d have no problem. “However, an issue arises when pipeline sections do not receive a constant dose, as N. fowleri can survive intermittent doses of chlorine,” says Puzon. “In addition, the N. fowleri associated with the pipe wall biofilm is even more resistant due to the added protection from the biofilm.”
When mingled with biofilm, N. fowleri can survive bursts of chlorine in concentrations up to 20 mg/L. Temperature can also make a vital difference.
“Pipe wall biofilm develops in all water distribution systems, but can be significantly worse in pipeline exposed to warmer temperature and low water flows, which both contribute to the loss of chlorine,” says Puzon.
Recommended doses are based on how chlorine dissipates at room temperature. In warmer climates, where an above ground pipe – or hose – can heat water to well over 30 degrees Celsius, chlorine dissipates faster, potentially offering N. fowleri enough respite to survive.
These amoeba aren’t the only variety we need to be paying attention to. “In addition, it is known that other amoeba, such as Acanthamoeba and Vermamoeba, are much more chlorine resistant than N. fowleri. These amoebae are also able to host pathogenic bacteria such as Legionella and non-tuberculous mycobacterium intracellularly – termed amoeba resistant bacteria – and aid in their dissemination through high chlorinated sections of the water distribution system.”
Fortunately, armed with such information, we can do something about it.
Doubling down on the deadly amoeba
Current Australia Drinking Water Guidelines dictate that finding a single N. fowleri cell in a litre of drinking water is sufficient to require immediate action, boosting free chlorine to at least 0.5 mg/L throughout the whole network.
Puzon explains that his research has shown maintaining this level at over 1 mg/L in areas where N. fowleri is known to occur is enough to not only kill it and its nasty amoeba cousins already present but changes the biofilm and prevents it from recolonising.
“The research has provided water utilities with a target concentration of chlorine that has been demonstrated to remove N. fowleri from both the bulk water phase, but also the pipe wall associated biofilm.”
After three years of testing, it appears this approach is effective—the study’s test site in West Australia remains free of the amoeba. Puzon has shared his results nationally and internationally and is now working with water and health authorities in Australia to inform their water treatment processes.