Globally waterborne pathogens kill 1.8 million people and cause about 4 billion cases of illness annually, and New Zealand is not immune to this risk. For example, contamination of water-supply bores in 2016 in Havelock North caused 8320 cases of illness.
To keep drinking water safe for human consumption, water-supply bores and water supply systems must be free of waterborne pathogens which are often present in contaminated surface-water and groundwater. This is where the use of pathogen surrogates comes in as an investigation tool to tackle the problems.
New Zealand has a higher incidence of drinking-waterborne cryptosporidiosis than other developed countries due to high livestock densities. Cryptosporidiosis is caused by the protozoa Cryptosporidium parvum/hominis, which are shed in the faeces of infected humans and animals. These parasites are hard to kill by conventional disinfection, so preventing water contamination in the first place is the best strategy to prevent disease.
Likewise, the bacterium Legionella pneumophilia, which lives in freshwater and engineering water systems (for example cooling towers and hot water tanks), causes a particularly nasty form of pneumonia called Legionnaires’ disease. There is no vaccine and the mortality rate for hospitalised patients is 5%–30%.
Rotavirus is another leading cause of gastroenteritis, particularly in children. It is among the most infective pathogens - just one viable rotavirus particle may cause infection. It's why rotavirus is used as a model virus for microbial risk assessment for drinking and recreational water, as well as for determining the safe setback distances between water-supply bores and on-site wastewater disposal.
By mimicking the physiochemical properties of the target pathogens, abiotic surrogates (non-living synthetic particles coated with biomolecules such as vitamins, proteins and amino acids) can be used to investigate pathogen removal and transport in freshwater environments, and to assess the performance of water and wastewater treatment systems.
Led by Dr Liping Pang, a Science Leader at ESR, the research group has been able to label the surrogates with unique synthetic DNA tracers. Surrogates’ DNA degradation can mimic pathogen inactivation to some degree. DNA-tagged surrogates can be analysed sensitively and rapidly using the quantitative polymerase chain reaction (qPCR). DNA tracer labelling also allows effective tracking of the surrogates from multiple contaminant source locations and pathways.
The transport and reduction of pathogens in water systems are largely determined by their size, shape, buoyant density, surface-charge and hydrophobicity (how attracted to or repelled by water they are).
Liping’s teams have created surrogates that mimic the properties of the nano-sized rotavirus, micro-sized Legionella bacteria and slightly larger Cryptosporidium protozoa.
These pathogens and surrogates are small. Rotavirus is 0.15% the diameter of human hair!
How surrogates are made
The Cryptosporidium surrogates are carboxylated latex microspheres coated with glycoprotein (the major type of protein that Cryptosporidium oocysts produce on their cell walls but can be found in many safe sources) or biotin (vitamin B7).
The Legionella surrogates are rod-shaped, DNA-encapsulated, amino acid-modified alginate-calcium carbonate microparticles.
The rotavirus surrogates are DNA-labelled, protein-coated silica nanoparticles.
Experiments in New Zealand and overseas have validated these new surrogates’ performance against the actual pathogens.The research findings demonstrate that the new surrogates significantly outperform existing surrogates.
In a pilot study conducted at the Invercargill water treatment plant, the Cryptosporidium surrogates were used to evaluate the effectiveness of filter materials used in New Zealand treatment plants for protozoan removal, including anthracite coal, pumice sand, and engineered ceramic sand (ceramic sand performed best). These new surrogates have also illustrated that turbidity (cloudiness), a key test of water clarity and existing proxy for water quality, may not be a reliable indicator of protozoan removal.
ESR researchers also evaluated different types of domestic water filters for Cryptosporidium removal, including activated carbon, silver-impregnated carbon, pleated paper, polypropylene and polyester cartridges that had pore sizes 1-2 micrometre. Although both carbon filters outperformed all the others, only the 1 micrometre activated carbon filters removed the protozoan surrogates to the Australian/New Zealand Standard.
Now the race is on to expand these surrogates’ usage and to advance surrogate technology.
Alongside overseas collaborators, the research team is working on creating and validating a new generation of surrogates made from food-grade biocompatible and biodegradable natural biopolymers, expanding their applications across real-world operational water systems.
Proof-of-concept studies suggest that the new pathogen surrogates show great promise as new tools for water applications. The surrogate technology approach has opened a new avenue for assessing pathogen removal and transport in water systems without the risk and expense that accompany work with actual pathogens. The research findings will facilitate improved management systems and engineering approaches to reduce waterborne infection risks and safeguard public health in Aotearoa New Zealand and around the world.