GENETIC testing has revealed that Sydney's cryptosporidium outbreak may have stemmed from a single infected person.
The finding means it is unlikely that an infected animal or environmental conditions, such as the weather, are to blame.
Since the beginning of last month, when the latest outbreak appeared, 628 people have fallen ill, compared with 482 in all of last year.
Macquarie University researchers say their testing is experimental, and unlikely to identify the origin of the current outbreak, but they hope their work will lead to the development of a world-leading, inexpensive automated test to quickly identify parasite strains involved in future outbreaks, allowing sources to be rapidly traced and infections contained.
Michelle Power, of Macquarie's biological sciences department, said cryptosporidium outbreaks were difficult to trace using existing technology.
Two key parasite "species" infected people - Cryptosporidium hominis, only contracted by direct human to human contact, and Cryptosporidium parvum, which can also be transmitted by animals. For each type there were about 40 different strains.
However, virtually nothing was known about which strains were responsible for most outbreaks. A decade after the contamination of Warragamba Dam, which forced Sydneysiders to boil their water, "we still just know that it was cryptosporidium", Dr Power said.
"To put it simply, we're looking for the easiest way to fingerprint the organisms."
With the new test it should be possible to compare results from individual patients to seek common patterns.
"We could look to see if they had all eaten at the same restaurant, or been to the same day-care centre or swimming pool."
The NSW Health Department has provided hundreds of fecal samples from the latest outbreak for testing by a Macquarie University parasite researcher, Liette Waldron.
"We have analysed about 250 samples from the current outbreak, and they are all the same strain" of Cryptosporidium hominis, Dr Power said.
If it had originated from different sources she would have expected to see multiple strains. When sporadic cases, reported over the past 12 months, were analysed they identified 24 strains, involving both the human and animal species.
Dr Power said people who were infected, but not necessarily appearing ill, usually had 5000 to 10,000 parasites per gram of faeces. However, some samples from the latest outbreak contained more than 1 million. "The more parasites you see, the more nasty it is."
Over the next two years the project, funded by the Australian Research Council and the Health Department, will "fingerprint" parasites from up to 4000 cryptosporidium patients.