Giardia is a parasite that causes a severe gastrointestinal illness, called giardiasis, in humans.
This is the third most common cause of human disease by a microbial agent in New Zealand, with infection rates up to five times higher than those observed in similar countries.
University of Otago researchers found a largeproportion of giardia samples taken from infected dairy calves and human sufferers in Otago over a two-year period were genetically indistinguishable.
Cattle had not previously been considered a significant reservoir of giardia strains that could infect humans because genetic studies involving dairy herds overseas had found the animals predominantly carried a livestock-specific strain.
However, study co-author and Otago zoology PhD student Cynthia Winkworth said that, to date, the livestock-specific strain had not been detected in New Zealand North or South Island dairy herds.
In fact, recent research showed that the giardia isolated from the animals consisted of the two human-infecting strains, she said.
The findings did not prove giardia was being transmitted from calves to humans, or vice versa, but did highlight a clear cause for concern.
"The significant degree of genetic overlap we detected suggests giardia present in dairy calves may indeed be a potentially significant reservoir of strains capable of causing infections in humans," she said.
Given New Zealand's comparatively high rates of giardiasis compared to otherwise similar countries, and the increasing amount of land being converted to dairying, further research on this potential link was warranted.
The Otago University research appeared recently in the international journal Applied and Environmental Microbiology.
Ms Winkworth supported efforts by Fonterra, through the Clean Streams Accord, and the Otago Regional Council, to separate dairy cows from streams through fencing.
"It's a great first step that makes a huge difference," she said in an interview.
The university research findings had highlighted the importance of evaluating effective waterway protection schemes to reduce potential risks arising from dairying.
She had, accordingly, also been evaluating the effectiveness of waterway edges planted with New Zealand native grassland species at removing giardia present in farm-field runoff, by comparison with fence-only protection.
Late this year or early next year, she expected to know what protective effect the native species were having.
The research was funded by the Tertiary Education Commission and the Miss E. L. Hellaby Indigenous Grasslands Research Trust.