''People don't want to get duck itch again. They think it's going to end up as bad as when they first got it,'' Dr Davis, of Waimate, said.
Cercarial dermatitis, or duck itch, has been Dr Davis' self-confessed ''labour of love'' since the 1980s.
He obtained his PhD in zoology from Otago University in 2000, and regular visits to his Bremner Bay holiday home in Wanaka over the past three decades have been used to conduct unfunded research on the parasites, which cycle between the native New Zealand scaup duck and lymnaeid snail and can invade human skin, causing an inflammatory immune response.
Since mid-January, he has been carrying out the ''tedious'' job of collecting about 1800 tiny lymnaeid snails from the bottom of Lake Wanaka. Among those, he found just eight that were shedding duck itch-causing parasites which could be used in his trials. He then put a call out to the Wanaka Lake Swimmers club for subjects for his study and has had about 10 volunteers participate, the ''absolute minimum'' required. His aim is to verify the results of a European study that found a commercial sunscreen which contains repellent for jellyfish stings deterred the parasite from penetrating human skin.
He applied three different sunscreens to patches on volunteers' forearms. The three patches and a control patch not pre-treated with sunscreen were then exposed to lake water containing the parasite. Dr Davis is carrying out a ''double blind'' study, which means the sunscreens have been dispensed from a pharmacy in plain bottles, so neither he nor the participants know which is which.
Based on preliminary findings, two of the sunscreens appear to completely repel the parasite, while the third results in fewer itchy spots than the control patch, which could suggest even a simple sunscreen works, to some extent, Dr Davis said.
He will later have a colleague at the University of New Mexico determine which parasites featured in his trials - Trichobilharzia longicauda, which infects the host through the blood vessels, or Trichobilharzia regenti, which infects through the nerves and has caused paralysis in mice and neurological impacts in birds.
Dr Davis believed medical researchers should be studying what happened when T. regenti entered the human body.
''I haven't got the facilities or the funds to do all of it myself. It requires good lab facilities and a dedicated group.''