The distressing "duck itch" usually makes itself felt during the summer holiday weeks when water temperatures rise and more people get into the lake.
It has been a low-key journey of discovery for Dr Davis, who has done much of his research work at the kitchen table of his modest concrete block holiday bach in Aubrey Rd, opposite the popular Bremner Bay swimming beach.
Dr Davis has received modest council grants for his study in the past, but now he does it for love.
The former US Navy pilot and flight instructor who used to fly fighter jets and was deployed to the US Deepfreeze III base in the Antarctic in 1958-59 has lived in New Zealand since 1974.
He married Annette McKay from Southland.
The couple raised a family and they now live on a Waimate farm.
They bought their Wanaka bach in the 1980s and have suffered from duck itch many times.
Relief can be obtained through topical remedies such as calamine lotion.
But Dr Davis, who retrained as a science teacher and later obtained his PhD in zoology from Otago University in the 1990s, would like to stop duck itch in the first place.
He knows of people who cut short their holidays because the itch made their children so miserable.
So he is intent on finding a biological control for a blood-dwelling avian parasite that lives on a small duck (the New Zealand scaup; Aythya novaeseelandiae; papango).
Every summer holidays he diligently researches the parasites and their hosts - a tiny water snail about 2mm-4mm big, and the scaup.
Some holidays, he has worked for 18 hours a day, seven days a week.
"I am not doing if for self aggrandisement. I think it is important," he said.
He has tried many things.
In 1982, he spread modest amounts of a chemical in the lake hoping to kill the snails, in which the parasites live for part of their life cycle.
In 1990, he did 100 transcepts across Bremner Bay, collecting and studying snails.
Dr Davis says he has identified a biological control for the duck itch parasite but its effectiveness is very limited because the controlling parasite cannot hatch properly in cool water.
Dr Davis says he would like to harvest the control parasite from mascerated Canada geese, incubate them at 28degC, toss them in the lake and see what happens.
Over the years, Dr Davis has published papers and given presentations on duck itch.
He has established links with scientists at the University of New Mexico, to whom he sends samples for DNA analysis.
Now, after following up suggestions by a scientist from that university, Dr Sara Brant, Dr Davis believes he has stumbled across a development: another parasite may also be causing duck itch.
This second parasite, trichobilharzia regenti, has been reported by scientists in the Czech Republic but had never been noticed in New Zealand before because no-one had been looking for it, Dr Davis said.
The second parasite is different from the one Dr Davis was initially studying.
"Instead of using the blood vessels and liver of the bird for its development, it uses the peripheral nerves to travel from the skin to the central nervous system," he explained.
While research in the Czech Republic had shown T. regenti could cause paralysis in mice, it was not clear what effect it might have in humans, as there was no medical research.
And one could not tell from itching skin which parasite caused it.
Dr Davis has not spoken with neurologists, but is asking himself if T. regenti could cause other unexplained maladies in humans.
He also wonders what effect it might have on the life expectancy of scaup.
"I feel that it is an important parasite that should be investigated further," he said.
He intends to start that process in April, once the nesting season is over and he is allowed to harvest more scaup.
Duck itch: Scratching the surface
What is duck itch?
A skin rash caused by an allergic reaction to bird and mammal parasites. Immature larval forms of parasitic flatworms are released from infected snails into fresh and salt water, such as lakes, ponds and lagoons. The larval forms penetrate a person's skin rather than attacking their usual host, a duck.
How to treat it
• Rinse skin with isopropyl (rubbing) alcohol and coat with calamine lotion.
• Antihistamines or mild corticosteroid cream can help.
• Scratching can cause abrasions and skin infections, which may require antibiotics.
• Oral steroids may be appropriate in severe cases.
• Other treatments include cool compresses, bath with baking soda, baking soda pastes and colloidal oatmeal baths.
• Dr Norman Davis suggests using the Cancer Society's combination sunscreen and insect repellent containing Deet. He hasn't tried this on himself.
How to avoid it
• Swim in a pool or pick your beach carefully. Swim where there are no onshore winds.
• Avoid shallow warm water, because the parasites (and the snails) are in greatest concentration there.
• This is often why small children seem to get worse attacks.
• Briskly towel off immediately after leaving water or wash in fresh water.
How to kill avian parasites
• Reduce the possibilities of birds getting infected.
• Try killing snails in small ponds and lakes with copper sulfate or copper carbonate.
• Place a drug for treatment of the parasite into food bait for the aquatic birds.
• Keep any vegetation in the pond or lake to a minimum so snails do not have an ideal growing environment.
• Do not attract birds by feeding them near swimming areas.
Source: New Zealand Dermatological Society
