What is Duck Itch?
Duck Itch, or schistosome cercarial dermatitis is a reaction that some people get when parasitic flatworms penetrate the skin.
The parasite is in the genera of world-wide avian schistosomes that complete their life cycle in two stages.
The schistosome family includes parasites infecting 200+ million humans throughout the world.
The primary definitive host in New Zealand is the New Zealand Scaup, Aythya novaeseelandiae.
Where these waterfowl are found, there will be Duck Itch. Other waterfowl are infected as well.
The secondary intermediate host is the lymnaeid snail, Austropeplea tomentosa known previously as Lymnaea tomentose.
In New Zealand, two avian schistosomes have been found concurrently in the scaup and individually in the snail.
They each follow different paths in the Scaup.
Trichobilharzia longicauda cercariae penetrate the bird’s skin, become schistosomules, enter the blood vessels, and travel to the heart, lungs, and the liver.
They become adult male or female worms and mate in the blood vessels of the liver and mesentery. The female migrates in the blood vessels to the intestine to deposit embryonated eggs which pass out of the bird with faeces. Upon reaching the water the eggs hatch to release a single miracidium, which searches for a snail.
Trichobilharzia regenti cercariae penetrate the bird’s skin, become schistosomules, enter the peripheral nerves and travel to the central nervous system and the brain. They become adult in the meninges and then migrate to the nasal tissue where embryonated eggs are deposited. When the bird puts its bill in the water, the eggs hatch and the miracidia then search for their snail host. This parasite, only recently found in New Zealand, is neuropathogenic and has been shown experimentally to cause paralysis in ducks and in mice.
The development of each of these parasites in the host snail is identical.
Upon encountering a snail, the miracidium penetrates the snail’s foot and migrates to the heart. In the heart, it becomes a mother sporocyst (bag of spores), which become daughter sporocysts to migrate into the hepatopancreas where they each produce another series of daughter sporocysts. They are nourished by absorbing the digested food of the snail. Finally, several tens of sporocysts produce thousands of furcocercariae. Upon shed of the parasites, the snail is exhausted of nutrients and dies.
Furcocercariae look like microscopic clear tadpoles with forked tails. They are virtually invisible, but can be seen in a vial of water held up to light. They are shed by the snail in great numbers to swim to the water surface in search of their primary host to complete the life cycle.
In humans and other mammals, the parasite is killed by the immune system, and may cause dermatitis in individuals who have been previously sensitized. This sensitivity will rarely disappear; it usually gets worse on subsequent exposure.
Anecdotal and research reports indicate that, in some humans, these parasites may penetrate beyond the skin in their attempt to complete their life cycle. Further medical research is certainly warranted. Further research is also warranted into the effects of these parasites on waterfowl.
For further information and any questions, please contact:
Dr. Norm Davis, PhD
61 Hodges Road RD 7 Waimate 7977
Ph: 03 689 7877,
Cell phone : 021 129 6822
Email: davisn @xtra.co.nz