© 2019 The Touchstone Project

Duck Itch

The most important thing you need to know first:

What can be done to prevent or to reduce swimmer’s itch?

Use SPF50+ sunscreen (before you enter the water) with phenoxyethanol in it as an ingredient. Read the label or ask. This has been shown in Wanaka exposure trials to prevent the disease. An alternative sunscreen might be one with DEET, however it is not recommended for children.

• Avoid swimming for long periods in shallow water.

• Avoid swimming in areas where swimmer’s itch is a problem and where there is an onshore wind.

• Post appropriate signs on beaches where swimmer’s itch is an annual problem

• Do not encourage birds to stay in your area by feeding them

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

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, (a small black duck) 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.

Duck Itch

Three Images Showing the Duck / Schistosome life cycle

Here's how it works...

It's a little yucky... It's actually like a real life 'Alien' (the movie) life cycle...

The two avian schistosomes each follow different paths in the scaup (duck).

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.


Will swimmer’s itch spread?

No, a papule forms only where a cercaria has entered the skin of a person. If the person is exposed to more cercariae, additional papules will form.

What are the symptoms of swimmer’s itch?

People who have never had Duck itch may not react on the first exposure. However, their immune system will become sensitized to react on subsequent exposure. Sensitized people develop swimmer’s itch when the parasite enters their skin. The immune system kills it there and, in the process, histamines are produced which attract white blood cells to consume the residue. This is what causes the papule and itch. Usually within 30 minutes, a small red spot appears at the site where the parasite penetrated. This red spot will continue to increase in size for the next 24-30 hours. The raised, reddened spot is then called a papule. It will continue to itch for up to a week. Papules are limited to areas of the body that are exposed to water because cercariae can not live out of the water. Towelling will not do any good because the cercariae penetrate the skin while the person is in the water.

Why do children often develop the most severe cases of swimmer’s itch?

They usually spend more time in the water, their skin may be more sensitive, and young children have a tendency to stay near the water’s edge where cercariae may concentrate.

When does the first outbreak of swimmer’s itch occur? How long can it last?

Often during the first warm period in the spring, usually in late November or early December. Duck itch has been reported at Lake Alexandrina as late as March.

If an outbreak of swimmer’s itch occurs in a particular lake or region of a lake, how long might it remain a problem?

There is no way to determine how long an outbreak will last. On some lakes, swimmers are infected once; in other lakes, it persists for the entire summer. Research has shown that there is another parasite, an echinostome, of waterfowl which infects the same snail and may kill either the snail or the duck itch parasites in the snail. This happens naturally, but the dynamics of this parasite’s egg production and development means that this parasite is not available to infect the snail until late Fall. In the Fall, the echinostome has been shown to be dominant over the schistosome in the snail.

If swimmer’s itch occurs on a lake, does that mean that the lake is polluted?

No. Natural lake conditions promote the diversity of species, including the birds and snails that are potential hosts for the causative agents of swimmer’s itch.

Why may swimmer’s itch be a problem one year but not the next?

The following are factors that may determine whether swimmer’s itch may be a problem on a specific lake at a given time:

distribution and number of snails that can serve as intermediate hosts; distribution and number of bird hosts that can serve as hosts for the adult worm; wind direction; water currents; time spent in the water; time of day; sensitivity of the individual (especially if your name is Rachel).

How common is swimmer’s itch in New Zealand?

It is widely scattered throughout New Zealand, and appears to be spreading with the NZ Scaup as it extends its range. Farm ponds, boat harbours, popular lake camping ground sites and lake beaches with onshore winds all have reported Duck Itch.

Is swimmer’s itch found only in New Zealand?

No, cases have been reported from nearly every country in the world, except Antarctica.

Can cercariae be seen in the water?

No, it is impossible to observe microscopic larvae in the water while swimming. They are approximately 1/10 of a millimeter long and transparent.

How many hosts are there in the life cycle of the parasites that cause swimmer’s itch?

There are always two, a snail intermediate and vertebrate primary or definitive host, usually a bird.

The parasite must be transmitted from snail to bird and from bird to snail. It can never go from snail to snail or from bird to bird.

How many species of avian schistosomes can cause swimmer’s itch in New Zealand?

Three species have been identified. There may be several more, as yet unidentified. This question is difficult to answer for several reasons. First. there are a large number of birds that potentially can serve as hosts for the adult worm. Second, the adult worms are so small and so difficult to remove from the blood vessels that few people have attempted to work out the classification scheme. The life cycles are not known and the snail intermediate and bird hosts for a specific species of schistosome have not been identified.

Do all of these species of schistosomes use the same species of snails and birds as their hosts?

No, most species of schistosome use only one species of snail and one species of bird to complete their life cycles. In other words, they are quite host-specific. This is an important concept to remember when control measures are employed. Most snail intermediate hosts for avian schistosomes belong to one of two families: Lymnaeidae and Physidae. Some members of a small snail (Planorbidae) can also serve as intermediate hosts. All of these snails are in New Zealand.

What is the relationship of snails to swimmer’s itch?

Certain stages of the parasites that cause swimmers’ itch must cycle through snails. Larval stages develop and reproduce in the internal organs of the snail. Each day, thousands of these free-swimming cercariae emerge from the snail but do not feed and therefore will not live for more than 24 hours in the water.

Do all snails carry the organisms that cause swimmer’s itch?

No, but there are at least nine species reported elsewhere that can serve as intermediate hosts for the parasites. Most species of schistosome have only one snail species that can serve as its host.

Are birds important to the organisms that cause swimmer’s itch?

Yes, many species of birds and some rodent species can harbor the adult parasites within their blood vessels. In New Zealand, Some common hosts may include NZ scaup, mallards, grey ducks, Canada geese, swans, NZ shovelers, Great Crested Grebes and Australian Coots. Further research is required. Mice may also be infected.

What is the role of these birds and mammals in the life cycle of the parasites?

Perpetuation of the parasite life cycle.

Why should ducks, geese, and swans not be fed?

Three good reasons include:

it may propagate swimmer’s itch in the area where the birds are being fed

it may make the birds dependent on humans for survival

it may stimulate fecal deposits at the feeding site.

How do you determine which birds and snails carry the schistosomes causing swimmer’s itch on a particular lake?

Birds can be checked by dissection for avian schistosomes, by hatching the miracidia from recovered parasite eggs and exposing lab reared snails to see if they can be infected If the cycle can be reared in the laboratory, cercariae from the lab cultures can be compared to those that emerge from naturally-infected snails taken from areas on the lake where swimmer’s itch was a problem. The behavior, size, and morphology of each species of avian schistosome are unique to each species. Most recently, DNA analysis has played an important role in classifying these parasites.

How can one be sure that New Zealand Scaup play such an important role as bird hosts on so many lakes?

First, on Lake Wanaka where swimmer’s itch has been researched, virtually all of the NZ Scaup are infected. Second, the scaup usually harbor heavy infections compared to other species of bird hosts. Mallards, Canada geese and Grey ducks usually have very few worms.

Can a relatively few NZ Scaup have an important swimmer’s itch impact on a large recreational lake?

Absolutely, because scaup are heavily infected and because there is a high prevalence of infections in them. Also, scaup are very mobile. In addition, one must consider the dynamics of the life cycle. For example, if one miracidium infects a snail, that intermediate host (after approximately 30 days) will produce several thousand cercariae every day it lives. Fortunately, only a small percentage of the miracidia ever contact a suitable snail.

Are Canada geese and swans important hosts for swimmer’s itch?

Perhaps not, because the snail intermediate hosts for the schistosomes that cycle through them may be snails found typically in marshy areas where people do not swim. The stage that causes swimmer’s itch comes from the snail and not directly from the bird host.

What can be done to prevent or to reduce swimmer’s itch?

Use SPF50+ sunscreen with phenoxyethanol in it as an ingredient. Read the label or ask. This has been shown in recent Wanaka exposure trials to prevent the disease. An alternative sunscreen might be one with DEET, however it is not recommended for children.

Avoid swimming for long periods in shallow water.

Avoid swimming in areas where swimmer’s itch is a problem and where there is an onshore wind.

Post appropriate signs on beaches where swimmer’s itch is an annual problem

Do not encourage birds to stay in your area by feeding them

What can individuals do who have a bad case of swimmer’s itch?

See a doctor or chemist and ask for a something to relieve the itching. Topical creams and Calamine lotion will reduce the itching.

What types of control are used to combat swimmer’s itch?

Research has been done into the use of a molluscicide and the possible use of embryonated echinostome eggs broadcast in the early spring to control the snails and their schistosome parasites in selected areas of lakes. These methods may be effective, but are prohibitively expensive and would have to be repeated every season at great cost.

Can swimmer’s itch be eliminated completely?

Because of the complexity of the problem and because of the number of species that can cause swimmer’s itch, no method will eliminate every case of swimmer’s itch on a given lake.

Why are control measures so costly?

Control of swimmer’s itch must be done by professional people who have invested a lot of time and money into their education and training. In addition, most control efforts either are time-consuming or they require the purchase of costly chemicals like Bayluscide.

Is it legal to shoot NZ scaup or other species of waterfowl that harbor parasites that cause swimmer’s itch?

The NZ scaup is protected and specific permission is required to take them.

Why hasn’t the problem been researched more extensively?

Field and laboratory research on swimmer’s itch requires expertise in Parasitology, Ornithology, Malacology (study of mollusks), and Limnology—a rare combination of backgrounds for biologists.

Is there reason for optimism for swimmer’s itch control in the future?

Yes. Recent exposure studies here have shown that the use of a suitable off-the-shelf sunscreen should be effective in preventing cercarial penetration of human skin.