No. Cases of swimmer’s itch have been reported in nearly every state in the USA, as well as in all of the provinces of Canada. In North America swimmer’s itch is most common in the northern tier of states, including Alaska. In addition, swimmer’s itch has been reported from more than 30 countries.

The causative agent for swimmer’s itch is the larval, free-living, aquatic stage of a group of flatworms that are called schistosomes. Most species that cause swimmer’s itch use bird hosts for the adult parasite and aquatic snails as intermediate hosts for the larval stages. The life history of the dermatitis-producing worms is cyclic.

Eggs, released from the adult worms that reside in the blood vessels (usually veins around the intestine) of the bird host, make their way into the digestive tract of the bird and then pass out of the host with the feces. If an egg is deposited in water, within an hour it will hatch into a miracidium, which is free-swimming, but non-feeding. A miracidium has enough energy to keep moving for approximately 24 hours, and once it comes in contact with the proper snail species it will either penetrate into the snail via the integument or it may enter through the snail’s mouth. Once inside the snail, the miracidium elongates to form a reproductive sac called a sporocyst. This germinating structure will produce a second generation of sporocysts. After living in the snail for approximately a month, the sporocysts produce another larval stage, called a cercaria. This stage burrows out of the snail, becomes a second type of nonfeeding, swimming aquatic stage that must enter the proper bird host species to continue its life cycle. It does this either by penetrating the skin of the bird or by being ingested and then entering the blood vessels in the walls of the pharynx or esophagus. In the bird host, the parasite migrates through various organs of the bird and finally matures in the blood vessels. The resulting adult worm then begins producing large numbers of eggs which again are voided with the feces. Avian schistosomes usually complete their life cycle in two months, but that time varies slightly with each species.

No. It is impossible to observe cercariae in the water without the aid of a microscope. They are transparent and each one is approximately 1/80th of an inch long.

There are always two, a snail intermediate and vertebrate final 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.

There are at least 12-15 different species of avian schistosome in Michigan alone. The exact number is difficult to determine for several reasons. First, there are a large number of bird species that potentially can serve as hosts for the adult worms. 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. Third, for most species the life cycles are completely unknown. In other words, the snail intermediate host and the vertebrate host species for some avian schistosome species have not been discovered.

No. Most species of avian schistosomes are very host-specific, meaning they can use only one species of snail and one species of bird to complete their life cycle. This is an important concept to remember in the implementation of any swimmer’s itch control program.

Certain stages of the parasites that cause swimmer’s itch must cycle through snails. Larval stages develop and reproduce in the internal organs of the snail. Each day thousands of free-swimming cercariae emerge from each infected snail.

No. There are at least nine different species of snails reported in Michigan that can serve as intermediate hosts for parasites that cause swimmer’s itch.

Yes. Many species of birds, and even some rodent species, can harbor the adult parasites within their blood vessels. Some of the more common vertebrate hosts include common mergansers, mallards, Canada geese, swans, red-winged blackbirds, as well as muskrats and mice.

When cercariae contact a suitable bird or mammal host, they penetrate through the skin, migrate through various organs including the liver and lungs, and then reside in the blood vessels of the host, particularly the veins surrounding the intestine. There the parasites develop into extremely small and thin adult worms. The female worms (no bigger than a single hair of a paint brush) lay eggs that work their way into the host’s intestine. When the host defecates into the water, the egg hatches into the next stage, a miracidium. Like a cercaria, the miracidium is non-feeding and lives only 24-32 hours, depending on the water temperature.

Absolutely! Common mergansers are usually very heavily infected, have a high prevalence of infection, and are extremely mobile, capable of covering several miles of shoreline in a single day.

No. They usually are not important hosts for swimmer’s itch parasites because the snail intermediate hosts for the schistosomes that cycle through them are snails found typically in marshy areas where people do not swim. Remember, the stage that causes swimmer’s itch comes from the snail and not directly from the bird host.

Yes. First, snails could become infected by spring and fall migrants. Remember that it takes at least 60 days for the parasite’s life cycle to be completed. That means that spring birds could transmit the infection to the snails. Second, there are species of schistosomes that cycle through passerine birds such as red-winged blackbirds, grackles, etc. and one species that cycles through rodents. It is unusual to see major problems of swimmer’s itch caused by schistosomes in these hosts.