After the parasite enters, it dies and may cause dermatitis in individuals who have been previously sensitized. This sensitivity will rarely disappear; it usually get worse to subsequent exposures.
Usually within 30 minutes, a small red spot appears at the site where the cercaria penetrated.
This red spot will continue to increase in size for the next 24-30 hours. The raised, reddened spot is now called a papule. It will continue to itch for up to a week. Papules are limited to areas of the body that get exposed to water because cercariae can not live out of the water. For some species of schistosomes that cause swimmer's itch, toweling off may help; with other species, it will not do any good because the cercariae penetrated the skin while the person was in the water.
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 the eggs are deposited in water, they will hatch within an hour if conditions are right. The miracidium, an aquatic stage, is free-swimming, but nonfeeding. It has enough energy to keep it moving for about a day. Once the miracidium comes in contact with the proper snail it will either penetrate into the snail via the integument or it may enter through its mouth. Within the snail, the miracidium will elongate to form a reproductive sac called the sporocyst. This germinating structure will produce a second generation of sporocysts. In approximately a month, the sporocysts produce another stage, called the cercaria. This stage burrows out of the snail, becomes a second type of nonfeeding, swimming aquatic stage that must enter the bird host. 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 adult worms then begin producing large numbers of eggs which again are voided with the feces. Avian schistosomes usually complete their life cycle in two months, however, the specific time varies with each species.
All of these factors can change on an annual basis.
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.
Remember that most species of schistosomes have only one snail species that can serve as its host.
Second, common mergansers usually harbor heavy infections compared to other species of bird hosts. For example, the average number of miracidia that hatch from a gram of feces from common mergansers is more than 300. Mallards, Canada geese and wood ducks usually have less than 25% infected and only a couple of miracidia per gram of feces. Furthermore, the cercariae from the species of schistosomes that cycle through common mergansers, are much larger than average and emerge only from lymnaeid snails, particularly Stagnicola emarginata.
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.
