The Rest of the Story on the Real Life Cycle of SI

The real life cycle is between birds and snails. People just happen to swim in the middle of the life cycle of the cercariae who try to penetrate our skin, thinking, of course, that we are a bird, and will help them complete their cycle.

So here is Harvey's letter to me that thoroughly explains every aspect of the swimmer's itch life cycle, and thus really helps me to understand what is really going on. If you are a biology major, as I was, this is easy to follow.


Carolyn:

Obviously, people are not getting the life cycle.  Adult schistosome worms live in the mesenteric veins of the bird.  These threadlike adult worms produce eggs that make their way from the veins of birds intestine, to the lumen of the intestine.  The eggs exit with the bird fecal material.  Once in the water, stimiluated by a change in osmolarity pressure and the exposue to light, the miracidium hatches out of the egg and begins swimming.  The miracidia are positively geotaxic (this means that they go toward gravity or to the bottom of the lake) where they are most likely to come in contact with the right species of snail (not just any snail).  For most species, the miracidium penetrates into the tegument of the snail and migrates to the hepatopancreas (in snails this organ is the liver and the pancreas combined).  At this site, it begins reproducing at an incredible rate.

After about a month, the tube factories in the hepatopancreas of the liver, produce cercariae (up to 2,000 per day).  Keyed by light again, these larvae (called cercariae), exit the snail every day.  Unlike the miracdia, the cercariae are positively phototaxic and so they move toward the surface where they are more likely to encounter the next host, a specific species of bird.  In the Midwest, they birds are most often common mergansers. How they enter the bird is a mystery.  They may go in the mouth and enter the epidermis of the mouth, pharynx, etc. or they may enter through skin on the feet or under the feathers.  

In about a month, these cercariae become adult worms again in the veins of the duck.  It takes about two months for the whole cycle to be completed in snails and birds.  There is a day on both sides of these hosts where the parasite is aquatic.  Finally, if the cercariae come in contact with people, they often mistake it for a bird, try to enter the skin but only can make it through the epidermis and then die.  They do not have the right enzymes to go through human skin.  In the right bird, they would enter the skin, go in a blood vessel, migrate to the heart, liver and then mesenteric veins of the bird where they become adult worms.

Incidentally, once they enter the skin (usually in the water), there is no amount of toweling or showering that will remove them. Some people believe that they can go into the water with the swimmer's itch parasites and that they will not get it if they shower or towel down. That is not correct.  I get a lot of contacts about that.   ....


Harvey Blakespoor, PhD