The official name for swimmer’s itch is cercarial dermatitis or schistosome dermatitis. The worms causing the issue are trematodes in the family Schistosomatidae. The more familiar malady from worms in this family is schistosomiasis or snail fever. Schistosomiasis inflicts harm to hundreds of millions of people worldwide each year. The problems caused by the schistosomiasis parasite, or more specifically, the parasite eggs, include abdominal pain, liver enlargement, kidney damage, bladder fibrosis, prostate issues, and infertility. The world’s major regions affected by snail fever are tropical Africa, the Middle East, Indonesia, the Philippines, China, Brazil, and the Caribbean.
Fortunately, those issues do not come with swimmers’ itch, though that same symptom of redness and itching can occur in those inflicted with schistosomiasis. The species responsible for swimmer’s itch is likely to be Austrobilharzia variglandis, while worms in the genus Schistosoma, primarily Schistosoma mansoni, are largely messing with humans in the disease schistosomiasis.
The life cycle of these worms involves snails and waterfowl, typically ducks, and thus we can see the reason it’s often called duck itch, or in the case of full-blown schistosomiasis, snail fever. In their swimming state, they don’t look like a worm, this is the period when they bump into the human epidermis. The general life cycle is shown in Figure 1.
Cue some science:
In 2010, a few researchers found that the exotic snail, Haminoea japonica, a relatively new arrival to San Francisco Bay, housed parasites. The snail is known as the Japanese Bubblesnail and is a small (see further issues with its size below) tiny-shelled snail. The news should not be shocking; new species are always arriving. The shocker was that this snail houses a swimmer’s itch parasite. San Francisco Bay is now the proud new home to annual swimmer’s itch outbreaks.
The description of this snail that I can piece together:
It is typically referred to as a sea slug, though it has a small, thin, oval, non-spired shell. The mantle completely covers the shell. The shell is right-handed. The length of the whole animal is up to 33 mm. Shell length is about 12 mm. Animal color is light-brown with dark spots. Orange and sometimes red spots are scattered along the body.
This snail has poecilogonous development (say that ten times fast). This means that developing eggs can either hatch directly as crawling youngsters or hatch as veligers entering the plankton for up to 30 days before settling down to a crawling lifestyle. A single mass of eggs can have individuals that do both of these patterns. This is a fascinating bet–hedging life-history pattern. I love it.
This snail species is associated with the Pacific Oyster (Crassostrea gigas). Prime importance for the Maine coast is culturing oysters, a relatively new thing. Do those oysters come with a parasitic price?
The major coastal change in the temperate environment is bound to be the emerging infectious diseases. Some will influence humans in minor ways, and others directly. Swimmer’s Itch is annoying as hell to get but is not deadly. Incidence of this type of infection does harken for the future, and emerging infectious diseases are fast approaching.
Sources and Further Readings:
Brant SV, Cohen AN, James D, Hui L, Hom A, and Loker ES. 2010. Cercarial dermatitis transmitted by exotic marine snail. Emerging Infectious Diseases 16: 1357– 1365.
Gibson GD & Chia FS. 1995. Developmental variability in the poecilogonous opisthobranch Haminaea callidegenita: Life-history traits and effects of environmental parameters. Marine Ecology Progress Series, 121, 139–155.
iNaturalist. Japanese Bubble Snail.
https://www.inaturalist.org/guide_taxa/248800#ref5
World Health Organization. Schistosomiasis. Fact sheet. Updated February 2016
https://web.archive.org/web/20161119053826/http://www.who.int/mediacentre/factsheets/fs115/en/