What is it?

The Life Cycle

Digenetic trematodes are the parasites responsible for causing swimmer’s itch. They are a unique group of parasitic flatworms that have almost exclusively evolved mechanisms to undergo their larval development within snails. There are over 18,000 different species of trematode, and many of them infect organisms by producing a free-swimming, infectious stage called cercariae that actively seeks out the next host and penetrates its skin (1). For swimmer’s itch causing parasites, these next hosts are often aquatic organisms such as waterfowl, rodents, amphibians, fish, or other mammals that often interact with the water. The trematode further develops in the definitive host into adult worms that will produce eggs to be released into the environment and hatch to infect snails, thereby continuing the life cycle.The infection that occurs in the definitive host can be substantial, causing extensive pathology in a variety of organ systems that are defined by the species of infecting trematode.

When most of these parasites encounter humans, however, they are able to penetrate the skin, but do not initiate infection because they are killed by an inflammatory response characterized by reddening and swelling of the skin, itching, burning, formation of blisters, and possibly secondary infection (2). Often, following the first exposure, an individual will be prone to having an allergic response to following exposures, making the symptoms experienced worse.

1. Gibson, D.I., Jones, A., and Bray, R.A. (2002) Keys to the Trematoda, CABI Pub. and The Natural History Museum, Wallingford

2. Brant, S.V., and Loker, E.S. Schistosomes in the southwest United States and their potential for causing cercarial dermatitis or ‘swimmer’s itch’. J Helminthol. 2009 83:191-8.


The Rash We Call “Swimmer’s Itch”




The rash we refer to as “Swimmer’s Itch” or “Cercarial Dermatitis” is the result of an allergic reaction in the skin caused when the cercariae, or larvae of the trematode parasite, enters human skin. Note the individual red bumps resulting from where each individual cercaria has penetrated. The severity of the rash can vary from person to person, depending on several factors, including level of exposure (how may parasites were in the water directly near you) and exposure history (past exposure episodes/cases of swimmer’s itch). For some individuals, multiple exposures can cause heightened allergic responses and therefore increased rash severity. The rash can last several days up to several weeks. It is very important to not scratch, despite the itchiness, because scratching can lead to a bacterial infection. For most cases of swimmer’s itch, it is recommended to use an everyday type of itch cream such as Calamine Lotion or a mixture of Baking Soda and water. If these remedies do not work, it is recommended to try a Cortizone cream. If these treatments do not work or conditions worsen, please inform your doctor.



Swimmer’s Itch Rash vs. Toxic Cyanobacteria (Blue-Green “Algae”) Rash


It is a common misconception that Swimmer’s Itch is caused by Blue-Green “Algae”. It is no doubt easier to correlate something you can see with the naked eye, such as massive blooms of Cyanobacteria (commonly called Blue-Green “Algae”) to something you cannot see, such as microscopic parasites. In hopes to clarify this confusion, here are links to some information on Blue-Green “Algae” Cyanobacteria and the rash that can be caused by it. We hope that this information and the pictures provided will help to show the distinct differences between these two rashes as both Swimmer’s Itch and Cyanobacterial blooms are both very common throughout Alberta and even exist within the same bodies of water. If you have any questions related to these differences, please e-mail us at info@swimmersitch.ca.



The Cyanobacteria “Blue-Green Algae” Rash

Note how this rash is more flush and a larger area of skin is covered as opposed to the swimmer’s itch rash that has defined red bumps spread out across the body.


Toxic Algae in Florida Waters Final 06 07 2011_Page_2_Image_0004

Image Credit: Florida Water Coalition (2014), www.floridawatercoalition.org