Swimmer’s Itch

Swimmer’s itch is a skin condition caused when swimming larvae (cercariae) of several species of parasitic flatworms trigger an allergic reaction. The parasite’s life cycle depends on two host species: in Crystal Lake the common merganser duck (Mergus merganser) is its definitive host and Stagnicola emarginata is its intermediate (snail) host. When the cercariae attempt to enter a person’s skin rather than a duck’s they die, causing inflammation and papules.

Scientifically known as schistosome cercarial dermatitis, swimmer’s itch has been recognized in Michigan inland lakes since at least the 1920s, although only in recent years has it become a significant problem for recreation. The CLWA has been seeking solutions to swimmer’s itch since 2006 when complaints intensified on Crystal Lake.

In 2013 the waterfront staff at the Congregational Summer Assembly (CSA) began maintaining daily records that have provided invaluable raw data. SEE THE DATA HERE

In order to find better methods of combating swimmer’s itch, since 2016 the CLWA has supported scientific research to increase knowledge of the biology of the parasitic cycle and the factors influencing swimmer’s itch. This research has produced annual assessments of the rate of snail infection in the lake, DNA analyses defining the species of the hosts, and better understanding of the parasite’s behavior.

READ THESE REPORTS IN THE SIDEBAR TO THE LEFT

In 2017, with the assistance of appropriations from the State of Michigan, the CLWA hired Swimmer’s Itch Solutions LLC (SIS) to trap merganser broods and relocate them to other water bodies with a compatible environment – but where swimmer’s itch is not present. TO SEE A VIDEO OF THEIR METHODS, CLICK HERE A limited study of the snail infection rate in summer 2018 already reflected a substantial decline in comparison to the 2016 assessment. The public was invited to report cases of swimmer’s itch to a SIS website.

In 2019 CLWA’s own team, which had been trained by Swimmer’s Itch Solutions, took over the trapping and relocation of mergansers, while SIS has continued to monitor snail infection rates and carry out additional research. [REPORTS IN THE SIDEBAR]

Metrics of the swimmer’s itch problem — snail infection levels and cases (measured by reports to the SIS website, and more scientifically, by the annual study at the Congregational Summer Assembly beach) – showed dramatic decreases in all years after merganser broods were relocated (2018-2022).

However, in 2022 highly pathogenic avian influenza (“bird flu”) arrived in Michigan, prompting the Michigan Department of Natural Resources to suspend permits for the relocation of mergansers. The suspension continued in 2023, was lifted briefly for the summer of 2024, but put in place once again in 2025 and has continued through 2026.

As expected, both SI metrics returned to pre-relocation levels after only one summer (2022) of common merganser broods growing up on the lake. Snail infection levels rose dramatically as did SI case reports to the SIS website and at the CSA beach. Once again metrics fell the summer after the resumption of relocation in 2024, but rose sharply in 2026.

These data show that the relocation of merganser broods works to lower the incidence of swimmer’s itch on Crystal Lake. SIS published the results of their research in peer-reviewed scientific articles in 2024 and 2025.

CLWA is continuing its research to monitor the incidence of swimmer’s itch and the levels of snail infection in Crystal Lake as it advocates for the resumption of merganser relocation. It shares its knowledge of SI behavior to advise the public on best methods to avoid infection. Various barrier skin products have been suggested, but none has yet been scientifically demonstrated to be effective.