What's in the water? Understanding E. coli, cyanobacteria and duck itch

Every summer, questions about water quality begin circulating around our lakes. Is that cyanobacteria? Was there an E. coli advisory? Is that rash duck itch? These three issues are often confused, but they are entirely different. They have different causes, different health risks, and different ways to reduce your chances of exposure. Knowing the difference can help you make informed decisions while enjoying the lake and better understand what different advisories/warnings actually mean.

E. coli is a bacteria that lives in the intestines of people and animals. Its presence in the water indicates fecal contamination, which can come from waterfowl, wildlife, pets, failing septic systems, or stormwater runoff following heavy rain. Because E. coli is an indicator of fecal pollution, elevated levels increase the risk of gastrointestinal illness and other infections. Public beaches are routinely monitored by NHDES and swimming advisories are issued when bacteria exceed state health standards.

Cyanobacteria, sometimes called blue-green algae, are naturally occurring aquatic microorganisms. Under the right conditions, especially warm water, abundant sunlight, and excess nutrients such as phosphorus, they can multiply rapidly and form blooms.

Some blooms produce toxins that can affect people and pets through skin contact or by swallowing contaminated water. If you suspect a cyanobacteria bloom, avoid contact with the water and report it using the NHDES Bloom Report Form. If the suspected bloom is on Lake Winnipesaukee, the LWA also encourages you to contact us so we can help document conditions and coordinate with NHDES as appropriate. As NHDES advises, "When in doubt, stay out."

Duck itch, also known as swimmer's itch, is different from both E. coli and cyanobacteria. It is not caused by bacteria or algae, but by microscopic parasites that naturally cycle between aquatic snails and waterfowl. If the parasites come into contact with people, they may penetrate the outer layer of the skin. Because humans are not a suitable host, the parasites quickly die, triggering an allergic reaction that causes the itchy rash known as duck itch. Is it kind of gross? Yes. Are there long-term side effects?

Not usually. Duck itch is not contagious and typically resolves on its own. It is most common in shallow shoreline areas where snails and waterfowl are abundant. Towel drying immediately after leaving the water, avoiding areas where duck itch is known to occur, and not feeding waterfowl can all help reduce your risk.

Although these are three separate issues, they can all be influenced by how we care for the lake. Stormwater runoff, aging septic systems, and nutrient pollution can contribute to poor water quality, while healthy shorelines and responsible stewardship help reduce pollution entering the lake.

Before heading to the beach, check the NHDES Healthy Swimming Mapper at des.nh.gov/water/healthy-swimming for current advisories/warnings, pay attention to posted signs, and use your own observations.

Understanding the difference between E. coli, cyanobacteria, and duck itch helps everyone enjoy our lakes safely while becoming better stewards of the resource we all share.

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Bree Rossiter is the associate director of the Lake Winnipesaukee Alliance, the only nonprofit dedicated to protecting the water quality and natural resources of Lake Winnipesaukee and its watershed through science, education, advocacy, and collaborative solutions. Her work focuses on advancing long-term watershed protection initiatives and connecting science with practical solutions to help preserve the lake for future generations. To learn more, visit winnipesaukee.org.