Geese are the likely culprits behind the closure of a popular Bellevue beach this week.

High levels of Escherichia coli, a common bacteria found in the feces of warm-blooded animals in the water, caused public health officials to shut down the swimming area of Meydenbauer Bay Park on Friday, according to James Apa, a spokesperson for Public Health – Seattle & King County.

While most strains of E. coli are actually helpful and found normally in the guts of healthy animals such as dogs, cats, waterfowl and in humans, some strains are known to cause illness and sometimes death, particularly in young children and older adults.

Closures of swimming areas in King County are most often due to high levels of E. coli, according to Apa, who said the presence of it can indicate the presence of other disease-carrying organisms, which live in the same environment.

“Fecal coliform is a proxy measure for the kinds of bacteria that are more likely to make people sick at high enough concentrations,” Apa said Friday.

It is incorrect, however, to say the beach was closed because of an E. coli outbreak, according to Daniel Nidzgorski, an ecologist with the King County Department of Natural Resources and Parks.

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“In fact, E. coli are mostly harmless bacteria, part of the “good gut bacteria” in people, pets, and wildlife. Very few strains of E. coli make people sick, but those few are the ones that make the news,” he said Friday.

Public health officials are notified when E. coli is found at certain levels at any of the almost 30 King County beaches tested weekly by the King County Department of Natural Resources and Parks. Officials then decide whether to close the waterfronts to all swimmers and waders.

Fecal coliform can enter waterways by failing septic systems, sewer overflows, inadequate treatment of municipal waste, runoff from animal pastures, or inadequate human waste disposal.

When tests rule out human and canine fecal matter, officials are left to suspect geese — usually present in large numbers around Puget Sound waterways.

At most lakes in the region, established bird populations are suspected to be the source of the bacteria, according to Nidzgorski.

During a wave of beach closures last year, Nidzgorski explained that while tests to determine whether E. coli is from people or pets are accurate, tests for waterfowl are not.

“We think, but can’t really prove, that geese and ducks are responsible for many of the beach closures around here,” he said at the time.

Residents who live near lakes can help combat geese droppings by planting native vegetation along the shoreline. The buffer of plants will discourages waterfowls, which prefer open grassy areas and succulent lawns, according to the public-health department.