What began as an E. coli outbreak in three children who swam in Lake Nokomis has now grown to 49 people who showed symptoms of bacterial infection, the Minnesota Health Department said Friday.

The south Minneapolis lake’s two beaches will be closed for the rest of the season out of an “abundance of caution,” Minneapolis Park and Recreation Board officials said.

It is the largest waterborne E. coli outbreak in several decades, according to Trisha Robinson, supervisor of the waterborne diseases unit at the Health Department. “Fortunately we do not see a lot of beach outbreaks,” she said.

The outbreak that sickened nearly 200 people who were near Big Island in Lake Minnetonka over the July 4th weekend was most likely caused by a virus, Robinson said.

The Lake Nokomis beaches were first shut down Tuesday after the Health Department determined that the children were infected with Shiga toxin-producing E. coli, a strain that can cause serious illness.

The Park Board has posted signs and even strung yellow caution tape along the beaches.

“Even though the beach is closed, we have a small crew of lifeguards whose sole purpose is education and telling people to stay out of the water,” said Dawn Sommers of the Park Board.

Despite the beaches being closed, other activities at the lake remain the same, including boating and using the park’s trails.

News stories about the infections prompted some people to call the Health Department. Health officials also reached out to swimmers who were known to have visited Lake Nokomis recently, such as those who registered for swimming classes or participated in a triathlon.

The 49 people sickened, including adults and children, all developed diarrhea after taking a dip in the lake sometime between July 16 and Aug. 11. None were hospitalized.

“It is quite likely that that number will change,” Robinson said, as more cases are reported to the Health Department.

Because exposures occurred over nearly a month, disease detectives suspect that one or more ill people introduced fecal matter into the water. Even swallowing a small amount of infected water can lead to illness.

“It is still important that anybody that is sick with diarrhea shouldn’t be going swimming anywhere,” Robinson said.

Any Lake Nokomis swimmers who have developed infection symptoms, which include diarrhea, often bloody, as well as stomach cramps or low-grade fever, should see a health care provider, health officials said. Most people become sick within five days of exposure, but it can take longer in some cases.

Swimmers are also asked to complete an online survey if they were at Lake Nokomis between mid-July and mid-August.

This year has seen a record number of Minneapolis beach closures, but park officials said seven beaches are open through Labor Day, including two beaches at Lake Harriet. The beach at Wirth Lake reopened Thursday.

In addition, the Park Board has dozens of pools that are open, including wading pools located at the Lake Hiawatha, Bossen and Keewaydin parks.

The Lake Nokomis outbreak is the first time in two decades that a Minneapolis lake beach was closed due to illness.

Most beaches close when tests show the presence of E. coli, which can be caused when rains wash animal waste and other bacteria sources into the lakes.

“We’re very proactive when it comes to closing just because we have so many other water resources that we can direct people to,” said Sommers.