A 9-year-old boy died Tuesday from a suspected rare form of meningitis caused by a freshwater amoeba, and Lily Lake in Stillwater has been closed to swimming in the aftermath, the Minnesota Department of Health reported Tuesday night.

If confirmed by the federal Centers for Disease Control and Prevention, the case will be the second in the state in three years. A 7-year-old girl who died of the disease in 2010 also had been swimming in the shallow Stillwater lake.

On Tuesday evening, sheriff's deputies were posting no-swimming signs on the beach and going door to door alerting neighbors to the closure.

Officials from the Health Department believe the boy had become ill with primary amebic meningoencephalitis (PAM), a rare form of meningitis caused by an amoeba known as Naegleria fowleri that is associated with freshwater areas. The amoeba causes a very rare but severe brain infection that is often fatal. The organism infects humans by entering the body through the nose, when people do such activities as swimming and diving in warm freshwater.

Richard Danila, the assistant state epidemiologist, said he hopes to have the cause of death confirmed this week by the CDC.

The boy, who was not identified Tuesday, had gone swimming at a number of locations in Washington County in the two weeks before he became ill. Authorities said they believe Lake Lily was the likely source of infection.

Naegleria is commonly found all over the world. However, infections are very rare. Forty cases were reported in the United States from 2001 to 2011, nearly all in the South.

"Our case in 2010 was by far the northernmost case," Danila said Tuesday night. "We said two years ago that this might be a harbinger of things as we are getting warmer, the water temperatures clearly are getting warmer. We used to think this was only in the far South, but it could happen anywhere because temperatures are increasing and water temperatures are increasing."

A mid-July reading at Lily Lake logged a water temperature of 85 degrees, Danila said.

The presence of algae, common in the late summer, isn't enough of a red flag for swimmers, Danila said. But people should be wary of warm, brackish water. "It has to do with temperature more than anything," Danila said.

He reiterated the infection is incredibly rare. "You have to imagine, every single day millions of people swimming in hundreds of thousands of lakes across the country," he said.

"Swimming is a very healthy summertime activity, and we do not want to discourage people from swimming," Danila said earlier in a written statement. "Rather, simply avoid swimming, diving or other activities in obviously stagnant water when temperatures are high and water levels are low."

Other precautions include keeping your head out of the water, using nose clips, pinching the nose shut and avoiding stirring up sediment.

The infection has an incubation period of up to two weeks, Danila said. Symptoms include headache, confusion, slurred speech and blurred vision.

Annie Elizabeth Bahneman, the girl who died from the disease in 2010, had been swimming in several bodies of water, including Lily Lake, before her death. She died four days after she fell sick with vomiting and a headache.

More information is at www.startribune.com/a1611.

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