The US Lakes Where Brain-Eating Amoeba Has Been Found

Naegleria fowleri is a microscopic, free-living amoeba found in soil and warm freshwater, including lakes, rivers, ponds, and hot springs. If water containing the organism travels up the nose, it can cause primary amebic meningoencephalitis, or PAM, a rare infection that destroys brain tissue. The organism is often associated with the South, but confirmed detections and infections have occurred much farther north. CDC recorded 180 diagnosed US cases from 1937 through 2025, with between zero and eight cases in any single year. There is also no rapid, standardized test used to determine whether every recreational lake contains the amoeba. The lakes below stand out because Naegleria fowleri itself has been identified in environmental samples, rather than the lake simply being named as a possible exposure site after an infection.

Lake Pontchartrain, Louisiana

Lake Pontchartrain has some of the strongest environmental evidence for Naegleria fowleri in a large US water body. Researchers collected surface water at 10 recreational sites around the lake between May 2016 and February 2017. Of 158 samples successfully analyzed, 56 contained the target DNA for N. fowleri, a detection rate of 35.4 percent. The organism was found at every sampling site during the study period, with measured concentrations ranging from 11.6 to 457.8 gene copies per 100 milliliters of water.

A later study examined the lake bottom and found N. fowleri in 51.25 percent of sediment samples. That matters because sediment can provide habitat for the amoeba and may be disturbed in shallow water. Lake Pontchartrain is technically a brackish estuary rather than a conventional freshwater lake, yet its relatively low salinity still allowed researchers to repeatedly detect the organism in both water and sediment.

Lake Granbury, Texas

Researchers surveying Lake Granbury in late summer 2010 found Naegleria fowleri across a surprisingly large portion of the reservoir. Water was collected from 42 locations during hot, dry conditions, when the average water temperature measured about 32.1 degrees Celsius, or 89.8 degrees Fahrenheit. N. fowleri DNA appeared at 28 of the 42 sites, meaning roughly two-thirds of the reservoir locations tested positive. Estimated concentrations reached as high as 26 cell equivalents per 100 milliliters.

The study also discussed two confirmed PAM cases from 1991 and 2010 that might have resulted from swimming in Lake Granbury. Researchers were careful not to describe the reservoir as the proven source because the exact place and time of exposure could not be established. The environmental testing itself, however, provided direct evidence that the amoeba was widely distributed in the reservoir during the sampling period.

Lake of Three Fires, Iowa

Lake of Three Fires is one of the clearest recent examples of health officials tracing an infection back to a recreational lake and then finding the organism in the water. In 2022, a Missouri resident developed PAM after swimming at Lake of Three Fires State Park in southwest Iowa and later died. Iowa health officials temporarily closed the swimming beach while environmental testing was carried out with assistance from the CDC.

Iowa Health and Human Services and the Department of Natural Resources subsequently announced that testing had confirmed Naegleria fowleri in the lake. The beach later reopened with signs explaining the presence of the amoeba and ways swimmers could reduce exposure. Officials did not begin routine testing of other Iowa lakes because there is no rapid standardized test that can reliably determine whether a natural water body is safe from N. fowleri at any particular moment.

Baldwin Lake, Florida

Baldwin Lake was the site of a concentrated run of Naegleria fowleri detections during the same Orange County investigation. Direct testing found the amoeba's DNA in both water and sediment samples collected in July, August, and September 2013. Researchers also successfully cultured N. fowleri from a July water sample. Overall, 20 percent of the samples collected from Baldwin were positive, the second-highest detection frequency among the repeatedly studied lakes.

The timing was striking because earlier samples from the lake had been negative. From December through early July, investigators repeatedly failed to detect the organism before the series of positives began later in the summer. By the October sampling event, both water and sediment were negative again. The sequence shows why a single test cannot describe conditions in a lake for an entire swimming season. N. fowleri populations can change substantially within the same body of water over a period of weeks.

Big Carnelian Lake, Minnesota

Sampling at Big Carnelian Lake showed that Naegleria fowleri can occur well outside the states traditionally associated with PAM. Washington County researchers sampled 10 Minnesota lakes annually between 2011 and 2014, testing both recreational water and bottom sediment. In 2011, Big Carnelian was positive in both sample types. Neither the water nor sediment produced another detection during the next three years of the survey.

The lake is about 457 acres and reaches a maximum depth of roughly 66 feet, so it differs considerably from the shallow, intensely heated ponds often associated with discussions of the amoeba. The researchers did not report a PAM case linked to Big Carnelian. Its importance comes instead from the environmental sampling, which demonstrated that the organism was present during at least one Minnesota summer and reinforced evidence that its US range extends beyond the southern states.

Lake Jessamine, Florida

Lake Jessamine was another clear positive in the Florida Department of Health and CDC survey. Direct molecular testing detected Naegleria fowleri in water collected in June 2013, then detected it again during the October sampling event. A sediment sample collected in June also produced a positive culture result. Those findings are useful because they came from different sample types and more than one laboratory method.

Jessamine covers roughly 292 acres and had a mean depth of about 16 feet in the study data. Unlike Baldwin, it was not sampled every few weeks throughout the entire investigation, so researchers could not reconstruct the same detailed seasonal pattern. Even so, repeated positive water tests separated by several months, coupled with the culture result from sediment, provide direct evidence that N. fowleri occurred in the lake environment during 2013.

Demontreville Lake, Minnesota

Demontreville Lake was one of five Washington County lakes where researchers detected Naegleria fowleri during the particularly warm summer of 2011. The positive result came from sediment, while the water sample collected at the same lake was negative. Annual sampling in 2012, 2013, and 2014 did not produce another detection.

That difference between water and sediment is scientifically important. N. fowleri can occur in material on the lake bottom even when a water sample taken above it does not detect the organism. CDC advises swimmers not to dig into or stir up sediment in shallow warm freshwater partly for this reason. Demontreville also helps demonstrate the limitations of describing a lake simply as positive or negative. The amoeba was found during one sampling period, then went undetected during three subsequent years of the same surveillance program.

Lake Conway, Florida

Lake Conway was one of the larger lakes sampled in the Florida investigation, covering about 1,773 acres with a reported mean depth of 23 feet. Water and bottom samples collected during the first two major sampling rounds were negative for Naegleria fowleri. In October 2013, however, investigators successfully detected the organism by culturing a sediment sample after immunomagnetic separation, a laboratory technique used to isolate the target amoeba from other material.

No Lake Conway water sample tested positive during the study, so the evidence is more limited than at Baldwin or Jessamine. The positive culture still establishes that viable N. fowleri was recovered from the lake environment. It also supports the broader pattern seen in several investigations: bottom sediment can reveal the organism when testing of the overlying water does not.

Stay Safe In Warm Freshwater

Finding Naegleria fowleri in a lake does not mean swimmers are likely to become infected. PAM remains extraordinarily rare in the United States, despite enormous numbers of people entering freshwater every summer. Infection occurs when water containing the amoeba enters the nose and the organism travels toward the brain. Swallowing contaminated water does not cause PAM.

CDC recommends taking greater care during hot periods when freshwater temperatures are high and water levels are low. Swimmers can reduce exposure by keeping water out of the nose, holding the nose shut or using a nose clip when jumping or diving, and avoiding digging or stirring up sediment in shallow warm water. Because there is no rapid standardized test that can declare a natural lake free of Naegleria fowleri, CDC advises treating warm freshwater as a possible habitat for the organism rather than relying on whether a particular lake has previously produced a positive sample.