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A 9-year-old Virginia boy has died of primary amoebic meningoencephalitis, the second case of the rare illness this month and the third this summer.

A 16-year-old Florida girl died last week of the illness, which is caused by infection with the amoeba Naegleria fowleri. A Louisiana man died in June of the same disease.

The illness is rare and nearly untreatable, according to the CDC. From 2001 through 2010, there were only 32 cases reported in the U.S. While several drugs appear to work in the lab, there is little evidence they can rescue infected people.

Indeed, the agency that a review found 121 cases from 1937 through 2007, but only one survivor. A in the New England Journal of Medicine said a 9-year-old California girl was successfully treated after contracting the infection while swimming in hot springs in the San Bernardino National Forest.

The two most recent cases were also linked to swimming in warm, fresh water, where N. fowleri thrives, but news reports said the Louisiana man contracted the infection after flushing his sinuses with tap water.

Virginia state epidemiologist Keri Hall, MD, said that an autopsy confirmed the boy's death was caused by the amoeba, but did not give further details. News reports, however, said he had died August 5 after a visit to a Virginia fishing camp the week before.

N. fowleri invades the central nervous system through the cribriform plate and can be found in the subarachnoid and perivascular spaces, according to the CDC. The subsequent inflammation of the olfactory bulbs progresses rapidly to the cerebral hemispheres, brain stem, posterior fossa, and spinal cord.

One clinical difficulty is that symptoms, which occur within seven days of exposure, are the same as those of fulminant bacterial meningitis, including headache, fever, anorexia, vomiting, signs of meningeal inflammation, altered mental status, and coma.

As well, the cerebrospinal fluid mimics what is seen in bacterial meningitis, meaning that primary amoebic meningoencephalitis is typically diagnosed only at autopsy.

The CDC said the key to diagnosis before death is "clinical suspicion based on history," especially if a previously healthy patient has been exposed to fresh, warm water within a week of symptoms. Other signs include the appearance of bacterial meningitis, with a predominantly basilar distribution of exudate, on a CT scan of the head.

The CDC said successful therapy in the three reported cases appeared to be the result of early diagnosis, followed by intravenous and either intrathecal or intraventricular amphotericin B, combined with supportive care.

The California girl was also given miconazole intravenously and intrathecally along with oral rifampin, according to the 1982 NEJM report.

In the Florida case, Courtney Nash was unsuccessfully treated with amphotericin B, fluconazole, rifampin, and azithromycin.

Virginia epidemiologist Hall said swimmers should be aware of the risk, especially during periods of no rain and high temperatures and avoid swimming in stagnant or slow-moving bodies of water.

"It's important that people be aware of these safe swimming messages," Hall said in a statement.

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