Primary amoebic meningoencephalitis (Naegleria fowleri)

Naegleria fowleri lives independently in the environment and does not need a human host. The CDC describes it as an amoeba found in warm fresh water around the world. It can cause primary amoebic meningoencephalitis, an infection of the brain and its coverings that begins after the organism travels up the nose and along the nerves to the brain.

The word primary distinguishes it from other amoebic brain infections that arise after the organism has spread from elsewhere in the body. PAM is an environmental accident: people become infected through exposure to a particular kind of water, not through contact with a patient.

The CDC says most infections follow swimming, diving or other activity in warm fresh water. A second, less common route is using tap water that contains the amoeba for sinus or nasal rinsing, for example with a neti pot. Swallowing contaminated water does not cause PAM, and the amoeba cannot be passed between people.

The risk is tied to warm, still fresh water such as lakes, rivers and hot springs, and to water systems that are not adequately disinfected. It is not a risk of well-maintained, chlorinated swimming pools or sea water.

• Hold your nose shut, use nose clips or keep your head above water in warm fresh water and hot springs.

• Avoid digging in or stirring up sediment in shallow warm fresh water.

• Use distilled, sterile or previously boiled and cooled water for nasal rinsing, not water straight from the tap.

The CDC lists early symptoms as headache, fever, nausea and vomiting. Later features include a stiff neck, confusion and hallucinations. Because the early stage looks like bacterial meningitis, it is usually treated and investigated as such first.

The CDC reports that the illness usually causes death within 1 to 18 days of the start of symptoms and typically after about 5 days. Time is therefore critical, which is why clinicians who hear about recent warm fresh-water exposure pass that history to the laboratory and the infection specialists immediately.

Between 1962 and 2024 the CDC recorded 167 cases in the United States with only 4 survivors, a death rate of 97.6 per cent. Its clinical guidance later refers to five well-documented survivors in North America, all of whom received aggressive combinations of drugs that included amphotericin B.

The same pages stress how uncommon the disease is. In the United States, fewer than 10 people a year develop PAM despite very large numbers of people swimming in fresh water. For a UK reader, the practical risk from ordinary swimming holidays is extremely low, although the infection has been reported across the world in warm regions.

Diagnosis depends on finding the amoeba in cerebrospinal fluid or brain tissue, or on molecular tests, alongside a history of exposure. Clinicians who suspect PAM contact national reference laboratories and infection specialists urgently.

The CDC describes treatment as a combination of drugs rather than a single agent, including amphotericin B given into a vein and into the spinal fluid, rifampicin, azithromycin, posaconazole, miltefosine and dexamethasone to control brain inflammation. No treatment has been shown to reliably work because the disease progresses so fast, and decisions sit with intensive care and infectious diseases teams. Nothing here is a treatment recommendation.

A sudden severe headache with fever, vomiting, a stiff neck, sensitivity to light, drowsiness or confusion is an emergency whatever the cause. Call 999 or go to A&E, and tell the team about any recent swimming in warm fresh water or nasal rinsing with tap water, including when and where it happened.

Meningitis of any type can deteriorate within hours, so do not wait for a rash or for symptoms to settle overnight.