How to Avoid Waterborne Parasites When Swimming in Tropical Lakes and Beaches

Recent deaths linked to a brain-eating amoeba in artificial lakes in Mérida, Mexico, have renewed attention on a question many international travelers ask before a tropical trip: how dangerous is swimming in lakes, rivers and hot springs, and what actually protects you? According to the US Centers for Disease Control and Prevention (CDC) and other health sources, the two parasitic or amoebic threats most relevant to swimmers in warm fresh water are Naegleria fowleri, which causes a rare but almost always fatal brain infection, and schistosomiasis, a parasitic worm disease common in parts of Africa, Asia, the Middle East, South America and the Caribbean. The ocean and chlorinated swimming pools are not risk factors for either.

What happened in Mexico

In September 2026, a person died of primary amebic meningoencephalitis (PAM) in Mérida after exposure to water at an artificial lake in a commercial plaza, according to The Yucatán Times; the outlet did not report the person's age or occupation. Following the death, state health authorities tested nine artificial water bodies across the city; six came back positive for amoebas of the Naegleria genus, which includes Naegleria fowleri, while three — the central lake at the Cabo Norte complex, the Acuaparque lake and the Animaya Zoo lake — tested negative. By October 15, 2026, officials had closed six artificial lakes — Plaza La Isla, the east and main lakes at the Cabo Norte complex, the Centenario Zoo lake, the Gran Parque La Plancha lake and the West Ecological Park lake — installed containment barriers and begun sanitation work with researchers from the Autonomous University of Yucatán (UADY) and the Center for Research and Advanced Studies (Cinvestav), the outlet reported.

Yucatán state health officials said infections occur only when contaminated water enters the nasal passages, and stressed that the amoeba is not transmitted person-to-person or through drinking water, according to the same report. Parks and zoos in the area remained open to visitors even as water contact was suspended, and an interinstitutional task force continued monitoring other bodies of water in the region.

What Naegleria fowleri actually is

Naegleria fowleri is a free-living, one-celled organism that occurs naturally in soil and warm fresh water — lakes, rivers, ponds and hot springs — worldwide, according to the CDC. It has also occasionally been found in poorly maintained swimming pools and in tap water. Infection happens only when contaminated water is forced up the nose, typically during swimming, diving or other water sports; the organism then travels along the olfactory nerve to the brain, where it destroys tissue and causes PAM. Swallowing water containing the amoeba does not cause infection, and the disease cannot spread between people.

Early symptoms — headache, fever, nausea and vomiting — usually appear within about a week of exposure and resemble bacterial meningitis, the CDC states. As the disease progresses, patients can develop a stiff neck, confusion, loss of balance and hallucinations. Most people who develop PAM die within one to eighteen days of symptom onset, and the disease often causes coma and death within five days. The CDC reports that of 167 known US cases between 1962 and 2024, only four people survived, putting the fatality rate above 97%. Despite the severity, actual cases are very rare: the CDC counts fewer than 10 PAM cases per year in the United States, with risk rising in warmer months when water temperatures climb and water levels drop.

How to reduce the risk in warm fresh water

The CDC says the only way to be certain of avoiding Naegleria fowleri is to stay out of warm fresh water altogether. For travelers who do swim, the agency recommends treating all lakes, ponds, rivers and hot springs as though the amoeba could be present, since it cannot be tested for in real time at most destinations.

1. Hold your nose shut or wear a nose clip whenever you jump or dive into fresh water.
2. Keep your head above the surface in hot springs and other naturally heated (geothermal) water.
3. Avoid digging in or stirring up sediment in shallow fresh water, since the amoeba is more likely to concentrate there.
4. Limit swimming in warm fresh water during periods of high heat, when water temperatures rise and water levels fall.
5. Choose properly maintained, chlorinated pools or ocean swimming where Naegleria fowleri is not a concern.

Schistosomiasis: the other major freshwater parasite risk

The second major parasitic threat tied to swimming in tropical fresh water is schistosomiasis, caused by parasitic flatworms released into water by infected freshwater snails. Unlike Naegleria fowleri, this parasite enters the body by penetrating the skin rather than through the nose, and infection can occur through wading, bathing or other contact with contaminated fresh water. The CDC states that more than 200 million people worldwide are infected with schistosomiasis, calling it the second most dangerous parasitic disease after malaria. The agency lists the highest-risk areas as southern and sub-Saharan Africa, with additional transmission in North Africa, parts of southern China and Southeast Asia, the island of Corsica, parts of the Middle East, and, in the Americas, Brazil, Suriname and Venezuela, plus lower-level transmission in some Caribbean islands such as Puerto Rico and the Dominican Republic.

The CDC states there is no vaccine for schistosomiasis, and the only reliable prevention is avoiding swimming, wading or bathing in fresh water in regions where the disease is common. If freshwater contact for bathing is unavoidable, the CDC advises boiling the water vigorously for at least one minute, treating it with chlorine at roughly 1 milligram per liter and waiting 30 minutes, or letting stored water stand for one to two days before use; the agency notes that iodine treatment alone does not reliably make water safe from this parasite.

Comparing the two main risks

Practical checklist for travelers

• Ocean swimming and properly chlorinated pools carry no risk of either Naegleria fowleri or schistosomiasis, per the CDC.
• Ask local health authorities or your accommodation whether nearby lakes, lagoons or rivers have had recent water-quality alerts, as Mexican authorities did in Mérida in 2026.
• In warm freshwater lakes, ponds or hot springs anywhere in the world, use a nose clip or hold your nose when jumping or diving in.
• In countries where schistosomiasis is endemic, avoid wading, bathing or swimming in natural fresh water, including seemingly clean irrigation canals, ponds and slow-moving rivers.
• Seek medical care immediately if severe headache, fever, stiff neck or confusion develop within about a week of freshwater exposure, and tell the clinician about the water exposure.