Photo/Illutration A sign urging people to refrain from swimming was set up near the Todoroki no Taki waterfall in Amakusa, Kumamoto Prefecture, after many people fell sick. (Junki Watanabe)

AMAKUSA, Kumamoto Prefecture—The recent illnesses of dozens of people here underscored the risks to health lurking in river water.

According to the Kumamoto prefectural government, seven high school students and others complained of diarrhea and vomiting a couple of days after they played in the Shimotsu-Fukaegawa river near the Todoroki no Taki waterfall on Aug. 13.

Forty-one people contacted public health centers about similar symptoms.

The number of individuals, including children, who received treatment for the illness at medical institutions in Amakusa reached 124 by Aug. 27.

Although none had severe symptoms, one was temporarily hospitalized.

The prefectural government said on Aug. 27 that the norovirus was the most likely cause of the illness.

Medical institutions and the prefecture examined stool samples of 16 patients and detected the norovirus in six of them. Prefectural government tests on four patients uncovered the genotype of the virus.

Water samples taken from the area around the waterfall on Aug. 19 and 23 were examined, and the norovirus was detected in the waterfall basin and upstream.

However, the prefecture said the genotype was different from that detected in the patients' stool samples.

“There is a high possibility that the norovirus was the cause, but it could not be confirmed,” a prefectural government official said.

The prefectural government set up a warning sign about the health hazard near the 5-meter-tall waterfall, which lies along a mountain trail and flows into a river that runs underneath a cable-suspended bridge.

According to a 22-year-old part-time worker near the waterfall site, the area is popular among junior and senior high school students because they can jump into the river from rocky ground above the waterfall.

But there were hardly any visitors on Aug. 23.

RIVER DISEASE

Two years ago in Fukuoka Prefecture, five people in their late teens and 20s suffered fevers, vomiting and headaches after playing in a river. Three were confirmed to have contracted leptospirosis, a disease caused by bacteria, which had been initially listed as a possible cause of Amakusa cases.

People can contract leptospirosis while swimming in or drinking water from a river contaminated with urine or feces of infected mice and other wild animals, said Kenji Sakai, an internist based in Fukuoka.

According to the National Institute of Infectious Diseases, 273 individuals were infected with leptospirosis in 29 prefectures between 2016 and 2022.

Okinawa Prefecture topped the list with 171 infections, or 60 percent of the total, followed by Kagoshima Prefecture with 17 cases.

Okinawa prefectural officials said 245 people were infected during the 18 years to 2020, 72 percent of whom contracted the disease from rivers.

Giardiasis, a parasitic disease, and infections from the Yersinia bacterium, could also pose health risks in rivers. Both had been named as a possible cause of Amakusa symptoms by some medical specialists.

PRECAUTIONARY MEASURES

Osamu Doi, a doctor who heads the Kumamoto branch of the Japanese Alpine Club, said the number of deer, boars and other wild animals has sharply increased in mountains across the Kyushu region.

“I see their bodies and feces while climbing. It is only natural for them to flow into rivers and become a source of infection,” he said.

Doi advises people to avoid drinking river water or entering rivers with open wounds or injuries.

(This story was written by Junki Watanabe and Ryutaro Ito.)

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