A waterborne, brain-destroying species of amoeba has killed 10 people in Karachi, Pakistan, this year, prompting local water officials to increase the amount of chlorine in the city’s water supply and advise residents to use sterilized water when performing a Muslim ritual which involves cleansing the nostrils.

Misbahuddin Farid, managing director of the Karachi Water and Sewerage Board, said the origin of the amoebas had not been determined, according to a report in the Dawn newspaper.

The amoeba, known by the scientific name Naegleria fowleri, is virtually impossible to detect in the water and its presence in humans is determined by spinal tests, according to information on the water board’s website.

According to the U.S. Centers for Disease Control and Prevention, the Naegleria fowleri amoeba causes a severe brain infection called primary amebic meningoencephalitis. The disease is usually fatal.

The amoeba most commonly enters the body through the nasal passages and usually infects people who have been swimming in warm freshwater lakes and rivers, according to the CDC.

But it can exist in improperly chlorinated swimming pool or heated tap water, the CDC says.

Only one of the 10 victims of the current outbreak in Pakistan had visited a swimming pool, according to another report from the Dawn newspaper, so authorities are focusing on the Muslim ablution ritual, or waddu.

“Special care should be taken when doing waddu, brushing teeth or cleaning the nasal area,” the water board’s website says.

Two people in the U.S. state of Louisiana died from the amoeba last year after apparently contracting it when rinsing their sinuses with a neti pot, a small vessel used to pour warm water into one nostril and out the other.

Three other Americans died last year after contracting the amoeba while swimming.

The Karachi water board said it would increase chlorine content in the water supply for the city of more than 15 million people after tests of more than 900 water samples taken around the city found 22% to be unchlorinated or have insufficient levels of the chemical, according to a report in the Dawn newspaper.

Saghir Ahmed, minister of health for Sindh state, in which Karachi is located, said residents should be safe if they followed the water board’s advice.

“There is no reason to panic and citizens should stay calm and take precautions,” Ahmed was quoted by Dawn as saying.