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Disease Control: Foiling Naegleria fowleri

Disease Control: Foiling Naegleria fowleri

The dreaded ‘brain-eating amoeba’ has thus far claimed seven lives in Karachi, causing residents to anxiously look for ways to protect themselves and their loved ones. The concern is real, as this amoeba has killed 29 people over the past three years with 14 deaths recorded last year.

Naegleria fowleri is sensitive to chlorine. Hence, chlorine alone is the only safeguard available. In view of the poor performance of the water service providers, citizens have taken it upon themselves to chlorinate their household water tanks. Pharmacies in many parts of Karachi have in fact rationed the purchase of chlorine and water purification tablets to a few items per customer.

Though it is the responsibility of the Karachi water agency (Karachi Water and Sewerage Board) to ensure adequate chlorine residual of 0.5 mg/L (milligrams per litre), the re-emergence of Naegleria fowleri in Karachi show that the residual chlorine is inadequate.

There is no straight forward answer to working out chlorine requirements, as there are a number of variables. These include the strength of the chlorine product, the nature of solutes and sediments in the tank and their volumes, how frequently the tank is filled up, turbidity, water temperature and. The pH scale measures how acidic or basic a substance is. It ranges from 0 to 14. A pH of 7 is neutral. A pH less than 7 is acidic, and a pH greater than 7 is basic).

Chlorine disinfecting ability is dependent on pH. When pH is 8 and above, the ability of chlorine to kill germs reduces. Ideal pH levels are 6.5 to 7.5. At pH 7, the water is neutral (i.e., neither acidic, nor alkaline). 

As the pH value goes down below 7, the water will become increasingly acidic. Likewise, the water will become more alkaline, as the pH value increases beyond 7.

Since we mostly get water from KWSB, we generally assume that the pH of this water is around 7.

If you want to be sure, pH can be easily measured by pH meters which are available in the market. They are scaled from 0 to 14. 

Despite these variables, an attempt is made to provide sample calculations. The residents should adjust the calculations according to size of their tank. Chlorine needs at least 30 minutes contact time with water to disinfect. The calculations are for using household bleach solution and the chlorine tablet.

Sample calculations

Size of tank (assumed) = 10 ft x 10 ft x 10 ft = 1,000 cubic feet = 1,000 x 6.23 = 6,230 gallons

= 6,230 gallons x 4.54 = 28,284 litres, say 28,300 litres

Using liquid household bleach (sodium hypochlorite solution)

Available chlorine in bleach = 4 per cent (typical)

This is equal to 4 grams chlorine/litre = 40,000 mg/L (mg = milligrams)

Chlorine dose = 4 mg/L

(Chlorine dose for clear water is 2 mg/L, and 4 mg/L for cloudy water. Since Karachi water is not always clear and the household tank may also be dirty, hence 4 mg/L dose is taken)

Chlorine required = 4 mg/L x 28,300 litres = 113,200 mg

Available chlorine in bleach = 40,000 mg/L

Net chlorine required = chlorine required divided by available chlorine

= 113,200 mg/40,000 mg/L = 2.82 litres, say three litres

Thus three litres of bleach (4pc chlorine) will be required to be added to the 10 ft x 10 ft x 10 ft tank on day one. Assume 50pc of the water in the tank is used on day one, add only 1.5 litres on day two, and so on.

This is the case, when the chlorine in water is zero. However, in practice, this may not be so and the water does have some chlorine in it. It is advised that only half of the worked-out chlorine requirements may be added, that is half of three litres = 1.5 litres. The dose should vary according to the chlorine odour in water. If there is strong chlorine smell, the dose should be reduced accordingly. Likewise, if no chlorine odour is detected, the dose may be increased.

Chlorine testing kits are available which can measure chlorine levels. In the absence of those kits, people should rely on chlorine odour in water. Low-to medium-level chlorine odour in water would indicate presence of chlorine in water of adequate levels.

Using chlorine tablets

Weight of 1 tablet = 8.68 grams = 8,680 mg

Available chlorine in 8.68 g is 5000 mg = 0.57pc

At 4 mg/L dose, chlorine required is 113,200 mg (see above)

No. of tablets required = 113,200 mg/8,680 mg = 13

Considering chlorine availability in tablet, the no. of tablets required = 13/0.57 = 22.8, say 23 tablets.


Chlorine irritates skin, the airways and eyes; it can cause tightness in the chest, difficulty in breathing, sore throat and wheezing. To prevent any such reaction it is advisable to wear gloves, masks and goggles while preparing a chlorine solution. Chlorine gas is heavier than air and will remain at ground level until it is moved away by air currents. Therefore, chlorine must be handled in well-ventilated areas.

The Atlanta-based US Centers for Disease Control and Prevention (CDC) guidelines for safe nasal rinsing can be found here:

CDC’s brief on Naegleria fowleri is available here: (

There are no drugs for treatment, but there are some recommended strategies — cooling the body to reduce brain swelling, and using some experimental drugs — see here:

F.H. Mughal earned his Master of Engineering in Water and Wastewater Engineering from the Asian Institute of Technology, Bangkok, in 1975.

Published in Dawn, Sunday Magazine, June 7th, 2015

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