Bacteria in Zambezi likely cause of fish disease

An infected fish caught in the Zambezi River has holes probably caused by worms Copyright: Moses Magadza

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[WINDHOEK] Namibian scientists have found dangerous levels of bacteria in the backwaters of the Zambezi River, where a mysterious and seemingly fatal disease outbreak among fish was recently detected.

Fishermen in Katima Mulilo, Namibia, began reporting serious sores on fish in early October, which is thought to affect up to four out of every five fish caught (see ‘Deadly infection hits Zambezi fish’).

Baffled scientists have sampled fish and water from areas where the infection was reported, and tested them for pathogenic bacteria.

The results showed dangerously high levels of coliforms — a group of bacteria that indicate the presence of disease-causing microbes — as well as E. coli, which can be toxic and cause severe illness, at all sites.

They also found salmonella — a deadly bacterium that causes food poisoning and diarrhoea — at one site.

The scientists suggest that sewage leaking into the river from the Caprivi region in Namibia could have caused the high levels of bacteria in the Zambezi. The river is Africa’s fourth largest and runs from Namibia into the Indian Ocean, supporting some 40 million people.

To protect people’s health, Namibia has declared a raft of measures, including a ban on fishing in the Zambezi from midnight today (21 December) until 31 January 2007, pending the outcome of further laboratory tests.

The fisheries ministry will deploy two patrol boats to enforce the ban.

In Zambia, the main country through which the Zambezi runs, a ban was not necessary as the fishing season does not start until 28 February. Zambia has also conducted tests but is yet to announce results.

In Botswana, a team of experts conducting tests in the Chobe tributary, where fishing is not allowed as it is part of a national park.

The infection causes blisters and sores, and eats away at the fins and tails of multiple fish species — notably breams, minnows and catfish — eventually killing them, although the fatality rate is unknown.

“The fish samples from each site were analysed for bacteria and for overall coliform,” Abraham Iyambo, Namibia’s fisheries minister said at a briefing.

Safe levels of coliforms should not exceed 10 per 100 millilitres of water.

But the scientists detected coliform levels above 2,419 per 100 millilitres in water at four backwater sites in Namibia: the Kasaya Channel, Chobe River, Chisambilo Lake and the area between Mpukano Channel and Nankuntwe.

Similarly, coliform levels in fish samples from the same areas were exceedingly high.

Salmonella was also detected at Mpukano, while E. coli was detected at all the sites sampled, with particularly high levels at Mpukano Channel and Nankuntwe.

Iyambo attributed the presence of the bacteria to leakage of sewage systems, and suggested that the wounds seen on some infected fish could be due to worms, such as roundworms and tape worms, that thrive in sewage systems.

He said people were in danger of suffering from diarrhoea, typhoid, amoebiasis, and schistosomiasis.

“We recommend to the community that water used for drinking purposes should be boiled for at least five minutes or more,” said lyambo, advising people to cook food thoroughly and to only use deep latrines to avoid further pollution.

He said other countries that share the Zambezi River would be contacted over the matter.