Zambezi fish disease mystery cracked

Diseased fish caught in the Zambezi River Copyright: Moses Magadza

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[WINDHOEK] Scientists have identified the mystery disease that killed fish in parts of the Zambezi River last year.

Researchers have identified the disease as Epizootic Ulcerative Syndrome (EUS), caused by a fungal pathogen. Infected fish develop large sores and die from secondary infections.

The researchers say this is the first known outbreak of the disease in Africa.

But they still don’t know how the pathogen got into the Zambezi, which flows through eight southern African countries.

EUS also affects fish in Australia, the United States, and many countries in Asia. When EUS broke out in Asia in the 1970s, approximately 80 per cent of the fish population perished.

In December last year fishermen in Namibia reported finding sores on fish caught in the Zambezi (see Deadly infection hits Zambezi fish). The government imposed a two-month ban on fishing to safeguard the public. A similar ban was imposed in Botswana, and was only lifted at the end of March this year.

Shaft Nengu, a member of the research team and Botswana’s assistant director of fisheries, said the spread of the disease downstream is inevitable. The research team has warned that the disease could become pandemic, damaging aquaculture, fisheries and aquatic biodiversity.

“We do not have the capacity to establish the extent of the outbreak. We are trying to come up with a regional effort to respond to this outbreak and have written to all countries that share the Zambezi River to support this initiative,” Nengu said.

Nengu said the fungal pathogen does not pose any human health implications, but that fish exhibiting sores, which could harbour secondary infections, should still be treated with caution.

Zimbabwean microbiologist Percy Chimwamurombe said that it is “presumptuous” to say that people will not be affected, and a coordinated regional response is essential to determining the extent and effects of the outbreak.

“There is need for a concerted public awareness campaign given the possibility of secondary infections, which can be terrible,” he said.

Fish biologist Ben van der Waal, from the Integrated Management of the Zanbezi/Chobe River System Fishery Resource Project and also a member of the research team, said there was nothing anybody could do to eradicate the outbreak now that it was in a natural setting.

He warned that fish losses during the first few years of the outbreak may be colossal, and that it would take many years to adapt to the disease.

“In Asia it took about 20 years for the outbreak to go down to endemic levels,” he said.

The team of scientists included experts from the United Nations Food and Agriculture Organization, Thailand’s Inland Aquatic Animal Health Research Institute, and the Network of Aquaculture Centres in the Asia-Pacific.