Red spot disease confirmed in Hartbeespoort Dam

The faculty of veterinary sciences of the University of Pretoria has identified Epizootic Ulcerative Syndrome (EUS), or red spot disease in fish in the Hartbeespoort Dam after fishermen reported ulcers and red spots on fish, as well as abnormal mortalities, since January this year.

Veterinarians Drs Ken Pettey, Gillian Taylor and Johan Steyl assisted with collection of samples of affected fish, and basic water analyses and it was confirmed that the fish were infected with EUS. According to Dr Gillian Taylor of the department of Paraclinical Sciences at the veterinary faculty, this is the second case of EUS disease reported on the Crocodile River system, but the first ever confirmed within the dam itself.

The outbreak has been reported to the fisheries and forestry department as well as the World Animal Health Organisation. “Because of its low occurrence so far within South Africa, yet its potential to rapidly spread through multiple fish species within a water body, with potential impact upon recreational use as well as food fish, it is a controlled disease in SA, meaning that any positive outbreaks are reportable,” she says in a report.

“This disease is rare in SA. First identified in Asia, it was confirmed in Africa in the Zambezi river system in 2006 (Huchzemeyer et al, 2012), and there have been isolated cases thereafter in the Western Cape, Northern Cape and one in Northwest Province. It is thought to have been introduced into South Africa through unregulated movement of fish from infected countries. Botswana, Namibia, Zambia and Zimbabwe have all been affected by this disease.”
EUS is caused by an “atypical water mould” (Aphanomyces invadens )similar to a fungus. It is also known as “Red-Spot Disease”. It typically seems to appear after seasonal water changes, like a drop in water temperatures to 18-220C or after heavy rainfall or flooding. Dormant spores within the water body are triggered to release large numbers of swimming zoospores. These selectively target already damaged skin or existing wounds on fish. So, fish already under stress from poor water quality, high parasite levels or other diseases, are most susceptible. The zoospores rapidly attach to the fish, and grow branching “arms” that invade the host muscle primarily causing severe muscle damage and ulcers ranging from 5-25mm in diameter.
This disease spreads quickly through the water body between fish. All fish that are affected are thought to succumb, however with time resistance to the disease may develop. High mortality levels are typical of the disease.

Many different species of wild and farmed freshwater and estuarine warmwater fish are susceptible including catfish, cichlids, barbs, bream and bass. Common carp and Nile Tilapia are considered resistant while catfish are highly sensitive. Young fish tend to be targeted.

Within a large open natural water body, no treatment of the fish or system is practically possible. Containment of the disease is also challenging within such a system, however implementation of basic biosecurity and awareness of the disease will go a long way in keeping it under control and preventing spread to other water bodies along this river system.

Management of this disease needs to be a coordinated group effort between all parties on the dam, as well as up and downstream of the dam.

Take note of the folloiwing precautionary measures:

  • Report all fish with sores and high numbers of fish death
  • No live fish should be moved from Hartebeespoort dam to another water body. Spores can potentially be transferred between water bodies like this and so the disease would spread.
  • All dead fish need to be promptly removed from the water as they pose an increased risk of spreading the disease to healthy fish. They need to be destroyed in a bio-secure manner: incineration is best.
  • All dying fish in the dam need to be removed and humanely dispatched.
  • Any equipment in contact with dam water can potentially transfer the disease. Disinfect all equipment and boats.