Fish for Development baseline study to map the disease occurrence in tilapia farms on Lake Volta

The Norwegian Veterinary Institute and the Fisheries Commission, in collaboration with the University of Ghana, conducted a baseline survey of pathogens in tilapia farms on Lake Volta as part of the Fish for Development, Ghana (gha-17/0005) project. Thirty farms were selected for inclusion, representing key operation types (hatchery, nursery, grow-out) and farm sizes. Farm visits and sampling was conducted between 25.08.21 and 07.12.21.

Samples included tissues/eggs in RNAlater® for virological analyses, tissues in 10% buffered formalin for histopathological analyses and samples on various growth media for bacteriological analyses. In addition, farm data was collected via a questionnaire. Protocols developed and validated for the detection of Infectious Spleen and Kidney Necrosis Virus (ISKNV) and Tilapia Lake Virus (TiLV) in Ghanaian farmed tilapia by the University of Ghana West African Centre for Cell Biology of Infectious Pathogens (WACCBIP), were used to test the survey samples.

Bacteriological assessments were conducted by a combination of bacterial colony morphology, staining and MALDI-TOF analysis (mass spectrometry), with additional characterisation through 16S sequencing for Streptococcus agalactiae isolates. Histopathological assessments were conducted by the NVI, with digital slides to be shared with Ghana.

ISKNV was found to be widespread and was detected in 80% of sampled farms. It was primarily detected in smaller fish, however, adult fish, broodstock and eggs were also found positive. The detection of ISKNV in eggs provides an opportunity to break the infection circle before frys are transferred to grow-out facilities. Not all ISKNV-positive fish displayed clinical signs of infection. In one group of fry, typical histological signs of ISKNV associated disease were found, while PCR detected no virus in cohabiting fry (though ISKNV was detected in some of the tested adults from the same farm). Co-infections with ISKNV and bacterial pathogens were found in a number of farms. On the other hand, TiLV was not detected in any of the analysed samples.