Co-Infections of Tilapia Lake Virus, Aeromonas hydrophila and Streptococcus agalactiae in Farmed Red Hybrid Tilapia

SIMPLE SUMMARY: Tilapia is a freshwater fish that is commercially cultured around the world. However, intensification of tilapia culture often results in diseases, occasionally with co-infections of multiple pathogens. This paper reports the first case of red hybrid tilapia that naturally co-infected with Tilapia Lake Virus (TiLV), Aeromonas hydrophila and Streptococcus agalactiae in Malaysia. In January 2020, a tilapia farm in Selangor, Malaysia, reported a mass mortality of adult red hybrid tilapias, with 70% mortality. Bacterial isolation, PCR and sequencing analysis confirmed the presence of TiLV, A. hydrophila and S. agalactiae in the affected fish. As tilapia is widely cultured throughout the world, detection of multiple infections might signal a potential threat to the industry. ABSTRACT: A high death rate among red hybrid tilapias was observed in a farm in Selangor, Malaysia, in January 2020. The affected fish appeared lethargic, isolated from schooling group, showed loss of appetite, red and haemorrhagic skin, exophthalmia and enlarged gall bladders. Histopathological assessment revealed deformation of kidney tubules, and severe congestion with infiltrations of inflammatory cells in the brains and kidneys. Syncytial cells and intracytoplasmic inclusion bodies were occasionally observed in the liver and brain sections. Tilapia Lake Virus (TiLV), Aeromonas hydrophila and Streptococcus agalactiae were identified in the affected fish, either through isolation or through PCR and sequencing analysis. The phylogenetic tree analysis revealed that the TiLV strain in this study was closely related to the previously reported Malaysian strain that was isolated in 2019. On the other hand, A. hydrophila and S. agalactiae were closer to Algerian and Brazilian strains, respectively. The multiple antibiotic resistance index for A. hydrophila and S. agalactiae was 0.50 and 0.25, respectively. Co-infections of virus and bacteria in cultured tilapia is a new threat for the tilapia industry.