Coinfection of tilapia lake virus and Aeromonas hydrophila synergistically increased mortality and worsened the disease severity in tilapia (Oreochromis spp.)

Global tilapia aquaculture has been experiencing serious disease problems for several years, and a breakthrough was made in 2014 when a novel virus called tilapia lake virus (TiLV) was linked to the observed high mortalities in tilapia. Notably, previous studies from Thailand, Egypt, and Malaysia identified TiLV along with various well-known pathogenic bacterial species. In this study, we have further investigated the significance of TiLV-bacterial concurrent infections in farmed tilapia. First, field studies across different locations in Thailand were performed to assess the extent of TiLV-bacterial coinfections in natural farm-raised tilapia. From a total of 52 cases, 15% were attributed to a single TiLV infection, 14% were positive for either Streptococcus species (spp.) or Aeromonas spp. without TiLV, and in 31% of the fish, TiLV was found coinfecting with bacterial species, the majority of which was credited to a TiLV-Aeromonas spp. coinfection. To further examine the impact of a TiLV-Aeromonas spp. coinfection on the severity of disease in tilapia, we co-challenged tilapia with TiLV, Aeromonas hydrophila, and combinations thereof under laboratory conditions. We found that the coinfection between TiLV and 10(7) colony forming units (CFU)/fish of A. hydrophila resulted in 93% cumulative mortality compared to 0%, 34%, and 6.7% in the control, single TiLV and single A. hydrophila infection, respectively. Serious histopathological findings were found in the dual challenged fish, presenting syncytial hepatitis and severe loss of hepatic sinusoid and glycogen storage, as well as red blood cell depletion and vaculaotion of lymphotyces in the spleen. Analysis of the bacterial load recovered from the fish revealed a high level of bacteria from the co-challenged fish compared to those only challenged with A. hydrophila. Collectively, our results show that TiLV is commonly found concurrently with well-known pathogenic bacteria such as Aeromonas spp. and that such infectious agents seem to synergistically exacerbate the disease severity in tilapia. This is an important finding because it has significant implications for implementing the most effective disease management strategies. Future work trying to decipher the details of the interaction between TiLV, bacteria and the tilapiine immune system will be crucial to better understand disease progression and pathogenesis.