Pathological effects and immune modulation in host during Tilapia Parvovirus (TiPV) outbreak in cage and wetland Tilapia farms

Viral diseases arising in farmed fish are an ongoing challenge to the aquaculture industry, causing severe mortality and economic losses. Recently, there has been a spike in the incidence of a viral disease caused by Tilapia Parvovirus (TiPV) inflicts irreparable damage, and large-scale fish kills in the farmed tilapia species. We investigated a case of disease outbreak and severe mortality in cage and wetland farms of tilapia in West Bengal and Odisha, India. The symptomatic fish showed clinical signs, including hemorrhage, discoloration, ulcer, and redness in the body surfaces. Further analysis revealed that Tilapia Parvovirus was associated (validated by PCR, phylogenetic analysis, and cell line assay) with the infection and mortality of tilapia. The virus was detected in gill, heart, spleen, liver, and kidney samples collected from apparently healthy (asymptomatic) and symptomatic tilapia samples from cage and wetland farms. At the same time, negative results were found in the brain and skin tissue samples. The histological analysis revealed that TiPV induces severe damage invariably in almost all studied tissue, including the liver, kidney, spleen, gill, heart, and brain of tilapia samples. The viral quantification analysis showed that the viral genome was higher in the liver, spleen, and heart than in the tilapia samples' gill, kidney, or brain tissue. Furthermore, the study indicated that TiPV infection has a significant effect on the health of tilapia. The tilapia exhibited an immune reactivity toward TiPV infection (upregulation of chemokine receptors, CRs and interleukin 1β, IL-1β), the majority of the studied immune genes (interleukin 8, IL-8; Toll-like receptors 7, TLR7; tumour necrosis factor α, TNF-α; major histocompatibility complex II, MHC II and nuclear factor kappa B, NF-kB) were significantly downregulated in the kidney, spleen and liver tissue samples of symptomatic tilapia. Further, the in vivo challenge assay confirms that the isolated TiPV is a novel parvovirus pathogen that causes massive mortality in tilapia. The lessons learned from the first cellular and molecular description associated with TiPV epidemiology from wetland and cage farms of tilapia could be critical to developing the current state of the tilapia farming industry. Additionally, a holistic approach is needed to develop management measures to control the virulence and risk factors of TiPV.