Detection and Histopathological Characterization of Megalocytivirus pagrus 1 (ISKNV) in Nile Tilapia (Oreochromis niloticus) under Experimental Infection and Density Stress
Abstract
The project encompasses the investigation of the presence of Megalocytivirus pagrus 1 (subtype ISKNV) in fish (Oreochromis niloticus), which has serious implications for animal health, public health and agribusiness. We aimed at the prospection and detection of ISKNV with molecular and histological techniques. Viral fish diseases caused by viruses of the family Iridoviridae, to which ISKNV belongs, are distributed worldwide and represent a real threat to the expansion of the global aquaculture system, due to their pathogenicity, which causes significant economic losses. Thus, the improvement and application of existing diagnostic techniques become necessary to control or prevent the advance of this virosis among countries and in Brazil. In addition, studies in our field that aim to detect and characterize them in a more comprehensive way are scarce. We were able to evidence the presence of the virus through clinical signs during the experiment, detection of genetic material by conventional PCR and by the presence of inclusion bodies, identified by histopathological analysis. We observed a higher infection rate in the period of 14 days after infection by intraperitoneal viral inoculation, as well as greater viral proliferation in the treatment submitted to stress, however without resulting in mortality.
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Introduction
The expansion of aquaculture in recent decades, driven by the increase in global demand for fish, has promoted significant changes in production systems, especially with the increase in stocking density and the intensification of management. In Brazil, tilapia (Oreochromis niloticus) stands out as the main cultivated species, representing approximately 42% of national production, which exceeded 700 thousand tons in 2024 (Associação Brasileira de Aquicultura, 2024). On a global scale, aquaculture production intended for human consumption reached around 94 million tons in 2022, evidencing the economic relevance of the sector (FAO, 2024).
However, the intensification of production systems favors the occurrence and dissemination of infectious diseases, since high population densities and controlled environmental conditions may act as predisposing factors for epizootic outbreaks. In this context, viral agents assume a prominent role, especially due to their high transmission capacity and potential to cause expressive economic losses (Wang et al., 2011).
Among the viruses of sanitary importance in aquaculture, members of the family Iridoviridae stand out, especially the genus Megalocytivirus, responsible for systemic infections in several species of freshwater and marine fish (Koda et al., 2018). Megalocytivirus pagrus 1 (subtype ISKNV) is considered an emerging pathogen, with wide geographical distribution and capacity to infect more than 50 fish species. The infection may result in significant mortality, especially in initial stages of development, such as fingerlings and juveniles.
Conclusion
The results of this study confirm that experimental infection with Megalocytivirus pagrus 1 (ISKNV) was successfully established in Nile tilapia, as detected by PCR and corroborated by characteristic histopathological alterations, especially in the kidney and spleen. The integrated diagnostic approach combining PCR and histopathology proved effective for detecting viral infection.
The absence of mortality, even in the groups subjected to high-density stress, indicates that, under the experimental conditions evaluated, the infection did not progress to lethal outcomes during the analyzed period, despite the presence of clinical signs and evident tissue lesions. This finding suggests that factors such as exposure time, viral load, and environmental conditions may influence the expression of ISKNV pathogenicity. A longer observation period or higher viral dose may be necessary to induce mortality.
A trend toward greater viral dissemination was observed in the groups subjected to density stress (T4: 61.1% positivity vs. T3: 21.4% on day 14; p = 0.04), indicating that intensive farming conditions may act as modulators of infection dynamics, favoring viral transmission even in the absence of increased mortality. High-density stress may impair the immune response and increase contact among individuals, facilitating viral spread.
Thus, the results obtained contribute to the understanding of ISKNV infection dynamics in tilapia and reinforce the importance of combined diagnostic strategies, especially in intensive production systems, where stress factors may influence the dissemination of pathogenic agents.
Limitations: The study did not perform qPCR to quantify viral load, did not measure stress biomarkers, and the observation period was limited to 14 days. Future research should address these limitations by incorporating qPCR, stress biomarker measurement, longer observation periods, and viral dose quantification.
Future Research Directions: Future studies should investigate the effects of different viral doses, longer observation periods, and other stress factors on ISKNV infection dynamics. The development of qPCR assays for viral load quantification and the measurement of stress biomarkers (cortisol, glucose) would provide more comprehensive data on the mechanisms underlying stress-induced susceptibility to ISKNV.
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