Fish Iridoviridae: infection, vaccination and immune response

Each year, due to climate change, an increasing number of new pathogens are being discovered and studied, leading to an increase in the number of known diseases affecting various fish species in different regions of the world. Viruses from the family Iridoviridae, which consist of the genera Megalocytivirus, Lymphocystivirus, and Ranavirus, cause epizootic outbreaks in farmed and wild, marine, and freshwater fish species (including ornamental fish). Diseases caused by fish viruses of the family Iridoviridae have a significant economic impact, especially in the aquaculture sector. Consequently, vaccines have been developed in recent decades, and their administration methods have improved. To date, various types of vaccines are available to control and prevent Iridoviridae infections in fish populations. Notably, two vaccines, specifically targeting Red Sea bream iridoviral disease and iridoviruses (formalin-killed vaccine and AQUAVAC^® IridoV, respectively), are commercially available. In addition to exploring these themes, this review examines the immune responses in fish following viral infections or vaccination procedures. In general, the evasion mechanisms observed in iridovirus infections are characterised by a systemic absence of inflammatory responses and a reduction in the expression of genes associated with the adaptive immune response. Finally, this review also explores prophylactic procedure trends in fish vaccination strategies, focusing on future advances in the field.