Iridovirus infections in finfish – critical review with emphasis on ranaviruses

Abstract

Viruses in three genera of the family Iridoviridae (iridoviruses) affect finfish. Ranaviruses and megalocytiviruses are recently emerged pathogens. Both cause severe systemic disease, occur globally and affect a diversity of hosts. In contrast, lymphocystiviruses cause superficial lesions and rarely cause economic loss. The ranavirus epizootic haematopoietic necrosis virus (EHNV) from Australia was the first iridovirus to cause epizootic mortality in finfish. Like other ranaviruses, it lacks host specificity. A distinct but closely related virus, European catfish virus, occurs in finfish in Europe, while very similar ranaviruses occur in amphibians in Europe, Asia, Australia, North America and South America. These viruses can be distinguished from one another by conserved differences in the sequence of the major capsid protein gene, which informs policies of the World Organisation for Animal Health to minimize transboundary spread of these agents. However, limited epidemiological information and variations in disease expression create difficulties for design of sampling strategies for surveillance. There is still uncertainty surrounding the taxonomy of some putative ranaviruses such as Singapore grouper iridovirus and Santee-Cooper ranavirus, both of which cause serious disease in fish, and confusion continues with diseases caused by megalocytiviruses. In this review, aspects of the agents and diseases caused by ranaviruses are contrasted with those due to megalocytiviruses to promote accurate diagnosis and characterization of the agents responsible. Ranavirus epizootics in amphibians are also discussed because of possible links with finfish and common anthropogenic mechanisms of spread. The source of the global epizootic of disease caused by systemic iridoviruses in finfish and amphibians is uncertain, but three possibilities are discussed: trade in food fish, trade in ornamental fish, reptiles and amphibians and emergence from unknown reservoir hosts associated with environmental change.

Ranavirus Ecology and Evolution: From Epidemiology to Extinction

“…High contact rates with abundant hosts in captivity and during transport may be resulting in evolution of ranavirus virulence ( f , credit = Jonathan Kolby) systems is rarely well documented. Most of what is known about the ecology and epidemiology of ranaviruses in fi shes comes from studies of Epizootic haematopoietic necrosis virus (EHNV) in Australia and largemouth bass virus (LMBV; a strain of Santee Cooper ranavirus ) in the Southeast USA (Whittington et al 2010 ). EHNV outbreaks have caused rapid (2-3 weeks) die-offs affecting tens to thousands of juvenile redfi n perch ( Perca fl uviatilis ) in the early summer, with a few adults also affected (Langdon et al 1986 ;Langdon and Humphrey 1987 ).…”

“…Most of what is known about the epidemiology, geography, and host range of ranaviruses comes from investigations of obvious die-offs, sporadic surveillance efforts in small numbers of populations and time points, and a few larger-scale surveillance efforts focused on a handful of species of economic importance or conservation interest (Grizzle and Brunner 2003 ;Gray et al 2009a ;Whittington et al 2010 ;Miller et al 2011 ;Duffus et al 2015 ). Duffus et al ( 2015 ) reviewed the known distribution and host range of ranaviruses.…”

Virus genomes and virus-host interactions in aquaculture animals

“…For example, lymphocystis disease virus (LCDV), has been identified as the causative agent of lymphocystis disease in more than 100 different seawater and freshwater fish species [22,23]. In particular, diverse ranaviruses (genus Ranavirus) have been reported to infect about 70 amphibian species from at least 14 families, more than 100 fish species and dozens of reptiles; hence, some experts believe that ranaviruses infect not only frogs but also numerous different aquatic vertebrates, and are, therefore, promiscuous pathogens of cold-blooded vertebrates [24][25][26][27][28][29][30][31][32][33].…”

Interferon regulatory factor 10 (IRF10): Cloning in orange spotted grouper, Epinephelus coioides, and evolutionary analysis in vertebrates

“…However, the diseases outbreaks have severely influenced the mariculture industry of this fish [32,33]. The infection of viral and bacterial pathogens, for example, iridovirus and Vibrio spp., has been widely reported, with the occurrence of heavy mortalities and large economic losses [34e36].…”