Megalocytivirus Infections in Fish, with Emphasis on Ornamental Species1
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FA182 Megalocytivirus Infections in Fish, with Emphasis on Ornamental Species1 Roy P. E. Yanong and Thomas B. Waltzek2 Introduction ornamental freshwater and marine fishes. In the early 1990s, RSIV was first observed in Japan and since What are the megalocytiviruses? then has been reported primarily in Asian marine finfish. Today, RSIV is reportable to the World The megalocytiviruses are an important group Organization for Animal Health (OIE) and the United (genus) of fish viruses in the family Iridoviridae (the States Department of Agriculture-Animal and Plant iridoviruses). Megalocytiviruses cause systemic Health Inspection Service (USDA-APHIS). The third infections that can result in moderate to heavy losses subgroup TRBIV has been reported predominantly in in many different species of freshwater and marine Asian flounder species. New viral isolates from other fishes in both cultured and wild stocks. In some fish species are currently being evaluated by disease outbreaks, 100% losses have occurred in scientists to determine their relationships to these under one week. Megalocytiviruses have been three main groups. This publication provides disease, reported in fish in the United States as well as other diagnostic, and management information on parts of the world, especially Asia. megalocytiviruses in fish for producers, wholesalers, retailers, and others who work with fish and may be Currently, nearly all isolates from diseased fish unaware of this disease. appear to be strains of the same virus species. Isolates have been divided into three major subgroups based Which fish species are susceptible? on their genetic similarities and differences: a) infectious spleen and kidney necrosis virus (ISKNV); Ornamental finfish species known to be b) red sea bream iridovirus (RSIV); and c) turbot susceptible to megalocytiviruses are listed in Table 1. reddish body iridovirus (TRBIV). Many popular aquarium fish are on this list, including freshwater angelfish (Pterophyllum Strains closely resembling ISKNV have been scalare), other cichlids (Family Cichlidae), reported to cause disease in numerous species of swordtails, sailfin mollies, and other common 1. This document is FA182, one of a series of the Program in Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Original publication date December 2010. Visit the EDIS Web site at http://edis.ifas.ufl.edu. 2. Roy P. E. Yanong, associate professor and extension veterinarian, Tropical Aquaculture Laboratory, Ruskin, FL 33570, Program in Fisheries and Aquatic Sciences, School of Forest Resources and Conservation, UF/IFAS, Gainesville, FL 32611; and Thomas B. Waltzek, Department of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616. The Institute of Food and Agricultural Sciences (IFAS) is an Equal Opportunity Institution authorized to provide research, educational information and other services only to individuals and institutions that function with non-discrimination with respect to race, creed, color, religion, age, disability, sex, sexual orientation, marital status, national origin, political opinions or affiliations. U.S. Department of Agriculture, Cooperative Extension Service, University of Florida, IFAS, Florida A. & M. University Cooperative Extension Program, and Boards of County Commissioners Cooperating. Millie Ferrer- Chancy, Interim Dean Megalocytivirus Infections in Fish, with Emphasis on Ornamental Species 2 live-bearers (Family Poeciliidae) and a number of (coelomic distension), ulceration, hemorrhages species of gourami (Family Osphronemidae and (including pinpoint hemorrhages on the skin and Family Helostomatidae). gills), pale gills/anemia, fin erosion, white feces, and heavy mortalities. Numerous marine and freshwater food and game finfish species have also been shown to be Although studies are ongoing, one research susceptible to infection by megalocytiviruses group has suggested that the virus may target and be including: jacks and pompanos (several species, spread by white blood cells or other immune cells in Family Carangidae), mackerels and tuna (several the body (Lee et al. 2009). This may explain why the species, Family Scombridae), grouper (several virus can be found throughout the body in severe species, Family Serranidae), cobia (Rachycentron cases. During necropsy, megalocytovirus-infected canadum), largemouth bass (Micropterus salmoides), fish are often observed to have damage to many barramundi (Lates calcarifer), redfish (Sciaenops internal organs (i.e., tissue death, necrosis) especially ocellatus), hybrid striped bass (Morone saxatilis x the spleen (which is often enlarged), kidney, and M. chrysops), and gray mullet (Mugil cephalus). A liver. Other organs and tissues, including muscles, more complete list is available online at OIEs gonads, heart, gills, and the gastrointestinal tract, may website also be affected. Some fish may have amber or http://www.oie.int/fileadmin/Home/eng/ hemorrhagic fluid visible within the body cavity. Internationa_Standard_Setting/docs/pdf/ 2.3.07_RSIVD.pdf. Concurrent (i.e., occurring at the same time) infections with bacteria, parasites, or fungi may result The first megalocytivirus discovered in a wild in additional disease signs and complicate the North American fish was recently described from the diagnostic process. Unfortunately, most sick fish threespine stickleback (Gasterosteus aculeatus, often have more than one disease contributing to their Family Gasterosteidae) in Canada, where outbreaks conditions. Determining the presence or absence of in 2007–2008 resulted in low mortality. DNA megalocytovirus in a group of diseased fish is studies demonstrate that the cause of this outbreak is important when making decisions about the yet another unique strain of the megalocytivirus disposition of the entire exposed population. (Waltzek et al. 2010), and is not one of the three types mentioned above. How do I tell if my fish has a megalocytivirus infection? Within what temperature range can megalocytiviruses infect fish and Special diagnostic tests are required to confirm a cause disease? diagnosis of megalocytivirus in fish. Only a fish health professional or fish disease diagnostic Megalocytiviruses have been reported to cause laboratory has the equipment and expertise to make disease at water temperatures ranging from this determination. Appropriate diagnostic tests 20–32°C (68–89.6°F). include histopathology, in which a preparation of thin fixed tissue sections are placed on a glass slide, What are typical external and internal stained, and examined under the microscope (figure signs of disease? 1); virus isolation, which requires growing the virus in cell culture; testing for evidence of antibodies Fish infected with megalocytivirus develop made by the fish against the virus; use of an electron clinical signs of disease that are non-specific, microscope to look for viral particles themselves meaning that they are similar to clinical signs (figure 2); and use of polymerase chain reactions observed with many other diseases. These include (PCR) or DNA probes (pieces of genetic material lethargy, loss of appetite, darkening, abnormal that will bind specifically to megalocytiviruses), in swimming (including spinning) or position in the order to reveal evidence of viral DNA. Because these water, increased respiration, distended body cavity are highly specialized tests, not all laboratories are Megalocytivirus Infections in Fish, with Emphasis on Ornamental Species 3 equipped to run them. Contact your local fish health contribute, and therefore a complete review of the professional for diagnostic assistance. facility including water quality records, movement of animals in or out, and a review of husbandry When a fish health professional or pathologist procedures (including nutrition, stocking rates, examines tissues prepared for histopathology, the cleaning and disinfection, and other related most distinctive finding is the presence of large, biosecurity and system management protocols), be bright blue (basophilic) or pale, foamy cells (figure carried out as part of a disease investigation. A 1). These enlarged, infected cells have become virus thorough necropsy carried out by a fish health factories and are typically found in the spleen, professional of representative fish showing clinical kidney, or liver. Other associated pathology, signs of disease should be performed. including dead tissue (necrosis), can be found in just about any organ. Can megalocytivirus infections be confused with any other diseases? Yes. Megalocytivirus infections in fish can cause disease signs that are similar to many other infectious diseases, especially those caused by bacteria or other viruses. They may also resemble disease signs caused by environmental toxins or poor water quality. A thorough fish disease investigation is required to confirm the presence of megalocytovirus infection and determine whether other factors are contributing to the disease outbreak. What factors contribute to Figure 1. H&E stain. Posterior kidney from a variable development and spread of platyfish (Xiphophorus variatus) with a megalocytivirus megalocytivirus disease? infection. Note large, bluish/purple (basophilic) cells, identified with arrows, which are filled with viral particles. As with other viral diseases, environmental and Photo credit: Roy P. E. Yanong procedural stressors