Erythrocytic Inclusion Body Syndrome: a Light and Electron Microscopic Study of Infected Erythrocytes of Chinook Oncorhynchus Tshawytscha and Coho 0
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DISEASES OF AQUATIC ORGANISMS Vol. 12: 229-233.1992 Published April 23 Dis. aquat. Org. NOTE Erythrocytic inclusion body syndrome: a light and electron microscopic study of infected erythrocytes of chinook Oncorhynchus tshawytscha and coho 0.kisutch salmon Patty Michakl, Charlie E. Smith2, Kathleen ~opper' 'Washington Department of Fisheries. Olympia, Washington 98504, USA 'US. Fish & Wildlife Service, Fish Technology Center, Bozeman. Montana 59715, USA ABSTRACT: Investigat~onof natural infections of viral ery- al. (1989) demonstrated the effect of water tempera- throcytic inclusion body syndrome (EIBS) were conducted at ture on EIBS; an increase in temperature accelerated Washington Department of Fisheries (MIDF) salmon hatch- eries. Comparisons were made of morphological changes the progression of the disease but decreased the seen in stained blood cells by light microscopy with those disease severity, and horizontal transmission of EIBS seen by electron microscopy, and it was determined whether was also demonstrated. erythrocytic inclusions were of viral ongin. Blood was collect- The most common clinical sign of the disease is ane- ed from 10 to 12 fish for each of 8 stocks of salmon. Blood mia which may vary from mild to severe (Leek 1987). smears and whole blood were processed for microscopic eval- uation. By light microscopy infected erythrocytes typically Often EIBS is associated with other diseases such as had single, pale blue (Leishman-Giemsa stain) inclusions. bacterial kidney disease, coldwater disease and fungus Erythrocytes in many stages of development were common, infections (Piacentini et al. 1989).The disease is clini- and In some stocks immature erythrocytes dccounted for 50 to cally diagnosed by microscopic examination of stained 75 "/o of cells present. Electron micrographs showed infected cells with 1 to 3 viral inclusions. Intact inclusions contained blood smears for the presence of typical intracyto- densely packed icosahedral virions. Dense-staining inclu- plasmic inclusion bodies in erythrocytes. The staining sions, by light microscopy, were seen infrequently and by method of choice is pinacyanol chloride (Yasutake electron microscopy appeared to be accumulations of 1987), although inclusions can also be observed when membrane-bound material and not viral inclusions. blood smears are stained by other methods including Leishman-Giemsa (LG) (Yasutake & Wales 1983). The purpose of this investigation was to compare Viral erythrocytic inclusion body syndrome (EIBS) morphological changes seen in stained blood cells by was first documented to occur, as a natural infection, in light microscopy with those seen by electron micros- 1982 at Little White Salmon National Fish Hatchery, copy and to determine if erythrocytic inclusions were Cook, Washington, USA, in juvenile spring chinook of viral origin, associated strongly with viral particles, salmon Oncorhynchus tshawytscha (Leek 1987). Since or contained material with no viral association. this observation EIBS has been found throughout Materials and methods. Between April 1988 and Washington and Oregon and in parts of Idaho and April 1989, juvenile chinook salmon (13 to 50 g) or California in hatchery-reared salmonids (Piacentini coho salmon Oncorhynchus kisutch (22 to 32 g) reared 1989). at WDF Columbia Basin hatcheries were sampled The virus, an icosahedral particle 70 to 80 nm in when they had a high prevalence of intracytoplasmic diameter, has not been replicated in vitro (Piacentini inclusions in their erythrocytes. Blood was collected 1989). A method for artificial infection has been devel- from 10 to 12 fish for each of 8 stocks of fish (Table 1). oped and used to determine the stages of the infection Wet mounts of blood were occasionally examined in process. Five stages were identified: incubation, inclu- the field using phase contrast microscopy. This method sion body formation, cell lysis, recovery and return to was used to find fish with erythrocytes containing normal (Piacentini et al. 1989). In addition Piacentini et numerous cytoplasmic inclusions. 0 Inter-Research/Printed in Germany 230 Dis. aquat Org. 12 229-233, 1992 Table 1 Oncorhynchus tschawytscha, 0, kisutch. Chinook was again removed and Millonig's phosphate buffer and coho stocks sampled for light and electron m.icroscopic pH 7.2 was added. Samples were refrigerated until examination prepared for transrnisslon electron microscopy. Results and discussion. Examination of wet mounts Hatchery Species Brood year Size of blood by phase contrast microscopy showed that Grays River Coho 1986 32 g cytoplasmic inclusions were easily observed (Fig. 1). Lewis River Coho 1986 32 g Mitochondria usually appeared as thin, elongate rods. Cowlitz Coho 1987 *2 g However, on end, they appeared round, but generally Kalarna Falls Coho 1987 23 g smaller than viral inclusions. A stained blood smear Washougal Coho 1987 24 g prepared from the identical fish that was examined Klickitat Spring chinook 1987 23 g Tucannon Spring chinook 1987 50 g using phase contrast had substantially fewer inclu- Lyons Ferry Fall chinook 1987 13 g sions, indicating that cytoplasmic inclusions and infec- ted erythrocytes may be fragile and possibly destroyed while making blood smears. This may result in an ob- Fish were sacrificed using MS222, the caudal ped- served lower prevalence than the population actually uncie severed, and biood collected in heparinized is experiencing. hematocrit tubes. Blood smears were made and the After LG staining, infected erythrocytes usually remainder of the blood from the fish dispensed into exhibited single, pale blue inclusions; however several microcentrifuge tubes. Blood smears were air-dried, small inclusions scattered throughout the cytoplasm, fixed in absolute methanol, stained with Leishman- usually in close proximity to each other, were occasion- Giemsa stain (LG) and examined by light microscopy. ally seen (Fig. 2). Erythrocytes in many stages of devel- LG was used because it allows for differentiation of cell opment were common, some of which were atypical in types and stages of cell development. Blood in the size and shape as characterized by spindle-shaped microcentrifuge tubes was centrifuged at 12500 X g cells, bilobed nuclei and/or nuclear segmentation. In for 5 min to pellet the cells, the serum was removed, some stocks immature erythrocytes accounted for 50 to and 2.5 % glutaraldehyde (in Millonig's phosphate 75 % of the cells present, and erythroblasts were occa- buffer solution) fixative was added to the pellet for sionally seen. Fish that had fewer inclusions and 30 min. The pellet of fixed cells was then teased away abundant immature erythrocytes in later stages of from the edge of the centrifuge tube using a fine point development were thought to be recovering from the wood skewer to assure complete penetration of the infection. Degeneration of erythrocytes was also ap- fixative. The glutaraldehyde was removed and fresh parent, the cytoplasm was often vacuolated and the fixative applied for 24 h at 4 "C. The glutaraldehyde nuclei sometimes swollen or pyknotic. Polymorpho- Fig. 1. Oncorhynchus tshawytscha. Wet mount of blood using phase contrast microscopy from Lyons Ferry fall chinook showing erythrocytic inclusion body (arrow) and erythroblasts (E) in circulating blood Michak et al.: Erythrocytic inclusion body syndrome 23 1 Fig. 2. Oncorhynchus kisutch. Two erythrocytes from Lewis River coho salmon stained with LG that contain small inclu- sions scattered throughout their cytoplasm (arrows). Note lymphocyte (L) adjacent to one Fig. 3. Oncorhynchus kisutch. Two macrophages. Note large degenerate nucleus (arrow) that has been engulfed by cell on nuclear leucocytes showed hypersegmentation of the right. From Lewis River coho salmon stained with LG nuclei, and macrophages were common often contain- ing cytoplasnlic debris (Fig. 3). n~icroscopyobservations of multiple inclusions of vary- Electron micrographs of blood cells showed infected ing sizes, 0.4 to 1.6pm, per cell. erythrocytes usually with 1 to 3 viral inclusions per cell We have shown that typical erythrocytic cytoplasmic (Fig. 4). Occasionally as many as 6 or more were pres- inclusions in EIBS infected fish were of viral origin. ent (Fig. 5). Intact inclusions contained densely packed Samples that had atypical dense staining inclusions by virions, icosahedral in shape with an electron-dense light microscopy had inclusions by electron micros- staining core (Fig. 6). Individual virions measured 70 to copy that appeared to be degenerate membrane- 80 nm in diameter. Microtubules were often present bound material. within inclusions, or in close proximity to them (Fig. 7). Virions were often observed within membrane-bound Acknowledgements. We thank Andy Blixt for the electron structures and were frequently associated with cellular photomicrographs and Pat Chapman for assistance with membranes (Fig. 8). It was not determined if virions sampling and editorial review, Beth MacConnell for assis- were being released from inclusions, or being formed tance with developing a whole blood preservation technique and coordination and transport of samples, John Morrison for on their membranes. Occasionally, virions were free in phase contrast photographs, and Bob Rogers and Dr Jim the cytoplasm of erythrocytes or had combined within Winton for editorial review. We also acknowledge Bonneville large cytoplasmic vacuoles (Fig. 9). Power Administration for providing funding for this work. Degenerate erythrocytes, both mature and imma-