Transmission of Loma salmonae (Microsporea) to chinook salmon in sea water Michael L. Kent, Sheila C. Dawe, David J. Speare Abstract poissons devraient considerer cette possibilite lors de Transmission studies were conducted to determine if l'elaboration et de l'implantation de leur plan Loma salmonae was transmissible in sea water. strategique pour controler les infections. Transmission of L. salmonae to chinook salmon (Traduit par Dr Therase Lanthier) (Oncorhynchus tshawytscha) held in sea water was achieved by exposing fish to macerated, infected gill Can VetJ 1995; 36: 98-101 tissue. Fish were exposed in seawater in a flow- through aquarium, and the infection was detected as Introduction soon as 5 wk after exposure. Heavily infected fish L oma (formerly Pleistophora) salmonae infects the exhibited numerous xenomas in the branchial arter- gills and other vascularized tissues of wild and ies, central venous sinusoids, and within the blood hatchery-reared salmonids in fresh water throughout channels of the lamellae. The pathological changes the Pacific Northwest (1-7). Severe gill infections were similar to those seen in pen-reared salmon with have been reported in rainbow and steelhead trout L salmonae infections. The infection was not observed (Oncorhynchus mykiss) and kokanee salmon (0. nerka) in Atlantic salmon (Salmo salar), Pacific herring (1). Hauck (4) observed high mortality in chinook (Clupea pallasi, family Clupeidae), or shiner perch salmon (0. tshawytscha) due to systemic infections by (Cymatogaster aggregata, family Embiotocidae), a Loniia sp. (presumably L. salmonae). Although the experimentally exposed using identical methods. gill is the primary site of infection, parasites and asso- This study suggests that L. salmonae is transmissible ciated lesions can occur elsewhere, including the heart, to chinook salmon in seawater netpens. Fish farmers spleen, kidney, and pseudobranch (8). and fish health specialists should consider this pos- Loma salmonae is considered to be a freshwater par- sibility when developing and implementing strategies asite, but infections can persist after fish are trans- to control the infection. ferred to seawater (9). In seawater netpens, severe gill lesions associated with the infection have been observed Resume in coho salmon (0. kisutch) (8,10) and chinook salmon Transmissibilite de Loma Salmonae (Micro- (1 1). Morbidity associated with L. salmonae infections sporea) au saumon chinook en eau salee has increased recently in pen-reared chinook salmon in Une etude a ete effectuee afin de determiner la trans- British Columbia, according to reports from fish farmers missibilite de Loma salmonae au saumon chinook and fish health specialists. garde en eau salee dans un aquarium 'a flux 'a debit Experimental transmission studies with other continu. La transmission de L. salmonae au saumon microsporidia of fishes indicate that most are directly chinook (Oncorhynchus tshawytscha) a ete accomplie transmitted and do not require intermediate hosts, en exposant le poisson a du tissu macere d'ouies although aquatic crustaceans may act as vectors for infectes. L'infection a ete decelee 5 semaines apres some microsporidia (12). In a previous study of pen- l'exposition. Les poissons fortement infectes demon- reared coho salmon, it was clear that some fish car- traient plusieurs xe'nomes dans les arteres branchiales, ried the infection from fresh water, but it was incon- dans les sinusoides veineux centraux et dans les vais- clusive whether the infection was transmitted from seaux des lamelles. Les changements pathologiques infected to uninfected fish within the seawater pens etaient semblables a ceux observes chez les saumons (8). A high prevalence of the infection was recently infectes gardes dans un bassin. observed in chinook salmon at one netpen farm in L'infection n'a pas ete observee chez le saumon de British Columbia, where all the fish had been reared on, l'Atlantique (Salmo salar), le hareng du Pacifique presumably, L. salmonae-free wellwater during their (Clupea pallasi, famille Clupeidae) et la perchaude freshwater phase of development. This suggested that (Cymatogaster aggregata, famille Embiotocidae) lors many, if not all, of these fish contracted the infection in des essais experimentaux. Cette etude suggere que seawater. L. salmonae est transmissible au saumon chinook The purpose of this study was to determine if garde' en eau salee dans des cuvettes en filet. Les L. salmonae is transmissible to chinook salmon in sea- pisciculteurs et les specialistes des pathologies des water and to describe the pathological changes in the gills due to L. salmonae, in the absence of other concurrent Department of Fisheries and Oceans, Pacific Biological Station, diseases of the gills that are frequently observed in Nanaimo, British Columbia V9R 5K6 (Kent, Dawe) and pen-reared salmon. Atlantic salmon (Salmo salar) is Department of Microbiology and Pathology, Atlantic Veterinary the most important species reared in seawater netpens in College, University of Prince Edward Island, 550 University British Columbia. Shiner perch (Cymatogaster aggre- Avenue, Charlottetown, Prince Edward Island CIA 4P3 gata) and Pacific herring (Clupea pallasi) are the most (Speare). common nonsalmonid wild fishes that enter and inhabit Funded in part by the British Columbia Ministry of Agriculture, these netpens. Therefore, the susceptibility of these Fisheries and Food. fishes to L. salmonae was also investigated. 98 Can Vet J Volume 36, February 1995 Materials and methods The transmission experiments were done at the Pacific Biological Station (PBS), Nanaimo, British Columbia in tanks with flow-through seawater (salinity 28 to 30 parts/ thousand). The seawater for this laboratory is pumped from a depth of approximately 30 m and is processed through sand filters that remove particles > 25 pm. Approximately 5 g of chinook salmon gill tissue containing numerous L. salmonae xenomas (collected from a commercial netpen site) were placed in sterile saline, chopped finely, and introduced to the aquar- ium. The water flow to the tank was turned off for 2 h to enhance contact of the parasite with the fish. Fish were exposed 3 times on alternate at days the beginning of Figure 1. Loma salmonae infection in chinook salmon gill at each experiment. 8 wk postexposure. Arrowheads = intact xenomas. Epithelial At the end of each experiment, fishes were euthanized hyperplasia (E) associated with chronic inflammation and with an overdose of tricaine methanesulfonate eosinophilic material with fibrin (F) in central venous sinusoids. (MS-222, Syndel Laboratories, Vancouver, British Hematoxylin and eosin. Bar = 500 pm. Columbia). Four gill arches were collected from each fish and fixed in Davidson's solution (13). Microsections were prepared from each arch, stained with hematoxylin and and for the (Figure 1). Ten control fish maintained under the same eosin, examined presence of L. salmonae conditions did not and associated lesions. Some sections were also stained exhibit the infection. with modified Russell-Movat stain The general architecture of the gills from heavily pentachrome (14) and infected fish was altered Martius scarlet blue stain (15) to verify the presence of markedly due to multifocal dis- fibrin. tribution of xenomas and associated inflammatory responses in and around blood vessels and blood chan- nels. Numerous xenomas were observed within the Experiment 1 afferent and efferent arteries Twenty chinook salmon smolts (avg wt 22 were in the branchial arch and g) the central venous sinusoids (CVS) of the filaments divided equally into 2 groups of 10 each. Each group was and placed in a 40 L and 1 was (Figures 1 2). Many of the xenomas appeared to have tank, group exposed as a focal attachment to the described. Water temperature was maintained at 1 °C to endothelium, and spores were 15C throughout the study. Both exposed and control fish entirely bound by the margin of the xenoma. Many were examined at 5 and 8 wk after the xenomas protruded into the lumen of the vessel, while initial exposure. others extended outward through the tunica adventitia Experiment 2 (Figure 2). Some of the latter extended through the basement membrane of the overlying filament epithelium. Twenty chinook salmon (avg wt 30 g) and 20 Atlantic The salmon (avg wt 80 g) were maintained in a 725 L tank regional distribution of the xenomas favored the dis- tal two-thirds of each filament. The host reaction to and exposed to infected gill tissue. An additional 20 chi- intact nook salmon, 20 Pacific herring (approx avg wt xenomas varied from 1) no detectable response, 10 g) and 10 shiner perch (approx avg wt 20 g) were like- to 2) minimal aggregation of thrombocytes and mono- wise exposed and maintained in another 725 L tank. nuclear cells associated with the intraluminal portion of the xenoma, to 3) features of 2 above, plus a mild peri- Control chinook salmon were maintained in a third vascular 725 L tank. The Atlantic salmon were stock fish that had cuff of mononuclear cells and fibroblasts. been reared on fresh water at the PBS prior to the study, Xenomas were also common within lamellae. whereas the Pacific herring and shiner perch were Although in many cases the structure of the lamella "wild-caught" from Departure Bay, Nanaimo, British was obliterated by xenomas, the xenomas appeared to Columbia. Fishes in the tank containing the herring develop within both pillar cells and the endothelial and perch were also fed chopped frozen euphasid cells of the marginal channel. Fibrin was commonly shrimps daily, because it was noted that the herring found in pillar channels, but little inflammation was did not feed well on the commercial salmon diet. This associated with xenomas in this location. experiment was terminated 7 wk after the initial expo- Approximately 25% of the xenomas in the branchial sure, and all fish from the three tanks were killed and arch and filaments were in stages of degeneration. As in examined by histotogy. previous reports (8,10), degenerated xenomas were associated with severe fibrinoid arteritis, periarteritis, and hyperplasia of the overlying epithelium.