British Columbia

White sturgeon (Acipenser transmontanus) are 1 of 5 Canadian species of sturgeon and are considered among the largest freshwater fish in North America. These fish range from the Aleutian islands in Alaska to Monterey Bay, California. Although white sturgeon most commonly inhabit freshwater environments, this species is anadromous and has been reported in coastal foreshores of British Columbia. White sturgeon have been recorded in the Fraser River system, the Taku River, the Skeena River, the Columbia River above Revelstoke, Duncan Lake on Vancouver Island, and, possibly, Okanagan Lake (1).

As an alternative to the harvest of wild fish and to attempt enhancement of this species, aquaculture facilities were established to propagate captive stocks of white sturgeon for value added market products (table food market, comprised of steaks weighing between 2 and 3 kg and caviar), fingerling supply, and the aquarium pet trade. In British Columbia, there are currently 3 white sturgeon culture facilities with captive stock derived from the Fraser River system. The first attempt at artificial spawning in British Columbia occurred in 1991, with limited success. Artificial spawnings in 2000 and 2001 each produced between 50 to 80 000 eggs, thus enabling initial captive rearing of juveniles. Although domestic juveniles, fertilized eggs, and fry had been transported from one facility to the other two, to the best of our knowledge, there has been no importation of white sturgeon from sources outside the province (D. Kieser, Department of Fisheries and Oceans, personal communication).

In the fall of 2001, in 1 aquaculture facility, fingerling white sturgeon from the 2001 spawn exhibited chronic inappetence, lethargy, and listlessness. Representative moribund and freshly dead fish between 8 and 10 cm in total length were presented to the Animal Health Centre, Abbotsford, British Columbia for diagnostic evaluation. The fish were maintained in 4, 1.3 m³ circular fiberglass tanks, supplied with mechanically and biologically filtered and ozonated recirculated water from a 600 m³ fresh water recirculation culture system. Effluent water from these tanks was not returned to the recirculation system. Culture fish were fed to satiation with a commercial diet. On initial presentation, moribund fish were dispatched by an overdose of tricaine methanesulfonate (500 mg/L); swabs of the kidney, spleen, and liver were collected and plated on tripticase soy and Cytophaga agars for bacterial culture. Gross and cytologic examination of gill wet mounts and skin scrapes were unremarkable, and bacterial culture yielded scant growth of Aeromonas hydrophila. Representative fish were fixed in Davidson's solution for 48 to 72 h; then, sagittal sections of the head and coelomic cavity were prepared for histologic examination.

Microscopic examination disclosed consistent lesions within virtually all of the examined fish. Throughout the epidermis of the cranium, as well as within the gills, oropharygnx, and, to a much lesser extent, nasal pit, there were variable numbers of solitary to small clusters of Malpighian and mucosal cells, respectively, that were moderately to markedly enlarged and featured abundant amounts of densely basophilic, finely granular to homogeneous cytoplasm (Figure 1). The adjoining epithelial cells were compressed and featured occasional hyperplasia, dysplasia, and scattered karyorrhectic debris. There were no other significant or consistent lesions within the remaining tissues.

Figure 1. High magnification photomicrograph demonstrating a cluster of enlarged Malpighian cells (arrows) within the epidermis of a white sturgeon fingerling. Hematoxylin and eosin stain, bar = 50 μm.

Paraffin embedded blocks were processed for electron microscopy by conventional techniques; ultrastructural examination revealed numerous virions within hypertrophic integumentary and mucosal epithelia (Figure 2). The measurements were as follows: outer capsid vertex to vertex = 305 nm; outer capsid side to side = 261 nm; inner capsid vertex to vertex = 209 nm; and inner capsid side to side = 192 nm.

Figure 2. Electron micrograph of WSIV-infected Malpighian cell within the epidermis of a white sturgeon fingerling. Hexagonal shaped virions are abundant in the host cell cytoplasm. Bar = 1 μm.

Based on the clinical history and the histologic and ultrastructural features of the affected fish, a diagnosis of white sturgeon iridovirus (WSIV) was adduced. This condition was originally reported in farmed white sturgeon stocks in California (3) and the virus was subsequently detected in production facilities situated in Idaho and Oregon, (4). Although epizootics involving cultured white sturgeon have resulted in up to 95% mortality of 200 000 fish (3), infections, as in this case, are typically more protracted and result in a chronic, debilitating condition, characterized by suboptimal growth performance and reduced survival of fry and fingerlings (5). Adults appear refractory to clinical disease and are likely subclinical carriers with intermittent viral shedding (4).

White sturgeon iridovirus is considered enzootic within wild white sturgeon stocks in the Sacramento- San Joaquin, Columbia, Snake, and Kootenai River systems; introduction of the virus into production facilities has been attributed to inadvertent selection of carrier fish, when obtaining brood or seed stock; vertical transmission associated with contaminated ova; or horizontal dissemination by influent water to hatchery facilities (4). In this case, as, purportedly, no fish have been imported from the USA and culture facilities are geographically remote to the Fraser Valley, infection was likely derived from sturgeon, or their progeny, obtained originally from the Fraser river system.

Currently, there are no treatments available for WSIV, and research efforts have focused on the development of sensitive diagnostic techniques, as well as the epizootiology of the disease, to establish control and preventative measures to minimize the impact of the disease. In a recent investigation into risk factors associated with WSIV infection (5), brood stock selection, particularly in enzootic regions, improved growth performance and the survival of progeny. Measures to reduce site viral exposure, such as by increased water flows and disinfection of fomites, as well as implementation of an all in, all out management scheme and minimization of stress in subyearling stock, significantly reduce the risk of subsequent infections (5).

Detection of this condition in British Columbia expands the geographic distribution of this virus in North American farmed stocks and suggests possible infection of wild sturgeon within the Fraser River system. As the Kootenai River system extends into southwest Alberta, there is a possibility of more widespread infection of wild white sturgeons stocks within Canada than is currently appreciated, and efforts to further characterize the distribution of this pathogen are recommended.