D. J. Whitaker

A review of the myxosporean genus Kudoa Meglitsch, 1947, and its impact on the international aquaculture industry and commercial fisheries

Abstract The genus Kudoa Meglitsch, 1947 (Myxozoa: Myxosporea) is comprised of myxosporean parasites with four valves, each of which contains a polar capsule. Species within this genus are typically histozoic parasites of marine teleosts. However, since the establishment of the genus, a few coelozoic species have been described. Presently, there are 44 identified species within the genus. This genus is of concern to both aquaculture and commercial fisheries because several of its species either produce unsightly macroscopic cysts in the musculature or are associated with post-mortem myoliquefaction, and thus reduce the market value of the infected fish products. With the emergence of marine aquaculture, concerns regarding Kudoa infections have increased significantly in recent years. Based upon the literature gathered, a taxonomic review of the genus is warranted to verify the host range, geographical distribution, and validity of several of its species. The use of molecular systematics to answer these questions has been initiated. Furthermore, PCR tests have been developed using small-subunit rDNA sequence for a few Kudoa species, which will be useful for resolving the life cycles of these important marine parasites.

PARVICAPSULA MINIBICORNIS N. SP. (MYXOZOA, MYXOSPOREA) FROM THE KIDNEY OF SOCKEYE SALMON (ONCORHYNCHUS NERKA) FROM BRITISH COLUMBIA, CANADA

A new species of Myxosporea Parvicapsula minibicornis is described from the kidney of adult sockeye salmon (Oncorhynchus nerka) that had recently returned from the Pacific Ocean to Weaver Creek, a tributary of the Fraser River, British Columbia, Canada. Spores are ovoid, symmetrical, with 2 pyriform polar capsules at the anterior pole. The posterior pole has 2 small, pointed projections. Mean spore dimensions are length 11.0 microm, thickness (perpendicular to suture plane) 6.8 microm, and width (in sutural plane) 7.5 microm. This myxozoan is compared to other described Parvicapsula species and a Parvicapsula sp. from netpen-reared coho salmon (Oncorhynchus kisutch).

Natural and laboratory transmission of the marine myxozoan parasite Kudoa thyrsites to Atlantic salmon.

Abstract Experiments were conducted to evaluate routes of transmission and subsequent development of Kudoa thyrsites (Myxozoa: Myxosporea) in Atlantic salmon Salmo salar. Atlantic salmon that were held for a 2-week exposure period at the seawater net-pen site and then transferred to freshwater tanks developed K. thyrsites infections as evidenced by the presence of myxospores in the somatic musculature at 3 months postexposure. Attempts to transmit the parasite directly from fish to fish by intubation of fresh myxospores were unsuccessful. However, the parasite was transmitted to Atlantic salmon by intraperitoneal injection of blood collected from a coho salmon Oncorhynchus kisutch infected with K. thyrsites. The waterborne infective stage of K. thyrsites was not removed by filtration of seawater entering the tanks at the Pacific Biological Station.

Myxosporean Kudoa thyrsites: A Cause of Soft Flesh in Farm-Reared Atlantic Salmon

Abstract Smoked fillets of Atlantic salmon Salmo salar that were reared in sea-pens at a fish farm in British Columbia were found with softened and discolored areas. Examination of the affected tissue revealed infection with a parasite, the myxosporean Kudoa thyrsites. The smoking process (at 50°C for about 10 h) allowed proteolytic enzymes produced by the parasite to lyse areas of the fillet, resulting in poor-quality flesh. Inspection and culling of heavily infected fillets should reduce the immediate possibility of an inferior product reaching the market. Further work is necessary to define the extent of affected farm sites and to develop other handling and processing techniques for infected Atlantic salmon.

Kudoa thyrsites (Myxozoa: Myxosporea) Infections in Pen-Reared Atlantic Salmon in the Northeast Pacific Ocean with a Survey of Potential Nonsalmonid Reservoir Hosts

Abstract The seasonality of the infective stage of Kudoa thyrsites was investigated via natural exposure of Atlantic salmon Salmo salar held in seawater net-pens in Departure Bay near Nanaimo, British Columbia, an area for which this parasite is enzootic. Atlantic salmon were exposed for various 8-week periods throughout the year, and it was determined that the infections were readily contracted in the summer and fall but not through the winter and early spring months. The progression of K. thyrsites infections was followed in populations of Atlantic salmon held in either seawater net-pens or seawater tanks for periods of up to 20 months. The majority of postsmolts contracted infections within a few months after their transfer to the seawater net-pens, and sporulation of the parasite occurred 4–6 months posttransfer. Mature myxospores could be detected as early as 1,000 degree-days postexposure (p.e.); however, the majority of infections did not progress to sporulation until 2,000 degree-days p.e. The pre...

Prevalence of Kudoa thyrsites in sexually mature and immature pen-reared Atlantic salmon (Salmo salar) in British Columbia, Canada

Abstract The prevalence of Kudoa thyrsites, a cause of post mortem soft flesh in Atlantic salmon (Salmo salar), was determined for 17 harvests of Atlantic salmon (Salmo salar) raised in British Columbia. Fish were divided into two categories, sexually immature and sexually mature (grilse), using criteria typically employed at processing plants, i.e., external morphology. The prevalence of the parasite was, with the exception of a few harvests, undetectable in the immature fish sampled between January and April of 1995. In contrast, the prevalence of K. thyrsites in grilse sampled from the same harvests ranged from 0% to as high as 76.9%. On average sexually mature fish harvested between January and April 1995 were 13 times as likely to be infected with K. thyrsites as their sexually immature counterparts. In May, the prevalence of K. thyrsites increased in the sexually immature fish populations. A similar increase was not observed in the sexually mature fish from the same harvests. Examination of scale rings revealed that this apparent increase in prevalence of K. thyrsites in sexually immature fish was partially due to the misclassification of `reconditioned grilse (i.e., fish that have undergone sexual maturation and then had reabsorbed their gonads and lost their external sexual characteristics) as immature fish at the processing plants. It may, therefore, be possible to decrease the prevalence of K. thyrsites in the premium and downgraded fish at the end of the harvest year by removing sexually reconditioned fish from the processing line prior to the removal of their gonadal tissue or by harvesting fish on farms before they undergo sexual maturation.

Evaluation of a nondestructive diagnostic test for Kudoa thyrsites in farmed Atlantic salmon (Salmo salar)

Abstract Postharvest myoliquefaction (soft flesh) associated with Kudoa thyrsites infections is a concern to the Atlantic salmon (Salmo salar) industry in the Pacific Northwest. Infection with this parasite is not macroscopically visible in salmon, and thus infected fish are not readily detectable on the processing line. Identification of infected fish relies on either histological or whole-mount evaluation of muscle tissue. A nondestructive, inexpensive diagnostic test for detection of K. thyrsites infection in the musculature of Atlantic salmon was evaluated in this study. The results indicated that the presence or absence of K. thyrsites in the hyohyoideus ventralis muscle of the operculum, as detected in wet-mount preparations, is a good indicator of the presence or absence of the parasite in the body musculature (fillets) of Atlantic salmon. The sensitivity and specificity of the diagnostic test were 79.0% and 94.6%, respectively. The sensitivity of the test was increased to 93% when the test was used to detect only heavily infected fillets. Therefore the presence or absence of K. thyrsites spores in the hyohyoideus ventralis muscle is a good indicator of heavily infected fish fillets, but is slightly less accurate at detecting fillets with lighter infections of K. thyrsites.