Long‐term epidemiological survey of Kudoa thyrsites (Myxozoa) in Atlantic salmon (Salmo salar L.) from commercial aquaculture farms

Abstract

Kudoa thyrsites (Myxozoa) encysts within myocytes of a variety of fishes. While infected fish appear unharmed, parasite-derived enzymes degrade the flesh post-mortem. In regions of British Columbia (BC), Canada, up to 4-7% of fillets can be affected, thus having economic consequences and impacting the competitiveness of BC's farms. K. thyrsites was monitored in two farms having high (HP) or low (LP) historical infection prevalence. At each farm, 30 fish were sampled monthly for blood and muscle during the first year followed by nine samplings during year two. Prevalence and intensity were measured by PCR and histology of muscle samples. In parallel, fillet tests were used to quantify myoliquefaction. Infections were detected by PCR after 355 and 509 degree days at LP and HP farms, respectively. Prevalence reached 100% at the HP farm by 2265 degree days and declined during the second year, whereas it plateaued near 50% at the LP farm. Infection intensities decreased after 1 year at both farms. Blood was PCR-positive at both farms between 778 and 1113 degree days and again after 2000 degree days. This is the first monitoring project in a production environment and compares data between farms with different prevalence.

Descriptive multi-agent epidemiology via molecular screening on Atlantic salmon farms in the northeast Pacific Ocean

“…1), a region in which the risk of K. thyrsites infection is acknowledged to be high 44 . This myxozoan parasite is economically important due to post-mortem myoliquefaction seen in infected fish, but it is not considered a pathogen 45 , and it is unclear why higher genecopy levels would be observed in dead/dying fish. K. thyrsites may merely replicate faster in stressed fish (in this case due to transport).…”

Acquired Protective Immunity in Atlantic Salmon Salmo salar against the Myxozoan Kudoa thyrsites Involves Induction of MHIIβ + CD83 + Antigen-Presenting Cells

“…Additionally, the persistence of these cells in fish demonstrating protection against reinfection suggests they also play a role in the acquired protective immune response. This work challenges an earlier paradigm in which stages of the infection prior to myocyte rupture went undetected by the host (3,34).…”

“…As the myocyte became degraded, the plasmodium gradually lost integrity and myxospores eventually were released and subsequently ingested by phagocytes, including MHII␤ ϩ cells. The presence of macrophages (M) within infected salmon myocytes has been previously reported at a low level (34). Here, infiltrating MHII␤ ϩ cells were frequently observed within plasmodia, and as the infection progressed, most plasmodia had deteriorated and MHII␤ ϩ cells were observed with engulfed myxospores.…”

“…Migratory (extrasporogonic) stages within teleost circulatory systems have been described in several myxozoans (74,75). Indeed, in Atlantic salmon infected with K. thyrsites, weakly infective circulating extrasporogonic stages have been detected (13,34), which may interact with antigen-presenting cells (APCs) to activate the host immune response. There is evidence of stage-specific surface antigens in K. thyrsites (76), raising the possibility of differential stimulation of defense responses during early and late stages of parasite development.…”

Infectious agent detections in archived Sockeye salmon (Oncorhynchus nerka) samples from British Columbia, Canada (1985–94)

“…These three were highly prevalent in samples from the Thompson region of the Fraser River. While the relationship between the two freshwater parasites is obvious (i.e., same alternate host), we speculate that the relationship with K. thyrsites may arise due to a higher density of this parasite in waters east of Vancouver Island due to its higher prevalence in this area (e.g., (Marshall et al, 2016). Interestingly, we detected a disproportionate abundance of K. thyrstes in fish that would have migrated through this same region from as early as 1985, suggesting that Pacific salmon from this region were disproportionately exposed to this parasite before the expansion of Atlantic salmon aquaculture in BC.…”