Post-mortem Sporulation ofCeratomyxa shasta(Myxozoa) After Death in Adult Chinook Salmon

Abstract

Ceratomyxa shasta (Myxozoa) is a common gastrointestinal pathogen of salmonid fishes in the Pacific Northwest of the United States. We have been investigating this parasite in adult Chinook salmon ( Oncorhynchus tshawytscha ) in the Willamette River, Oregon. In prior work, we observed differences in the pattern of development of C. shasta in adult salmon compared to juvenile salmon. Adult salmon consistently had large numbers of prespore stages in many of the fish that survived to spawn in the fall. However, myxospores were rarely observed, even though they were exposed and presumably infected for months before spawning. We evaluated the ability of C. shasta to sporulate following fish death because it is reported that myxosores are common in carcasses of Chinook salmon. We collected the intestine from 30 adult salmon immediately after artificial spawning and death (T0). A total of 23 fish were infected with C. shasta based on histology, but only a few myxospores were observed in 1 fish by histology. Intestines of these fish were examined at T0 and T7 (latter held at 17 C for 7 days) using quantified wet mount preparations. An increase in myxospore concentrations was seen in 39% of these fish, ranging between a 1.5- to a 14.5-fold increase. The most heavily infected fish exhibited a 4.6-fold increase from 27,841 to 129,352 myxospores/cm. This indicates, supported by various statistical analyses, that under certain conditions presporogonic forms are viable and continue to sporulate after death in adult salmon. Considering the life cycle of C. shasta and anadromous salmon, the parasite may have evolved 2, non-mutually exclusive developmental strategies. In young fish (parr and smolts), the parasite sporulates shortly after infection and is released into freshwater from either live or dead fish before their migration to seawater, where the alternate host is absent. The second strategy occurs in adult salmon, particularly spring Chinook salmon, which become infected upon their return to freshwater in the spring or early summer. For several months throughout the summer, only prespore stages are observed in most fish, even at the time of spawning. But once the fish dies, environmental conditions experienced by C. shasta change and viable presporogonic stages are induced to sporulate. As the post-spawned fish occur in the upper reaches of rivers, the myxospores would be released in a freshwater environment that would provide a reasonable opportunity for them to encounter their freshwater polychaete hosts, which reside downstream.

Production of Ceratonova shasta Myxospores from Salmon Carcasses: Carcass Removal Is Not a Viable Management Option

“…These data indicated that myxospores were found primarily in carcasses from adult salmon that had senesced and died days after spawning. It is possible that the pansporocyst stage (presporogonic) responds to changes in the microenvironment associated with decomposing tissue (e.g., lytic enzymes, oxygen and pH changes) to complete sporogenesis (see also Kent et al 2014). Myxospore increase in fish carcasses postmortem has also been observed in another Pacific Northwest river, the Williamette River (Kent et al 2014).…”

Ceratonova shasta: a cnidarian parasite of annelids and salmonids

“…Disease severity is directly related to temperature and may culminate in death, which can occur as early as 15 days post-exposure in fish held at 15°C (Ray et al ., 2012). Myxospores typically mature during terminal stages of infection with the majority released when fish die (although there is a notable exception described below in which spores are shed from apparently healthy fish), possibly triggered by factors produced as tissues become necrotic (Kent et al ., 2014).…”

“…Chinook and coho salmon are semelparous, dying after they return to freshwater to spawn. Infections in adult salmon by both genotypes I and II are characterized by release of spores only upon the death of the fish, and the process of sporulation continues post-mortem in adult Chinook salmon (Kent et al ., 2014; Foott et al ., 2016). This provides explanation for earlier studies on the effects of C. shasta on pre-spawn mortality where fish that were killed for egg taking had a much lower apparent infection prevalence (18% myxospore positive) compared to fish that died prior to spawning (65% myxospore positive) (Chapman, 1986).…”

Using a mechanistic framework to model the density of an aquatic parasiteCeratonova shasta

“…Developmental strategies of C. shasta differ between infections in adult and juvenile salmonid hosts; in adults, release of myxospores occurs after death ( i.e. , post-spawn), while in smolts and parr the parasite is released shortly after infection ( Kent et al, 2014 ). The parasite released from infected adult carcasses is dispersed downriver where deposition overlaps with habitat for the invertebrate host.…”