Effects of freezing, drying, ultraviolet irradiation, chlorine, and quaternary ammonium treatments on the infectivity of myxospores of Myxobolus cerebralis for Tubifex tubifex

Ceratomyxa mennani n. sp. (Myxosporea: Bivalvulida) parasitizing the gallbladder of the dusky grouper Epinephelus marginatus (Serranidae) from Tunisian waters

Ceratomyxa mennani n. sp. is a new coelozoic Ceratomyxa species found in the gallbladder of Epinephelus marginatus from the Gulf of Tunis, Tunisia. Mature plasmodia were disporic, ovoid in shape measuring 9-12 μm in width and 11-14 μm in length. Mature myxospores were slightly crescent-shaped with almost straight posterior margin, measuring 5.8 ± 0.2 (5.4-6.1) μm in length and 12.7 ± 0.3 (11.9-13.0) μm in thickness. The two valves were unequal with rounded ends. Polar capsules were spherical, equal in size with 2.1 ± 0.2 (1.9-2.6) μm in diameter. The binucleated sporoplasm filled the entire cavity of the myxospore. Molecular analysis of SSU rDNA sequences indicated that C. mennani n. sp. was distinct from all other Ceratomyxa sequences in GenBank. Phylogenetic analysis revealed that C. mennani n. sp. clustered with Ceratomyxa species infecting Epinephelinae fishes. Seasonal prevalence of infection over one year was significantly higher in winter and the lowest in autumn. This is the third report of Ceratomyxa species infecting the gallbladder of Epinephelus marginatus from Tunisia and the first study to include molecular data.

Elimination of Myxobolus cerebralis in Placer Creek, a Native Cutthroat Trout Stream in Colorado

Placer Creek, a tributary of Sangre de Cristo Creek in Colorado's San Luis Valley, supported an allopatric core conservation population of native Rio Grande Cutthroat Trout Oncorhynchus clarkii virginalis during much of the 20th century. After the failure of gabion barriers in the late 1990s, Brook Trout Salvelinus fontinalis infected with Myxobolus cerebralis invaded from Sangre de Cristo Creek. By 2005, whirling disease (WD) and competition from Brook Trout reduced Rio Grande Cutthroat Trout numbers to less than 10% of the total trout population. New barriers were constructed in 2006 and the stream was treated with rotenone in 2007 and 2009 to eliminate all fish prior to the reintroduction of Rio Grande Cutthroat Trout. Results of WD research studies in Montana, California, and Colorado indicated it might be possible to break the life cycle of the parasite in some situations. Our management interventions included (1) reducing the fish population in the stream to zero for approximately 14 months, (2) introducing lineage V and VI Tubifex tubifex worms, which are not susceptible to M. cerebralis, and (3) eliminating a small off-channel pond that provided optimal habitat that sustained a localized high-density population of lineage III T. tubifex, the oligochaete host susceptible to M. cerebralis. Electrofishing during the fall of 2009 and spring of 2010 indicated the drainage was devoid of fish. Fry, juvenile, and adult Rio Grande Cutthroat Trout were stocked in September and October of 2010 and 2011. Approximately 975,000 lineage V and VI T. tubifex were introduced into Placer Creek between 2010 and 2012 as possible oligochaete competitors for the lineage III worms. The off-channel pond was filled in, and the surface was reseeded in April 2012. No evidence of M. cerebralis infection was detected among more than 280 Rio Grande Cutthroat Trout tested between July 2012 and July 2016, indicating the parasite had been eradicated from the Placer Creek basin upstream of the barriers.

Whirling disease revisited: pathogenesis, parasite biology and disease intervention

Whirling disease (WD) is an ecologically and economically debilitating disease of rainbow trout Oncorhynchus mykiss caused by the actinosporean spores of the parasite Myxobolus cerebralis. M. cerebralis has a complex, 2-host life cycle alternating between salmonid fish and the oligochaete host Tubifex tubifex. The parasite alternates between 2 spore forms as transmission stages: an actinosporean triactinomyxon spore that is produced in the oligochaete host and a myxosporean spore that develops in the salmonid host. Waterborne triactinomyxon spores released from infected T. tubifex oligochaetes attach to the salmonid host by polar filament extrusion elicited by chemical (nucleoside) and mechanical (thigmotropy) stimuli-a process which is rapidly followed by active penetration of the sporoplasms into the fish epidermis. Upon penetration, sporoplasms multiply and migrate via peripheral nerves and the central nervous system to reach the cartilage where they form trophozoites which undergo further multiplication and subsequent sporogenesis. M. cerebralis myxospores are released into the aquatic environment when infected fish die and autolyse, or when they are consumed and excreted by predators. Myxospores released into the water are ingested by susceptible T. tubifex where they develop intercellularly in the intestine over a period of 3 mo through 4 developmental stages to give rise to mature actinospores. In this article, we review our current understanding of WD-the parasite and its alternate hosts, life cycle and development of the parasite in either host, disease distribution, susceptibility and resistance mechanisms in salmonid host and strategies involved in diagnosis, prevention and control of WD.

Assessment of the Long-Term Viability of the Myxospores of Myxobolus cerebralis as Determined by Production of the actinospores by Tubifex tubifex

While whirling disease was first observed in Rainbow Trout Oncorhynchus mykiss in 1893, the complete life cycle of Myxobolus cerebralis (Mc), the causative agent of the disease, was not understood until 1984, when it was shown to involve two obligate hosts, a salmonid fish and the aquatic oligochaete Tubifex tubifex (Tt). The viability of the triactinomyxon (TAM) actinospores produced by Tt has been well studied, and is known to be temperature dependent and measured in days and weeks. Assertions that Mc myxospores produced by infected fish remain viable for years or even decades were made during the mid-20th century, decades before the Mc life cycle was described. Moreover, the duration of myxospore viability has not been well studied since the life cycle was elucidated. In a series of time-delay treatments, we assessed the long-term viability of Mc myxospores by exposure to Mc-susceptible Tt oligochaetes and quantified TAM production. As the time delay between inoculation and incubation of Mc myxospores in sand and water and exposure to Tt oligochaetes increased, TAM production decreased exponentially. Production among the 15-d time-delay replicates was reduced 74.7% compared with the 0-d treatment. Likewise, total TAM production was reduced 94.5, 99.4, and 99.9%, respectively, in the 90-, 120-, and 180-d time-delay treatments. Linear regression analysis of our data and the absence of TAM production among replicates of Mc myxospores held at 5°C for 365 d prior to exposure to Mc-susceptible Tt oligochaetes indicate that the long-term viability of Mc myxospores is less than 1 year under the conditions of this study.

Disinfection of contaminated equipment: evaluation of benzalkonium chloride exposure time and solution age and the ability of air-drying to eliminate Flavobacterium psychrophilum

Disinfection of equipment that comes in contact with fish can help to minimize the spread of Flavobacterium psychrophilum (the etiological agent of bacterial coldwater disease) within and among fish culture facilities. We present the results of three studies that evaluated the potential use of benzalkonium chloride and air-drying to kill surface-attached F. psychrophilum. In the first study, we established a vat with a 600-mg/L benzalkonium chloride solution and sampled this solution 0, 14, 35, 56, 70, and 84 d after creation. The solution was kept outdoors and subjected to typical hatchery use. Plastic test strips were dipped in a solution containing F. psychrophilum and were then immersed in benzalkonium chloride for 0, 1, 10, 30, or 60 min. The strips were then rinsed with sterile water and streaked across a plate containing tryptone yeast extract salts (TYES) medium. No culturable bacteria were detected from any strips immersed for 10, 30, or 60 min. Bacteria were detected on 17% of the strips that were immersed for 1 min. The age of the benzalkonium chloride solution had no effect on disinfection ability. In the second study, plastic strips were immersed in a solution containing F. psychrophilum and then were dipped in a 600-mg/L benzalkonium chloride solution for 10 s. The strips were then air-dried for 1 h and were streaked onto TYES medium. No bacterial growth was observed from any strips in the second experiment. The third study determined whether air-drying alone was sufficient to kill F. psychrophilum. Plastic strips were dipped in a solution containing F. psychrophilum; were allowed to dry at room temperature for 0, 24, 48, or 96 h; and were then streaked across TYES medium. Bacteria were cultured from strips representing each drying interval, indicating that air-drying times of 96 h or less are insufficient to kill F. psychrophilum.

Potential for dissemination of the nonnative salmonid parasite Myxobolus cerebralis in Alaska

Myxobolus cerebralis, the myxozoan parasite responsible for whirling disease in salmonids, was first introduced into the United States in 1958 and has since spread across the country, causing severe declines in wild trout populations in the intermountain western United States. The recent detection of the parasite in Alaska is further evidence of the species' capability to invade and colonize new habitat. This study qualitatively assesses the risk of further spread and establishment of M. cerebralis in Alaska. We examine four potential routes of dissemination: human movement of fish, natural dispersal by salmonid predators and straying salmon, recreational activities, and commercial seafood processing. Potential for establishment was evaluated by examining water temperatures, spatial and temporal overlap of hosts, and the distribution and genetic composition of the oligochaete host, Tubifex tubifex. The most likely pathway of M. cerebralis transport in Alaska is human movement of fish by stocking. The extent of M. cerebralis infection in Alaskan salmonid populations is unknown, but if the parasite becomes dispersed, conditions are appropriate for establishment and propagation of the parasite life cycle in areas of south-central Alaska. The probability of further establishment is greatest in Ship Creek, where the abundance of susceptible T. tubifex, the presence of susceptible rainbow trout Oncorhynchus mykiss, and the proximity of this system to the known area of infection make conditions particularly suitable for spread of the parasite.