Many marine fish species including olive flounder, black rockfish, sea breams and sea bass are being cultured in Korea, and the production quantity has been increased continuously.
Parasites can serious disease, often resulting in mass epidemics in marine fish culture, where large numbers of fish are concentrated under sometimes stressful environmental conditions. The following are diseases causing parasites in economically important cultured fish in Korea.
Two important parasitic diseases in cultured marine fish in Korea are scuticociliatosis in olive flounder and gill monogenean diseases in rockfish. Several parasitic disease including white spot disease, ichthyobodosis, amyloodiniasis, myxosporian diseases and copepodid disease have occurred, also. Among them, the most important protozoan diseases caused by Uronema marinium, a causative the infection mechanism of the parasite is and important problem to solve.
Scuticociliatosis
In recent years, the number of scuticociliatosis outbreaks has explosively increased in Korea's flounder fry industry. This parasitic disease often results in massive deaths and considerable economic loss.
Scuticociliatosis invades the flounders' skin, fins, muscles, peritoneal cavity, kidney, pancreas and brain, often leading to death of the host. Heavily infected fish are difficult to treat, in particular when the ciliates enter the brain. To control the ciliates, a 2-hour 200-ppm formalin dip is recommended once a day for six days.
Based on the arrangement and shape of the buccal structure, scuticociliates isolated from cultured olive flounder Paralichthys olivaceus belonged to the family Uronematidae and showed many characteristics of Uronema marinum. There was variation in the morphometry of clinical isolates taken from different organs of infected flounder. However, the isolates did not show any significant difference in morphometry under cultured conditions. The ciliates were easily maintained in vitro medium to which antibiotic agents had been added and which had been enriched with the raw brain tissue of a healthy olive flounder. The ciliates propagated in a wide range of both temperature(6 to 30°C) and salinity(10 to 35ppt).
Our present study, in vitro efficacy of chemicals on the scuticociliate at low salinity was investigated. All chemicals showed synergistic effects to be parasiticidal effects at salinity in 5ppt compared to chemicals at salinity in 33ppt. At 5ppt, ciliates were killed completely within 1.5-h by exposure to at 50 ppm formalin (37% formaldehyde), at 100ppm hydrogen peroxide (30% solution) and at 100ppm copper sulfate (20% solution). The formalin was the most effective chemical against the parasites at low salinity. This study examined effects of bath treatment of 3 chemical agents including formalin, hydrogen peroxide and sodium chloride, and freshwater against scuticociliates infecting olive flounder. Although 100 ppm formalin and freshwater did not completely eliminate the scuticociliates within the internal organ of fish, chemicals were effective to prevent scuticociliatosis from spreading. It confirms the efficacy of the chemical with treating the diseased fish for at least 4 consecutive day
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Gill monogenean diseases
The important gill monogenean parasite in cultured marine fish in Korea is Microcotyle sebatisc.
Several monogenean parasitic disease including Bivagina tai, Heteraxine heterocerca, Heterobothrium tetrodonis have occurred in cultured marine fish, also.
M. sebatis is a pathogenic polyopisthocotylean monogenea of cultured rockfish. Gill hyperplasia is seen in heavy infections and results in serious deformation of gill lamellae. Heavily infected fish may also exhibit blood changes that included low hematocrit values and depletion of hemoglobin.
The prevalence of the parasite on the 2004 year-class rockfish was 100% from November 2004 to June 2005, then, decreased slightly during the warm months. Between January and March 2004, abundance of parasite increased, reaching a peak in March, then decreased significantly at April. The abundance of the parasite re-decreased significantly at August.
White spot disease
An obligate parasite, Cryptocaryon irritans responsible for the white spot disease of marine fish is known to develop in the temperature regime over 19°C Recently, however, we found white spot disease of olive flounder, Paralichthys olivaceus during winter at water temperatures ranging between 12 to 16°C in Korea.
In our present study we isolated a C. irritans-like ciliate from the affected fish and investigated its reproductive characters to compare the newly found ciliate with typical C. irritans. The newly found ciliate had an additional process in the reproductive stage, characterized by a budding before palintomic division, and it showed higher ability of tomitogenesis at lower temperature (16°C) than at high temperature (24°C). Nevertheless, the present ciliate had still common with typical C. irritans as to clinical, histopathological, and morphological characters, suggesting that it is a new strain of C. irritans, adapted to lower water temperature.
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A Flounder infected with white spot disease, N Cryptocaryon irritans and histological feature |
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Marteilioides chungmuensis
The parasite induced ovary necrosis and prohibited normal growth of fertilized eggs. Oysters infected by this parasite show grossly visible ovary deformations, with lump like hypertrophy that renders them unmarketable. Therefore, the parasite is considered one of the most serious problems for oyster production in Korea. The parasite has been found in oysters from almost all of the oyster culture areas of Korea, although infection levels vary with region and season. Infection levels along the southern coast have increased since 1990 and oyster industry in this area is facing increasing production challenges due to poor seed collection.
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A: The external view of an oyster with an advanced M. chungmuensis
B: Histological observations of M. chungmuensis infected in C. gigas. Sporants in a oocyte are noted.
C: TEM observation of primary cell of M. chungmuensis.
Early developmental stage of M. chungmuensis(M) adjacent to the nucleus(N) of the host cell(ova)
in the gonad of C. gigas.
D: TEM observation of M. chungmuensis. Ova of C. gigas containing two primary cells of M. chungmuensis adjacent to its nucleus(N). The cytoplasm of both primary cells is disintegrating but the primary cell on the right has two sporonts(Sp), while the one on the left is unusual in having five sporonts, two of which are producing spores(Sp*).