Ecological factors influencing the infection levels of salmonids by Acanthocephalus opsariichthydis in Lake Yunoko, Japan, were analysed. The most important factor is the feeding habits of the definitive host. Brook trout, Salvelinus fontinalis, were heavily infected with this parasite as they fed on the intermediate host, Asellus hilgendorfi. By contrast, Biwa salmon, Oncorhynchus rhodurus, and kokanee, O. nerka, preyed mainly on fish and plankton, respectively, and hence, they were infected to a lesser extent with this parasite. The overlapping habitats of the definitive and intermediate hosts were noted. These are the factors accelerating greater contact between both the hosts. Most brook trout inhabited the littoral zone, to which A. hilgendorfi were restricted. However, Biwa salmon and kokanee were widely dispersed in the lake. A close distributional relationship between plant materials and A. hilgendorfi was also suggested.
A monorchiid cercaria was found in a brackish water clam Corbicula japonica collected from Lake Shinji, Shimane Prefecture, Japan, and a new name Cercaria corbiculae sp. nov. is proposed. The present species is a distome, pharyngeate cercaria with a slender long tail, no stylet, no eyespots, long ceca, three pairs of penetration gland cells, 16 flame cells, a saccate excretory vesicle deflected to left.
Pathological activities of Edwardsiella tarda strains were studied. Adhesive or invasive activity was first examined. But we failed to detect any specific adhesive pili or invasive function. These strains were found to have slimes. These slimes could include adhesins as well as the protective substances against host defence systems. These strains were found to produce hemolysins but not to produce Escherichia coli-like enterotoxin, proteolytic enzymes and phospholipases. But different from these functionally defined toxic substances, we found that all E. tarda strains were producing the functionally different two dermatotoxins. These toxins could be the major exotoxins of E. tarda responsible for its pathogenicity.
Edwardsiella tarda was found to produce two dermatonecrotic exotoxic substances for rabbit. These two toxins were functionally distinguished by their onsets of symptoms. The early reacting toxin was found to cause mainly erythema 3-8 hours after intracutaneous injection and this erythema lasted for three days. The late reacting toxin was found to cause oedema first and then necrotic erythema 5-7 days after injection. Both toxins were antigenic and their toxic functions were not observed in immune animals. Both toxins were suggested to be high molecular weight proteins by the following reasons. They were precipitated by ethanol. The early acting toxin precipitated mainly in 0-20% ammonium sulfate saturation and the late acting toxin precipitated mainly in 40-60% saturation. They were heat sensitive. They were sensitive to acid and alkaline treatment. Their activities were not reduced by chloroform treatment. The molecular weight of the early reacting toxin was found to be less than blue dextran and more than ferritin, and that of the late reacting toxin was found to be less than albumin.
The bactericidal activity of normal sera from five species of fish, eel, rainbow trout, carp, tilapia and black sea bream, was investigated. All sera examined indicated the bactericidal activity against Escherichia coli. After they were incubated with zymosan, the carp and black sea bream sera lost the activity completely, but the tilapia serum was not affected at all. EDTA entirely inhibited the bactericidal activity of all sera tested. While the carp and tilapia sera exhibited the bactericidal activity in the presence of EGTA, the others partially lost the activity. None of the sera used possessed antibody against E. coli. The bactericidal reaction of fish serum appeared to be owing mainly to the alternative pathway of complement, but the activation system of the alternative pathway of fish complement is probably different in each fish species. The bactericidal activity of the sera was lower against fish pathogens, Vibrio anguillarum and Edwardsiella tarda, than against E. coli.
Four hundred and forty five isolates of Edwardsiella tarda were collected on field surveys in eel culture ponds in Shizuoka Prefecture in 1980 and 1981. Two hundred and seventy isolates (61%) were classified, according to O-agglutination test, into four serotypes (A, B, C and D). There were no great differences in the composition of the serotypes among the following three kinds of samples; rectum contents of eel, eel pond water and pond sediments (A: 13-17%, B: 22 35%, C: 4-13%, D: 2-4%). However, serotype A was predominant in kidney samples (A: 72%, B: 0%, C: 3%, D: 13%). By experimental infection to eel, tilapia and loach, serotype A of E. tarda proved highly virulent to the fishes, especially to eel, in comparison with the other serotypes tested.
Four birnaviruses isolated from fishes in Taiwan were compared to the three known serogroups of infectious pancreatic necrosis virus (IPNV) by neutralization tests and analyses of the virion polypeptides and RNA. Two viruses isolated from different populations of Japanese eels (Anguilla japonica) in the Lukang (LKE) and Dan Sway (DSE) regions and another virus from tilapia (Tilapia mossambica) in the Lukang area (LKT) were closely related to the AB strain of IPNV. A fourth virus isolated from rainbow trout (Salmo gairdneri) in the Kukuan region (KRBT) was similar to VR-299 IPNV. This indicates that North American and European serotypes of IPNV have been introduced to Taiwan. These studies represent the first complete characterizations of birnaviruses isolated from fish in Taiwan.
In October 1982, a bacterial disease occurred among cultured young flounder, Paralichthys olivaceus, in the Kurida Bay, Kyoto Prefecture. The diseased fish showed abdominal inflation and accumulation of ascites. Bacteria isolated from the diseased fish were identified as Edwardsiella tarda. The pathogenicity of the isolate for flounder and yellowtail, Seriola quinqueradiata, was proved positive by intra-peritoneal and/or intramuscular injections.
In 1982 infections of cultured yellowtail (Seriola quinqueradiata) with ampicillin (ABPC) resistant strains of Pasteurella piscicida were observed in the southwest of Japan. Determining whether current strains of P. piscicida are resistant or sensitive to ABPC is very important in the selection of initial antibiotics therapy for this infection. The detection of β-lactamase in ABPC resistant strains was attempted by using the rapid β-lactamase test of ROSENBLATT and NEUMANN (1978). The production of β-lactamase was found first in ABPC resistant strains of P. piscicida. The test was useful to determine whether the strain was resistant or not. The rapid slide test can be, therefore, easily utilized as a rapid detection method for ABPC resistant P. piscicida in routine work.
From July through September 1982, an epizootic occurred in a population of young cultured red sea bream, Pagrus major, in Nagasaki Prefecture. Some moribund fish without any significant lesions (average body weight 10 g) were submitted to bacteriological examination. The organism isolated from the kidney or spleen of the specimens was identified as Pasteurella piscicida. The characteristics were identical to those of the isolates from pseudotuberculosis in cultured yellowtail, Seriola quinqueradiata, in Nagasaki Prefecture in the period 1980-1982. The present isolate showed pathogenicity not only for red sea bream but also yellowtail, and LD50 for red sea bream was between 0.001 mg and 0.01 mg cells per fish. The injection of the isolate caused consistently white spots in the internal organs of yellowtails, whereas their formation in red sea breams was scarcely observed.