We examined the effect of cortisol on the degranulation and number of eosinophilic granular cells (EGCs) in tilapia, Oreochromis niloticus. One hour after injection of stimulants (killed Escherichia coli or substance P, a neurotransmitter) into the swim bladder, the percent of the degranulated EGCs in the swim bladder membrane was determined. Peritoneal implantation of coconut oil containing cortisol (50 mg/kg body weight) 24 h before injection of stimulants significantly reduced the percent of the degranulated EGCs, compared with the control fish administered with coconut oil without Cortisol. Cortisol also suppressed the in vitro degranulation of EGCs stimulated with substance P or with tilapia normal sera containing zymosan. To investigate the effect of cortisol on the abundance of EGCs, the number of EGCs in the swim bladder membrane was counted. Density of EGCs was decreased after implantation of cortisol, compared with that of the control group. These results suggest that cortisol suppresses the degranulating activity of EGCs and decreases the density of EGCs in the tissue.
Out of 78 diseased guppies imported from Singapore to Japan, 43 fish were found infected with ciliates. Thus, pathological examination of guppies naturally infected with ciliates and identification of a ciliate sampled from guppies in Singapore were conducted. Most of the ciliates parasitized the inside of scales, muscles or internal organs, while some invaded into the eye socket and spinal cord. No damage was found in the intestinal epithelium. From these findings, it is considered that the main invasion route of these ciliates was from the scales of hosts to internal organs through muscles. As the results of morphological examination using silver impregnation, the examined ciliate was identified as Tetrahymena corlissi Thompson, 1955, which has been reported as a pathogen of guppies in North America.
Agglutinating activities of the skin mucus extract (SME) from Japanese eel were examined against bacteria isolated from water and skin of reared eels in an experimental aquarium. The SME agglutinated 60 out of 152 bacterial isolates from the rearing water (water group), and 40 out of 130 bacterial isolates from eel skin (skin group). The agglutinating activities tended to be higher against bacterial isolates in the water group than those in the skin group, suggesting the skin mucus agglutinins are efficient for excluding bacteria from the skin. Lactose, which is known as a specific carbohydrate recognized by the eel skin mucus lectin, inhibited agglutination against 36% bacterial isolates in the water group and 35% in the skin group. This result shows that SME contains more than one kind of bacterial agglutinins.
Early stages of Glugea plecoglossi (Microspora) are difficult to detect by normal light microscopy. In situ hybridization (ISH) protocol with DIG-labeled oligonucleotide probes designed from SSUrRNA of G. plecoglossi was established with the aim of detecting all stages. All developmental stages of G. plecoglossi in xenomas could be detected by ISH. First, we aimed to evaluate the suitability of rainbow trout as an experimental animal in place of ayu for G. plecoglossi. Immersion challenge tests showed that the susceptibility and location of G. plecoglossi did not differ between ayu and rainbow trout. Cyst forming sites were different among challenged trout by the three methods (intubation, immersion and intraperitoneal injection). In the intestinal epithelium of rainbow trout (Oncorhynchus mykiss) at 5 min post-intubation of G. plecoglossi spores, we found possible initial stages, which were positive in ISH and negative in Uvitex 2B staining. This finding suggests that G. plecoglossi spores discharging the polar tube pierced the gut epithelial cells, and injected the infective sporoplasm. The present study showed that ISH is a promising tool for studying transmission modes of G. plecoglossi in fish and that rainbow trout is a useful replacement host to identify the pathobiological characteristics of G. plecoglossi.
Effects of temperature, salinity and chlorine treatment on egg hatching of the monogenean parasite, Neoheterobothrium hirame, of the Japanese flounder, Paralichthys olivaceus, were examined. More than 85%of the eggs developed and hatched between 10 and 25°C, but at 30°C hatching rates were lower than 5%. They hatched similarly in a wide range of salinities from 1/3-to full-strength seawater (salinity : 11-33‰). In distilled water and double-strength seawater, they did not hatch at all. In 1.5-strength seawater, a decrease of hatching rates was observed. Eggs treated with sodium hypochlorite (available chlorine concentrations below 50ppm for 15min) showed hatching rates similar to those in the control. Even an extraordinary chlorine treatment (available chlorine concentration 100 ppm for 15 min) had a limited effect on the hatching rates.