| Grass carp (Ctenopharyngodon idellus) is an important aquaculture species, accounting forabout20%of gross freshwater fish product in China. However, during recent years, the grasscarp has suffered from more serious diseases, the mainly parasitic harm for grass carp areTrichodina, Dactylogyrus，Chilodonella cyprini, Ichthyophthirius muhifiliis, Gowkongensis. Inthis paper, a preliminary survey of the grass carp’s parasitic diseases in the ponds in threedifferent places of Guangdong Province, QingYuan Didixia village, ZhongShan Guangchengweiand GuangZhou Xilang was conducted, to find the relationship between the grass carp’sparasites and the water quality, physical and chemical factors as well as the number of ciliatedprotozoan. While in the course of the investigation, the characteristics and dynamics of theparasite-Dactylogyrus on the gills were studied; in different hardness and alkalinity water, theimpact of hatchability and vitality of theront of Ichthyophthirius multifiliis (Im) were studied.Histopathological changes of parasite was observed when Im infected grass carp juvenileunder laboratory conditions. HSP70gene was cloned from Im strain that collected on grass carpin Qingyuan and heat stress molecular mechanism of Im was preliminary studied. The mainresults and conclusions are as follows:1. During the period of investigation, Grass carp parasites was closely related to theseasonal change, from May to November which the temperature is high(15℃~32℃), Grasscarp mainly infected Trichodina and Dactylogyrus, at the climate changed season, Trichodinawas more likely to break out. Analysis of data showed that infection rate, abundance of GrassCarp’s Trichodina parasite in intensive cultivation were0.71and ‘++(5~10)’ respectively, in thenatural breeding were0.2and ‘+(1~5)’ respectively, the intensive cultivation higher than thenatural breeding. To survey GuangZhou Xilang ponds that was found， the Dactylogyrusinfection rate on the gills of grass carp is high, all over0.93, In July and August, the two highesttemperature months, the abundance of Dactylogyrus on gills reached33.5and31.5,respectively.The Dactylogyrus population standard variance average ratio, the negative binomial, diffusionindex are greater than one. That show the Dactylogyrus was aggregated in the host populations.During the period of investigation, except that the Dactylogyrus’ abundance on the the first gilllamellae in the right side is smaller than that of the left side, the rest are larger than the left.2. In the survey, a strain of Ichthyophthirius multhifiliis was found that in higher watertemperatures,28~30℃was still able to infect grass carp. To analyse the heat stress molecularmechanism, the HSP70gene ORF (open reading frame) of Ichthyophthirius multifiliis wascloned and its expression in following different experimental conditions of different temperaturewas analysed. Im HSP70gene ORF is1995bp, encoding a putative protein of664amino acidresidues. According to the gene structure analysis, Im HSP70secondary structure was mainly α-helix and randon coil-based, and spatial configuration included the N-terminal ATPase domainand C-terminal peptide-binding domain. Compared with other known HSP70protein, Im HSP70was most similar to that of Carchesium polypinum (78.08%identity). Phylogenetic analysis revealed that I. multifiliis clustered into a branch with C. polypinum, T. thermophila and S.histriomuscorum, whose classification status were the same. Im HSP70gene expressions in threedifferent development stages (theront, trophont and tomont) at three different temperatures(20℃,25℃and30℃) were detected by fluorescence quantitative RT-PCR. The resultsshowed as follows: the gene expression levels of Im HSP70were lowest in theront stage at allthree experimental temperature, the levels in trophont were higher than that in theront andtomont at20℃and25℃, while at30℃the levels in tomont were highest among the threestages. Additionally, the gene expression levels in theront and trophont stages were higher in30℃than that in25℃respectively.3. In the optimal temperature, the paper investigated the influence of hardness and alkalinityon cyst formation, hatchability, and activity power of theront under experimental condition. Theresults are as follows. When the water hardness and alkalinity were0mmol/L, the cyst wall ofIchthyophthirius muhifiliis up to70.5μm, when the hardness and alkalinity increased to4mmol/L, the cyst wall thinning to21.4μm. The hatch period in hardness, alkalinity of0.5mmol/Land0.1mmol/L was significantly shorter than other groups; the time of larvae’s keepingactivity power in hardness, alkalinity was0mmol/L group significantly lower than other groups;followed by hardness and alkalinity were4mmol/L and0.1mmol/L group.This paper shows that, the occurrence of grass carp parasites had a certain correlation withthe water temperature, alkalinity, hardness, etc of ecological factors, At30℃，the trophontHSP70expression levels was lower than that at20℃and25℃，but higher in tomont, whichshowed the cyst was the main way through adverse environment. Alkalinity and hardness in thewater had a certain correlation with the thickness of cyst wall, incubating time, the time oftheront’s keeping activity and hatch rate. However, the cross effects of the concentration of Mg2+, Ca2+and temperature affect Ichthyophthirius multhifiliis needs further study, Grass carpinfusorians parasite infection rates, abundance and the relationship of infusorians protozoa inwater also need further study. |
