小瓜虫对澳洲龙纹斑不同生长阶段苗种的感染与致病性研究

The experimental infection and pathogenicity of Ichthyophthirius multiifliis in the larvae and juvenile of Maccullochella peelii

为了解澳洲龙纹斑苗种人工感染小瓜虫的情况,本研究以小瓜虫感染3种生长阶段鱼体(平均全长分别为1.2 cm、2.3 cm和10.7 cm),记录其达到全部致死时间(LT_(100)),分别为10 d、13 d和13 d;半数致死时间(LT_(50))分别为6.4 d、8.8 d和6.7 d;最高感染强度分别为168个、150个和1 450个小瓜虫。分析表明其小瓜虫感染率、感染强度和死亡率与感染天数均呈显著正相关(p<0.01)。相同感染天数的澳洲龙纹斑驯食前和驯食期鱼苗小瓜虫感染强度差异不显著,鱼种阶段的小瓜虫感染强度则显著高于鱼苗阶段(p<0.01)。澳洲龙纹斑鱼苗阶段感染小瓜虫初期呈随机分布,随后均为聚集分布;鱼种阶段感染小瓜虫均呈聚集分布。Logistic模型可很好拟合澳洲龙纹斑苗种小瓜虫感染强度、感染率和死亡率曲线;感染率拐点出现时间均早于感染强度和死亡率拐点时间。本研究结果为澳洲龙纹斑苗种培育中防控小瓜虫病提供了实验依据。

To identify the pathogenicity of Ichthyophthirius multiifliis to larvae and juvenile of Murray cod （Maccullochella peelii）, the Murray codat three different growth phase were artificial infected with I.multiitliis. The results showed that 100% lethal time （LTl0o） of the larvae （average length of 1.2 cm）, the larvae （average length of 2.3 cm）, and the juvenile （average length of 10.7 cm） of Murray cod were 9, 13 and 13 days; the half-leathal time （LTs0） were 6.4, 8.8 and 6.7 days after infected by I.multiifliis, respectively. The highest intensity of infection by I.multiifliis were 168, 150 and 1,450 in the fry of Murray cod, respectively. The infection rate, infection intensity of I.multiifliis and mortality of the fry were significantly and positively correlated with the infection days （p〈O.O1）. With the same infection days, there was no significant difference of infection intensity among the larvae and juvenile of Murray cod, but infection intensity of juvenile stage was significantly higher than that of the larvae stage. Infection intensity of Lmultiifliisin the larvae of the fish was random distribution in the early period, then became aggregation distribution quickly; and it showed aggregated distribution in all the juvenile stage of Murray cod. The curves of the infection rate and infection intensity of I.multiifliis, and mortality of the fish were well fitted using Logistic regression model, which showed inflexion point of infection rate appeared earlier than inflexion point of the infection intensity and mortality.