Studies on rickettsia-like organism (RLO) disease of tropical marine pearl oyster——Epidemiological investigation of RLO disease in larvae populations of maricultured Pinctada maxima

The epidemiological investigations on the disease and death in mature eggs, embryonic developmental periods and larvae populations[including oocytes, fertilized ovum, early embryonic phase larvae (6 h), D-shaped phase larvae (24 h), early umbo phase larvae, umbo phase larvae, post umbo phase larvae and eyespot phase larvae] in the Luhuitou Pearl Oyster Mariculture Farm of Sanya City,Hainan Province in April 1995 showed that there were two peaks of mortality rates which occurred in early umbo phase or umbo phase and post umbo phase (Figs 3~5 and Table 1) respectively from preembryonic developmental periods to larvae phases. It indicated that the onset and death of Pinctada maxima larvae populations followed a pattern of outbreak. Between the prevalence, intensity of RLO infection and the mortality rates of larvae populations were of obvious positively correlations. Generally,every peak of RLO infection is always followed by a peak of mortality rate of larvae hosts, and the mortality rates of larvae hosts declined with the decreased RLO infection (see Figs 3~5). Under the transmission electron microscopy (TEM), no rickettsia-like organisms were discovered in oocytes, fertilized ovum and early embryonic phase larvae (6 h). The RLO inclusions occurred first in the D-shaped phase larvae (24 h) under histological examination. Absences of RLO in transmission electron examination of oocytes of RLO-infected adult females, fertilized ovum and early embryonic phase larvae (6h) indicated that RLO may not be transmitted transovarially. But RLO for host infection may be transmitted by contact transmission since RLO inclusions were first identified regularly in D-shaped phase larvae (24 h), while these D-shaped phase larvae were still unable to take food during hatching 24 hours. In addition, the result of epidemiological investigation showed that no observable death occurred in D-shaped larvae populations, but early obvious death occurred in larvae populations in the seventh day after fertilization, in a rate of about 21.8%. It indicated that there was a incubation period from RLO infection for host (D-shaped phase larvae) to host onset and death occurred clinically.