Prevalence of Perkinsus sp. in the Manila clam Ruditapes philippinarum along northern coast of Yellow sea in China.
Epizootic mortality of the Manila clams, Ruditapes philippinarum, was investigated along the northern coast of the Yellow sea in China (July to October 2000), and the disease relativity to temperature and salinity was assessed. Perkinsus sp. was detected in the Manila clams from seven sites along the coast. Ray's fluid thioglycollate medium (FTM) technique and Choi's 2 mol/l NaOH digestion technique were used for the detection and quantification of Perkinsus sp. Cross-sections of the clams were embedded in paraffin, sliced to 3 µm, and stained with hematoxylin and eosine to observe the presence of trophozoites. Perkinsus cells were found mostly in connective tissues of gills, gonads, the mantle and digestive gland; they were rarely observed in feet and siphons. The trophozoites were often enclosed in amorphous granules. Among the infected clams, infection prevalence fell within a range of 20-100%. Average infection intensity at each sampling site varied from 3 to 3 565 936 Perkinsus spores per individual or 2 to 1 670 615 spores per gram of tissue. The clam's mortality was the highest in the Shichengdao because the Perkinsus cells in the individual was the greatest. A size-dependent Perkinsus sp. infection was found in the Manila clam collected from Donggang and Shichengdao. Perkinsus infection intensity was slight. These results showed that the mortality of the Manila clam was relative to the infection intensity of Perkinsus sp., and the Perkinsus sp. was further confirmed to be responsible for mass mortality of Manila clams along the northern Yellow sea coast. Temperature and salinity were considered to be major environmental parameters limiting spatial distribution and the occurrence of Perkinsus infection. Temperature over 20°C and salinity between 20-32 were suitable for the occurrence of outbreaks. This study reports on the spatial distribution, the infection and infection intensity of Perkinsus sp. in mollusks found in Chinese coastal waters.