Preliminary Evidence for the Effect of Heavy Metal Cations on the Susceptibility of Hard Clam (Meretrix lusoria) to Clam Birnavirus Infection

The hard clams (Meretrix lusoria) that are cultured in Taiwan have suffered high mortalities each spring and/or summer since 1969. Environmental factors, pollution and infectious diseases have been implicated, but no single factor has been shown to lead directly to large-scale death of the cultured hard clam. In the present study, attempts were made to examine the effects of heavy metals on the susceptibility of the hard clam to birnavirus infection. First we determined the stability of the clam birnavirus in sea, brackish, fresh waters at 4, 15 and 26°C and in solutions containing different concentrations of Zn2+, Cd2+, Cu2+, and Hg2+. For each heavy metal cation, two concentrations which had no influence on clam birnavirus infectivity were selected and used to treat the hard clams before and after virus infection. In experiment I, clams were immersed in 105.0 TCID50 /ml virus solution for 24 h and subsequently exposed to one of the heavy metals. Cumulative mortalities of clams were 20∼52% in most experimental groups although mortalities reached 91% in infected clams exposed to high concentrations of copper. In experiment II, groups of 60 clams were exposed to one of the heavy metals for 7 days and then infected with the virus, while controls were only expose to heavy metals. Mortalities of clams in the experimental groups reached 65∼90% within 28 days, while only 10% mortalities were observed in the groups that were exposed to either heavy metals or virus infection alone.