Hy. Chou et al., PRELIMINARY EVIDENCE FOR THE EFFECT OF HEAVY-METAL CATIONS ON THE SUSCEPTIBILITY OF HARD CLAM (MERETRIX LUSORIA) TO CLAM BIRNAVIRUS INFECTION, Gyobyo kenkyu, 33(4), 1998, pp. 213-219
The hard clams (Meretrix lusoria) that are cultured in Taiwan have suf
fered high mortalities each spring and/or summer since 1969. Environme
ntal factors, pollution and infectious diseases have been implicated,
but no single factor has been shown to lead directly to large-scale de
ath of the cultured hard clam. In the present study, attempts were mad
e to examine the effects of heavy metals on the susceptibility of the
hard clam to birnavirus infection. First we determined the stability o
f the clam birnavirus in sea, brackish, fresh waters at 4, 15 and 26 d
egrees 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 a
nd used to treat the hard clams before and after virus infection. In e
xperiment I, clams were immersed in 10(5.0) TCID50/ml virus solution f
or 24 h and subsequently exposed to one of the heavy metals. Cumulativ
e mortalities of clams were 20 similar to 52% in most experimental gro
ups although mortalities reached 91% in infected clams exposed to high
concentrations of copper. Tn experiment II, groups of 60 clams were:
exposed to one of the heavy metals for 7 days and then infected with t
he virus, while controls were only expose to heavy metals. Mortalities
of clams in the experimental groups reached 65 similar to 90% within
28 days, while only 10% mortalities were observed in the groups that w
ere exposed to either heavy metals or virus infection alone.