In farm samples, WSSV infections of L. vannamei showed infection
loads as high as 9.8×106 and 2.1×105copies/ng DNA in summer and
autumn, respectively; however, there was no statistically significant
difference in infection loads between samples collected in the two
seasons (PN0.05). It is interesting to compare these results with those
of F. chinensis sampled at the same time, as the latter showed 7.5×106
and 3.5 WSSV copies/ng DNA in summer and autumn, respectively
(Jang et al., unpublished data). Although infections in the summer
samples of the two species were very similar, those in autumn were
greatly different (i.e., a mean value about 5.7×104 times higher in L.
vannamei than F. chinensis). This result agrees well with those of a
previous study, in which L. vannamei was found to be much more
tolerant of WSSV than F. chinensis (Rosenberry, 2002). It can be
explained that F. chinensis, which sustains a lower infection load, can
survive only until harvest season; meanwhile, L. vannamei can live for
a longer period with a higher viral load, because of its high viral
tolerance. Similar observations have been reported by Jang et al.
(2007), who cultured L. vannamei and F. chinensis under the same
conditions in neighboring ponds and found a much higher mortality
rate in the F. chinensis ponds than in the L. vannamei ponds after a
WSSV outbreak had occurred. This is also one of the major reasons