the moribund juveniles contained 105e106 copies ng1 DNA. A
minimum of 105 WSSV copies are required to transmit WSSV in
shrimp by immersion and results in disease outbreak. L. vannamei
with load of less than 105 copies mg1 tissue could survive to
harvest [6]. However, the pre-infected shrimp with WSSV could
make shrimp more susceptible to environmental stress or other
pathogens, such as Vibrio spp. and virus [9e12]. Under the field
conditions, multiple infections by more than one pathogen can
cause much greater losses in shrimp culture than any single
infection. However, there is limited information available on multiple
infections. Our previous study demonstrated that shrimp in
multiple infection groups with WSSV and Vibrio anguillarum
showed higher mortality, and death occurred more quickly than
those infected by a single pathogen. The WSSV load increased
rapidly during a time-course replication analysis by TaqMan realtime
PCR [1]. In order to better understand why multiple infections
caused higher mortality and led to greater losses in
aquaculture, the defense mechanism of shrimp against multiple
infections is evaluated.