which was only slightly lower than the
dose received by the Poly(I:C)-VHSV 104 group, which effectively
protected treated fish. Therefore, we concluded that the formalintreated
VHSV in the first vaccine trial worked as a live vaccine at
103.0 to 104.0 TCID50 mL1 and provided effective protection to 80%
of treated fish.
In contrast, the FT-VHSV in vaccine trial II was completely
inactivated by 7 days of formalin treatment, so that there was no
protective effect conferred by exposure the live virus. Vinay et al.
[12] reported that fish immunized with a formalin-inactivated VHS
vaccine without squalene and aluminum hydroxide adjuvants
showed 37% relative percent survival (RPS) 4 weeks postvaccination.
At 108.8 TCID50 fish1
, the VHSV dosage used by
Vinay et al. [12] was approximately 1000-fold that used in this
study. The higher efficacy in their study may have been the result of
their use of higher antigen concentrations in smaller fish, in comparison
with the fish and antigen concentrations used in this work.
We also inactivated concentrated VHSV (1010.3 TCID50 mL1
) via
24 h of treatment with 0.3% formalin; however, this treatment
failed to inactivate the virus (data not shown). Taken together, these
findings show that the protective effects of formalin-killed vaccines
should be verified before widespread use in fish.
In the ELISA results, the high antibody titers were shown in the
high-dose antigen inoculated groups in the Poly(I:C)-VHSV and
DEPC-FT VHSV groups. However, the protective effects observed in
the Poly(I:C)-VHSV and DEPC-FT VHSV groups following VHSV
challenge were different. The Poly(I:C)-VHSV 104 group generally
had low titers of antibodies against VHSV, but had a 100% survival
rate. In contrast, the DEPC-FT VHSV 106 group had high titers of
antibodies against VHSV, but had a survival rate of only 8%. These
findings show that the degree of induction of an antibody response