Fish and Shellfish Immunology, 2011, vol. 30, n. 3
Rainbow trout antibodies (Abs) binding to recombinant fragments (frgs) derived from the protein G of the
viral haemorrhagic septicemia virus (VHSV)-07.71 strain, could be detected by ELISA (frg-ELISA) in
sera from trout surviving laboratory-controlled infections. Abs were detected not only by using sera from
trout infected with the homologous VHSV isolate but also with the VHSV-DK-201433 heterologous
isolate, which had 13 amino acid changes. Sera from healthy trout and/or from trout surviving infectious
haematopoietic necrosis virus (IHNV) infection, were used to calculate cut-off absorbances to
differentiate negative from positive sera. Specific anti-VHSV Abs could then be detected by using any of
the following frgs: frg11 (56-110), frg15 (65-250), frg16 (252-450) or G21-465. While high correlations
were found among the ELISA values obtained with the different frgs, no correlations between any frg-
ELISA and complement-dependent 50 % plaque neutralization test (PNT) titres could be demonstrated.
Between 4 to 10 weeks after VHSV-infection, more trout sera were detected as positives by using
heterologous frg-ELISA rather than homologous PNT. Furthermore, the percentage of positive sera
detected by frg11-ELISA increased with time after infection to reach 100 %, while those detected by
complement-dependent PNT decreased to 29.4 %, thus confirming that the lack of neutralising Abs does
not mean the lack of any anti-VHSV Abs in survivor trout sera. Preliminary results with sera from field
samples suggest that further refinements of the frg-ELISA could allow detection of anti-VHSV trout Abs
in natural outbreaks caused by different heterologous VHSV isolates.The homologous frg-ELISA method
could be useful to follow G immunization attempts during vaccine development and/or to best
understand the fish Ab response during VHSV infections. The viral frgs approach might also be used with
other fish species and/or viruses.
