Serological analysis of historical field samples reveals major inconsistency between PCR and antibody ELISA for establishing KHV infection status of groups and individual koi

Koi herpesvirus (KHV) is the causative agent of a highly infectious and notifiable disease of C. carpio L. Serology has the potential to identify koi or carp that have been previously exposed to KHV and which may be possible carriers of the virus. In the present study, sera (n = 162) from groups of farmed koi carp, previously screened for KHV using a variety of molecular methods as part of a surveillance program in Asia from 2008 to 2010, were subsequently tested here individually by ELISA using plates coated with purified virus (American isolate KHV-I, H361). Only 31% of koi from PCR-positive KHV fish groups or populations associated with KHV disease (n = 59/162) were seropositive when screened in the ELISA at a serum dilution of 1/200, in contrast to 52.9% of seropositive koi that were KHV-negative by PCR (n = 103/162). Furthermore >34% of those seropositive/PCR negative fish had titres of >1/400 (moderate-strong responders). This field data highlights the concerns related to carp populations that have been screened for KHV using molecular methods alone and supports the need for serology to accompany molecular testing in carp for this notifiable virus.