Development and validation of a 3ABC indirect ELISA for differentiation of foot-and-mouth disease virus infected from vaccinated animals. 
Non-structural protein (NSP) 3ABC antibody is considered to be the most reliable indicator of present or past infection with foot-and-mouth disease virus (FMDV) in vaccinated animals.  An indirect ELISA was established, using purified His-tagged 3ABC fusion protein as antigen, for detection of the antibody response to FMDV NSP 3ABC in different animal species.  The method was validated by simultaneous detection of the early antibody responses to NSP and structural protein (SP) in FMDV Asia 1 infected animals.  The performance of the method was also validated by detection of antibody in reference sera from the FMD World Reference Laboratory (WRL) in Pirbright, UK, and comparison with two commercial NSP ELISA kits.  The results showed that the antibody response to SP developed more quickly than that to NSP 3ABC in FMDV infected animals.  In contact-infected cattle, the antibody response to NSP 3ABC was significantly delayed compared with that to SP antibody.  The early antibody responses to SP and NSP 3ABC in FMDV inoculated cattle and contact-infected or inoculated sheep and pigs were generally consistent.  In pigs, 3ABC antibody was linked to the presence of clinical signs; however, in sheep, subclinical infection was detected by the development of 3ABC antibodies.  Therefore, the antibody responses to 3ABC varied between host species.  Eight out of 10 positive serum samples from FMD WRL were tested to be positive at cutoff value of 0.2.  The rate of agreement with the ceditest FMDV-NS and the UBI NSP ELISA were 98.05% (302/308) and 93.2% (287/308), respectively.  The prevalence of 3ABC antibodies reached 71.4% in some diseased cattle herds.  The further work is required to evaluation the performance of this method in different animal species and different field situations.