The practical importance of relating field strains of foot-and-mouth disease (FMD) virus to vaccine strains had been stressed by many workers (Rweyemamu and Hingly, 1984; Rweyemamu et al., 1984). The use of two dimensional serological tests has been established as the most relevant methods for showing antigenic relationship based on ‘r’ value, which is basically a ratio of activity of serum against heterologous (field) strains and activity of serum against homologous (vaccine) virus strain. Micro complement fixation test (mCFT) and virus neutralization test (VNT) have been the most widely used techniques for characterization of FMD virus types, but CFT has limited use in assessing antigenic relationship because it detects a wide spectrum of antigenic epitopes not relevant to protection. Sandwich ELISA (sELISA) has been used to characterize FMD virus types by different workers (Mishra et al., 1995; Pattnaik et al., 1990; Tosh et al., 1995; Wani et al., 1997). Similarly, sELISA also detects wide spectrum of antigen not relevant to protection. The liquid phase blocking ELISA (LPBE) a test comparable to the VNT (Hamblin et al., 1986), detects antibodies that can be correlated with protection (Van Maanen and Terpstra, 1989). The LPBE was also reported equally sensitive but more reproducible than VNT (Hamblin et al., 1986) and the test has been used for comparison of vaccine strains and field isolates of FMD virus (Kitching et al., 1988; Samuel et al., 1990). In the present study the three tests viz. mCFT, sELISA and LPBE were compared based on the r value of the FMD virus type O field isolates and a reference vaccine virus strain.

In the present investigation, the r values obtained the unidirectional and reverse directional testing of the 10 isolates of FMD virus type O by the three tests viz. mCFT, sELISA and LPBE were compared (Table 1, 2).

The unidirectional testing revealed lower r values of all the isolates except one by the mCFT compared to those with the sELISA and LPBE. The r values of the 6 isolates with the LPBE was also lower than the r value of the isolates with sELISA. The r value of the field isolates was the highest with sELISA and the lowest with the mCFT in the unidirectional testing (Table 1). In the reverse directional testing when the reference virus strain was tested against the antiserum of different field isolates, almost similar r values were observed with all the tests except that the one isolate in mCFT and three isolates in LPBE showed lower r-values (Table 2).

The results of r values when compared both in unidirection and reverse direction testing by the tests revealed antigenic relationship of all the field isolates with the reference strain except one isolate (G/8/02), which showed lower r value by the LPBE in both the directions. However, other workers observed that the r value between 0.20 to 0.39 by the LPBE had a significant difference from the reference strain, but protection might have been satisfactory if a potent vaccine was used (Samuel et al., 1990). In our study, the r value >1.00 was found with one isolate each in the unidirection and reverse direction testing by the sELISA. Earlier workers (Kitching et al., 1988; Kumar et al., 1999) have also reported r value >1.00, probably on account of broader antigenic spectrum of heterologous system (Kitching et al., 1988).

Calculation of r value has been used in the past for antigenic characterization of FMD virus strains by different serological tests as well as to compare the sensitivity of these test. The sensitivity of the sELISA has been reported to be more then that of CFT for antigenic characterization of FMD virus stains (Ouldridge et al., 1982; Ouldridge and Rweyemamu, 1983). On comparing the mCFT sandwich ELISA and micro VNT, higher r values have been reported with sandwich ELISA followed by micro VNT and mCFT (Wani et al., 1996). On the other hand, the r values obtained by CFT, ELISA and VNT have been reported to be almost similar with no significant difference between the results obtained with sandwich ELISA and two dimentional micro VNT for differentiation of FMD virus type Asia-1 strains (Mishra et al., 1995). A number of other workers (Gleeson et al., 1994; Lunt et al., 1994) have also used LPBE to characterize FMD virus and to assess the antigenic relationship of field isolates with vaccine strain. The present study also showed the usefulness of sELISA and LPBE for antigenic characterization of field isolates of FMD virus.

References