The non-structural proteins (NSPs) of Foot-and-mouth disease virus (FMDV) have received conside-rable attention in recent years with the search for improved serological tests for the virus (1, 5, 7, 13, 14). The virus neutralization test (VNT) (6) and liquid phase blocking ELISA (LPBE) are currently the prescribed tests by the OIE (Office Internatinal des Epizooties). However,these tests require each serotype to be tested separately,are time consuming to perform,require virus containment facilities and cannot differen-tiate vacci-nated from convalescent animals. The development of a simple,quick to use test that covered all 7 serotypes as differentiating vaccina-ted from convalescent animals would be a major advance in the epidemiological tool for FMDV diagnosis.
The NSP s are only expressed in animals with replicating virus,therefore only animals that have been infected with live virus should develop antibo-dies to these proteins (1, 2, 7, 11).The currently used inacti-vated vaccines had been purified to remove cellular proteins and NSPs and should not induce antibodies to these proteins. However,in practice,vaccinated animals do pro-duce antibodies to some of the NSPs such as 3D (13) and antibodies to others,such as 2C,were found to rapidly fall below detectable levels (10). The 3ABC proteins appear to be the most promi-sing protein as diagnostic antigens (3, 8, 12, 15).
The following paper described a comparison of the Ceditest kit and UBI kit with the FMDV 3ABC-Ⅰ-ELISA kit developed at the Lanzhou Veterinary Re-search Institute. From the results,it could be concluded that the 3ABC antibody was the most reliable single indicator of infection. Some reference indicated that the immune response to the 3ABC appeared early after infection and the antibody to 3ABC could be detected longer than antibody to any other NSPs (4). It can be concluded that the FMD 3ABC-Ⅰ-ELISA kit developed in China will be a useful tool in FMD control and eradication program-mes facilitating the detection of virus circula-tion in FMD vaccinated populations.
The specificity (Sp) of the FMDV 3ABC-Ⅰ-ELISA kit was evaluated with sera collected from non-vaccinated and vaccinated animals. A total of 60 sera from non-vaccinated cattle were tested for antibodies to 3ABC,observing that 1 out of 60 sera gave a positive reaction (Table 1). In addition,249 sera,collected from vaccinated cattle were also tested,indicating that 8 out of 249 gave a positive reaction (Table 1).
Table 1. Specificity of the FMDV 3ABC-Ⅰ-ELISA kit
Thus,the specificity of the FMDV 3ABC-Ⅰ-ELISA kit was over 96% when used to examine sera from naive and vaccinated cattle (Table 1). For the calculation of specificity,results in the "doubtful" range were counted as positive.
The sensitivity (Se) of the assay was evaluated by examining 66 serum samples from experimen-tally infected cattle. All sera gave a positive reac-tion,thus the sensitivity of the assay was 100%.
The specificity and the sensitivity of the Ceditest® kit and UBI® kit were evaluated using the same sera. Sixty-seven out of 249 vaccinated sera were not tested because of limited Ceditest® kit (Table 2).
Table 2. Specificity and sensitivity of the Ceditest® kit and UBI® kit
The sensitivity of Ceditest® kit and UBI® kit was evaluated by examining 66 sera from experimentally infected cattle (Table 2). As the table showed,the specifi-city of the Ceditest kit exceeded 96% when used to examine vaccinated or non-vaccinated sera. By comparison,the specifi-city of the UBI® kit was extremely high though the sensitivity was low.
The results showed that the UBI® kit had a low sensitivity,the sensitivity was only 81.8%,54 out of 66 sera gave positive results,12 out of 66 sera gave negative results,most of these sera were bled after the 12th month post-infection. In contrast,the Ceditest® kit had a high degree of sensitivity.
The results obtained from the FMDV 3ABC-Ⅰ-ELISA kit was compared with those of two foreign ELISA kits (Table 3 and Table 4). 302(70+232) out of 308 showed the same results as shown in Table 3,corresponding to a coincidence rate of 98.05%. In contrast,only 354(55+299) out of 375 showed the same results as that indicated in Table 4,corresponding to a coincidence rate of 94.4%. One possible reason is that the UBI® kit detects the anti-3B antibody which exists in sera of cattle not more than 364 days,so some experimentally infected sera gave negative results (Table 4). However,anti-3ABC antibodies could be detected in experimentally infected cattle up to 742 days post-infection (1).
Table 3. Comparison of FMDV 3ABC-Ⅰ-ELISA kit with ceditest® kit
Table 4. Comparison of FMDV 3ABC-Ⅰ-ELISA kit with UBI® kit