Relationship between virus neutralization and serum protection bioassays for IgG and IgM antibodies to foot-and-mouth disease virus.

The time interval between administering the serum and the virus was found to influence the results of the in vivo mouse protection test for foot-and-mouth disease antibodies. In particular, for both IgG and IgM antibodies to strain A12 virus, the mouse protection index increased from zero to a maximum at about 6 h and remained high for at least five days. Variations in the antiserum concentration, on a log scale, had a proportional effect on the mouse protection index, if between 1 and 3. The constant of proportionality was unity for IgM and 2 for IgG antibody. Comparison with in vitro neutralization tests revealed essentially parallel neutralization curves. The lower serum titre in the protection test, if computed for less than 10(3) LD50/dose, was accounted for by the simple dilution of the inoculated serum into the volume of the mouse. Consequently, in the low titre range, the same virus-antibody reaction and its effect are operable in each of the two tests. Analysis of literature data in which both the in vivo protection test and the in vitro neutralization test results were available on the same sera showed consistency with the above conclusions for both cattle and swine sera. The protection test had a highly atypical survival pattern occurring at antibody concentrations expected to neutralize more than 10(3) LD50/dose. The resulting in vivo dampening effect on virus titre is postulated to be caused by the excess antibody of the passive immunity test interfering with the spread of infection. The effect is analogous to an anomaly caused by not removing the inoculum in quantal tissue culture assays and it prevents quantification of antibody levels in strong sera.