Resolution of block neutralization test curves into components of the foot-and-mouth disease virus system

Classic neutralization studies by Fazekas de St. Groth and Webster (8) on mixtures of influenza viruses and mixtures of rabbit antisera are reinterpreted in terms of a percentage contaminant in the stock used for the dilution series. A very small amount of a different virus changes the shape of quantal assay curves considerably, but even a large amount of a different antiserum has negligible effect on the shape and merely shifts the curve along the serum dilution axis. These conclusions are the reverse of the authors, who only considered an absolute amount of another component in all dilution tubes. An artificial mixture of 2.7 percent O8 in O1 foot-and-mouth disease virus strains was tested against anti-O1 serum assaying in suckling mice. The small amount of O8 virus greatly altered the shape of the neutralization curve in the direction expected from the reanalysis of the influenza literature data. Results from artificial mixtures are used to explain what were hitherto anomalously broad neutralization curves for some other foot-and-mouth disease strains given by Booth et al. (1) Many of the virus stocks studied can now be postulated as a natural mixture of related virus strains. In fact, the O1 and O8 stocks used might also be of themselves mixtures. These virus strains also exhibit still a further test complication in that the virus-antibody reaction appeared to shift away from complexes on dilution immediately prior to assay.