Strains of the Virus of Foot-and-Mouth Disease Recovered from Outbreaks in Mexico

The present series of papers deals with studies on strains of the virus of foot-and-mouth disease from outbreaks in Mexico. The work was undertaken at the request of the Bureau of Animal Industry of the United States Department of Agriculture on behalf of a joint Mexican-American Commission to whom all the results recorded here have been reported. During the experiments recourse has been made to many new methods that have been developed at this institute, to which some preliminary reference should perhaps be made. The necessity of having a quantitative appreciation of a biological problem is becoming increasingly apparent especially in approaching problems of immunity. No great reliance can be placed on the results of “field” vaccination experiments, for example, in estimating the real value of an immunizing agent as it is not possible to analyse satisfactorily the many factors involved. This can be done only under controlled experimental conditions. Again before satisfactory interpretations can be made of the results of experiments carried out under controlled conditions it is necessary to have reliable “standard methods.” The main program of the scientific staff of this Institute for the last eight years has been directed with this object in view. As a result it is now possible to measure the potency of virus suspensions of bovine origin such as are used for vaccine production by a method of titration involving the simultaneous inoculation of a number of serial dilutions of virus in the bovine tongue. Sufficient observations are made for the calculation of the 50% end-point. This method guarantees the greatest accuracy commensurate with the use of a very limited number of animals. In serum neutralization tests correlated with electrophoretic studies and the development of methods for concentrating antibodies it has been shown that by application of the above titration method it is possible to estimate with a greater degree of accuracy than was formerly attainable, the neutralizing and protective properties of bovine antisera and to detect low levels of antibody. For quantitative estimates of this sort it is obvious that there must be basic information on the relative susceptibility of the test animals employed. In the case of foot-and-mouth disease where cattle are the experimental animals of choice for the study of virus of bovine origin, this susceptibility must vary within wide limits according to the source.