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

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Abstract
Methods used in preparing a crystal violet blood virus foot-and-mouth disease vaccine were descr. and the means of testing this vaccine in cattle were outlined. Vaccine prepared with a stock Vallee type A virus, while very effective in protecting cattle against the homologous virus, conferred little protection against the Mex- ican strains tested. Conversely, vaccine prepared with Mexican strains of foot-and-mouth disease virus possessed little immunizing capacity for the stock Vallee type A virus although they were very effective in protecting cattle against the Mexican strains. Immunization expts. with Mexican strains of virus were rendered difficult of interpretation due to the low invasiveness of these strains for cattle. The results emphasize the necessity, in attempting prophylactic control of Mexican virus infection, of testing vaccines prepared from Vallee A strains, other than selected Mexican strains, under controlled exptl. conditions, before using them in the field.