Isolation of a variant strain of foot-and-mouth disease virus (type O) during passage in partly immunized cattle

Abstract

Foot-and-mouth disease virus of type O (strain Israel 1/63) was passaged serially fourteen times in cattle previously vaccinated with increasing doses of formol-treated vaccine of the homologous strain. Primary and secondary lesions developed in the majority of the animals. Virus from the 14th passage, which proved to be capable of reinfecting animals only 34 and 35 days after infection with the original strain, differed from the original strain in complement-fixing properties and in sensitivity to antiserum. These changes were of a degree indicative of subtype variation.Virus titres in the saliva, in the tongue vesicles and in the foot vesicles of partly resistant cattle were very great and would have constituted a danger to other stock under field conditions.The experimental data confirm previous observations on the infection of partly resistant cattle and on the isolation of variant strains during passage of FMD virus in tissue cultures containing specific antiserum.