Foot-and-mouth disease virus of Type SAT 1 (strain Turkey 323/62) was passaged serially 34 times in cattle previously vaccinated with increasing doses of formol-treated vaccine of the homologous strain. Primary vesicles developed in all the partly immunized animals and secondary lesions occurred in the majority. Virus from the 34th passage was capable of reinfecting a steer only 76 days after primary infection early in the passage series. Virus isolated from the second infection of this animal differed from that isolated from the primary infection in complement-fixing properties and in sensitivity to antiserum, and these differences were of a degree indicative of subtype variation.
The variant strain was transmissible by contact, and virus titres in tongue and foot lesions of partly immunized animals were of the same order as those encountered in susceptible cattle.
These observations suggest one way in which variant strains may arise in the field.
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