The development of an attenuated foot-and-mouth disease virus vaccine by modification and cloning in tissue cultures of BHK 21 cells

Summary

An attenuated strain of foot-and-mouth disease virus, type SAT1, has been developed by serial passage in tissue cultures of BHK 21 cells and cloning by plaque isolation. Clones of attenuated virus were obtained after 48 passages in culture. Uncloned virus was still pathogenic at the 90th passage but was attenuated by the 150th passage.

Tests of the immunising properties of a clone prepared after the 82nd passage of the strain in over 100 cattle and 36 sheep showed that approximately 80% of the cattle given a dose of 10^8.0MID₅₀ and all of the sheep given 10^7.5MID₅₀ were protected against challenge. The vaccine strain was not transmitted to in-contact control cattle.

Advantages of this method of attenuation are:

• 1. Early separation of attenuated from pathogenic clones of virus makes possible the rapid development of an attenuated strain vaccine.

• 2. Screening large numbers of virus clones by inoculation of cattle tongues simplifies the selection of those which have the first necessary attribute of any live vaccine, viz. safety for the species to be immunised.

• 3. Attenuation of virus in BHK 21 cells results in strains capable of rapid growth to high titres. The same tissue culture systems provides a convenient method for the large scale manufacture of vaccine.