Minute quantities of foot-and-mouth disease virus (FMDV) were suspended in 500-, 1,000- and 2,000-ml volumes of virus growth medium and inoculated into primary bovine kidney cell cultures grown as monolayers in 5-liter Povitsky bottles. After 48 hr of incubation, the low concentrations of FMDV, containing from 10-3 to 10-1.7 plaque-forming units (PFU) per milliliter of inoculum, multiplied to approximately 106 PFU per milliliter of fluid. The presence of extracellular virus after multiplication was verified by assaying the fluid in 4-oz prescription-bottle cultures. Minimal quantities of FMDV were able to infect the cells and replicate despite the large volume of the suspending fluid. This technique may be applicable for the detection of trace amounts of FMDV in clinical samples and vaccines. Skin samples of unknown infectivity in limited tests gave identical positive or negative results in both steers and Povitsky cultures, but were consistently negative when tested in suckling mice and in prescription-bottle tissue cultures.
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