Relationship Between Plaque Assay and the Mouse Assay for Titrating Rift Valley Fever Virus

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Abstract
Discussion and summary RVFV produced plaques that were well defined and characteristic of the small-plaque variant in monolayer cultures of L-MA Clone 1–1 tissue cells under a double agar overlay. Plaque counts made on days 3 and 4 were not statistically different, although plaque size was increased by the fourth day. A linear relationship was observed between plaque counts and virus concentration under various treatment conditions, permitting precise virus assays. The results were further extended to include comparison of slopes between MICLD50 and PFU. Although the titers for MICLD50 were consistently higher, there were no statistical differences in or between slopes of the two assay systems. This observation permitted accumulated data comparisons between the two assay systems to be combined into one linear regression program for development of a model for estimating MICLD50 from PFU. The model now provides a technique for observing infectivity in two independent assay systems with certain accuracy at a greatly reduced cost to an experimenter or diagnostic and experimental laboratories where such determinations are routine. To insure that the relationship portrayed in the model continues to hold after additional passage of the virus in tissue culture lines, it will be necessary to perform studies at regular intervals to confirm the relationship between PFU and MICLD50. This procedure will serve to spot any changes in mouse pathogenicity, unaccompanied by decrease in PFU. For the virus system under study, a large reduction in cost has been provided utilizing the model described. The particular model, and the parameter estimates obtained, while not necessarily applicable to other virus strains gives some hope that similar comparative studies may provide a basis for like reduction in cost for routine assays of other viruses. Based on their own experience, the authors recommend such studies where the number of assays is appreciable.

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