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Titration of foot-and-mouth disease cattle challenge virus in cattle tongue has been the standard for many years in many countries, although titration in animals has been replaced by in vitro methods for all other applications. The objective of the analysis was the replacement of in vivo titration of cattle challenge virus by in vitro titration. Using data from 32 in vivo titration experiments together with the in vitro titration results of the same samples obtained by plaque count on primary lamb or pig kidney cells, as well as data from the virus isolation control chart used in the laboratory, we show that the reproducibility of the in vitro titration is much higher than that of the in vivo titration. The titer on primary kidney cells was on average 1.4 log10 higher than the titer determined by titration in cattle tongue (PFU/ml compared to bovine ID50/ml), but the difference varied among different strains. The study also shows that the probability of infection in cattle tongue is high even when a lower challenge dose is used, which makes the variability between strains less important. Based on these results, we propose to change the standard dose for cattle challenge from 104 bovine ID50 to 105.4 PFU, and to replace the in vivo cattle tongue titration method with the in vitro titration method.
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