Statistical procedures for calculating the residual virulence of Brucella abortus strain 19 (S19) and Brucella melitensis strain Rev 1 vaccines in mice: theoretical basis and practical applications.

Regular control of the biological quality of live Brucella abortus strain 19 (S19) and B. melitensis strain Rev 1 vaccines is essential for the successful management of ruminant brucellosis in affected countries. The reference procedures recommended by the OIE (World organisation for animal health) and the European Pharmacopoeia include the determination of residual virulence, expressed as the recovery time 50 (RT50), of the tested (problem) vaccine in a reference mouse model compared with the RT50 of the corresponding reference strains in the same assay. The underlying statistical procedure applied is based on a parallel line assay and a classical probit model. In practice, the currently recommended procedure for calculating the RT50 is based on a graphical method which has never been described in detail. This paper provides a full description of this graphical method with the aim of making the technique comprehensible and accessible to all interested biologists. The procedure is somewhat cumbersome and very few laboratories apply the OIE and European Pharmacopoeia recommendations on a regular basis. Moreover, since this reference graphical method shows some statistical inconsistencies, a dedicated internet interface has been developed to perform RT50 calculations and is now available free of charge on the web (www.afssa.fr/interne/rev2.html).

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