The invention provides Streptococcus vaccine strains (e.g. S. equi which is causative of strangles) comprising the following modifications in their genomes: (i) attenuation of one or more essential biosynthetic genes, (ii) attenuation of one or more genes which encode a haemolytic toxin, or protein involved in the production thereof, plus preferably any one, two, or most preferably three of the following modifications: (iii) attenuation of one or more genes which encode a protein responsible for immune evasion (iv) modification of one or more genes such as to permit serological discrimination of the vaccine strain based on analysis of a protein encoded by said genes, and (v) attenuation of one or more genes which encode an enzyme responsible for recombination repair of the genome. The invention provides inter alia live attenuated vaccine strains which generate an immune response to multiple protective epitopes presented in a context resembling the live pathogen.
