The clinical manifestations in human disease range from superficial skin-infections and tonsillitis, to severe necrotising fasciitis and bacteraemia.

[2][3][4] Several different animal species are susceptible to infection by S. dysgalactiae, but bovine mastitis and infectious arthritis in lambs (joint ill) have been most frequently reported.

The names are derived from Greek; Streptococcus meaning chain forming (Streptos) rounded berry-like bodies (kokkos), referring to their usual appearance under a light-microscope.

[1][2][13] Recently, Streptococcus dysgalactiae subspecies equisimilis has been linked to post-streptococcal glomerulonephritis and acute rheumatic fever.

However, the bacterium is frequently encountered as a colonizer of healthy animals, especially in the alimentary tract and genital region.

[19] S. dysgalactiae has been isolated from infectious polyarthritis in several animal species, including piglets, lambs, calves and goats.

Second-line agents include macrolides and clindamycin, although increasing resistance, due to both efflux and target modification, has been documented in some geographic regions.

[28][29] The correlation of group carbohydrate specificity with the proposed species S. dysgalactiae and S. equisimilis, however, were not explored in detail.

The Lancefield classification soon became the preferred laboratory identification method for streptococci, and the names S. dysgalactiae and S. equisimilis fell into disuse.

Although no official taxonomic gold standard exists, the most current and widely supported definition was published by Vieira et al. in 1998.

The rest are classified as S. dysgalactiae subspecies equisimilis, are (mostly) beta-haemolytic and can harbour carbohydrate antigens of Lancefield group A, C, G or L. However, a recent study indicates that the Streptococcus dysgalactiae subspecies equisimilis strains of animal and human origin are genetically divergent, and future taxonomic reclassifications are conceivable.

Although many laboratories currently identify bacteria by mass-spectrometry (Matrix Assisted Laser Desorption/ionization Time Of Flight MALDI TOF MS), phenotypic testing is still widely used.

Unlike Streptococcus pyogenes (harbouring Lancefield group A antigen), S. dysgalactiae is PYR-negative and Bacitracin resistant.

It is ubiquitous in Streptococcus dysgalactiae subspecies equisimilis of human origin, and its hypervariability in the 5'-terminal region forms the basis for categorization into separate emm-types.

The M-protein, the most extensively studied Streptococcus dysgalactiae subspecies equisimilis virulence factor, has been documented to facilitate both adherence to and internalization into host cells.

[1][42] Other adhesins have also been described, including the genes gfba, fnB, fbBA, fnBB, lmb and gapC; all mediating binding to fibronectin.

[47][48] These properties may explain the tendency of recurrent bacteraemia observed in human cases caused by Streptococcus dysgalactiae subspecies equisimilis .

In order to establish infection, the bacteria need to escape the host immune response, and in streptococci, a varied arsenal of bacterial strategies have been described.

[1] Furthermore, Streptococcus dysgalactiae possesses protein G, a virulence factor binding circulating immunoglobulins, and thus interfering with the host antibody response.

[49] DrsG, a virulence protein abrogating the effect of antimicrobial peptides secreted by human immune cells, is also harboured by a subset of strains of Streptococcus dysgalactiae subspecies equisimilis.