Streptococcus dysgalactiae

Streptococcus dysgalactiae is a gram positive, beta-haemolytic, coccal bacterium belonging to the family Streptococcaceae. It is capable of infecting both humans and animals, but is most frequently encountered as a commensal of the alimentary tract, genital tract, or less commonly, as a part of the skin flora. The clinical manifestations in human disease range from superficial skin-infections and tonsillitis, to severe necrotising fasciitis and bacteraemia. The incidence of invasive disease has been reported to be rising. 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. Streptococcus dysgalactiae is currently divided into the subspecies Streptococcus dysgalactiae subspecies equisimilis (SDSE) and Streptococcus dysgalactiae subspecies dysgalactiae (SDSD); the former mostly associated with human disease, and the latter almost exclusively encountered in veterinary medicine. Their exact taxonomic delineation, however, is a matter of ongoing debate (See taxonomy). 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. Dys (bad) galactiae (milk) alludes to their propensity to cause bovine mastitis. Equi (horse) similis (like) infers similarity to the closely related species, Streptococcus equi. Streptococcus dysgalactiae was long believed to be non-pathogenic to humans. However, an increasing incidence of S.dysgalactiae infections has been documented, and in some geographic regions, the rate of invasive infection has even surpassed that of Streptococcus pyogenes. The age distribution of invasive cases among humans is clearly skewed towards the elderly, whereas the healthy carrier state appears to have the inverse relation to age. People with chronic maladies, including cancer and diabetes, are also especially susceptible to infection. These opportunistic traits have been proposed as one of the mechanisms underlying the observed increasing frequency of invasive disease. Furthermore, a male predominance has been noted, presumably due to a higher burden of comorbidity. The incidence of non-invasive disease in human does not appear to be increasing. Streptococcus dysgalactiae subspecies equisimilis (SDSE) is a commensal in human alimentary tract and genital tract. Occasionally it is isolated from skin, but usually in relation to a chronic skin condition or some breach of the epithelial barrier. Non-invasive disease manifestations include predominantly tonsillitis and superficial skin infections. Additionally, it has long been recognized as a potential cause of cellulitis/erysipelas. However, the role of SDSE in cellulitis might have been previously underestimated, and it was linked to a majority of the cellulitis cases in a recent study. The clinical presentation among invasive disease is also dominated by skin and soft tissue infections, including a small subset of patients presenting with severe necrotizing fasciitis. Moreover, it is an important cause of bone and joint infections, and this disease manifestation is reported to be increasing. Less commonly it can present as pneumonia, endocarditis, genital or intra-abdominal infections. Primary bacteraemia, infection without identifiable focal origin, comprises approximately 20% of the reported cases. Recently, SDSE has been linked to post-streptococcal glomerulonephritis and acute rheumatic fever. These immunologic sequelae have previously only been associated with Streptococcus pyogenes.Streptococcus dysgalactiae subspecies dysgalactiae (SDSD) is almost exclusively an animal pathogen. However, a few casuistic reports of human zoonotic infection have been documented.