Streptococcus spp. infections

Infections by Streptococcus spp. in livestock cause a great variety of primary or secondary suppurative conditions (Table 1), including strangles, mastitis, genital tract infections, cervical lymphadenitis, septicaemia, endocarditis, meningoencephalitis and arthritis.

The name ‘Streptococcus’ was first coined by Billroth and Ehrlich in 18774 for the chain-forming coccus found in infected wounds. Since then the Streptococcus spp. have been extensively studied and a large number of species have been identified. Their taxonomy is in a state of flux following new findings that resulted from hybridization studies.^{12, 24} There are currently 55 recognized species of Streptococcus¹⁰ and 27 of Enterococcus. 10 Streptococcus dysgalactiae and S. equisimilis have been shown to be the same species,²⁶ which has been divided into S. dysgalactiae subspecies dysgalactiae for animal isolates, and S. dysgalactiae subspecies equisimilis for human isolates, regardless of the type of haemolysis produced. Streptococcus zooepidemicus and S. equi are also regarded as one species; they are divided into S. equi subspecies zooepidemicus, S. equi subspecies equi and Streptococcus equi subspecies ruminatorum ¹⁰, Fernandez The species in this chapter, however, are referred to by their established names.

Molecular genetic analysis has been used to characterize new genera in this group. The pathogenicity for livestock of these new genera still has to be established, except in the case of Globicatella sanguinis which has been associated with meningoencephalitis in lambs.²⁷ Others of interest are Gemella, isolated from pyogenic conditions,⁸ Alloiococcus from middle ear infections in humans,¹ Aerococcus from urinary tract infections,² Carnobacterium and Vagococcus which cause disease in fish,²⁸ and Facklamia which causes purulent conditions.¹⁶

In South Africa, S. equisimilis and S. zooepidemicus affect livestock species most commonly (Tables 2 to 4) while S. agalactiae is the most important cause of bovine mastitis. The isolation of S. dysgalactiae in cattle, and S. suis and S. porcinus in pigs, is also regularly reported (Tables 2 to 4). Streptococcus equi, the cause of strangles in equids, and to a lesser extent S. porcinus and S. suis, are the only streptococci which infect livestock and which are relatively host-specific.

Streptococcus spp. are facultatively anaerobic, Gram positive, spherical or ovoid cocci that usually occur in chains, but may also be single or in pairs. They are catalase negative and ferment sugars without the production of gas.¹²

Streptococci grow best on media enriched by blood or tissue extracts at 37 °C, suggesting that they are adapted to a parasitic rather than a saprophytic existence. Surface colonies are small and transparent to semi-opaque, and a wide zone of complete (beta) haemolysis is evident around the colonies of most pyogenic streptococci. The pathogens in serological groups B and D are often either non-haemolytic or cause alpha haemolysis.²³ Alpha-haemolytic strains, which cause greening of blood agar, were previously placed in the ‘Viridans Group’ of Sherman, but this is now not considered to be a reliable taxonomic feature.¹² In most veterinary diagnostic laboratories Group C streptococci are identified on the basis of their fermentation reactions.⁷

Streptococcus spp. are traditionally divided into serologic groups (Lancefield groups) based on their dominant carbohydrate cell wall antigens. These group-specific antigens (or C-substances) are either polysaccharides associated with the cell wall (e.g. Groups A, B, C, E, F and G) or teichoic acids (e.g. Groups D and N) situated in the region between the cell membrane and the inner surface of the cell wall.¹² There are 20 Lancefield groups, designated A to H and K to V. The use of this system of classification is being abolished, as its main value was to distinguish between pathogenic beta haemolytic streptococci. It could not be used, however, to classify all streptococcal species, such as the alpha haemolytic viridans streptococci and the non-haemolytic streptococci.^{15, 20} To a large extent these latter groups comprise strains found normally in the oral cavity and upper respiratory tract, and seldom cause disease.¹²

Depending on the Streptococcus spp. involved, the virulence of the bacterium is determined by a number of factors including cell wall-associated components such as the capsule, fimbriae, M-protein, lipoteichoic acids, and peptidoglycan, and other extracellular products such as nucleases, proteinase, neuraminidase, nicotinamide adenine dinucleotidase (NADase), streptolysins O and S, hyaluronidase, and streptokinase (fibrinolysin).^{3, 12, 19} Abscesses or purulent infections are produced as a result of the response of neutrophils to the products of the bacteria. While antibodies directed against the protein antigens are protective, those formed in response to carbohydrate antigens are not.²³

Only a few of the streptococcal species form a capsule, and when they do, for example in the case of S. pyogenes, the capsule is composed of hyaluronic acid¹⁹ and is formed during the early stages of growth.¹² The capsule is not important in terms of pathogenesis except in the case of S. pneumoniae (which rarely causes disease in animals), where the capsular polysaccharide is the virulence factor and enables S. pneumoniae to evade phagocytosis in the absence of type-specific antibody.¹⁹

Table 1 Most...