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 common diseases caused by Streptococcus spp. in livestock and humans. (Adapted from Carter & Thoen,⁷ and Schleifer et al. ²⁴)

STREPTOCOCCUS SPP.	ANIMAL SPECIES	DISEASE
PYOGENIC STREPTOCOCCI
Group A streptococci	Humans	Septic sore throat, scarlet fever, pyoderma, lymphadenitis and erysipelas
S. pyogenes
	Cattle	Mastitis
Group B streptococci
S. agalactiae	Cattle, sheep and goats	Mastitis
Group C streptococci
S. equi	Horses	Strangles
S. zooepidemicus	Horses	Endometritis, cervicitis, placentitis, embryonal loss, abortion, secondary respiratory infections, pneumonia, abscesses, omphalophlebitis, neonatal septicaemia and polyarthritis
	Cattle	Mastitis, abscesses, neonatal septicaemia and pneumonia
	Goats	Mastitis and abscesses
	Sheep	Abscesses, neonatal septicaemia and pneumonia
	Pigs	Cervical lymphadenitis, neonatal septicaemia and arthritis, abortion and abscesses
S. equisimilis	Pig	Neonatal purulent arthritis, lymphadenitis (cervical nodes), genital infections, mastitis-metritis-agalactiae syndrome, abortion, stillbirths and weak piglets at birth
	Horses	Suppurative conditions, abscesses, neonatal septicaemia, polyarthritis and enteritis, secondary infection to viral infection of the respiratory tract and endometritis
S. dysgalactiae	Cattle	Mastitis and neonatal septicaemia
	Pigs	Metritis, pneumonia, neonatal septicaemia, polyarthritis and genital infections
	Sheep	Mastitis, abscesses, neonatal septicaemia and polyarthritis
Group D enterococci
Enterococcus faecium	All livestock	Meningoencephalitis, sporadic cases of valvular endocarditis and urinary tract infections
E. avium
E. avium
E. gallinarum
E. casseliflavus
E. durans
E. hirae
E. malodoratus
E. mundtii
OTHER STREPTOCOCCI
S. uberis	Cows	Mastitis
S. porcinus (Groups E, P, U and V)	Pigs	Cervical lymphadenitis (jowl abscesses), neonatal septicaemia, pneumonia, infertility, arthritis, pleuritis and peritonitis
	Cows	Mastitis
S. suis	Pigs	Septicaemia, meningitis, arthritis, pharyngitis, endocarditis, abscesses, lymphadenitis (cervical nodes), genital tract infections (vaginitis, endometritis), abortion and stillbirths
GEMELLA
G. mobillorum	All species	Abscesses

Table 2 Common Streptococcus spp. isolated from pigs from 1977 to 2002 at the general diagnostic laboratory, Onderstepoort Veterinary Institute, South Africa

	S.EQUISIMILIS	S.SUIS	S.PORCINUS	S.ZOOEPIDEMICUS	S.DYSGALACTIAE	TOTAL NUMBE OF CASES
Mastitis-metritis agalactia syndrome	59	7	8	3	2	79
Septicaemia	68	37	17	6	1	129
Pneumonia	71	76	54	8	5	214
Abcessation	13	6	1	1	1	22
Polyserositis	3	5	3	–	1	12
Abortion	28	6	4	1	2	41
Arthritis	32	1	5	1	2	41
Lymphadenitis	8	1	4	1	1	15
Pyoderma	10	2	5	–	–	17
Meningitis	4	3	1	–	–	8
Cystitis	2	–	–	2	–	4
Other	4	6	5	–	4	19

Table 3 Percentage of isolations of Streptococcus spp. made from livestock species from 1977 to 2002 at the general diagnostic laboratory, Onderstepoort Veterinary Institute, South Africa

STREPTOCOCCUS SPP.	HORSES	PIGS	CATTLE	SHEEP	GOATS
S. zooepidemicus	80,1	3,8	4,3	39,7	52,4
S. equisimilis	9,3	49,3	10,2	9,3	9,5
S. dysgalactiae	1,1	3,3	34,9	26,5	–
S. suis	–	25,4	14,9	13,2	23,8
S. porcinus	–	16,9	1,6	1,3	–
S. agalactiae	0,2	0,8	12,5	0,7	–
S. uberis	0,5	0,2	21,6	5,9	14,3
Streptococcus G	–	0,3	–	3,3	–
S. equi	8,8	–	–	–	–

Peptidoglycan, the main component of the cell wall, has similar systemic effects to those produced by the endotoxins of Gram-negative bacteria¹² (see  Escherichia coliinfections).
Streptolysin O is produced by all Group A and some Group B, C, F and G streptococci. It causes beta haemolysis as well as the destruction of leukocytes, macrophages and platelets in certain animal species.^{12, 23} The effects of streptolysin S are similar to, but less pronounced than, those of streptolysin O.

Hyaluronidase is produced by S. suis and streptococci in Groups A, B and C. It causes lysis of the ground substance of connective tissue and may play a role in the local spread of infection. Streptokinase (fibrinolysin) produced by Group A and some members of Groups C and G, is a fibrinolytic protease and interferes with the formation of a fibrin barrier around lesions, thereby promoting the spread of streptococcal infections.²³ Streptodornase, which degrades DNA, is the collective term for the four distinct deoxyribonucleases (A, B, C and D) of which type B is most common.^{3, 23} Nucleases, proteinase and neuraminidase are produced by Group A streptococci. M-protein (in S. pyogenes), which is also antiphagocytic, is a determinant of virulence.

Epidemiology

Streptococcus spp. inhabit the mucous membranes of the upper respiratory, alimentary and lower urogenital tracts of healthy animals as commensals. They may also be found in feed, faecally contaminated water, and in the environment contaminated by purulent exudates which develop as a consequence of streptococcal infections. They are destroyed by the usual strengths of disinfectant but may survive for months in dust in buildings.²⁹ Some species, such as Enterococcus casseliflavus, thrive as epiphytes on plants.¹⁸

Bacteria gain entrance to the tissues of the body through the mucous membranes of the respiratory, genital and gastrointestinal tracts, wounds in the skin, and the umbilicus. Infections may spread per continuitatum, ascend or descend in the duct systems or lumens of hollow organs of tissues that contain them, or lymphogenously or haematogenously to distant organs or tissues.⁹

Table 4  Isolates of Streptococcus spp. made from a variety of conditions in livestock from 1977 to 2002 at the general diagnostic laboratory, Onderstepoort Veterinary Institute, South Africa

CONDITION	HORSES	PIGS	CATTLE	SHEEP	GOATS	TOTAL NUMBER OF CASES
Metritis	282	83	8	6	–	379
Septicaemia	25	132	42	50	11	260
Pneumonia	21	216	21	31	5	294
Abcessation	25	23	20	12	–	80
Abortion	70	43	21	3	1	138
Arthritis	10	42	7	4	1	64
Nasal discharge	72	–	–	–	–	72
Polyserositis	–	12	–	2	–	14
Lymphadenitis	9	16	3	–	–	28
Skin infections	13	17	–	3	1	34
Meningitis	–	9	3	8	1	21
Cellulitis	7	–	3	-	1	11
Cystitis	–	5	–	–	–	5
Myositis	1	1	1	1	-	4
Otitis	2	-	-	-	-	2

Infection with many of the Streptococcus spp. may occur in animals of either sex and of all age groups. Exceptions to this occur when infection is dependent on the existence of certain predisposing circumstances, such as metritis that occurs particularly in the post-partum period, mastitis in the lactating udder, and omphalophlebitis in neonates, while in some infections, such as in the case of equine strangles, an age-dependent immunity seems to play a role, since the condition is infrequently encountered in the older age group.^{5, 30} As many of the streptococcal infections in livestock in South Africa occur in neonates and in the genital tract of adult animals, there is an increased incidence of these infections in spring.¹³

Pathogenic streptococci selectively colonize specific anatomical sites. The infections result either in the development of subclinical carrier states (for example, the tonsils in the case of S. pyogenes in humans, S. equi in horses, and S. suis in pigs) or in clinical disease. In mares and sows, S. zooepidemicus and S. equisimilis occur commonly in the vagina, and occasionally in the cervix and uterus (especially immediately post-partum). Streptococcus zooepidemicus may be part of the normal flora of the vagina and clitoral fossa in mares, and occasionally also of the skin and upper respiratory tract. In boars and stallions respectively, S. equisimilis and S. zooepidemicus may be part of the normal flora of the preputial mucosa, but their prevalence in this locality is much lower than in the female genital tract of sows and mares.¹⁴

Streptococcus agalactiae is an obligate intramammary parasite of the mucosa of the lactiferous ducts of cattle, sheep and goats.¹¹ It may also be found, however, in the mouth and vagina, and on the skin of the udder of domestic ruminants.¹² Streptococcus uberis may be isolated from many sites of the body, including the skin, tonsils, vagina, rectum, teats and udder, and in the faeces of cattle.^{6, 11} Enterococci such as E. faecalis, E. faecium, and G. mobillorum occur normally in the intestinal tract of all animals. They are opportunistic pathogens usually associated with abscesses in various internal organs and tissues following the occurrence of trauma, omphalophlebitis, or septicaemia.^{14, 15, 22, 23, 25}

Mixed infections of streptococci with one or more other organisms, such as Salmonella serovars, Klebsiella spp., pathogenic strains of Escherichia coli, Pasteurella spp., Actinobacillusspp., Corynebacterium spp. and Chlamydia psittaci, are sometimes encountered.^{13, 21}

Most streptococci (with the exception of enterococci) are sensitive to penicillin, which is the therapeutic drug of choice.^{3, 17}

References

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