Streptococcus suis infections

M-L PENRITH AND M M HENTON

GENERAL INTRODUCTION: GRAM-POSITIVE COCCI Streptococcus spp. infections

Introduction

Streptococcus suis is a common and potentially pathogenic commensal organism in pigs that can cause septicaemia, meningitis, purulent arthritis and valvular endocarditis, and is also associated with bronchopneumonia, genital infections and abortions. Sporadic infections with S. suis have been reported in other species including humans.

Outbreaks of disease associated with an alpha-haemolytic Streptococcus were first described in the Netherlands in 1951, followed by the UK in 1954.^{10, 28} Disease caused by S. suis has been described in most countries where pig production is highly developed, and is regarded as a major problem in the pork industry. Disease incidence is rising,^{35, 42} apparently in response to increased intensification of pig production. The infection can cause unacceptably high mortality levels among neonates and weaners, but the major economic loss results from high medication costs and reduced growth.

Aetiology

Early isolates were assigned to Lancefield groups R, S, R/S and T, as well as Lancefield group D; groups R, S and R/S were designated as serotypes 1, 2 and 1/2 respectively on the basis of capsular antigens.⁵⁵ In 1987 the organism was formally described as a species genetically distinct from other streptococci, including Lancefield group D.³² Thirty-four capsular serotypes have subsequently been recognized,^{14, 15, 16, 23} as well as two intermediate serotypes, 1/255 and 1/14.¹⁹ Many more strains within these serotypes have been identified.^{3, 35} Serotyping, based on capsular antigens, is not possible in all laboratories owing to the complexity of the organism. Not all of the serotypes have been associated with disease.

Streptococcus suis is a catalase negative, facultatively anaerobic, Gram-positive coccus, usually occurring in pairs. It is weakly beta-haemolytic on blood agar. No haemolysis or alpha-haemolysis may be seen after 24 hours of culture, but beta-haemolysis is usually evident after 48 hours of growth. In some cases it may be necessary to scrape away the small grey colonies to observe the haemolysis on the underlying agar.

Streptococcus suis ferments lactose, salicin, trehalose and inulin, but not arabinose, mannitol and sorbitol. Rattinose fermentation is variable. Arginine, esculin and starch are hydrolysed. Growth occurs at 37 °C, but not at 10 °C or 45 °C.

Epidemiology

Disease caused by S. suis has been reported in Europe, North and South America, the Far East, Australia, New Zealand⁴⁵ and South Africa.²⁰ Most of the serotypes are apparently widespread. Owing to the complexity of the organism, detailed epidemiological studies are unusual. Serotyping of isolates from target organs of diseased pigs, in particular the brain, has indicated that serotype 2 is most often implicated in epidemics worldwide. However, several other serotypes have also been demonstrated to cause epidemics, notably 1, 1/2, 3–8, 9, 10, 14, 15, 17 and 27.^{12, 19, 33, 34, 39, 49} Serotypes 2, 4 and 14 have been associated with disease in humans.¹⁶ Although some serotypes are apparently isolated more frequently from particular age groups, considerable overlap occurs.³⁹ Clinical and pathological manifestations of infection with different strains and serotypes do not differ materially,⁴⁰ although differences in the frequency and distribution of lesions have been reported.⁴⁹ During an epidemic, in order to establish the aetiology, it is necessary to isolate the bacterium from diseased organs. Genomic fingerprinting has revealed that clusters of several closely related strains within a serotype may be involved in an epidemic.³⁵ Epidemic strains may persist for long periods in a herd.⁴⁶ Other strains, and even other serotypes, may be isolated from non-target organs, especially tonsils and nasal cavity, of sick and healthy pigs in the same herd.³⁵ However, infection of herds with multiple serotypes has rarely been reported.⁴²

Intensive pig farming, where large numbers of pigs are in close contact, appears to favour outbreaks of disease caused by S. suis. Early weaning, accompanied by strategic medication, has proved effective in the prevention of many important diseases of pigs^{1, 4} but does not eliminate S. suis infection.^{4, 42} The removal of piglets weaned before three weeks of age to another site and the mixing of litters may precipitate S. suis infection,^{4, 39, 42} because piglets are exposed to different serotypes or strains when litters are mixed at a time when they lose maternally conferred immunity and lack active immunity.⁴⁷

It is generally supposed that the disease is introduced into herds by carrier pigs.^{7, 42, 45} Isolation of epidemic strains of a serotype from healthy pigs in an affected herd is, however, apparently rare.³⁵ Horizontal transmission, via the oronasal route, appears to be usual, but some evidence exists for vertical transmission.⁴³

During epidemics, younger pigs are usually affected. Septicaemic S. suis infection has been reported in both neonates and pigs from one to six months of age, with higher mortality among neonates. On ‘high health’ farms where early weaning is practised, mortality among neonates is often between 4 and 14 per cent.⁴⁶ The organism is usually the primary or sole pathogen in cases of septicaemia and meningoencephalitis. Respiratory infections associated with it occur...