S suis is a significant pathogen of swine. Like a number of swine pathogens, the bacteria can be readily found in tonsillar tissue and feces of clinically normal pigs.
Etiology and Pathogenesis
S suis is a Lancefield group D streptococcus that is characterized as facultative anaerobic, gram-positive and nonmotile, and orients in chains of varying lengths. S suis produces α−hemolysis (incomplete hemolysis) on blood agar and is catalase negative. It has a worldwide distribution, and 35 serotypes are recognized. Of these, the number of serotypes expressing virulence is small and dependent on geographic location. Serotypes 1–9 make up about 70% of S suis isolates in laboratories; serotype 2 is the most prevalent worldwide. Although most weaned piglets carry S suis, few carry serotypes capable of inducing disease after weaning.
S suis is found in the upper respiratory tract, particularly the tonsils and nasal cavities, but the organism is also present in the genital and alimentary tracts of pigs. Clinical infections are seen mainly in weaned pigs (2–5 wk postweaning), growing pigs, and less frequently in suckling piglets.
Asymptomatic carriers serve as a source of infection for their pen mates after they are mixed and commingled in the nursery. Most studies on virulence factors have been completed with type 2 serotypes. Type 2 virulent and avirulent serotypes exist, but characterization of virulence factors is incomplete. Capsular polysaccharide is so far the only proven virulence factor (C-substance). Muramidase-released protein (MRP) and extracellular factor (EF) constitute virulence-related proteins that may be active in type 2 infections; however, there is no single predictor of pathogenicity.
Epidemiology and Transmission
S suis is present in all parts of the world in swine intensive areas. Type 2 is responsible for the majority (90%) of infections in diseased pigs in most countries. Most clinically healthy pigs are carriers of multiple serotypes of S suis. Piglets become colonized with S suis from vaginal secretions during parturition or while nursing. Transmission between herds occurs by the movement and mixing of healthy carrier pigs. Their introduction into a non-infected herd usually results in the subsequent onset of disease in weaned pigs and/or growing pigs. However, some infected herds that show no illness may develop clinical disease in the presence of other predisposing factors such as stress and co-infections with other pathogens. Disease outbreaks due to S suis infection have been frequently reported with co-infections of porcine reproductive and respiratory syndrome virus. S suis can also be transmitted via fomites and flies. The importance of other animal species or birds as reservoirs or vectors of the infection is unknown.
The earliest sign is usually fever, which may occur initially without other obvious signs. It is accompanied by a pronounced septicemia that may persist for several days if untreated. During this period, there is usually a fluctuating fever and variable degrees of inappetence, depression, and shifting lameness. In peracute cases, pigs may be found dead with no premonitory signs. Meningitis is the most striking feature and the one on which a presumptive diagnosis is usually based. Early nervous signs include depression, incoordination and adoption of unusual stances, which soon progress to inability to stand, paddling, opisthotonos, convulsions, and nystagmus. Endocarditis is also a frequent finding in older piglets. Affected pigs may die suddenly or show signs of dyspnea, cyanosis, and wasting. Swollen joints and lameness are indicative of polyarthritis, and signs of respiratory disease may be observed in some outbreaks.
Lesions are mainly seen in weaned and growing pigs and are associated with lymphadenopathy, meningitis, arthritis, serositis, and endocarditis. Lesions may include fibrinopurulent exudates in the brain, swollen joints, fibrinous serositis, and cardiac valvular vegetations. Splenomegaly and petechial hemorrhages indicating septicemia may be observed. Lesions of septicemia, meningitis, or polyarthritis may be seen in suckling piglets.
Presumptive diagnosis is generally based on history, clinical signs, age of animals, and gross lesions. Isolation and serotyping of the infectious agent and evaluation of microscopic lesions in affected tissues confirms the diagnosis. Serology is not routinely available. Genetic characterization is done in some laboratories and is particularly useful for epidemiologic studies.
Differential diagnoses include polyserositis caused by Haemophilus parasuis or Mycoplasma hyorhinis; meningitis caused by H parasuis; endocarditis caused by Erysipelothrix rhusiopathiae; septicemia caused by H parasuis, Actinobacillus suis, Escherichia coli, E rhusiopathiae, or Salmonella choleraesuis; and polyarthritis caused by other streptococci, staphylococci, E coli, or A suis.
Treatment, Control, and Prevention
Prompt recognition of the early clinical signs of streptococcal meningitis, followed by immediate parenteral treatment of affected pigs with an appropriate antibiotic, is currently the best method to maximize pig survival. The early stages of meningitis may be difficult to detect, so groups of pigs should be observed frequently after weaning on farms where S suis infections are a problem. Resistance of isolates to penicillin has been reported, but extended spectrum beta-lactams such as ampicillin and amoxicillin appear to retain some effectiveness in treating affected pigs. In addition to the beta-lactams, other antibiotics approved in the USA for treating S suis infections include ceftiofur and fluoroquinolones. Medication of the drinking water may be accomplished with florfenicol, which is labeled for treating S suis infections in pigs. Amoxicillin is also commonly administered in the drinking water as a prophylactic measure. Administration of an anti-inflammatory preparation is recommended to reduce inflammation of the affected tissues and improve the overall condition of pigs with S suis meningitis. Treatment of sows with antibiotics prior to farrowing to reduce pathogen transmission to piglets has been investigated with mixed results.
Vaccines have proved ineffective in preventing outbreaks. S suis is one of several bacterial pathogens that have been able to defeat efforts to eradicate it using early weaning of nursing pigs.
Streptococci are susceptible to the action of aldehyde, biguanide, hypochlorite, iodine, and quaternary ammonium disinfectants.
Human infections with S suis can result in septicemia, meningitis, permanent hearing loss, endocarditis, and arthritis. Mortality has been reported to approach 7% in some cases. The majority of cases in people have been attributed to S suis serotype 2. Transmission to humans occurs via contamination of skin wounds or mucous membranes by blood or secretions from infected pigs. Swine producers, slaughter plant employees, and veterinarians are at greatest risk for infection. The disease is considered to be underdiagnosed and underreported in most countries.
Last full review/revision March 2012 by Darryl Ragland, DVM, PhD