STREPTOCOCCUS SUIS
LEVELS: Likely to occur: Effective control measures not fully understood; Occupational exposure risk: Non-foodborne transmission pathway(s) that are strongly associated with occupational exposure and can lead to human infection; Unlikely to be effective: One or more pathways of farm-to-farm transmission exist that cannot be controlled by on-farm biosecurity; Moderate: Clinical signs not unique but existing tests available at local/regional laboratory(s); Substantial: Unsustainable acute or chronic losses related to severe clinical signs in a high prevalence of animals; Negligible: Little or no market disruption when disease occurs on one or more farms; High risk: Resistance to antibacterial or antiviral treatments is, or can be expected to be a common problem; High risk: Antibacterial or antiviral treatments commonly used on affected groups, or for prophylaxis; Available but with uncertain efficacy: Limited treatments available in US or are only effective in some situations; Available but uncertain efficacy: Commercial or autogenous vaccines exist in the US but protection may be inconsistent; Not feasible: Eradication extremely unlikely
OVERVIEW
Streptococcus suis is the most important streptococcal swine pathogen worldwide, causing septicemia, meningitis, polyarthritis, polyserositis, and valvular endocarditis primarily in 5-10 week old pigs. Currently 29 "true" S. suis serotypes are recognized, with serotype 2 predominating in Eurasia (ST1 highly virulent) while North America has more serotype diversity (ST25, ST28). The organism resides in upper respiratory tract (tonsils, nasal cavities), genital tract, and saliva of healthy carrier pigs. Transmission is vertical (sow to piglet) and horizontal (pig-to-pig, aerosol). Disease expression requires interplay of strain virulence, environmental factors, management factors, host immunity, and coinfections (especially PRRSV). S. suis is an important ZOONOTIC pathogen causing meningitis with hearing loss in humans; in Southeast Asia it is among the most common causes of adult bacterial meningitis and is primarily foodborne through consumption of raw/undercooked pork. Two major outbreaks in China (1998, 2005) caused >50 deaths each with toxic shock-like syndrome. Control relies on antimicrobial metaphylaxis, autogenous bacterin vaccination, and management of predisposing factors. Eradication is not economically feasible.
FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Likely to occur: Effective control measures not fully understood
Major foodborne zoonosis in Southeast Asia: (1) Consumption of raw pork: "S. suis is a major cause of a foodborne disease...the well-documented risk of consuming contaminated undercooked pork in those countries is compounded by the common practice of consuming raw pork products"; (2) Survival in meat: "S. suis can survive in carcasses at 4°C (39°F) for 6 weeks, contaminated chilled or frozen meat poses risk long after being butchered"; (3) Common meningitis cause: "in countries such as Vietnam and Thailand, human S. suis is among the most frequent causes of bacterial meningitis in adults."
NON-FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Occupational exposure risk: Non-foodborne transmission pathway(s) that are strongly associated with occupational exposure and can lead to human infection
Well-documented occupational zoonosis: (1) Occupational risk: "pig farmers, abattoir workers, people who transport pork, meat inspectors, veterinarians, and butchers being commonly affected"; (2) Skin wound transmission: "Infection is primarily through direct contact of small skin wounds with S. suis-infected pigs or contaminated pork"; (3) Clinical disease: "Meningitis is the most common manifestation...hearing loss"; sepsis with toxic shock-like syndrome also described; (4) Major outbreaks: Two Chinese outbreaks (1998, 2005) each caused >50 deaths; (5) Case fatality: "varies from less than 3% in most Western countries to 26% in some Asian countries."
EFFECTIVENESS OF ON-FARM BIOSECURITY IN PREVENTING FARM-TO-FARM TRANSMISSION
Level: Unlikely to be effective: One or more pathways of farm-to-farm transmission exist that cannot be controlled by on-farm biosecurity
Multiple transmission routes: (1) Carrier introduction: "Transmission of virulent strains between herds usually occurs by the introduction of carrier pigs"; (2) Vertical transmission: "Sows infect their piglets during farrowing via contamination from vaginal colonization and probably through the respiratory route"; (3) Aerosol transmission: "Aerosol transmission without nose-to-nose contact has been confirmed"; (4) Environmental survival: "Viability in water at 4°C (39°F) is retained for 1–2 weeks...In experimentally inoculated feces, S. suis survives at 0°C...for 104 days."
DIFFICULTY OF DETECTING AND CONFIRMING INFECTION
Level: Moderate: Clinical signs not unique but existing tests available at local/regional laboratory(s)
Multiple diagnostic challenges: (1) Clinical overlap: "difficult to differentiate from Glaesserella parasuis infections"; (2) Biochemical limitations: "use of biochemical tests...can hardly differentiate 'true' S. suis from S. suis-like microorganisms"; (3) MALDI-TOF imperfect: "less than 50% of S. suis isolates (as identified by MALDI-TOF) were confirmed by genetic tools"; (4) PCR serotyping available: Reduces untypable isolates to <10%; (5) Sample selection critical: "Tonsils from clinically infected animals should not be used...different strains (other than that causing the disease) will be present."
FINANCIAL IMPACT ON FARM'S COST OF PRODUCTION
Level: Substantial: Unsustainable acute or chronic losses related to severe clinical signs in a high prevalence of animals
Major economic impact: (1) European cost estimates: "the average cost of S. suis per pig was recently established between 0.60 and 1.30 euros, depending on the country"; (2) Mortality potential: "in the absence of antimicrobials, mortality rates can reach 20%"; (3) Global distribution: "S. suis has been reported globally in both traditional and modern intensive swine operations"; (4) Most important streptococcal pathogen: Affects swine production worldwide.
EFFECT ON DOMESTIC OR EXPORT MARKETS
Level: Negligible: Little or no market disruption when disease occurs on one or more farms
No trade restrictions: (1) Endemic worldwide: Present in all swine-producing regions; (2) Not regulated for trade: Production and public health concern, not trade-limiting.
PATHOGEN'S ABILITY TO DEVELOP AND SPREAD RESISTANCE
Level: High risk: Resistance to antibacterial or antiviral treatments is, or can be expected to be a common problem
AMR concerns significant: (1) High resistance to some classes: "Very high rates of resistance to macrolides/lincosamides and tetracyclines are observed"; (2) Mobile genetic elements: "S. suis is considered a reservoir for antibiotic resistance...high risk of transmission of such resistance to other veterinary and human pathogens due to the presence of mobile genetic elements"; (3) Penicillin resistance emerging: "penicillin-resistant isolates (some of them isolated from humans) have been described."
AMR DEVELOPMENT DRIVEN BY DISEASE MANAGEMENT
Level: High risk: Antibacterial or antiviral treatments commonly used on affected groups, or for prophylaxis
Widespread antimicrobial use: (1) Metaphylaxis common: "prophylactic and metaphylactic use of antimicrobials (mainly penicillin and amoxicillin) is still used in many countries to control S. suis infections"; (2) Historical intensive use: "In the past, a wide range of antibiotics have been used in pig husbandry for targeted prophylactic and metaphylactic use"; (3) Resistance attributed to use: "attributed to the intensive use of antimicrobials in pigs."
AVAILABILITY OF EFFECTIVE TREATMENT OPTIONS
Level: Available but with uncertain efficacy: Limited treatments available in US or are only effective in some situations
Treatment effective if early: (1) Beta-lactams still effective: "emergence of resistance to antimicrobials normally used...among S. suis isolates derived from diseased pigs appeared to be limited"; (2) Early treatment critical: "Early recognition of nervous signs...followed by immediate parenteral treatment with an appropriate antibiotic with or without an anti-inflammatory agent maximizes pig survival"; (3) Effective drugs: Ampicillin, penicillin, trimethoprim/sulfonamide commonly used.
AVAILABILITY OF EFFECTIVE VACCINES OR BACTERINS
Level: Available but uncertain efficacy: Commercial or autogenous vaccines exist in the US but protection may be inconsistent
Autogenous vaccines commonly used with variable results: (1) Autogenous vaccines predominate: "Autogenous vaccines...are by far the most common product used as an alternative to the use of antimicrobials"; (2) Variable results: "autogenous vaccines used in the field are highly heterogeneous, since different laboratories will use completely different methodologies"; (3) Strain selection critical: "One of the most important aspects before the application of an autogenous vaccine in the field is also to choose the right isolate(s)"; (4) Sow vaccination limitations: May protect only to 5-6 weeks of age; (5) Few commercial vaccines: "Very few commercial bacterins are available."
FEASIBILITY OF ERADICATING THE DISEASE FROM THE US
Level: Not feasible: Eradication extremely unlikely
Eradication not economically justified: (1) Normal inhabitant: "S. suis is a normal inhabitant of the upper respiratory tract, is present in almost all herds and all animals"; (2) Cost prohibitive: "only depopulation and restocking with 'clean' pigs will ensure the eradication of the infection and in most herds, this cannot be justified economically"; (3) Carrier detection difficult: "The problem is on how determining whether a pig or herd is 'clean'"; (4) Control preferred: "it would appear reasonable to direct resources toward control measures rather than eradication."