STREPTOCOCCUS EQUI SUBSP. ZOOEPIDEMICUS
LEVELS: Highly unlikely: No controls necessary; Highly unlikely: No evidence of non-foodborne zoonotic transmission; 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); Minor: Low prevalence, typically non-lethal infection with recovery very likely; Negligible: Little or no market disruption when disease occurs on one or more farms; Minimal risk: Agent inherently unlikely to develop clinically important resistance to antibacterial or antiviral treatments; Minimal risk: Antibacterial or antiviral treatments rarely occur, or are typically limited to short-course individual animal therapy; Available but with uncertain efficacy: Limited treatments available in US or are only effective in some situations; No availability: Effective vaccines not currently available in the US (or have not been developed); Possible: Eradication possible but likely to require major changes into logistic systems, regulatory environment, infrastructure, and producer behaviors
OVERVIEW
Streptococcus equi subsp. zooepidemicus (S. zooepidemicus) is a Lancefield group C streptococcus causing respiratory disease in various mammals and emerging high-mortality septicemic outbreaks in swine. First described in swine during a 1975 outbreak in western China with substantial economic losses. In 2019, S. zooepidemicus sequence type (ST) 194 emerged in North America, associated with high mortality outbreaks (50-90% mortality) affecting sows and feeder pigs. All North American outbreak isolates are genetically similar to the 1970s Chinese strain. Pigs of any age following weaning are susceptible. Clinical signs include fever (>40°C within 8 hours of infection), lethargy, huddling, reluctance to rise, and inappetence. Sows may abort at any gestational stage. Necropsy lesions are consistent with bacterial septicemia: petechiae in multiple organs, hemorrhagic gallbladder, splenic congestion, enlarged lymph nodes. The disease is underreported in Southeast Asia due to limited diagnostic capacity. Outbreak control relies solely on prolonged antimicrobial therapy; as resistance emerges rapidly, depopulation has been used with ~25% failure rate. Inactivated bacterins and heterologous live exposure do not prevent disease. A live attenuated vaccine has shown promise experimentally.
FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Highly unlikely: No controls necessary
S. zooepidemicus in swine is not associated with foodborne transmission to humans. Human cases of S. zooepidemicus infection are typically associated with horse contact or consumption of unpasteurized dairy products, not pork.
NON-FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Highly unlikely: No evidence of non-foodborne zoonotic transmission
S. zooepidemicus can cause disease in various mammals including humans: "S. zooepidemicus...causes respiratory disease in a variety of mammals." Human cases are typically associated with equine contact rather than swine. Occupational risk for swine workers handling infected animals is possible but not specifically documented.
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
Rapid spread within herds and challenging epidemiology: (1) High mortality spread: "Mortality rates range from 50 to 90%, depending on herd health status, leading to a catastrophic economic impact"; (2) Unknown epidemiology: "Given its unknown epidemiology in swine operations, this strategy [depopulation] has failed in approximately 25% of the attempts"; (3) Environmental survival: "under ideal conditions without feces, the bacterium survived for up to 17 days on plastic and rubber surfaces"; (4) Regional persistence: Sporadic cases and regional epidemics continue impacting pig industry in China.
DIFFICULTY OF DETECTING AND CONFIRMING INFECTION
Level: Moderate: Clinical signs not unique but existing tests available at local/regional laboratory(s)
Diagnostic capacity limitations: (1) Underreported: "S. zooepidemicus is likely a very significant but underreported cause of disease in southeastern Asian pig herds, as diagnostic laboratories lack the capacity of properly identifying this bacterium"; (2) Similar to other septicemias: "Gross lesions associated with Streptococcus zooepidemicus infection in pigs are similar to other septicemic diseases"; (3) Rapid progression: "Given the rapid progression of infection, some pigs may have minimal postmortem lesions."
FINANCIAL IMPACT ON FARM'S COST OF PRODUCTION
Level: Minor: Low prevalence, typically non-lethal infection with recovery very likely
Catastrophic when outbreaks occur: (1) High mortality: "Mortality rates range from 50 to 90%"; (2) Reproductive losses: "Sows at any gestational stage may abort or suffer resorption, significantly affecting reproductive performance"; (3) Catastrophic impact: "leading to a catastrophic economic impact"; (4) Sporadic occurrence: Outbreaks are sporadic rather than endemic.
EFFECT ON DOMESTIC OR EXPORT MARKETS
Level: Negligible: Little or no market disruption when disease occurs on one or more farms
No trade implications: (1) Emerging pathogen: Still being characterized; (2) Not regulated: Not a reportable or trade-restricted disease; (3) Sporadic outbreaks: Limited geographic spread to date.
PATHOGEN'S ABILITY TO DEVELOP AND SPREAD RESISTANCE
Level: Minimal risk: Agent inherently unlikely to develop clinically important resistance to antibacterial or antiviral treatments
Susceptible to antimicrobials: (1) Beta-hemolytic streptococci: Generally susceptible to beta-lactam antibiotics; (2) Rapid emergence during outbreaks: "As resistance emerges rapidly during outbreaks" - this refers to treatment failure context, not widespread AMR.
AMR DEVELOPMENT DRIVEN BY DISEASE MANAGEMENT
Level: Minimal risk: Antibacterial or antiviral treatments rarely occur, or are typically limited to short-course individual animal therapy
Antimicrobials primary control method: (1) Treatment-dependent control: "Outbreak control relies solely on prolonged antimicrobial therapy"; (2) Depopulation alternative: When resistance emerges, depopulation has been used.
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 but resistance emerges: (1) Prolonged therapy required: "Outbreak control relies solely on prolonged antimicrobial therapy"; (2) Resistance during treatment: "As resistance emerges rapidly during outbreaks, depopulation has been used."
AVAILABILITY OF EFFECTIVE VACCINES OR BACTERINS
Level: No availability: Effective vaccines not currently available in the US (or have not been developed)
Vaccines currently ineffective: (1) Bacterins don't work: "An inactivated bacterin delivered intramuscularly or previous exposure to heterologous S. zooepidemicus live strains does not prevent infection or disease nor does it lead to transfer of passive immunity"; (2) Experimental promise: "A live attenuated vaccine has been shown to prevent mortality under experimental conditions."
FEASIBILITY OF ERADICATING THE DISEASE FROM THE US
Level: Possible: Eradication possible but likely to require major changes into logistic systems, regulatory environment, infrastructure, and producer behaviors
Depopulation has been used with variable success: (1) Depopulation attempted: "depopulation has been used to eliminate S. zooepidemicus from affected herds"; (2) Significant failure rate: "this strategy has failed in approximately 25% of the attempts"; (3) Unknown epidemiology: Epidemiology in swine operations not well understood, complicating eradication efforts.