STAPHYLOCOCCUS AUREUS (ABSCESSES AND MRSA)
LEVELS: Highly unlikely: No controls necessary; Occupational exposure risk: Non-foodborne transmission pathway(s) that are strongly associated with occupational exposure and can lead to human infection; Highly effective: Routine on-farm biosecurity measures are effective in preventing farm-to-farm transmission; 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; High risk: Resistance to antibacterial or antiviral treatments is, or can be expected to be a common problem; 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); Not feasible: Eradication extremely unlikely
OVERVIEW
Staphylococcus aureus is the only staphylococcal species besides S. hyicus consistently isolated from lesions in pigs. It causes sporadic individual animal disease including abscesses, septicemia, mastitis, vaginitis, metritis, osteomyelitis, and endocarditis. S. aureus is commonly found in swine facilities and on healthy pig skin but is not a common pathogen and is not associated with herd outbreaks. Of significant public health importance, swine are a reservoir for livestock-associated methicillin-resistant S. aureus (LA-MRSA), primarily multilocus sequence types ST398, ST9, and ST5. Nasal and skin colonization without clinical infection is common in pigs. People in contact with pigs have increased risk of LA-MRSA nasal carriage, though these livestock-adapted strains appear less virulent for humans than traditional healthcare-associated MRSA strains. LA-MRSA colonization in pigs represents a One Health concern requiring continued surveillance. Clinical disease in pigs is typically sporadic, affecting individual animals, with abscesses in skin, bones, joints, and internal organs found at slaughter.
FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Highly unlikely: No controls necessary
S. aureus infections in pigs are not associated with foodborne transmission to humans. Colonization rather than clinical infection is the primary concern.
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
Significant LA-MRSA colonization concern: (1) Swine as reservoir: "Swine have been identified as a reservoir for methicillin-resistant S. aureus (MRSA)"; (2) Occupational carriage: "People who spend considerable time in contact with infected pigs are at an increased risk for testing positive for LA-MRSA from nasal swabs relative to the general population"; (3) Pig-to-human transmission: "isolates from pigs and employees from the same farm were indistinguishable, suggesting transmission between pigs and humans on farm"; (4) Reduced virulence: "these strains appear to be less virulent and infrequently cause disease in colonized people compared with other human MRSA strains."
EFFECTIVENESS OF ON-FARM BIOSECURITY IN PREVENTING FARM-TO-FARM TRANSMISSION
Level: Highly effective: Routine on-farm biosecurity measures are effective in preventing farm-to-farm transmission
Individual animal disease with limited spread: (1) Not herd outbreaks: "S. aureus is not a common pathogen and is not associated with herd outbreaks"; (2) Individual cases: "Most cases occur in individual animals, and animal-to-animal spread is rare"; (3) Contact transmission: Direct contact with infected animals required.
DIFFICULTY OF DETECTING AND CONFIRMING INFECTION
Level: Moderate: Clinical signs not unique but existing tests available at local/regional laboratory(s)
Laboratory confirmation required: (1) Culture-based diagnosis: "Diagnosis of S. aureus is based on culture from suggestive lesions"; (2) Characteristic growth: "Aerobic growth on sheep blood agar of small (1–2 mm diameter), yellow to white, opaque colonies with double-zone hemolytic pattern is highly indicative"; (3) MALDI-TOF available: Further identification by biochemical testing or MALDI-TOF; (4) Typing methods: "multilocus sequence typing (MLST) or protein A gene sequence typing (spa typing)" for LA-MRSA surveillance.
FINANCIAL IMPACT ON FARM'S COST OF PRODUCTION
Level: Minor: Low prevalence, typically non-lethal infection with recovery very likely
Individual animal disease: (1) Sporadic occurrence: "S. aureus is not a common pathogen"; (2) Fatal in neonates: "Neonatal septicemia may be fatal or result in stunted piglets"; (3) Slaughter findings: "abscesses in bones, joints, heart valves, liver, kidney, lymph nodes, and other internal organs. Abscesses may be observed at slaughter in otherwise normal pigs."
EFFECT ON DOMESTIC OR EXPORT MARKETS
Level: Negligible: Little or no market disruption when disease occurs on one or more farms
No trade restrictions, but public health surveillance: (1) LA-MRSA surveillance: Public health concern for occupational exposure; (2) Not regulated for trade: Disease not trade-limiting; (3) One Health concern: Ongoing monitoring for antimicrobial resistance.
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
MRSA is the primary concern: (1) MRSA prevalence: "Colonization by MRSA has also been reported in livestock, including pigs"; (2) Common resistance: "many isolates are resistant to common drugs such as penicillin, ampicillin, chlortetracycline, oxytetracycline, and spectinomycin"; (3) LA-MRSA genotypes: "nasal and skin colonization without clinical infection in pigs is common and caused by the multilocus sequence types ST398, ST9, and ST5."
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
Limited antimicrobial use: (1) Individual treatment: "Since S. aureus infection is an individual animal problem, there is usually no need to treat the entire group"; (2) Sporadic disease: Not requiring routine antimicrobial use for prevention.
AVAILABILITY OF EFFECTIVE TREATMENT OPTIONS
Level: Available but with uncertain efficacy: Limited treatments available in US or are only effective in some situations
Treatment options exist: (1) Surgical drainage: "Treatment of individual abscesses can be accomplished by surgical drainage and antimicrobial therapy"; (2) Parenteral treatment: "Prompt parenteral treatment is recommended to minimize development of extensive and fatal abscesses"; (3) Effective drugs: "Ceftiofur, enrofloxacin, and trimethoprim/sulfamethoxazole are good choices."
AVAILABILITY OF EFFECTIVE VACCINES OR BACTERINS
Level: No availability: Effective vaccines not currently available in the US (or have not been developed)
Limited vaccine use: "The use of bacterins has been described, but they are not widely available or extensively used."
FEASIBILITY OF ERADICATING THE DISEASE FROM THE US
Level: Not feasible: Eradication extremely unlikely
Eradication not feasible: (1) Commensal organism: "commonly found in swine facilities and on the skin of healthy pigs"; (2) Environmental persistence: "S. aureus is somewhat resistant to disinfectants, especially when protected by organic materials"; (3) Ubiquitous colonization: LA-MRSA colonization widespread in pig populations.