BRACHYSPIRA PILOSICOLI (PORCINE INTESTINAL SPIROCHETOSIS)
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; Highly effective: Routine on-farm biosecurity measures are effective in preventing farm-to-farm transmission; Easy: Distinct clinical signs and/or existing test(s) available at local/regional laboratory(s); Moderate: Manageable losses related to endemic (population) or chronic (individual) occurrence; Negligible: Little or no market disruption when disease occurs on one or more farms; Moderate risk: Evidence of development of clinically important resistance to antibacterial or antiviral treatments, or that development of such resistance is likely to occur over time; 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; No availability: Effective vaccines not currently available in the US (or have not been developed); Not feasible: Eradication extremely unlikely
OVERVIEW
Brachyspira pilosicoli is a weakly beta-hemolytic anaerobic spirochete that causes porcine intestinal spirochetosis (PIS) or porcine colonic spirochetosis (PCS), a milder enteric disease compared to swine dysentery. The condition was first described in 1980 when pigs challenged with strain P43/6/78T developed mucoid diarrhea with flecks of blood and colitis. PIS/PCS occurs mainly in weaners, growers, and recently mixed pigs, causing watery to mucoid diarrhea, poor condition, and decreased feed conversion efficiency. While mortality is rare, affected pigs show significant loss of condition and delays in reaching market weight. A distinctive pathological feature is the "false brush border" formed when spirochetes attach by one cell end to colonic epithelial cells. Unlike B. hyodysenteriae, B. pilosicoli has a broad host range including humans, dogs, birds, and other species, making it a confirmed zoonotic pathogen. Human infections occur primarily in immunocompromised individuals or those in communities with poor hygiene, causing chronic diarrhea and related conditions. B. pilosicoli shows considerable genetic diversity through homologous recombination, extensive genomic rearrangements, and varied genome sizes. Environmental persistence is notable—surviving 66 days in lake water at 4°C and up to 210 days in soil with pig feces. Disease expression is influenced by diet, with pelleted diets and high-viscosity intestinal contents increasing colonization risk.
FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Likely to occur: Effective control measures not fully understood
B. pilosicoli is a recognized zoonotic pathogen with waterborne transmission documented: (1) Human infection confirmed: "Brachyspira pilosicoli colonizes human beings who are usually either immunocompromised or live in developing communities where hygiene is poor and fecal contamination of water supplies may occur"; (2) Cross-species infectivity: "Strains of B. pilosicoli from humans can cause disease when inoculated into pigs and chickens"—and vice versa, implying bidirectional transmission potential; (3) Water contamination: "fecal contamination of water supplies may occur" as a transmission route; (4) Human disease manifestations: "Human infections have been associated with chronic diarrhea, failure to thrive in children, and irritable bowel syndrome with diarrhea"; (5) Systemic spread in humans: "B. pilosicoli has been isolated from the bloodstream of individuals with severe clinical disease or impaired immunity." While direct foodborne transmission through pork consumption is not specifically documented, the waterborne route and cross-species infectivity establish the zoonotic potential.
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
B. pilosicoli poses a recognized occupational/contact risk: (1) Zoonotic potential acknowledged: "The potential for transmission of B. pilosicoli from animals to humans exists"; (2) Genetic relatedness: "Isolates from pigs, dogs, birds, and humans can be closely genetically related"; (3) Cross-species disease: Human strains cause disease in pigs and chickens, suggesting reverse transmission also occurs; (4) Risk assessment: "the risk of healthy pig industry workers developing disease from contact with pigs is slight"—indicating risk exists but is low for immunocompetent individuals; (5) At-risk populations: Immunocompromised individuals and those in developing communities with poor hygiene are at higher risk.
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
Multiple biosecurity bypass routes exist: (1) Environmental persistence: Survives "66 days in lake water at 4°C"; "119 days in soil at 10°C"; "210 days in soil with 10% pig feces and in feces"; (2) Wildlife reservoirs: "Feral animals and birds may be a source of infection"; B. pilosicoli detected in chickens, effluent pond water, and wild ducks on pig farms; "an isolate from the pond belonged to the same genetic type as one from a pig"; (3) Multi-host pathogen: "A wide range of species may be naturally infected with B. pilosicoli"; (4) Intermittent shedding: Makes detection of carriers difficult; "shedding...may be intermittent and continue over weeks"; (5) Strain diversity: Multiple strains on a single farm with different properties complicate control. While B. pilosicoli is susceptible to common disinfectants, "organic matter reduces efficacy," and the environmental survival and wildlife reservoirs create biosecurity challenges.
DIFFICULTY OF DETECTING AND CONFIRMING INFECTION
Level: Easy: Distinct clinical signs and/or existing test(s) available at local/regional laboratory(s)
Multiple validated diagnostic methods are available: (1) Culture: Grows on CVS selective medium (BJ medium may inhibit growth); weak beta-hemolysis without the "ring phenomenon" of SD agents; (2) Biochemical: Hippurate positive (usually), indole negative (usually); (3) PCR: Multiple targets (16S rRNA, 23S rRNA, nox genes); duplex and multiplex PCRs available; real-time PCR for quantitation; (4) MALDI-TOF MS: Rapid species identification; (5) FISH: Specific probes allow tissue localization showing characteristic "false brush border"; (6) Histopathology: Pathognomonic attachment pattern visible. Diagnostic caveats: "clinically normal pigs may shed B. pilosicoli in their feces" so presence alone doesn't confirm PIS/PCS; "their significance needs to be interpreted in the context of a complete diagnostic investigation."
FINANCIAL IMPACT ON FARM'S COST OF PRODUCTION
Level: Moderate: Manageable losses related to endemic (population) or chronic (individual) occurrence
PIS/PCS causes moderate but persistent economic impact: (1) Production effects: "Pigs with PIS/PCS exhibit variable loss of condition, which leads to increased time to reach market weight and disrupts efficient production flow"; "decreased feed conversion efficiency and diminished weight gain"; (2) Subclinical losses: "Not all infected animals develop diarrhea, but subclinical infections may still depress growth rates"; (3) Morbidity: "Between 17 and 100% of pigs may become infected, with 17–67% developing diarrhea and 8–100% having colitis"; (4) Low mortality: "Mortality is rare in the field"; (5) Self-limiting but recurring: "Diarrhea is usually self-limiting and lasts 2–14 days, although some animals may relapse"; (6) Multiple strains complicate control: "The presence of multiple B. pilosicoli strains within certain farms might explain why PIS/PCS commonly recurs in convalescent animals." Overall impact is less severe than swine dysentery but creates ongoing production inefficiency.
EFFECT ON DOMESTIC OR EXPORT MARKETS
Level: Negligible: Little or no market disruption when disease occurs on one or more farms
PIS/PCS has minimal trade impact: (1) Not regulated: Not a WOAH-listed or nationally regulated disease; (2) Lower health significance: Considered less important than SD for herd health certification; (3) Widespread presence: Found in "most pig-producing countries"; (4) Mild disease: The relatively mild nature of disease reduces urgency compared to SD; (5) No specific trade restrictions: Positive status does not typically affect market access. The chapter notes that while PIS/PCS has "increased recognition," this is due to improved diagnostics and reduced antimicrobial use rather than market concerns.
PATHOGEN'S ABILITY TO DEVELOP AND SPREAD RESISTANCE
Level: Moderate risk: Evidence of development of clinically important resistance to antibacterial or antiviral treatments, or that development of such resistance is likely to occur over time
Antimicrobial resistance occurs but is less characterized than for B. hyodysenteriae: (1) Pleuromutilin resistance documented: "Resistance to pleuromutilins is associated with the tva(B) gene"; (2) Multiple resistance mechanisms: "numerous OXA-63 group class D β-lactamase genes have been described"; (3) Variable susceptibility: "tiamulin, valnemulin, carbadox, dimetridazole, and, to a lesser extent, lincomycin have had low MIC values" historically; (4) Tylosin resistance: "Fewer isolates are susceptible to tylosin"; (5) Tiamulin resistance: "resistance to several antimicrobials, including tiamulin, has been recorded"; (6) Less contemporary data: "there is little contemporary data available" on susceptibility patterns.
AMR DEVELOPMENT DRIVEN BY DISEASE MANAGEMENT
Level: High risk: Antibacterial or antiviral treatments commonly used on affected groups, or for prophylaxis
PIS/PCS treatment drives moderate antimicrobial use: (1) Outbreak treatment: "Affected pigs should be treated with water or fed medication at similar levels and durations as recommended for SD"; (2) Parenteral treatment: "may be necessary for severely ill pigs"; (3) Prophylaxis: "Antimicrobial therapy can be used...to prevent sudden increases in morbidity"; (4) Lower overall pressure: The milder economic impact means less intensive medication compared to SD; (5) Endemic management: Continuous flow systems increase PIS/PCS risk—"Replacing continuous flow systems with AIAO systems reduces the risk of infection." Overall antimicrobial use is moderate given the self-limiting nature of disease.
AVAILABILITY OF EFFECTIVE TREATMENT OPTIONS
Level: Available but with uncertain efficacy: Limited treatments available in US or are only effective in some situations
Multiple antimicrobials remain effective: (1) Primary drugs: Tiamulin, valnemulin, lincomycin (where susceptible); (2) Historical efficacy: "tiamulin, valnemulin, carbadox, dimetridazole, and, to a lesser extent, lincomycin have had low MIC values when tested against porcine B. pilosicoli isolates"; (3) Standardized testing: Same international ring trial-validated protocol available for MIC determination as for B. hyodysenteriae; (4) Limitations: Some drugs no longer available (carbadox, dimetridazole withdrawn in many countries); tylosin resistance common; emerging tiamulin resistance. Treatment generally effective but "the economic impact generally does not warrant such costly procedures" as aggressive eradication.
AVAILABILITY OF EFFECTIVE VACCINES OR BACTERINS
Level: No availability: Effective vaccines not currently available in the US (or have not been developed)
No effective vaccines exist: (1) Autogenous bacterin failure: "An autogenous bacterin induced systemic antibody titers, but the pigs still became colonized and developed diarrhea after experimental challenge"; (2) Antigenic diversity: "B. pilosicoli strains show considerable variability in their LOS"—making cross-protection unlikely; (3) Poor immune response: "Initial evidence suggests that B. pilosicoli is not recognized by antigen presenting cells as they fail to increase MHCII presentation"; (4) Long-term colonization: "The existence of long-term colonization suggests that the spirochete may be able to evade immune mechanisms"; (5) No commercial products: "Unfortunately, no effective vaccines are available for B. pilosicoli."
FEASIBILITY OF ERADICATING THE DISEASE FROM THE US
Level: Not feasible: Eradication extremely unlikely
Eradication is not practical or warranted: (1) Widespread distribution: Present in "most pig-producing countries"; (2) Multiple host species: "A wide range of species may be naturally infected"; pigs, dogs, birds, humans, and others; (3) Environmental persistence: Survives months in soil and water; (4) Wildlife reservoirs: Wild birds, feral animals as ongoing sources; (5) Zoonotic pathogen: Human reservoir complicates any animal-focused eradication; (6) Low economic justification: "The methods described for the elimination of SD also may be effective for PIS/PCS, but the economic impact generally does not warrant such costly procedures"; (7) Strain diversity: Multiple strains with different properties; genetic recombination generates ongoing diversity. The chapter explicitly notes that the mild economic impact doesn't justify eradication efforts.