ACTINOBACILLUS SUIS
LEVELS: Rarely occurs: Requires significant failure at one or more control points for transmission to humans; Highly unlikely: No evidence of non-foodborne zoonotic transmission; 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); 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; 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
Actinobacillus suis is a gram-negative, ubiquitous opportunistic pathogen that colonizes the upper respiratory tract of pigs. While originally associated with septicemia and death in suckling and recently weaned pigs, disease can occur in fattening pigs and adults, particularly in high-health status herds. A Canadian survey found 94% of tested herds were probably infected, though no clinical cases were observed, demonstrating the organism's commensal nature in most circumstances. A. suis produces Apx toxins similar to those of A. pleuropneumoniae (ApxI and ApxII homologs), though at lower levels, which may explain its generally lower virulence. Three clinical presentations predominate: (1) acute fulminant septicemia in suckling and recently weaned pigs, often with sudden death and petechial hemorrhages in multiple organs; (2) respiratory disease in grower-finisher pigs with hemorrhagic and necrotizing pleuropneumonia resembling APP infection; and (3) acute septicemia in adults with erysipelas-like skin lesions, fever, and potential abortions. Differential diagnosis from APP (particularly biotype II strains) and erysipelas is critical for appropriate management. Prevalence of disease is often greater in newly populated herds before animals develop immunity.
FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Rarely occurs: Requires significant failure at one or more control points for transmission to humans
A. suis is not recognized as a zoonotic pathogen, with only one documented human infection: "A case of human infection by A. suis or A. equuli has been reported recently in Italy." This represents an isolated case report rather than an established zoonotic risk pattern. The organism colonizes the porcine upper respiratory tract and causes localized or systemic disease in pigs; there is no evidence of foodborne transmission to humans through consumption of pork products.
NON-FOODBORNE ZOONOTIC TRANSMISSION POTENTIAL
Level: Highly unlikely: No evidence of non-foodborne zoonotic transmission
Human infection is exceptionally rare, with only a single case report documented in the literature. The chapter notes this occurred in Italy, without details on the transmission route. Given the ubiquitous nature of A. suis colonization in pigs (94% of herds) and the extensive occupational contact between humans and pigs, the extreme rarity of human cases indicates very low zoonotic potential. There is no evidence of occupational risk for veterinarians or pig workers despite constant exposure.
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
A. suis is an endogenous opportunistic pathogen rather than one typically introduced from external sources: (1) Ubiquitous colonization: "colonizes the upper respiratory tract of pigs"; 94% of herds infected; (2) Opportunistic disease: Clinical disease occurs when host defenses are compromised or in naive populations; "Prevalence of disease may be greater in newly populated herds before animals develop immunity"; (3) No environmental persistence documented: Unlike environmental pathogens, A. suis appears to require host-to-host transmission; (4) High-health herd vulnerability: "especially in high health status herds"—suggesting disease emerges from endogenous organisms when naive animals are introduced. Prevention focuses on allowing immunity development rather than preventing pathogen introduction. Standard biosecurity can prevent introduction of particularly virulent strains, but eliminating the commensal organism is impractical.
DIFFICULTY OF DETECTING AND CONFIRMING INFECTION
Level: Moderate: Clinical signs not unique but existing tests available at local/regional laboratory(s)
Clinical recognition requires differentiation from similar diseases: (1) Variable clinical presentation: Three distinct disease forms (septicemia in young pigs, respiratory disease in growers, septicemia with skin lesions in adults) each have different differentials; (2) Resembles other diseases: Septicemic form resembles S. suis and Klebsiella infections; respiratory form resembles APP pleuropneumonia; adult form with skin lesions resembles erysipelas; (3) Culture and identification: "Preliminary diagnosis is based on typical clinical signs and gross lesions. Diagnosis is confirmed by culture and identification of A. suis from tissues with typical microscopic lesions"; (4) Differentiation from APP critical: A. suis must be distinguished from APP biotype II strains; "complete biochemical identification along with species-specific PCR tests must be done"; A. suis isolates may cross-react with APP serotype 3, 6, or 8 antisera; (5) Strain-specific ELISA: "A strain-specific ELISA test has been shown to be useful to follow up maternal and active antibodies in an infected herd." The main challenge is recognizing A. suis as the cause rather than APP or erysipelas.
FINANCIAL IMPACT ON FARM'S COST OF PRODUCTION
Level: Moderate: Manageable losses related to endemic (population) or chronic (individual) occurrence
A. suis causes sporadic losses but is not typically a major endemic problem: (1) High subclinical prevalence: 94% herd infection rate with "no clinical cases observed" in survey; (2) Sporadic clinical disease: Disease episodes occur in specific circumstances—naive populations, high-health herds, immunocompromised animals; (3) Mortality when clinical: "sudden death may also be the sole sign"; septicemia can be fatal in young and adult pigs; (4) Growth effects: In grower-finisher respiratory form, morbidity affects growth but mortality is variable; (5) Abortion losses: Can occur in adult form; (6) Limited endemic losses: Unlike APP, A. suis does not typically cause sustained endemic disease with chronic lesions and ongoing production effects. Losses are primarily related to acute mortality events rather than chronic production impairment.
EFFECT ON DOMESTIC OR EXPORT MARKETS
Level: Negligible: Little or no market disruption when disease occurs on one or more farms
A. suis is not a regulated or reportable disease: (1) No trade restrictions: Not WOAH-listed; no international trade implications; (2) Ubiquitous distribution: Presence in 94% of herds means it's not a differentiating factor for herd health status; (3) Not typically part of health declarations: Unlike APP, A. suis-free status is not a standard certification requirement; (4) High-health herd consideration: May be relevant when establishing new herds or introducing genetics, but generally not a trade barrier. The clinical similarity to erysipelas (a regulated disease) could cause initial concern until properly diagnosed.
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
Limited data available on antimicrobial resistance: (1) Limited published data: "There are almost no data in the literature suggesting resistance of numerous A. suis strains to different antimicrobials and no clear breakpoints are available"; (2) Variable susceptibility reported: One study suggested decreasing resistance to ceftiofur, spectinomycin, sulfadimethoxine, and tiamulin over time (2006-2016), though "data should be interpreted cautiously"; (3) Related organism comparison: A. suis is closely related to APP, which shows moderate resistance concerns particularly to tetracyclines; (4) Opportunistic pathogen ecology: As a commensal organism, A. suis may be exposed to antimicrobials used for other purposes, potentially selecting for resistance.
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
A. suis treatment patterns depend on disease presentation: (1) Acute treatment needed: Septicemic forms require rapid individual treatment; "Antibiotic treatment should be applied early in the appearance of signs"; (2) High-health herd outbreaks: New populations may experience multiple cases requiring repeated treatments until immunity develops; (3) Sporadic nature limits overall use: Unlike endemic diseases requiring continuous medication, A. suis causes episodic outbreaks; (4) Autogenous vaccine use: "Vaccination has been implemented in some herds using autogenous vaccines"—may reduce antimicrobial needs where used.
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 are effective when treatment is timely: (1) Recommended antibiotics: "ceftiofur, gentamicin, and trimethoprim/sulfadiazine seem to be the antibiotics of choice, followed by ampicillin, sulfadimethoxine, and tiamulin"; (2) Early treatment critical: Given the septicemic and acute nature of disease, delays reduce treatment success; (3) Limited resistance documentation: No widespread resistance problems reported; (4) Challenge is timing: "Antibiotic treatment should be applied early in the appearance of signs"—sudden death presentations may precede any opportunity for treatment.
AVAILABILITY OF EFFECTIVE VACCINES OR BACTERINS
Level: No availability: Effective vaccines not currently available in the US (or have not been developed)
No commercial vaccine exists: (1) Autogenous vaccines only: "Vaccination has been implemented in some herds using autogenous vaccines with variable results"; (2) Limited efficacy data: In one documented case, "gilts and sows were vaccinated, although these animals already presented high titers of antibodies before vaccination"—suggesting natural immunity may be more important; (3) Variable results: "autogenous vaccines with variable results" indicates inconsistent protection; (4) Natural immunity develops: "Prevalence of disease may be greater in newly populated herds before animals develop immunity" suggests exposure leads to protective immunity. Prevention relies primarily on allowing immunity to develop naturally in newly populated herds rather than vaccination.
FEASIBILITY OF ERADICATING THE DISEASE FROM THE US
Level: Not feasible: Eradication extremely unlikely
A. suis eradication is not practical or warranted: (1) Ubiquitous commensal: 94% of herds infected; colonizes upper respiratory tract of pigs as normal flora; (2) Opportunistic pathogen: Disease results from host-pathogen imbalance rather than pathogen introduction; (3) No clear elimination strategy: Unlike primary pathogens, commensal organisms cannot be eliminated through standard eradication approaches; (4) Low disease burden: Sporadic clinical disease does not justify eradication efforts; (5) Natural immunity protective: Herds develop immunity over time, limiting disease impact. Management focuses on preventing clinical outbreaks in naive populations rather than eliminating the organism.