Pasteurella multocida Toxigenic Strains and Progressive Atrophic Rhinitis in Pigs
Introduction
Progressive atrophic rhinitis (PAR) is a globally significant infectious disease of swine characterized by irreversible turbinate bone atrophy, snout deformation, and impaired growth performance [68, 85, 86]. The primary etiological agent of PAR is toxigenic Pasteurella multocida, a Gram-negative coccobacillus that produces a potent 146 kDa dermonecrotic toxin known as Pasteurella multocida toxin (PMT) [1, 111]. While non-toxigenic strains of P. multocida are frequently isolated from the respiratory tract of healthy pigs, the presence of toxigenic strains, particularly those of capsular serogroups A and D, is strongly correlated with the development of progressive turbinate atrophy [2, 3, 4]. The disease is often exacerbated by co-infection with Bordetella bronchiseptica, which acts as a predisposing agent by damaging the nasal mucosa and facilitating colonization by toxigenic P. multocida [5, 6, 7]. This article provides a detailed review of the molecular pathogenesis, diagnostic methodologies, and control strategies for PAR, with a focus on the biophysical and genetic characteristics of toxigenic P. multocida strains.
Etiology and Taxonomy
Pasteurella multocida is a member of the family Pasteurellaceae. The species is subdivided into five capsular serogroups (A, B, D, E, F) based on capsular polysaccharide antigens, and 16 somatic lipopolysaccharide (LPS) serotypes [91, 94]. In swine, the capsular serogroups most frequently associated with respiratory disease and PAR are A and D [8, 2, 79]. Toxigenic strains are predominantly found within serogroup D, although serogroup A strains can also carry the toxin gene [9, 91]. The ability to produce PMT is conferred by the toxA gene, which is encoded within a lysogenic bacteriophage integrated into the bacterial chromosome [10, 11]. The presence of this phage is the defining genetic feature that distinguishes toxigenic from non-toxigenic isolates [10, 12, 11].
The Pasteurella multocida Toxin (PMT)
PMT is a 1285-amino acid protein that functions as a potent mitogenic toxin and a deamidase [1, 13]. The toxin is a single-chain polypeptide that is post-translationally processed but remains as a single functional unit [78, 111]. Its three-dimensional structure, elucidated by cryo-electron microscopy, reveals three distinct domains: an N-terminal receptor-binding domain, a central translocation domain, and a C-terminal catalytic domain [1].
Molecular Mechanism of Action
PMT enters host cells via receptor-mediated endocytosis [13]. Once internalized, the catalytic domain is translocated into the cytosol where it exerts its effects by deamidating a specific glutamine residue (Gln61) in the alpha subunit of heterotrimeric G proteins, particularly Gq, Gi, and G12/13 [1, 13]. This deamidation converts the glutamine to a glutamic acid, locking the G protein in an active, GTP-bound state. The constitutive activation of these G proteins leads to the sustained stimulation of multiple downstream signaling cascades, including the phospholipase C-beta (PLC-beta) pathway, the mitogen-activated protein kinase (MAPK) pathway, and the RhoA signaling pathway [1, 13, 14].
Cellular Effects of PMT
The sustained activation of these pathways has profound effects on host cell physiology:
Osteoclastogenesis and Bone Resorption: PMT is a potent inducer of osteoclast differentiation and activation, leading to the resorption of nasal turbinate bone [15, 16, 63, 103]. The toxin stimulates osteoclast formation through both RANKL-dependent and RANKL-independent mechanisms [15]. PMT-induced osteoclastogenesis requires the activation of mTOR signaling and is dependent on the presence of B cells [16, 17]. The resulting imbalance between bone formation and resorption is the direct cause of the progressive, irreversible turbinate atrophy characteristic of PAR [63, 68].
Modulation of Immune Responses: PMT manipulates the host immune system in multiple ways. It can modulate Toll-like receptor 4 (TLR4)-mediated immune responses through its effects on G protein signaling [18]. The toxin also influences T cell differentiation, promoting a Th2-biased response while suppressing Th1 responses [19]. Furthermore, PMT activates anti-apoptotic signaling pathways, promoting the survival of infected cells and potentially facilitating bacterial persistence [14].
Mitogenic Activity: PMT acts as a potent mitogen for a variety of cell types, including fibroblasts and osteoblasts, contributing to the cellular proliferation observed in the nasal mucosa during infection [1, 78].
Pathogenesis of Progressive Atrophic Rhinitis
The pathogenesis of PAR is a multifactorial process involving the interaction of toxigenic P. multocida, predisposing factors, and the host immune response.
Colonization and Predisposing Factors
Toxigenic P. multocida colonizes the nasal mucosa and tonsils of pigs [20, 21, 22]. However, efficient colonization and the subsequent development of PAR are often dependent on prior or concurrent infection with B. bronchiseptica [5, 6, 7]. B. bronchiseptica produces a dermonecrotic toxin that damages the ciliated epithelium of the nasal mucosa, thereby reducing mucociliary clearance and providing a more favorable environment for P. multocida adherence and proliferation [5, 112]. Other environmental factors, such as high ammonia levels in housing, can also predispose pigs to colonization by damaging the respiratory epithelium [108].
Development of Turbinate Atrophy
Once established in the nasal cavity, toxigenic P. multocida produces PMT, which is released locally [23]. The toxin diffuses into the underlying connective tissue and bone, where it directly stimulates osteoclasts to resorb the turbinate bones [63, 68]. This process begins in the ventral and dorsal scrolls of the ventral turbinates and progresses to involve the ethmoid turbinates [63]. The resulting bone loss is irreversible and leads to the clinical signs of snout shortening, twisting, and epistaxis [85, 86].
Clinical Signs and Economic Impact
Clinical signs of PAR are most commonly observed in growing pigs between 4 and 12 weeks of age [85, 86]. The cardinal signs include:
- Sneezing and serous to mucopurulent nasal discharge.
- Epistaxis (nosebleeds), often observed as blood-stained snouts.
- Shortening or twisting of the snout.
- Tearing and periocular staining due to obstruction of the nasolacrimal duct.
- Reduced growth rate and feed conversion efficiency.
The economic impact of PAR is substantial, resulting from reduced weight gain, increased feed costs, and increased susceptibility to other respiratory pathogens [85, 86, 107].
Molecular Epidemiology and Genotyping
The molecular epidemiology of toxigenic P. multocida has been extensively studied using a variety of genotyping methods.
Capsular and LPS Typing
Multiplex PCR assays targeting capsular biosynthesis genes are used to classify isolates into serogroups A, B, D, E, and F [94]. In pigs, toxigenic strains are most commonly serogroup D, but serogroup A strains can also carry the toxA gene [2, 9, 79]. LPS genotyping, which targets genes involved in LPS biosynthesis, further subdivides isolates into genotypes (e.g., L1-L8) [91]. Studies have shown that certain combinations of capsular and LPS types, such as D:L3 and A:L3, are frequently associated with toxigenic isolates [91].
Virulence-Associated Genes
In addition to toxA, P. multocida possesses a repertoire of other virulence-associated genes (VAGs) that contribute to its pathogenicity. These include genes encoding adhesins (ptfA, pfhA, fimA), iron acquisition proteins (exbB, exbD, tonB), protectins (ompA, ompH, oma87), and sialidases (nanB, nanH) [88, 90, 94, 98]. The presence and expression levels of these VAGs can vary between strains and are correlated with virulence potential [98]. For example, the tadD gene, part of the tad locus encoding a putative adherence/secretion system, has been associated with biofilm formation and colonization [96, 101].
Multilocus Sequence Typing (MLST)
MLST, which analyzes the sequences of seven housekeeping genes, has revealed significant genetic diversity among P. multocida isolates [89, 90, 104]. Certain sequence types (STs), such as ST50 and ST74, are commonly found in porcine isolates globally [89, 90]. MLST data have demonstrated that some STs can infect multiple host species, highlighting the potential for cross-species transmission [89, 97].
Diagnostic Approaches
Accurate diagnosis of PAR requires the detection of toxigenic P. multocida and the assessment of turbinate atrophy.
Detection of Toxigenic Strains
Several methods are available for detecting toxigenic P. multocida:
Polymerase Chain Reaction (PCR): PCR targeting the toxA gene is the most sensitive and specific method for identifying toxigenic strains [24, 25, 26, 73, 81]. Multiplex PCR assays can simultaneously detect toxA and other species-specific or virulence-associated genes [27, 25]. Real-time PCR (qPCR) allows for quantification of bacterial load.
Enzyme-Linked Immunosorbent Assay (ELISA): ELISA using monoclonal or polyclonal antibodies against PMT can detect the toxin directly in nasal swabs or bacterial culture supernatants [28, 29, 30, 70]. This method is commercially available and suitable for herd-level screening [28, 70].
Cell Culture Assay: The cytotoxicity of PMT can be assessed using cell lines such as Vero or embryonic bovine lung cells [67]. This assay is sensitive but requires cell culture facilities and is slower than PCR or ELISA.
Mouse Lethality Test: Historically, the intraperitoneal injection of bacterial culture supernatant into mice was used to detect the lethal effects of PMT [31, 70]. This method is now largely replaced by in vitro assays due to animal welfare concerns.
Assessment of Turbinate Atrophy
Postmortem examination of the nasal cavity is the definitive method for diagnosing PAR. The degree of turbinate atrophy is scored using a standardized system, such as the one described by the Danish system, where a score of 0 (normal) to 5 (complete atrophy) is assigned [68, 102]. Radiography and computed tomography (CT) can be used for antemortem assessment but are less practical for routine field diagnosis [110].
Diagnostic Workflow
The following Mermaid diagram illustrates a typical diagnostic workflow for PAR:
graph TD
A["Clinical Signs: Sneezing, Nasal Discharge, Snout Deformation"] --> B{Nasal Swab Collection};
B --> C[DNA Extraction];
C --> D[toxA PCR / qPCR];
D --> E{toxA Positive?};
E -- Yes --> F[Confirmed Toxigenic P. multocida];
E -- No --> G[Non-toxigenic or No P. multocida];
B --> H[Bacterial Culture];
H --> I[Isolate Identification];
I --> J[PMT ELISA / Cell Culture Assay];
J --> K{Toxin Detected?};
K -- Yes --> F;
K -- No --> G;
F --> L[Postmortem Examination];
L --> M[Turbinate Atrophy Scoring];
M --> N[Confirm PAR Diagnosis];
Control and Prevention
Control of PAR relies on a combination of management practices, antimicrobial therapy, and vaccination.
Management Practices
Good husbandry is essential for reducing the risk of PAR. This includes:
- All-in/all-out production systems to break the cycle of infection.
- Adequate ventilation to reduce ammonia levels [108].
- Minimizing stress and overcrowding.
- Early weaning and segregation of infected pigs.
Antimicrobial Therapy
Antimicrobials can be used to reduce the nasal carriage of P. multocida and B. bronchiseptica. However, the emergence of antimicrobial resistance is a growing concern [89, 94, 99, 100]. Susceptibility testing is recommended to guide treatment choices. Commonly used antimicrobials include tetracyclines, sulfonamides, and fluoroquinolones, although resistance to these classes has been reported [79, 82, 94, 99].
Vaccination
Vaccination is a key component of PAR control programs. Both bacterin-toxoid vaccines, which contain inactivated P. multocida cells and inactivated PMT toxoid, and subunit vaccines based on recombinant PMT fragments have been developed [69, 105, 106]. Vaccination of sows provides passive immunity to piglets via colostrum, protecting them during the critical early weeks of life [65, 69, 109]. The efficacy of vaccines depends on the antigenic match between the vaccine strain and the circulating field strains [93].
Conclusion
Progressive atrophic rhinitis remains a significant challenge for the global swine industry. The disease is driven by the potent mitogenic toxin PMT, produced by toxigenic strains of P. multocida. A deep understanding of the molecular mechanisms of PMT action, the genetic diversity of circulating strains, and the complex interactions between the pathogen, host, and environment is essential for developing effective diagnostic and control strategies. Continued surveillance of antimicrobial resistance and the development of broadly protective vaccines are critical for the sustainable management of this economically important disease.
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