Fowl Cholera (Avian Pasteurellosis): Etiology, Epidemiology, Clinical Signs, Pathology, Diagnostics, Treatment, and Control

Etiology

Fowl cholera, also referred to as avian pasteurellosis, is a contagious bacterial disease caused by the Gram-negative coccobacillus Pasteurella multocida [1, 2, 3]. The organism is classified into five capsular serogroups (A, B, D, E, F) based on capsular polysaccharide antigens, and into 16 lipopolysaccharide (LPS) genotypes (L1 to L16) based on outer core biosynthesis loci [4, 5, 6]. In avian species, capsular serogroup A and LPS genotypes L1, L3, and L4 are most frequently associated with outbreaks [7, 8, 9]. The pathogen possesses multiple virulence factors including a hyaluronic acid capsule that mediates antiphagocytosis, filamentous hemagglutinins (e.g., FhaB1), lipoproteins (e.g., PlpE), and a mitogenic toxin (PMT) that modulates host cellular signaling [10, 11, 12, 13]. The hyaD gene has been shown to contribute to capsular biosynthesis and virulence in avian strains [11]. Additionally, the stringent response (e.g., (p)ppGpp signaling) negatively regulates capsule production, influencing bacterial persistence in the host [6]. The genetic diversity of P. multocida is well documented through whole-genome sequencing, revealing sequence types (STs) such as ST20 in Australian poultry and wild waterbirds, and diverse capsular and LPS combinations in isolates from Bangladesh and other regions [14, 15, 4, 8, 5]. The question "fowl cholera is caused by which bacteria" is unequivocally answered: Pasteurella multocida.

Epidemiology

The disease occurs worldwide and is recognized as a major cause of mortality in domestic poultry, including chickens, turkeys, ducks, and geese, as well as in wild waterfowl and other avian species [1, 16, 17]. The term "fowl cholera in hindi" and "fowl cholera meaning in bengali" reflect the global distribution of the disease; vernacular names such as "murgi haiza" (Hindi) and "pakhir cholera" (Bengali) are used regionally. Outbreaks can reach 100% mortality in naive flocks, as documented in slow-growing broiler chickens [1] and in commercial turkeys, where coinfection with Mycoplasmoides gallisepticum exacerbates severity [18]. Transmission occurs horizontally via respiratory aerosols, contaminated feed and water, and fomites; carrier birds are a critical reservoir [19, 9]. Land cover and environmental factors influence outbreak occurrence, with case-case studies showing associations with specific habitat types [16]. Phylogenetic analyses indicate that certain P. multocida lineages, such as ST20, are widespread across Australian poultry farms and can spill over into wild waterbirds, underscoring a "poultry pandemic" dynamic [8]. The species is also a cause of "chicken bacteria outbreak" scenarios in intensive and backyard systems [2, 20]. While "avian cholera transmission to humans" is possible through bites or direct contact, human infections are rare and typically associated with non-avian strains; the zoonotic risk from avian P. multocida is considered low [3, 12].

Clinical Signs

Clinical presentation varies from peracute to chronic forms [1, 20]. In peracute fowl cholera, birds die suddenly without premonitory signs, often with high mortality within 24 hours [1]. Acute disease is characterized by fever, depression, anorexia, mucoid or sanguineous oral discharge, diarrhea, and cyanosis of comb and wattles [20, 17]. Respiratory signs such as dyspnea and rales may be present, particularly in turkeys [18, 7]. Chronic infections manifest as localized swelling of wattles, joints (arthritis), sternal bursae, and infraorbital sinuses [17]. In laying flocks, a drop in egg production is a common sign [20, 17]. Coinfection with M. gallisepticum can intensify respiratory signs and mortality [18].

Pathology

Gross lesions in peracute cases may be absent or limited to petechial hemorrhages on serosal surfaces [1, 17]. Acute cases exhibit multifocal hepatic necrosis (characteristic small, pale foci), splenomegaly, fibrinous pericarditis, peritonitis, and hemorrhagic enteritis [1, 7, 20]. Turkey poults often show severe pulmonary congestion and airsacculitis [18, 7]. Chronic cases present caseous abscesses in wattles, joints, and periarticular tissues [17]. Histologically, hepatic lesions consist of coagulative necrosis with heterophilic infiltration, and the spleen shows lymphoid depletion and fibrinoid necrosis [17].

Diagnostics

Definitive diagnosis relies on isolation and identification of P. multocida from affected tissues (liver, spleen, bone marrow, or exudate) using selective media (e.g., blood agar or MacConkey agar) [2, 20]. The bacterium is oxidase- and catalase-positive, and further characterization includes capsular serotyping (A, B, D, E, F) and LPS genotyping [4, 5, 9].

Molecular diagnostics have advanced significantly. PCR assays targeting the kmt1 gene are widely used for species confirmation [3, 21]. Loop-mediated isothermal amplification (LAMP) provides a field-deployable alternative with comparable sensitivity to PCR [21]. Whole-genome sequencing enables high-resolution typing, detection of antimicrobial resistance genes (e.g., blaROB-1, tetH, sul2), and phylogenetic tracking [14, 15, 22, 4, 5]. Serological tools include in-house indirect ELISA kits for detecting anti-P. multocida antibodies in chickens [23]. The diagnostic workflow is illustrated in Figure 1.

Figure 1: Diagnostic workflow for suspected fowl cholera cases

flowchart TD
    A[Clinical suspicion: sudden death, hepatic necrosis], > B[Postmortem examination]
    B, > C[Aseptic collection of liver, spleen, bone marrow]
    C, > D[Culture on blood agar + MacConkey]
    D, > E{Gram-negative, oxidase-positive coccobacilli}
    E, > F[Biochemical identification]
    F, > G[PCR: kmt1 gene]
    G, > H[Confirm P. multocida]
    H, > I[Capsular typing / LPS genotyping]
    H, > J[Antimicrobial susceptibility testing]
    J, > K[Antibiogram profiling]
    I, > L[Epidemiological typing: MLST, WGS]
    H, > M[Serology: indirect ELISA]

Treatment

Antimicrobial therapy is the primary intervention in acute outbreaks. Historically, tetracyclines, sulfonamides, and penicillin derivatives have been used [22, 2, 20]. However, antimicrobial resistance (AMR) is increasingly reported; multidrug-resistant P. multocida isolates from poultry and rabbits carry resistance genes such as tetH, blaROB-1, sul2, and aph(3''')-Ib [22, 3]. A study in Ethiopia found high resistance to tetracycline and sulfonamides among breeder chicken isolates [2]. Phenotypic drug resistance profiling is recommended to guide therapy [20]. Alternative strategies include phytotherapeutic agents; wild Egyptian artichoke extract has demonstrated in vitro antibacterial activity against P. multocida [24]. Supportive care includes separation of affected birds, electrolyte supplementation, and stress reduction.

Control

Biosecurity measures, including all-in-all-out management, rodent control, and disinfection, are foundational for preventing introduction and spread of P. multocida [19, 17]. Bacterins (killed whole-cell vaccines) and live attenuated vaccines are commercially available [25, 26]. Recent advancements in vaccine development include:

• Hydrogel inactivated vaccines that enhance immunoprotection in chickens [27].
• Gamma-irradiated vaccines formulated with adjuvants to induce antibody and cytokine responses [28].
• Subunit vaccines using recombinant lipoproteins (e.g., PlpE) with flagellin adjuvants to improve immunogenicity [29, 13].
• Recombinant viral-vectored vaccines using duck enteritis virus or turkey herpesvirus expressing OmpH protein, providing dual protection [30, 31, 32].
• Live attenuated strains (e.g., PMZ8) developed by serial passage, with reduced virulence and protective efficacy in ducks [25].
• Next-generation strategies including CRISPR/Cas9-edited vectors and LPS-truncated mutants [33, 34].

Vaccination programs should be coupled with regular surveillance, including molecular monitoring of circulating serotypes and AMR profiles [22, 8, 9]. The concept of a "poultry pandemic" requires coordinated regional control strategies, especially in free-range and organic systems where environmental exposure is higher [16, 9].

References

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