Fowl Cholera: Etiology, Epidemiology, and Control in Poultry

Introduction

Fowl cholera, also termed avian pasteurellosis, is a highly contagious and economically significant bacterial disease affecting domestic poultry and wild avian species worldwide [1, 2]. The disease is caused by infection with Pasteurella multocida, a Gram-negative coccobacillus that can produce peracute, acute, or chronic disease depending on host susceptibility and bacterial strain virulence [3, 27]. Mortality events may reach 100% in susceptible commercial flocks, as documented in slow-growing broiler chickens [3] and turkeys [4]. The term fowl cholera bacterial infection is commonly used interchangeably with avian pasteurellosis in clinical and diagnostic contexts [5].

Etiology

Fowl Cholera Is Caused by Which Bacteria

Fowl cholera is caused by the bacterium Pasteurella multocida [6, 7, 8]. This species is classified within the family Pasteurellaceae and is a facultative anaerobe that exhibits bipolar staining with Giemsa or Wright stain [6, 9]. The organism produces a polysaccharide capsule and lipopolysaccharide (LPS) that are critical for virulence and serotype classification [8, 32, 33].

Serotypes and Virulence Factors

P. multocida isolates are classified into capsular serogroups (A, B, D, E, F) and LPS genotypes (L1–L8) [8]. In poultry, serogroups A and D are predominant, although untypable capsule strains have been reported [25]. The LPS outer core locus undergoes phase variation, which contributes to immune evasion and outbreak persistence on free-range layer farms [33]. Hyaluronic acid capsule production is negatively regulated by the stringent response, and this regulation influences biofilm formation and survival [32].

Key virulence factors include the Pasteurella multocida toxin (PMT), a mitogenic toxin that modulates host cell signaling [27]; the hyaluronidase encoded by hyaD, which contributes to tissue invasion and pathogenicity [10]; and outer membrane proteins such as OmpH and lipoprotein E (PlpE) that are targets for vaccine development [11, 28]. The filamentous hemagglutinin B1 (fhaB1) gene, investigated as a putative adhesin, was found not to be essential for pathogenesis in turkey poults [12]. Multidrug-resistant strains carry an array of antimicrobial resistance genes, virulence-associated genes, and the kmt1 gene used for molecular detection [13, 14].

Genomic profiling has been performed on strains from different avian hosts, including ISA Brown chickens [6], Jinding ducks [9], layer chickens [25], and waterfowl [15]. Whole-genome sequencing of eight isolates representing all LPS outer core loci has facilitated understanding of serotype diversity [8]. The stringent response has been identified as a genome-wide regulator of capsule and metabolism [32].

Epidemiology

Host Range and Transmission

Fowl cholera affects chickens, turkeys, ducks, geese, and many wild bird species [4, 3, 15]. Outbreaks have been reported with high mortality in commercial turkeys coinfected with Mycoplasmoides gallisepticum [4] and in slow-growing broilers [3]. Waterfowl, including ducks, are both reservoirs and victims; Australian studies demonstrated that P. multocida sequence type 20 (ST20) is widespread in poultry and may infect wild waterbirds [15].

Transmission occurs horizontally via respiratory aerosols, contaminated feed and water, and fomites. Carriers recovered from outbreaks can shed the bacterium intermittently [1]. Compartmental models have been developed to simulate transmission dynamics within poultry farms [1]. Environmental risk factors include land cover type; a case-case study showed that occurrence of fowl cholera is associated with specific landscape features, possibly due to wildlife reservoir proximity [16].

Regional Nomenclature

In Hindi, fowl cholera is commonly referred to as murgi ka haiza, and in Bengali it is called pakhir haiza (ougha পাখির হাইজা). The term fowl cholera meaning in bengali reflects the colloquial association of the disease with cholera-like mortality in birds. These regional names are used in field surveillance and farmer education programs.

Clinical Signs

Fowl cholera presents in three main forms: peracute, acute, and chronic.

Peracute Form

Birds are found dead without premonitory signs. Mortality can reach 100% within 24 hours in susceptible flocks [3].

Acute Form

Affected birds exhibit fever, anorexia, depression, diarrhea, cyanosis of comb and wattles, and respiratory distress. Oral and nasal discharges may be observed. Death occurs 2–5 days post-infection [4, 3].

Chronic Form

Chronic infection manifests as localized inflammation of the wattles, sinuses, joints, and footpads. Synovitis and osteomyelitis are common in chronic cases. In layers, chronic fowl cholera can cause a drop in egg production [17].

Pathology

Gross lesions in peracute and acute cases include extensive petechiae and ecchymoses on the heart, epicardium, serosal surfaces, and abdominal fat [3]. The liver may be enlarged with multiple small necrotic foci (miliary necrosis). Splenic enlargement and pulmonary congestion are consistently observed [3, 18]. In turkeys, fibrinopurulent exudates in the pericardium and air sacs are common [4, 18].

Microscopically, acute cases demonstrate bacterial emboli in capillaries, leading to necrosis and inflammation of the myocardium and liver parenchyma [3, 18].

Diagnostics

Sample Collection and Culture

Samples include blood, liver, spleen, bone marrow, and swabs from oropharynx or exudates. Pasteurella multocida grows on blood agar and MacConkey agar, producing smooth, glistening colonies with a characteristic "mousy" odor [18, 7]. Gram staining reveals Gram-negative bipolar rods.

Molecular Diagnostics

Polymerase chain reaction (PCR) targeting the kmt1 gene is widely used for species-specific detection [14, 19]. Loop-mediated isothermal amplification (LAMP) offers a rapid, field-deployable alternative with comparable sensitivity to PCR [19]. Real-time PCR and high-resolution melting analysis can differentiate serotypes.

Serological Assays

Indirect enzyme-linked immunosorbent assays (ELISAs) have been developed for detection of anti-P. multocida antibodies in chicken flocks [20]. These assays use whole-cell antigens or recombinant PlpE and OmpH proteins [20, 11].

Antimicrobial Susceptibility

Antibiogram profiling is essential for guiding treatment due to widespread antimicrobial resistance [13, 7, 17]. Disk diffusion and minimum inhibitory concentration (MIC) determination against a panel of antibiotics (e.g., tetracyclines, penicillins, fluoroquinolones) are standard [13, 7, 17].

Mermaid Diagnostic and Control Decision Tree

flowchart TD
    A[Clinical suspicion of fowl cholera], > B[Sample collection: blood, liver, spleen, swabs]
    B, > C[Culture on blood/MacConkey agar & Gram stain]
    C, > D[Identify Pasteurella multocida: bipolar rods, colony morphology]
    D, > E{Confirm by PCR (kmt1) or LAMP}
    E, > F[Positive]
    F, > G[Perform serotyping & AMR testing]
    G, > H[Antimicrobial therapy based on susceptibility]
    G, > I[Vaccination if applicable]
    E, > J[Negative – consider other pathogens]
    H, > K[Biosecurity measures & flock monitoring]
    I, > K

Avian Cholera Transmission to Humans

Avian cholera transmission to humans is considered a rare zoonotic event. Human infection with Pasteurella multocida typically occurs through animal bites or scratches from cats or dogs, not through direct contact with poultry. Immunocompromised individuals may be at increased risk. Occupational exposure (e.g., poultry workers) has not been strongly associated with clinical disease, and the pathogen is not considered a major foodborne hazard. This zoonotic potential is noted for comparative host-range awareness, but the primary focus remains on avian infection and poultry health.

Treatment

Antimicrobial therapy is most effective when administered early in the acute phase based on in vitro susceptibility results. Commonly used agents include oxytetracycline, chlortetracycline, enrofloxacin, sulfonamides, and penicillin derivatives [13, 7, 17]. However, multidrug resistance is increasingly reported. Miao et al. characterized resistance genes in avian P. multocida isolates from China, revealing resistance to aminoglycosides, tetracyclines, sulfonamides, and beta-lactams [13]. Studies from Ethiopia and Egypt also documented high levels of resistance with multiple resistance determinants [7, 14, 17].

Alternative therapeutic approaches have been explored, including the use of plant extracts. Wahdan et al. demonstrated the in vitro antibacterial effect of wild Egyptian artichoke extract against P. multocida [21].

Control

Biosecurity

Strict biosecurity is the cornerstone of fowl cholera prevention. Measures include all-in/all-out production, rodent and pest control, cleaning and disinfection of housing and equipment, and preventing contact with wild birds [1, 16]. Water treatment and feed hygiene reduce the risk of oral transmission.

Vaccination

Vaccines against fowl cholera have evolved from traditional bacterins to next-generation formulations [5].

Conventional Bacterins

Inactivated bacterins (killed whole cells) adjuvanted with mineral oil, aluminum hydroxide, or saponin have been widely used [22, 35]. Ghadimipour et al. compared the immunogenicity of different adjuvants and found that oil-adjuvanted bacterins induced the highest antibody titers in chickens [35].

Live Attenuated Vaccines

Serial passage-derived attenuated strains such as PMZ8 have shown promise in ducks, providing protection without significant reversion to virulence [23].

Subunit and Recombinant Vaccines

Subunit vaccines based on lipoprotein E (PlpE) and outer membrane protein H (OmpH) induce robust immune responses when combined with adjuvants or flagellin [11, 28]. The signal sequence of PlpE contributes to its immunogenicity [28]. Recombinant viral vector vaccines have been developed using duck enteritis virus and turkey herpesvirus engineered via CRISPR/Cas9 and Cre-LoxP systems to express OmpH [26, 30, 31]. These bivalent vaccines can protect against both duck plague and fowl cholera [30].

Novel Adjuvant and Delivery Systems

Hydrogel-based inactivated vaccines have demonstrated enhanced immunoprotection in chickens [22]. Gamma-irradiated vaccines combined with various adjuvants induced specific antibody and cytokine responses [24]. Co-delivery of immune-modulating proteins with live vaccines can further improve efficacy [29].

Next-Generation Approaches

A comprehensive review by Tesfaye et al. summarizes the transition from conventional bacterins to DNA vaccines, recombinant proteins, and vectored platforms, emphasizing the need for cross-protective vaccines covering multiple serotypes [5]. Truncated LPS mutants have also been investigated as vaccines in ducks [34].

Conclusion

Fowl cholera remains a major threat to commercial poultry production worldwide. The causative agent, Pasteurella multocida, exhibits extensive genetic and serotypic diversity, complicating diagnosis and control. Advances in molecular diagnostics, including rapid LAMP assays and high-throughput sequencing, have improved detection and surveillance. Antimicrobial resistance is a growing concern, necessitating judicious antibiotic use and alternative therapies. Vaccination strategies continue to evolve, with recombinant vectored vaccines offering the potential for broader protection. Integrated biosecurity, flock management, and vaccination programs are essential for effective control of fowl cholera.

References

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