Fowl Cholera (Pasteurella multocida) in Poultry: Clinical Manifestations, Diagnosis, and Control

Introduction

Fowl cholera, also known as avian pasteurellosis, is a highly contagious bacterial disease of domestic and wild birds caused by Pasteurella multocida [1, 2]. The disease manifests in peracute, acute, or chronic forms and is associated with significant economic losses in poultry production worldwide [3, 4]. P. multocida is a Gram-negative, non-motile, facultatively anaerobic coccobacillus that possesses a polysaccharide capsule and expresses a diverse array of virulence factors [5, 6]. The bacterium is capable of infecting a broad host range including chickens, turkeys, ducks, and game birds [7, 8]. Understanding the fowl cholera bacterial pathogenesis, clinical presentation, and control measures is essential for veterinary practitioners and poultry health managers. This article provides a detailed, publication-grade overview of the disease, with a focus on molecular diagnostics and management strategies. A brief discussion of regional terminology, including fowl cholera in Hindi, is also included to reflect global veterinary contexts.

Etiology

Pasteurella multocida is classified into five capsular serogroups (A, B, D, E, F) and 16 lipopolysaccharide (LPS) genotypes based on the Heddleston scheme [9, 10]. Avian isolates predominantly belong to serogroups A, followed by D and F [8, 11]. The LPS outer core is a critical virulence determinant; phase variation in glycosyltransferase genes can alter LPS structure and influence outbreak dynamics [10, 12]. Several genes contribute to virulence, including hyaD (involved in hyaluronic acid capsule synthesis), fhaB1 (filamentous hemagglutinin), and lipoprotein E (plpE) [13, 14, 15]. The stringent response negatively regulates capsule production, linking metabolic stress to virulence [6]. Genomic profiling of avian strains has revealed high diversity, with sequence types such as ST20 being widespread in Australian poultry and wild waterbirds [16]. Multidrug-resistant strains carrying diverse resistance genes have been reported in Bangladesh, Ethiopia, and China [2, 17, 5, 11].

Epidemiology

Fowl cholera occurs in both commercial and backyard poultry flocks across all continents [1, 18]. Transmission occurs via direct contact, fomites, contaminated feed and water, and carriage by asymptomatic birds or wildlife [19, 16]. Outbreaks often follow stressor events such as changes in weather, overcrowding, or concurrent infections. For example, coinfection with Mycoplasmoides gallisepticum has been associated with high mortality in turkeys [3]. A compartmental model of cholera transmission in poultry farms highlighted the importance of rapid detection and culling to control spread [18]. Land cover characteristics, including proximity to wetlands and free-range access, have been linked to increased disease occurrence [19]. In free-range layer farms, phase-variable LPS genes enable immune evasion and persistent circulation [10]. Wild waterbirds can act as reservoirs, maintaining P. multocida ST20 clones that spill over into poultry [16].

Clinical Manifestations

The clinical presentation of fowl cholera depends on the virulence of the strain, host susceptibility, and route of infection [4, 20]. Peracute cases cause sudden death without premonitory signs, often in apparently healthy birds. Acute disease is characterized by fever (increased body temperature), depression, anorexia, mucoid or bloody diarrhea, cyanosis of the comb and wattles, and increased respiratory effort [3, 4]. In commercial slow-growing broilers, acute fowl cholera has been documented to cause 100% mortality within days [4]. Chronic infections manifest as localized swelling of the wattles, conjunctivitis, torticollis from middle ear infection, and lameness due to arthritis or osteomyelitis [17, 21]. The clinical signs may be indistinguishable from other septicemic diseases such as avian influenza or colibacillosis, necessitating laboratory confirmation [22].

Pathological Findings

Gross lesions in acute fowl cholera include petechial hemorrhages on the epicardium, serosal surfaces, and abdominal fat; enlarged and congested liver and spleen; and multifocal necrotic foci in the liver (so-called "cholera spots") [4, 5, 20]. In peracute cases, lesions may be minimal. Chronic cases show fibrinous polyserositis, caseous arthritis, and wattle edema containing yellow, caseous exudate [19]. Microscopic findings consist of fibrinonecrotic hepatitis, splenitis, and pneumonia with Gram-negative coccobacilli in tissue sections [3, 4].

Diagnosis

Bacteriological Culture and Isolation

Isolation of P. multocida from liver, spleen, bone marrow, or wattle exudate on blood agar or MacConkey agar is the gold standard [17, 21]. Colonies appear as small, gray, mucoid, and non-hemolytic after 18-24 hours of incubation at 37°C. Gram staining reveals bipolar rods (safety pin appearance) [5]. Biochemical identification is based on oxidase, catalase, and indole positivity, and lack of growth on cetrimide agar [17].

Molecular Diagnostics

PCR assays targeting the kmt1 gene (species-specific) and capsular typing genes are widely used for confirmation and serogroup determination [2, 23, 11]. Loop-mediated isothermal amplification (LAMP) assays offer comparable sensitivity to PCR with faster turnaround and minimal equipment requirements [23]. High-throughput sequencing enables whole-genome characterization for epidemiological tracking, AMR gene detection, and virulence profiling [1, 9, 24].

Serological Tests

In-house indirect ELISA kits have been developed to detect anti-P. multocida antibodies in chicken sera, facilitating serosurveillance and vaccine response monitoring [25]. However, serology cannot distinguish natural infection from vaccination [22].

Below is a decision tree for the diagnostic workup of suspected fowl cholera.

flowchart TD
    A[Suspected fowl cholera], > B{Clinical signs & mortality?}
    B, >|Peracute/acute| C[Necropsy: collect liver, spleen, bone marrow]
    B, >|Chronic| D[Collect wattle exudate, joint fluid, or sinuses]
    C, > E[Gram stain & culture on blood agar]
    D, > E
    E, > F[Biochemical identification]
    F, > G[PCR for kmt1 & capsular genes]
    G, > H[Report: confirmed P. multocida + serogroup]
    F, > I[Antimicrobial susceptibility testing]
    I, > J[Therapy guidance]
    G, > K[Optional: whole-genome sequencing for AMR/virulence]

Treatment and Antimicrobial Resistance

Antimicrobial therapy is often attempted in early acute cases and for treatment of individual valuable birds. Historically effective drugs include tetracyclines, sulfonamides, fluoroquinolones, and beta-lactams [2, 17, 21]. However, widespread resistance has been documented. Studies from Ethiopia, China, and Bangladesh have reported multidrug-resistant (MDR) P. multocida strains with resistance to oxytetracycline, sulfamethoxazole, and enrofloxacin [2, 5, 21]. Antimicrobial resistance genes such as tetR, sul2, and bla variants are frequently detected [2, 11]. The emergence of MDR strains necessitates routine antibiogram profiling before treatment selection [17, 21]. Alternative strategies such as probiotics and plant extracts (e.g., wild Egyptian artichoke extract) have shown in vitro inhibitory activity and reduction of mortality in broilers [26, 27].

Control and Vaccination

Biosecurity

Control of fowl cholera relies on strict biosecurity: all-in/all-out management, rodent and wild bird exclusion, disinfection of premises, and avoidance of stress factors [18, 19]. Free-range flocks require additional vigilance because of increased contact with wildlife reservoirs [19, 10].

Vaccination

Both inactivated (bacterin) and live attenuated vaccines are available. Inactivated vaccines formulated with adjuvants such as oil emulsions or gel 01 hydrogel induce humoral immunity and reduce mortality [28, 29]. Gamma-irradiated vaccines represent a promising approach, providing strong antibody and Th1/Th2 cytokine responses without the risks of chemical inactivation [30, 31]. Subunit vaccines based on lipoprotein E (PlpE) with flagellin adjuvant enhance immunogenicity [32, 15]. Live attenuated strains, developed through serial passage (e.g., PMZ8 in ducks), offer mucosal protection but may retain residual virulence [7]. Truncated LPS mutant vaccines have demonstrated efficacy in ducks by limiting bacterial colonization [33]. It is critical to differentiate field strain infection from vaccine-induced reactions using genotyping methods [22].

Antimicrobial Alternatives

Probiotic mixtures containing Lactobacillus and Bacillus species have been shown to reduce fowl cholera mortality in broilers by modulating the gut microbiota and enhancing innate immunity [27].

Fowl Cholera in Hindi

In the Indian subcontinent, fowl cholera is referred to as मुर्गी हैजा (Murghi Haiza) or फाउल कॉलेरा in Hindi. The disease is recognized as a significant constraint in village and smallholder poultry systems. Outbreaks are often seasonal and associated with monsoon rains. Recent studies on Indian native Nicobari chicken breeds reveal differential host immune responses and inherent disease tolerance that may inform future breeding strategies [34]. Genomic characterization of MDR isolates from Bangladesh underscores the regional importance of understanding genetic diversity for effective control [11]. The availability of affordable diagnostic tests and vaccines remains a challenge in rural areas where the term fowl cholera in Hindi is commonly used by local farmers.

Conclusion

Fowl cholera (avian pasteurellosis) remains a major cause of morbidity and mortality in poultry flocks globally. The causative agent, Pasteurella multocida, exhibits extensive genomic and serotypic diversity, complicating diagnosis and control. Molecular tools, including PCR, LAMP, and whole-genome sequencing, have enhanced our ability to rapidly identify outbreaks, track antimicrobial resistance, and differentiate field strains from vaccine isolates. Effective management requires an integrated approach combining biosecurity, targeted antimicrobial therapy guided by susceptibility testing, and strategic use of inactivated or live-adjuvanted vaccines. Continued research into novel vaccine platforms (e.g., gamma-irradiated, subunit) and non-antibiotic alternatives such as probiotics and phytogenic compounds is essential to mitigate the impact of this disease.

Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.

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