Avian Cholera (Fowl Cholera) in Poultry and Wild Birds: Etiology, Epidemiology, Clinical Signs, Pathology, Diagnosis, Treatment and Control

Introduction

Avian cholera, also termed fowl cholera, is a highly contagious bacterial disease of domestic poultry and wild birds caused by Pasteurella multocida [1, 2]. The disease is characterized by acute septicemia with high morbidity and mortality, although chronic localized infections also occur [3, 2]. P. multocida is a Gram-negative, non‑motile, facultative anaerobic coccobacillus that belongs to the family Pasteurellaceae [34]. The bacterium is classified into five capsular serogroups (A, B, D, E, F) and 16 lipopolysaccharide (LPS) genotypes based on the Heddleston scheme [4, 34]. In poultry and wild birds, serogroup A strains are most frequently isolated, while serogroups B and E are associated with hemorrhagic septicemia in mammals [5, 6]. The disease is of major economic importance to the poultry industry worldwide and also poses a conservation threat to susceptible wild bird populations [7, 6]. This article provides a detailed, evidence‑based review of the etiology, epidemiology, clinical signs, pathology, diagnosis, treatment, and control of avian cholera, with emphasis on recent molecular and immunological advances.

Etiology

The Causative Agent: Pasteurella multocida

Fowl cholera is caused by Pasteurella multocida, a bacterium that exhibits considerable genetic and antigenic diversity [34]. The species is divided into subspecies: P. multocida subsp. multocida, P. multocida subsp. septica, and P. multocida subsp. gallicida [34]. Capsular typing distinguishes five serogroups (A, B, D, E, F), with serogroup A being the predominant cause of avian cholera [5, 6]. LPS typing (Heddleston system) further differentiates 16 serotypes, and specific LPS outer core loci have been fully sequenced [4]. The complete genome sequences of multiple isolates representing all LPS outer core loci are now available, facilitating studies on virulence and vaccine development [4].

Virulence Factors

P. multocida possesses a range of virulence factors that contribute to its pathogenicity. The polysaccharide capsule is a key antiphagocytic factor [34]. LPS is a major endotoxin that triggers inflammatory responses [1, 8]. The bacterium also produces filamentous hemagglutinin (FhaB), although the fhaB1 gene has been shown not to be essential for pathogenesis in turkey poults [9]. Other virulence‑associated genes include those encoding outer membrane proteins (e.g., OmpH), lipoproteins (PlpE, VacJ), and iron‑acquisition systems [10, 11, 12]. The hyaluronidase gene hyaD contributes to virulence by facilitating tissue invasion [13]. Phase variation in glycosyltransferase genes of the LPS biosynthesis locus has been linked to outbreaks on free‑range layer farms, suggesting that LPS structure can modulate host adaptation [14].

Pathogenesis

After inhalation or ingestion, P. multocida colonizes the upper respiratory tract and then invades the bloodstream, leading to septicemia [1, 8]. The bacterium activates the MAPK‑NLRP3‑GSDMD signaling pathway in the liver, inducing pyroptosis in broilers [1]. In ducks, P. multocida causes liver injury through inflammatory, apoptotic, and autophagic pathways [8]. The LPS outer core transferase genes pcgD and hptE contribute differently to virulence in ducks, with pcgD being more critical for systemic infection [35]. A truncated LPS outer core mutant has been shown to be attenuated and immunogenic in ducks [15].

Epidemiology

Host Range and Geographic Distribution

Avian cholera affects a wide range of domestic and wild birds, including chickens, turkeys, ducks, geese, and many waterfowl species [3, 2, 6]. Outbreaks in wild waterbirds can cause mass mortality events [6]. The disease is distributed globally, with reports from all major poultry‑producing regions [5, 29, 31]. In Australia, P. multocida sequence type 20 (ST20) is widespread in poultry farms and may also infect wild waterbirds, indicating potential spillover [6]. In Bangladesh, multidrug‑resistant P. multocida type B:2 has been reported in fowl cholera outbreaks [29]. In China, isolates from ducks between 2017 and 2019 showed high genetic diversity [31].

Transmission and Risk Factors

Transmission occurs via direct contact with infected birds, contaminated feed, water, or fomites [7]. Carrier birds (recovered or asymptomatic) are important reservoirs [7]. Stress factors such as overcrowding, poor ventilation, nutritional deficiencies, and concurrent infections increase susceptibility [3, 7]. Coinfection with Mycoplasmoides gallisepticum has been associated with high mortality in commercial turkeys [3]. Mathematical models of cholera transmission in poultry farms highlight the importance of biosecurity and culling in controlling outbreaks [7].

Zoonotic Potential

Avian cholera transmission to humans is rare but documented. P. multocida can cause wound infections, bacteremia, and other opportunistic infections in humans, often following animal bites or scratches [16]. A case of P. multocida bacteremia following a scratch by an adopted Pekin duck has been reported [16]. However, the bacterium is not considered a primary human pathogen, and human‑to‑human transmission is not documented.

Clinical Signs

Acute Form

The acute form is most common and is characterized by sudden death with few premonitory signs [2]. Affected birds may show depression, anorexia, ruffled feathers, cyanosis of the comb and wattles, and oral or nasal discharge [2]. Mortality can reach 100% in susceptible flocks, as reported in commercial slow‑growing broiler chickens [2]. In turkeys, acute fowl cholera can present with high mortality and respiratory distress [3, 5].

Chronic Form

Chronic infections manifest as localized lesions, including swollen joints (arthritis), wattles (wattle edema), sinuses (sinusitis), and conjunctivitis [3]. Chronic fowl cholera may also present as otitis media or pneumonia [3]. These forms are more common in flocks with partial immunity or after antibiotic therapy.

Pathology

Gross Lesions

At necropsy, acute cases show generalized congestion and petechial hemorrhages on serosal surfaces, heart, and abdominal fat [1, 2]. The liver is often enlarged, friable, and may have multiple small necrotic foci [1, 8]. Splenomegaly and pulmonary edema are common [2]. In chronic cases, caseous exudate may be found in joints, wattles, and sinuses [3].

Histopathology

Histologically, acute cases reveal fibrinoid necrosis of blood vessels, multifocal hepatic necrosis, and bacterial emboli in various organs [1, 8]. In the liver, pyroptosis mediated by the MAPK‑NLRP3‑GSDMD pathway is observed [1]. In ducks, hepatic lesions include inflammatory cell infiltration, apoptosis, and autophagy [8].

Diagnosis

Clinical and Necropsy Findings

Presumptive diagnosis is based on clinical signs, high mortality, and characteristic gross lesions [2]. However, definitive diagnosis requires laboratory confirmation.

Bacteriological Culture

P. multocida can be isolated from blood, liver, spleen, bone marrow, or exudates on blood agar or MacConkey agar (it does not grow on MacConkey) [5, 17]. Colonies are small, gray, and mucoid. Identification is confirmed by Gram stain (Gram‑negative coccobacillus), oxidase and catalase positivity, and biochemical tests [17].

Molecular Diagnostics

PCR assays targeting the kmt1 gene (species‑specific) and capsular typing genes are widely used [17, 18]. Loop‑mediated isothermal amplification (LAMP) assays have been developed and compared with PCR for detecting P. multocida in poultry, showing comparable sensitivity and specificity with simpler equipment requirements [18]. Real‑time PCR can also quantify bacterial load. Whole‑genome sequencing provides detailed information on serotype, virulence genes, and antimicrobial resistance determinants [19, 4, 34].

Serological Tests

Indirect ELISA kits for detecting antibodies against P. multocida in chickens have been developed and optimized [20]. These are useful for monitoring vaccine responses and flock exposure. However, serology is less commonly used for acute diagnosis.

Differential Diagnosis

Avian cholera must be differentiated from other causes of acute septicemia in poultry, including highly pathogenic avian influenza (HPAI), Newcastle disease, avian colibacillosis, and avian coryza (infectious coryza caused by Avibacterium paragallinarum). The latter is particularly relevant when respiratory signs and facial swelling are present. For a detailed discussion of avian coryza, see the article on Avian Coryza in Poultry. Other differentials include fowl typhoid, pullorum disease, and pasteurellosis caused by other Pasteurella species.

Diagnostic Workflow

The following Mermaid diagram illustrates a recommended diagnostic algorithm for suspected fowl cholera outbreaks.

flowchart TD
    A[Clinical suspicion: acute mortality, cyanosis, respiratory signs], > B[Necropsy: petechiae, hepatic necrosis, splenomegaly]
    B, > C[Collect samples: liver, spleen, bone marrow, exudates]
    C, > D[Gram stain: Gram-negative coccobacilli]
    D, > E[Culture on blood agar, 37°C, 24-48h]
    E, > F[Colony morphology: small, gray, mucoid]
    F, > G[Biochemical identification: oxidase+, catalase+, indole+]
    G, > H[Confirm by PCR: kmt1 gene]
    H, > I[Capsular typing PCR (A, B, D, E, F)]
    I, > J[Antimicrobial susceptibility testing]
    J, > K[Optional: whole-genome sequencing for epidemiology]

Treatment

Antimicrobial Therapy

Treatment of affected flocks relies on prompt administration of antibiotics. Historically, tetracyclines, sulfonamides, and penicillin have been used [19]. However, antimicrobial resistance is a growing concern. A study on avian P. multocida isolates revealed multidrug resistance and the presence of resistance genes such as tet, bla, and sul [19]. Another study from Egypt and Saudi Arabia reported multidrug‑resistant strains from poultry and rabbits with diverse resistance genes [17]. In Bangladesh, multidrug‑resistant type B:2 strains have been identified [29]. Therefore, antimicrobial susceptibility testing is essential before selecting therapy [19, 17]. Commonly used antibiotics include oxytetracycline, enrofloxacin, and florfenicol, but resistance patterns vary regionally [19, 17].

Alternative and Adjunctive Therapies

Phage therapy has been explored as an alternative to antibiotics. A phage vB_PmuM_CFP3 specific for P. multocida showed lytic activity and potential for therapeutic application [21]. Probiotics have also been investigated; a novel multi‑strain probiotic reduced fowl cholera mortality in broilers [33]. Plant extracts, such as wild Egyptian artichoke extract, have demonstrated in vitro antibacterial activity against P. multocida [22]. These approaches may complement conventional therapy but require further field validation.

Control

Biosecurity and Management

Prevention relies on strict biosecurity measures: all‑in/all‑out production, rodent and wild bird control, disinfection of premises, and quarantine of new birds [7]. Carrier birds should be culled. Freezing chicken does not reliably kill P. multocida; the bacterium can survive freezing temperatures, so contaminated carcasses remain a source of infection. Proper cooking kills the bacteria, but freezing is not a control measure.

Vaccination

Vaccination is a cornerstone of fowl cholera control. Both inactivated bacterins and live attenuated vaccines are available [23, 24, 5, 25]. Inactivated vaccines often require adjuvants to enhance immunogenicity. Gel 01 hydrogel‑adjuvanted inactivated vaccine has shown immunoprotective effects in chickens [24]. Aluminum hydroxide‑adjuvanted vaccines incorporating recombinant PlpE multi‑epitope protein also provide protection [10]. Gamma‑irradiated fowl cholera vaccines, formulated with various adjuvants, induced antibody responses and cytokine expression in chickens [25]. A gamma‑irradiated mucosal vaccine has been proposed as a safe and effective candidate for chicken immunization [28]. Live attenuated vaccines, such as the serial passage‑derived strain PMZ8 in ducks, offer the advantage of mucosal immunity [23]. Subunit vaccines based on lipoproteins (PlpE, VacJ) and outer membrane proteins (OmpH) have been evaluated in ducks and chickens [11, 26, 12]. Flagellin has been used as an adjuvant to enhance the immunogenicity of a PlpE subunit vaccine [26]. Outer membrane vesicles displaying PlpE protein have also shown immunoprotective efficacy [11]. The choice of vaccine depends on the serotype involved, production system, and regulatory approval.

Integrated Control Strategies

A combination of vaccination, biosecurity, and antimicrobial stewardship is recommended. Mathematical modeling suggests that culling infected flocks and reducing contact rates are effective in controlling outbreaks [7]. In wild bird populations, management is challenging; reducing congregation at feeding sites and minimizing contact with domestic poultry can help [6].

Conclusion

Avian cholera remains a significant threat to poultry production and wild bird conservation worldwide. Advances in genomics, immunology, and diagnostics have deepened our understanding of P. multocida pathogenesis and epidemiology. The emergence of antimicrobial resistance underscores the need for prudent antibiotic use and alternative control strategies. Vaccination, biosecurity, and rapid molecular diagnosis are essential tools for managing this disease. Future research should focus on cross‑protective vaccines, phage therapy, and the ecology of P. multocida in wild bird reservoirs.

References

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