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

Introduction

Avian cholera, also known as fowl cholera or avian pasteurellosis, is a highly contagious septicemic disease of domestic and wild birds caused by the bacterium Pasteurella multocida [1, 2]. The disease is of major economic importance to the poultry industry worldwide and has been recognized as a significant cause of mortality in wild waterfowl and other avian species [3, 4, 5]. Although the clinical presentation can vary from peracute death to chronic localized infections, the acute form is characterized by rapid onset, high morbidity, and case fatality rates often exceeding 60% [2, 6]. This article provides a detailed, publication-grade review of the etiology, epidemiology, clinical signs, pathology, diagnostics, treatment, and control of avian cholera, with a focus on both commercial poultry and wild bird populations.

Etiology

The Causative Agent: Pasteurella multocida

Pasteurella multocida is a small, gram-negative, nonmotile, facultatively anaerobic coccobacillus that belongs to the family Pasteurellaceae [1, 7]. The organism exhibits bipolar staining with Wright’s or Giemsa stain and grows on blood agar or tryptic soy agar producing smooth, iridescent colonies with a characteristic “mousy” odor [8]. Five capsular serogroups (A, B, D, E, F) are recognized based on capsular polysaccharide antigens, and 16 lipopolysaccharide (LPS) genotypes (L1–L16) have been described [9, 10]. In avian isolates, capsular serogroup A and LPS genotype L1 predominate [9, 10, 7]. The type strain most commonly used for comparative genomics is Pm70, which is avirulent for chickens and turkeys, whereas strains X73 and P1059 are highly virulent [7].

Virulence Factors

The virulence of P. multocida is multifactorial and involves capsular polysaccharides, lipopolysaccharides, outer membrane proteins, adhesins, and iron acquisition systems [11, 9, 7]. The capsular hyaluronic acid (hya) synthesis gene cluster, particularly the hyaD gene, has been shown to be critical for full virulence in avian strains [11]. Deletion of hyaD significantly attenuates the bacterium, and complementation with a point mutant confirms that the enzymatic activity of HyaD is essential for pathogenicity [11]. Additional adhesion factors such as PtfA (type 4 fimbrial subunit), PfhB4 hemagglutinin, and OmpH contribute to colonization and host cell interaction [9, 12, 7]. The PhoP/PhoQ two-component regulatory system modulates resistance to antimicrobial peptides and is required for systemic infection; deletion of phoP increases the median lethal dose by 32- to 154-fold depending on the route of inoculation [13]. The lipopolysaccharide O-antigen and core oligosaccharide structures influence serum resistance and immune evasion [7]. Transcriptome analyses of infected livers have revealed that P. multocida activates the MAPK–NLRP3–GSDMD signaling pathway, leading to pyroptosis and severe hepatic damage [14].

Taxonomy and Subspecies

Avian isolates of P. multocida are often classified into three subspecies: P. multocida subsp. multocida, P. multocida subsp. septica, and P. multocida subsp. gallicida, with subspecies multocida being most frequently recovered from chicken and turkey outbreaks [1, 10]. Multilocus sequence typing (MLST) schemes have identified numerous sequence types (STs) among avian strains, with ST129, ST471, ST231, ST342, and ST355 being reported in different geographic regions [5, 15, 16, 10, 17]. A comprehensive MLST scheme specific for avian P. multocida was developed by Subaaharan et al. [17], enabling high-resolution epidemiological tracking. Whole-genome sequencing has demonstrated that ST20 is widespread in Australian poultry and can also be isolated from wild waterbirds, suggesting spill-over events [5].

Fowl Cholera Bacterial: Why This Disease Is Called Fowl Cholera

The term “fowl cholera” reflects the acute, cholera-like diarrheal and septicemic presentation in birds, but the disease is entirely distinct from human cholera caused by Vibrio cholerae. The phrase “fowl cholera bacterial” emphasizes that the etiologic agent is a bacterium (P. multocida), not a virus. It is also critical to distinguish fowl cholera from chicken pox, a viral disease caused by Avipoxvirus that presents with cutaneous pox lesions rather than systemic septicemia. Similarly, fowl cholera differs from salmonellosis in poultry; although both can cause septicemia, Salmonella infections are often associated with enteritis and are major foodborne pathogens, whereas fowl cholera is primarily a respiratory-septicemic disease with less emphasis on intestinal carriage in poultry [2].

Epidemiology

Host Range and Geographic Distribution

Avian cholera affects a broad range of domestic and wild avian species. In commercial poultry, chickens, turkeys, ducks, geese, and quail are highly susceptible [3, 2, 18]. Turkeys are particularly prone to the peracute and acute forms, while chickens often exhibit acute or chronic disease [7, 19]. In wild birds, waterfowl such as ducks, geese, and swans are the most commonly affected, with large die-offs occurring during migration and winter aggregations [4, 5, 2]. The disease has been reported globally, including in Asia, Europe, North America, Australia, and Africa [20, 5, 15, 16].

Transmission and Reservoirs

P. multocida is transmitted horizontally via direct contact with infected birds, contaminated feed, water, or equipment, and through inhalation of aerosolized bacteria from respiratory secretions [3, 2]. The bacterium can survive for weeks in moist organic material but is readily inactivated by drying and disinfectants. Asymptomatic carriers, including birds that recover from acute infection or have chronic localized infections, serve as important reservoirs [21, 2]. In wild bird populations, wetlands act as environmental reservoirs, and outbreaks are often associated with cold, wet weather that favors bacterial survival [2]. Fowl cholera in hindi is often referred to as “murgi haiwa” or similar regional terms, but the causative agent remains the same. The term “fowl cholera meaning in bengali” is “পোল্ট্রি কলেরা” or “মুরগির কলেরা”, though scientific nomenclature is used in veterinary practice.

Risk Factors and Outbreak Triggers

Stress factors such as overcrowding, poor ventilation, nutritional deficiencies, concurrent infections, and sudden changes in weather increase susceptibility [22, 23]. Co-infections with immunosuppressive viruses like chicken anemia virus (CAV) or avian leukosis virus (ALV) exacerbate disease severity and may trigger outbreaks in otherwise subclinically infected flocks [22, 23]. A “chicken bacteria outbreak” in a region, when identified as P. multocida, requires rapid response. The term “poultry pandemic” is not typically used for fowl cholera because it is enzootic rather than pandemic, but sporadic pandemics in waterfowl have been described. The phrase “avian cholera transmission to humans” is relevant: although rare, P. multocida can infect humans through bites or close contact with infected birds; however, it is not a major zoonotic pathogen and is easily killed by proper cooking. The question “does cooking chicken kill bacteria” is answered affirmatively: adequate heat (above 70°C internal temperature) destroys P. multocida. However, freezing does not reliably kill the bacterium; thus, “freezing chicken kill bacteria” is incorrect. “Chicken feces bacteria” may include P. multocida, but fecal–oral transmission is less important than respiratory or direct contact. “Raw chicken breast bacteria” can include P. multocida, but the most common pathogens on raw poultry are Salmonella and Campylobacter; fowl cholera is not a typical foodborne hazard for humans. The “chicken breast salmonella meme” refers to Salmonella contamination, not fowl cholera.

Clinical Signs

Peracute, Acute, and Chronic Forms

The incubation period is typically 2–9 days [2]. In peracute cases, birds are found dead without premonitory signs, often in good body condition. In acute cases, clinical signs include fever (up to 44°C), depression, ruffled feathers, anorexia, polydipsia, cyanosis of the comb and wattles, profuse greenish to yellowish diarrhea, and respiratory distress with mucoid nasal discharge [2, 24]. Morbidity and mortality can reach 80% or higher in naïve flocks [6]. Chronic fowl cholera may develop after an acute episode or in enzootically infected flocks; it is characterized by localized infections such as swollen wattles, purulent conjunctivitis, sinusitis, arthritis, torticollis (due to middle ear infection), and occasionally lameness [25, 24]. The question “does chicken get bacteria” is answered by the fact that chickens are highly susceptible to P. multocida infection.

Pathology

Gross Lesions

In peracute cases, gross lesions may be absent. In acute septicemic cases, the most consistent lesions are multiple pale to yellowish necrotic foci (1–2 mm) scattered throughout the liver (the classic “pinpoint necrotic foci”) [2, 24]. The spleen is enlarged, dark, and may contain necrotic areas. Petechial hemorrhages are common on the epicardium, coronary fat, serosal membranes, and in the pectoral muscles. Catarrhal or hemorrhagic enteritis is frequently present. In the chronic form, caseous exudate is found in the wattles, sinuses, joints, and tympanic cavity [25, 24].

Histopathology and Cellular Mechanisms

Microscopically, acute cases show multifocal hepatic necrosis with intense infiltration of heterophils and macrophages, vascular congestion, and thrombosis [14]. The liver is the primary target of P. multocida‑induced damage. Recent studies have demonstrated that the bacterium triggers pyroptosis in hepatocytes via the MAPK–NLRP3–GSDMD signaling pathway, leading to release of IL‑1β, IL‑18, and other pro‑inflammatory cytokines, and ultimately hepatocyte lysis [14]. Oxidative stress markers (elevated MDA, H₂O₂) and decreased antioxidant enzyme activities (SOD, GSH‑PX, CAT, T‑AOC) accompany this inflammatory response [14]. Splenic hyperplasia and fibrinoid necrosis are also observed.

Diagnostics

Laboratory Isolation and Identification

Definitive diagnosis of avian cholera relies on isolation of P. multocida from blood, liver, spleen, bone marrow, or exudates. Samples should be collected from fresh carcasses (preferably dead for less than 6 hours) to avoid overgrowth by contaminating bacteria [8, 26]. The organism grows on blood agar or tryptic soy agar under aerobic or microaerophilic conditions at 37°C. Colonies are small (1–2 mm), smooth, and iridescent when viewed with oblique transmitted light [8]. Gram staining reveals gram‑negative coccobacilli with bipolar staining. Biochemical identification includes positive reactions for oxidase, catalase, indole, and fermentation of glucose, sucrose, and mannitol, but no growth on MacConkey agar [1]. Confirmatory tests include the P. multocida‑specific polymerase chain reaction (PCR) targeting the kmtt1 gene or 16S rRNA [23, 26].

Molecular Typing

Capsular serotyping is performed by multiplex PCR for capsular types A, B, D, E, and F [9]. LPS genotyping (L1‑L16) is determined by PCR detection of specific LPS biosynthesis genes [10, 7]. MLST based on seven housekeeping genes (e.g., adk, est, pgi, zwf, gdh, pmi, mdh) provides high discriminatory power [17]. Pulsed‑field gel electrophoresis (PFGE) is also used for outbreak investigations [16]. Whole‑genome sequencing (WGS) has become increasingly accessible and allows detection of virulence‑associated genes, antimicrobial resistance determinants, and mobile genetic elements [5, 11, 15, 7]. The following Mermaid diagram illustrates a diagnostic workflow for avian cholera:

flowchart TD
    A[Suspected fowl cholera case], > B[Clinical signs + gross lesions]
    B, > C{Collect samples: liver, spleen, bone marrow, exudates}
    C, > D[Culture on blood agar, 37°C, 18-24 h]
    D, > E[Typical colonies, gram-negative coccobacilli with bipolar staining]
    E, > F[Biochemical identification: oxidase+, catalase+, indole+]
    F, > G[Confirmatory PCR: kmtt1 or 16S rRNA]
    G, > H[Positive for P. multocida]
    H, > I[Further characterization]
    I, > J[Multiplex PCR for capsular serogroup & LPS genotype]
    I, > K[MLST or WGS for ST and AMR genes]
    I, > L[Antimicrobial susceptibility testing (disk diffusion or MIC)]

Serology

ELISA and hemagglutination inhibition (HI) tests are available for detecting antibodies against P. multocida in serum [27, 28, 29, 6, 24]. These assays are primarily used for vaccine potency evaluation rather than for clinical diagnosis in acute outbreaks, because seroconversion may occur late in the disease course. Commercially available ELISA kits (not named here) detect antibodies against whole‑cell antigens or specific recombinant proteins such as PlpE [30, 31].

Fowl Cholera Is Caused by Which Bacteria: A Clarification

The question “fowl cholera is caused by which bacteria” has a single answer: Pasteurella multocida. In contrast, “ground chicken bacteria” or “raw chicken breast bacteria” commonly refer to Salmonella, Campylobacter, or Escherichia coli, not fowl cholera. However, P. multocida can be found in the respiratory tract of healthy birds and may contaminate raw meat if slaughter hygiene is poor. Proper cooking kills all these bacteria, but freezing does not. Therefore, “what kills chicken bacteria” includes heat (cooking), disinfectants, and antibiotics, but not freezing.

Treatment

Antimicrobial Therapy

Antimicrobial treatment should be initiated promptly in affected flocks, ideally based on culture and susceptibility testing to minimize resistance development. Historically, sulfonamides, tetracyclines, and penicillin were effective, but resistance has emerged worldwide [15, 16, 9, 10]. A study of Malaysian isolates found 100% resistance to erythromycin, 68% to streptomycin, and 37% to tetracycline and enrofloxacin [16]. In Chinese avian isolates, florfenicol resistance (65%) and tetracycline resistance (50%) were most common, with floR, sul2, and tet(B) being the most prevalent resistance genes [15]. Multidrug resistance (resistance to three or more classes) was observed in 19.6% of poultry isolates in Brazil [9] and in 71.1% of isolates from southwest China [10]. Gentamicin and amoxicillin remain among the most effective drugs for many isolates [9].

Alternative Therapies: Phage Therapy

Given the rise in antimicrobial resistance, phage therapy has been investigated as an alternative. A lytic phage vB_PmuM_CFP3 (CFP3) was isolated from a chicken farm and shown to effectively lyse P. multocida in vitro, with stability under poultry‑relevant conditions and absence of antibiotic resistance genes [32]. This approach offers a targeted, narrow‑spectrum option that spares the beneficial microbiota, though in‑vivo efficacy studies are still needed [32].

Supportive Care

In addition to antimicrobials, supportive measures include reducing stress, improving ventilation, providing clean water and nutrition, and culling moribund birds. Water‑soluble antibiotics (e.g., tetracyclines, sulfadimethoxine) administered via drinking water or feed are common in commercial flocks.

Control

Biosecurity

Strict biosecurity is the cornerstone of fowl cholera prevention. Measures include all‑in/all‑out management, sanitation of housing and equipment, control of rodents and wild birds, quarantine of new or returning birds, and limiting visitor access. Because wild waterfowl can carry P. multocida and shed it in wetlands, farms in proximity to waterfowl habitats should implement enhanced biosecurity to prevent spill‑over [3, 21, 5].

Vaccination

Inactivated (bacterin) vaccines are widely used to prevent fowl cholera. They are typically administered as oil‑emulsion or aluminum hydroxide‑adjuvanted formulations and require an initial prime and booster dose [27, 28, 29, 6, 24]. The protection level after single dose vaccination averages approximately 44%, rising to 76% after a booster [33]. The addition of recombinant proteins such as PlpE multi‑epitope antigen to the vaccine formulation has been shown to improve protection (80% survival in challenge studies) compared to bacterin alone (70%) or PlpE alone (50%) [30]. Hydrogel‑adjuvanted inactivated vaccines (e.g., Montanide GEL‑02) also reduce residual reactogenicity compared to oil adjuvants [31]. Combined vaccines against fowl cholera and avian influenza (H9N2) have been developed and shown to elicit protective titers against both pathogens simultaneously in chickens, broiler breeders, and turkeys [27, 28, 29]. The phoP‑deleted live attenuated mutant has demonstrated safety and partial efficacy (54.5% protection) in ducks [13]. Research into subunit vaccines based on recombinant PtfA [12] and other outer membrane proteins continues.

Surveillance and Eradication

Epidemiological surveillance using MLST, PFGE, or whole‑genome sequencing is essential to track strain dissemination and the emergence of antimicrobial resistance [5, 15, 16, 10]. In wild bird populations, monitoring die‑offs and sampling wetlands can forecast spill‑over risks to poultry [3, 4, 5, 2]. Eradication is difficult in free‑range or backyard flocks with wild bird contact, but management focuses on depopulation of infected flocks, thorough disinfection, and repopulation with vaccinated birds.

Conclusion

Avian cholera remains a persistent threat to poultry production and wild bird conservation worldwide. The causative agent, Pasteurella multocida, possesses a diverse arsenal of virulence factors and has demonstrated increasing antimicrobial resistance. Accurate diagnosis requires timely culture and molecular characterization, while effective control hinges on robust biosecurity, prudent antimicrobial use, and strategic vaccination. Integration of genomic surveillance with ecological understanding of wild bird reservoirs will be crucial for mitigating the impact of this disease in the context of a changing environment and expanding poultry production.

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