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

Etiology

Fowl cholera is a contagious bacterial disease of domestic and wild birds caused by Pasteurella multocida, a Gram-negative, non-motile, facultatively anaerobic coccobacillus belonging to the family Pasteurellaceae [1, 2]. The bacterium produces a polysaccharide capsule that is a critical virulence factor, and strains are classified into five capsular serogroups (A, B, D, E, F) and 16 somatic lipopolysaccharide (LPS) serotypes [3, 4]. In poultry, capsular serogroup A and somatic serotypes 1, 3, and 4 are most frequently associated with acute fowl cholera [5, 6, 7]. The complete genome sequences of P. multocida isolates representing all LPS outer core loci have been determined, revealing substantial genetic diversity in the genes governing LPS biosynthesis [4]. Phase variation in glycosyltransferase genes contributes to LPS heterogeneity and is linked to outbreaks on free-range layer farms [8]. The virulence repertoire includes the Pasteurella multocida toxin (PMT), a potent mitogenic protein that activates cellular signaling pathways, though its role in avian disease is less pronounced than in porcine atrophic rhinitis [9]. Filamentous hemagglutinin (FhaB1) is not essential for pathogenesis in turkey poults [10], whereas hyaluronic acid capsule production, regulated by the stringent response via the hyaD gene, is a key determinant of virulence [11, 12]. The stringent response negatively regulates capsule production, thereby modulating immune evasion [12]. Isolates from poultry frequently carry multiple antimicrobial resistance genes and virulence-associated genes, including those encoding adhesins, iron acquisition systems, and toxins [3, 13]. Capsular type B:2 strains with multidrug resistance have been reported in Bangladesh [13].

Epidemiology

Fowl cholera occurs worldwide and affects chickens, turkeys, ducks, geese, and many wild bird species [1, 14, 15]. Turkeys are particularly susceptible to acute outbreaks, often with high morbidity and mortality [14, 6]. In commercial slow-growing broiler chickens, mortality can reach 100% in acute cases [5]. Coinfection with Mycoplasmoides gallisepticum has been associated with very high mortality in turkey flocks [14]. The bacterium is shed in oral and nasal secretions and feces, and transmission occurs via direct contact, contaminated feed, water, and fomites [16, 17]. Asymptomatic carrier birds, including recovered birds and healthy birds harboring P. multocida in the pharyngeal tonsils, serve as reservoirs [18, 15]. Wild waterbirds may act as a source of infection for poultry, as demonstrated by the widespread sequence type ST20 in Australian poultry farms that also infects wild waterbirds [15]. Land cover characteristics, such as proximity to water bodies and forested areas, influence the occurrence of fowl cholera [19]. Compartmental models of transmission dynamics indicate that infection spreads rapidly through susceptible flocks in the absence of control measures [16]. Stress factors including overcrowding, poor ventilation, nutritional deficiencies, and concurrent infections predispose birds to clinical disease [17, 18].

Clinical Signs

The clinical presentation of fowl cholera ranges from peracute to chronic forms, depending on the virulence of the strain, host susceptibility, and route of exposure [5, 20, 17]. Peracute disease is characterized by sudden death in apparently healthy birds, often with no premonitory signs [5]. Acute fowl cholera presents with fever, depression, anorexia, ruffled feathers, cyanosis of the comb and wattles, oral and nasal mucous discharge, dyspnea, and diarrhea [20, 17]. Swelling of the wattles and conjunctivitis are common [5, 17]. Mortality can exceed 50% in untreated flocks [16, 18]. Chronic infection manifests as localized lesions including wattle edema, swollen joints (arthritis), sternal bursitis, torticollis due to otitis media, and conjunctivitis [17, 18]. In layers, a drop in egg production is observed [17]. Ducklings infected with serogroup A strains exhibit similar signs, including lameness and neurological symptoms [7].

Pathology

Gross pathological findings in acute fowl cholera include generalized congestion and hemorrhage on serosal surfaces, petechiae on the epicardium and abdominal fat, hepatomegaly with multiple pale necrotic foci, splenomegaly, and pulmonary edema [5, 6, 17]. The liver often shows pinpoint necrotic foci, a hallmark of the disease [5, 17]. Hemorrhagic enteritis and fibrinous pericarditis may be present [6]. Chronic cases show caseous exudate in wattles, joints, and tendon sheaths, and occasionally fibrinopurulent meningitis [18]. Histologically, acute lesions consist of multifocal hepatic necrosis with heterophilic infiltration, fibrin thrombi in small blood vessels, and bacterial emboli in parenchymatous organs [5, 6]. The presence of many short, bipolar-staining rods (safety-pin appearance) in tissue impression smears is suggestive of P. multocida infection [17].

Diagnosis

Definitive diagnosis of fowl cholera requires laboratory confirmation of P. multocida infection through bacterial isolation, molecular detection, or serological methods. A diagnostic algorithm is presented in Figure 1.

flowchart TD
    A["Suspected FC based on clinical signs & lesions"], > B["Collect samples: liver, spleen, bone marrow, wattle exudate"]
    B, > C{"Direct smear: Gram-negative coccobacilli, bipolar staining?"}
    C, >|Positive| D["Culture on blood agar & MacConkey agar, 37°C, 24h"]
    C, >|Negative| E["Consider other diagnoses (CRD, coryza, salmonellosis, avian influenza)"]
    D, > F{"Colonies: smooth, iridescent, non-hemolytic?"}
    F, >|Positive| G["Biochemical confirmation: oxidase +, catalase +, indole +"]
    F, >|Negative| H["Re-culture or use selective media"]
    G, > I{"Molecular confirmation"}
    I, > J["PCR: kmt1 gene or capsular typing PCR"]
    I, > K["LAMP assay: faster, same sensitivity"]
    J, > L["Final diagnosis: positive for P. multocida"]
    K, > L
    G, > M["Serology: indirect ELISA for flock antibody monitoring"]
    M, > L
    L, > N["Antimicrobial susceptibility testing (disk diffusion / MIC)"]
    N, > O["Implement control measures: biosecurity, vaccination, treatment"]

Figure 1. Diagnostic workflow for suspected fowl cholera in poultry.

Bacterial Isolation and Identification

P. multocida can be isolated from liver, spleen, bone marrow, heart blood, or wattle exudate of acutely ill or dead birds [20, 17]. Samples are streaked onto blood agar (5% sheep blood) and MacConkey agar and incubated at 37°C for 24 hours. Colonies are smooth, grayish, and non-hemolytic on blood agar; the bacterium does not grow on MacConkey agar [17]. Biochemical tests reveal positive reactions for oxidase, catalase, indole, and nitrate reduction, and fermentation of glucose and sucrose but not lactose [17]. Bipolar staining with methylene blue or Wright stain is characteristic [20, 17].

Molecular Detection

Conventional PCR targeting the kmt1 gene (species-specific) and capsular typing PCR are widely used for confirmation and typing [3, 21, 13]. Loop-mediated isothermal amplification (LAMP) assays offer comparable sensitivity and specificity to PCR with reduced turnaround time and simpler equipment requirements, making them suitable for on-site diagnosis [21]. Genomic profiling and whole-genome sequencing have been applied to characterize outbreak strains and track transmission dynamics [1, 4, 15]. Single nucleotide polymorphism analysis can differentiate closely related isolates [15].

Serological Assays

Indirect enzyme-linked immunosorbent assays (ELISAs) are employed for flock-level seromonitoring, particularly to assess vaccine-induced antibody responses [22, 23]. In-house ELISAs using whole-cell antigens have been developed and optimized for chickens, showing good sensitivity and specificity [22]. Subunit vaccines incorporating lipoprotein E with flagellin adjuvant have been evaluated using ELISA for immunogenicity [23, 24]. Gamma-irradiated vaccines also elicit measurable antibody responses detected by ELISA [25, 26].

Treatment

Antimicrobial therapy is most effective if initiated early in the course of disease, preferably based on in vitro susceptibility testing [20, 17, 18]. Commonly used antimicrobials include oxytetracycline, chlortetracycline, sulfonamides, trimethoprim-sulfonamide combinations, penicillin, and fluoroquinolones [20, 17]. However, antimicrobial resistance is widespread: studies from Ethiopia, Bangladesh, China, and Egypt have reported high frequencies of resistance to tetracyclines, sulfonamides, aminoglycosides, and β-lactams [2, 20, 3, 17, 13]. Multidrug-resistant (MDR) strains harboring resistance genes such as blaROB-1, tetH, tetB, strA, strB, and sulII are common [2, 3]. The emergence of MDR P. multocida complicates therapeutic management and underscores the need for routine antimicrobial susceptibility surveillance [2, 17]. Alternative control strategies include the use of probiotics: novel multi-strain probiotics have been shown to reduce fowl cholera mortality in broilers, likely through competitive exclusion and immune modulation [27]. Plant-derived compounds, such as wild Egyptian artichoke extract, exhibit in vitro antibacterial activity against P. multocida [28].

Control

Biosecurity and Management

Effective control of fowl cholera relies on stringent biosecurity measures to prevent introduction and spread of P. multocida [16, 18]. Key practices include all-in/all-out flock management, thorough cleaning and disinfection of houses and equipment, control of rodent and feral bird access, and segregation of different age groups [18]. Reduction of environmental stressors such as overcrowding, poor ventilation, and nutritional deficiencies is critical to reduce susceptibility [18]. Compartmental models suggest that culling of infected flocks and quarantine of contacts can substantially reduce transmission [16].

Vaccination

Vaccination is a cornerstone of fowl cholera control in high-risk areas or flocks with a history of outbreaks [29, 30, 6]. Both bacterins (killed whole-cell vaccines) and live attenuated vaccines are available [29, 31]. Bacterins are typically administered parenterally and provide serotype-specific protection; they require two doses and annual boosters [6, 31]. Adjuvants such as oil-in-water emulsions, aluminum hydroxide, and saponin can enhance immunogenicity [25, 31, 26]. Gamma-irradiated vaccines have been developed as safer alternatives to formalin-killed bacterins, as they preserve antigenic structure while inactivating the pathogen [25, 26]. Hydrogel-based inactivated vaccines incorporating gel 01 hydrogel induce robust immune responses and protection in chickens [30]. Live attenuated vaccines derived from serial passage (e.g., PMZ8 strain in ducks) confer strong protection but carry a risk of reversion to virulence [29]. Subunit vaccines targeting lipoprotein E (PlpE) with signal sequences or flagellin as an adjuvant have shown promise in inducing protective immunity in chickens and turkeys [23, 24]. Additionally, a strain with a truncated LPS outer core has been shown to be immunogenic and protective in ducks [32].

Integrated and Alternative Strategies

Integrated control combining vaccination, biosecurity, antimicrobial stewardship, and health monitoring is recommended [18]. The use of probiotics as feed additives can reduce colonization and mortality [27]. Plant extracts with antibacterial activity represent a potential alternative for treatment or prophylaxis [28]. Farm-level risk assessment and land cover management may help reduce the likelihood of outbreaks [19].

References

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