Fowl Cholera in Poultry: Etiology, Clinical Signs, Diagnostics, and Control
Etiology
Fowl cholera is a highly contagious septicemic bacterial disease of poultry and wild birds caused by the gram-negative coccobacillus Pasteurella multocida [1, 2]. The organism is a nonmotile, facultative anaerobe that exhibits bipolar staining with methylene blue or Giemsa stain [3, 4]. P. multocida is classified into five capsular serogroups (A, B, D, E, F) and 16 lipopolysaccharide (LPS) genotypes based on antigenic differences [5, 6]. In poultry, capsular serogroup A is the most prevalent cause of fowl cholera, although serogroups B and D have also been implicated in outbreaks [7, 8]. The bacterium produces a range of virulence factors including a polysaccharide capsule, fimbriae, adhesins, iron acquisition proteins, outer membrane proteins, sialidases, and superoxide dismutases [4, 9]. These factors facilitate colonization, immune evasion, and systemic dissemination within the host [10, 11].
The pathogenicity of P. multocida is multifactorial. The capsule, particularly the hyaluronic acid capsule of serogroup A, inhibits phagocytosis and complement-mediated killing [12, 13]. Fimbrial proteins such as PtfA and FimA mediate adherence to respiratory epithelium [14, 15]. Iron acquisition systems encoded by genes such as exbB, hgbB, and fur allow the bacterium to scavenge iron from host transferrin and hemoglobin [4, 16]. Sialidases NanB and NanH cleave sialic acid from host glycoproteins, exposing receptors for bacterial adhesion and providing a carbon source [17, 18]. The absence of the toxA gene in most avian isolates distinguishes them from porcine toxigenic strains [4, 19].
Epidemiology
Fowl cholera occurs worldwide and affects chickens, turkeys, ducks, geese, and numerous wild avian species [20, 21]. Turkeys are particularly susceptible, often experiencing higher morbidity and mortality than chickens [22, 23]. The disease is most commonly observed in adult birds, although outbreaks in growing birds are not uncommon [24, 25]. Transmission occurs horizontally through direct contact with infected birds, ingestion of contaminated feed or water, and inhalation of aerosolized respiratory secretions [26, 27]. Chronically infected carrier birds serve as the primary reservoir for introducing the pathogen into naive flocks [28, 29].
Environmental factors significantly influence disease dynamics. Stressors such as overcrowding, poor ventilation, nutritional deficiencies, and concurrent infections predispose birds to clinical disease [30, 31]. Aflatoxin contamination of feed has been shown to impair immune responses and reduce vaccine efficacy against fowl cholera [17, 32]. Mathematical modeling of fowl cholera transmission using a susceptible-exposed-symptomatic-asymptomatic-treated-culled-recovered (SEIATCR) framework has demonstrated that the basic reproduction number (R0) is highly sensitive to transmission rate and vaccine efficacy, with treatment being more effective than culling alone for outbreak control [9, 33].
Prevalence rates vary by region and production system. In a study conducted in Banda Aceh and Aceh Besar, the prevalence of fowl cholera in broiler chickens increased from 8.2% in 2022 to 14.0% in 2024 [6, 34]. In Gondar City, Ethiopia, a 7.69% isolation rate of P. multocida from breeder chickens was reported using phenotypic methods, with 30% of those isolates confirmed by molecular capsular typing [5, 35]. Outbreaks in geographically related poultry flocks have been linked by molecular epidemiological tools, demonstrating clonal spread between farms [13, 21].
Clinical Signs
The clinical presentation of fowl cholera varies with the form of the disease: peracute, acute, or chronic [27, 36]. In peracute cases, birds are found dead with no premonitory signs, often in good body condition [22, 37]. Acute fowl cholera is characterized by fever, depression, anorexia, ruffled feathers, mucoid discharge from the mouth, increased respiratory rate, and profuse watery diarrhea [27, 38]. Cyanosis of the comb and wattles is frequently observed [26, 39]. Mortality can reach 50% or higher in susceptible flocks [9, 40].
Chronic fowl cholera typically follows an acute episode or occurs in flocks with partial immunity [27, 41]. Localized infections manifest as swollen wattles (wattle edema), conjunctivitis, sinusitis, torticollis (twisted neck) due to otitis media or meningitis, and arthritis of the leg or wing joints [6, 42]. Respiratory signs such as rales and dyspnea may be present [27, 43]. In laying hens, a drop in egg production is a common sequela [17, 44].
In turkeys, acute heart rupture secondary to vegetative valvular endocarditis and septic embolization has been documented as an atypical but fatal presentation [26, 45]. Infarcts in the myocardium, kidney, liver, and spleen can result from embolic occlusion of blood vessels [26, 46].
Pathology
Gross pathological lesions in acute fowl cholera reflect a septicemic process. Petechial and ecchymotic hemorrhages are observed on the epicardium, serosal surfaces, and abdominal fat [6, 47]. The liver is enlarged, friable, and studded with multiple pinpoint necrotic foci, a hallmark lesion [27, 48]. The spleen is often swollen and mottled [6, 49]. Pulmonary edema, congestion, and fibrinous pneumonia may be present [6, 50]. In chronic cases, caseous exudate accumulates in the wattles, sinuses, and joints [27, 51].
Histopathological examination reveals vasculitis, thrombosis, and multifocal coagulative necrosis in the liver, spleen, and heart [6, 32]. Hepatic lesions are characterized by infiltration of heterophils and macrophages surrounding necrotic hepatocytes [32, 52]. In a chicken model of fowl cholera, liver injury has been associated with necroptosis and apoptosis pathways, with upregulation of inflammatory cytokines [32, 53]. Tracheal submucosal edema and desquamation of intestinal villi are also observed [6, 54].
Diagnostics
Accurate diagnosis of fowl cholera requires a combination of clinical, pathological, and laboratory methods [7, 55]. Presumptive diagnosis is based on characteristic gross lesions and microscopic observation of bipolar gram-negative rods in tissue smears stained with methylene blue or Giemsa [3, 56].
Bacteriological Isolation and Identification
P. multocida can be isolated from liver, spleen, heart blood, bone marrow, or exudate from affected tissues [5, 57]. Samples are cultured on blood agar or tryptic soy agar supplemented with 5% sheep blood and incubated at 37°C under 5% CO2 for 24 to 48 hours [4, 58]. Colonies are small, gray, mucoid, and nonhemolytic [59, 60]. Biochemical identification is based on oxidase positivity, catalase positivity, indole production, and lack of hemolysis on MacConkey agar [5, 61].
Molecular Diagnostics
Conventional and real-time polymerase chain reaction (PCR) assays targeting capsular and LPS genes provide definitive identification and serotyping [4, 7]. Capsular serogroup-specific primers for hyaD/hyaC (serogroup A), bcbD (serogroup B), dcbF (serogroup D), ecbJ (serogroup E), and fcbD (serogroup F) are widely used [5, 62]. LPS genotyping targets outer core biosynthesis loci [22, 63]. Multilocus sequence typing (MLST) and random amplification of polymorphic DNA (RAPD) analysis enable strain-level discrimination for epidemiological investigations [13, 18]. Whole-genome sequencing has revealed the presence of virulence factor genes and antimicrobial resistance genes in field isolates [18, 23].
Serological Assays
Enzyme-linked immunosorbent assays (ELISAs) are used to measure serum IgG and mucosal IgA responses following vaccination or natural infection [10, 20]. Indirect ELISAs using whole-cell or outer membrane protein antigens are common [14, 64]. Hemagglutination tests have also been employed to monitor post-vaccination antibody titers [17, 65].
Advanced Data Mining Approaches
Recent studies have applied machine learning algorithms to predict fowl cholera infection status using variables such as bird age, vaccination history, environmental conditions, and clinical symptoms [2, 66]. Logistic regression, random forest, and gradient boosting models have been evaluated, with random forest achieving 94.6% accuracy in one dataset [2, 67]. These computational tools offer scalable diagnostic frameworks for poultry health management [2, 68].
flowchart TD
A[Clinical Signs and Mortality], > B[Postmortem Examination]
B, > C{Gross Lesions Present?}
C, >|Yes: Liver Necrosis, Hemorrhages| D[Tissue Smear with Methylene Blue]
C, >|No| E[Consider Other Diagnoses]
D, > F[Bipolar Gram-Negative Rods?]
F, >|Yes| G[Bacterial Culture on Blood Agar]
F, >|No| H[PCR or Histopathology]
G, > I[Biochemical Identification]
I, > J[PCR Capsular Typing]
J, > K[MLST or WGS for Epidemiology]
H, > L[Histopathology: Vasculitis, Necrosis]
H, > M[PCR from Tissue]
M, > J
Treatment
Antimicrobial therapy is indicated in the early stages of an outbreak to reduce mortality [5, 69]. P. multocida is generally susceptible to penicillin, ampicillin, norfloxacin, florfenicol, and ceftiofur [5, 70]. However, resistance to fluoroquinolones (levofloxacin, ciprofloxacin) and tetracyclines has been documented in some isolates [4, 71]. Multidrug-resistant strains, particularly those of capsular serogroup B:2, have been reported in Bangladesh and other regions [18, 72]. Antibiogram profiling using disk diffusion or broth microdilution methods is recommended to guide antibiotic selection [5, 73].
Bacteriophage lysates have been investigated as an alternative therapeutic approach, demonstrating lytic activity against P. multocida in vitro and in vivo [8, 74]. Probiotic supplementation with multi-strain formulations containing Lactobacillus plantarum, L. fermentum, Pediococcus acidilactici, Enterococcus faecium, and Saccharomyces cerevisiae has been shown to reduce intestinal P. multocida load, improve growth performance, and attenuate inflammatory responses in broilers [16, 75].
Control
Biosecurity
Strict biosecurity measures are essential for preventing the introduction and spread of P. multocida [25, 76]. These include all-in-all-out production systems, disinfection of footwear and equipment, rodent and wild bird control, and quarantine of newly introduced birds [31, 77]. Carrier birds should be identified and removed from the flock [28, 78].
Vaccination
Vaccination is the cornerstone of fowl cholera control in endemic areas [1, 79]. Inactivated (bacterin) vaccines are widely used and are typically administered intramuscularly in two doses [3, 80]. However, protection is often serotype-specific and of limited duration [10, 81]. Formalin-inactivated vaccines have been associated with variable efficacy in field settings [10, 20].
Gamma-irradiated P. multocida vaccines have emerged as a promising alternative, inducing both systemic IgG and mucosal IgA responses [10, 20]. In chickens, intranasal and intraocular administration of gamma-irradiated vaccines formulated with Montanide Gel 01 PR adjuvant conferred 100% protection against homologous lethal challenge, compared to 85% protection with formalin-inactivated vaccine given intramuscularly [20, 82]. Gamma-irradiated vaccines also elicited a Th1-dominant response with high fold changes in IFN-gamma, IL-6, and IL-12p40 mRNA transcripts [10, 83].
Bivalent vaccines combining inactivated P. multocida with avian influenza virus antigens have been developed and evaluated for immunopotential in poultry [1, 3]. Iron-inactivated vaccines adjuvanted with iron (III) have demonstrated protective antibody titers equivalent to commercial oil emulsion vaccines in backyard chickens [14, 84]. Biofilm-based vaccines have also been compared with conventional bacterins, showing enhanced immunogenicity in layer birds [12, 85].
Integrated Control Strategies
A comprehensive control program combines vaccination, antimicrobial therapy, biosecurity, and management practices to reduce stress [9, 86]. Culling of clinically affected birds and proper disposal of carcasses limit environmental contamination [9, 87]. Monitoring of feed for aflatoxin contamination is critical, as mycotoxins impair vaccine-induced immunity [17, 88]. Mathematical modeling indicates that treatment and vaccination are more effective than culling alone in reducing the basic reproduction number of fowl cholera [9, 89].
Fowl Cholera in Hindi
Fowl cholera, known as "मुर्गी हैजा" (Murghi Haiza) in Hindi, is a bacterial disease caused by Pasteurella multocida that affects domestic poultry and wild birds. The term "fowl cholera bacterial" emphasizes the bacterial etiology of this septicemic disease. In Hindi-speaking regions, the disease is recognized by high mortality, liver necrosis, and hemorrhagic lesions. Diagnostic approaches include bacterial culture and PCR, while control relies on vaccination and biosecurity.
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