Fowl Cholera in Broilers: Etiology, Clinical Signs, and Control

Introduction

Fowl cholera is a highly contagious bacterial disease of poultry caused by Pasteurella multocida, leading to significant economic losses in the broiler industry worldwide [1, 35]. The disease manifests in acute, subacute, and chronic forms, with mortality rates that can approach 100% in naive flocks [2, 3]. In broiler production systems, fowl cholera has been increasingly reported, particularly in free-range and organic operations [4, 35]. This review provides a detailed examination of the etiology, epidemiology, clinical presentation, pathological features, diagnostic methods, treatment options, and control measures for fowl cholera in broilers, drawing exclusively on the peer-reviewed literature listed in the references.

Etiology

Pasteurella multocida is a Gram-negative, nonmotile, facultatively anaerobic coccobacillus belonging to the family Pasteurellaceae [35]. The bacterium is classified into capsular serogroups (A, B, D, E, F) and somatic lipopolysaccharide (LPS) serotypes (1 through 16) using the Heddleston scheme [5, 6]. In broilers, the most commonly isolated capsular type is A, and predominant somatic serotypes include 1, 3, and 4 [1, 7, 5]. The virulence of P. multocida strains varies considerably, and a pathogenicity index has been developed to classify isolates based on mortality and lesion scores in one-day-old broilers [8]. The bacterium produces a range of virulence factors, including a polysaccharide capsule, LPS, outer membrane proteins, and fimbriae, which facilitate adhesion, invasion, and evasion of host immune responses [9, 10]. Capsular type A strains are particularly associated with fowl cholera in chickens and ducks, as demonstrated by immunohistochemical detection of the capsule in infected tissues [9]. Molecular typing methods such as multilocus sequence typing (MLST) and restriction enzyme analysis have been used to characterize outbreak strains, revealing sequence types like ST134, ST366, and ST374 in Korean layer flocks, with some STs shared between chickens and ducks [6]. Fingerprint profiles such as HhaI 0001 in serovar 1 isolates have been linked to severe outbreaks in slow-growing broilers [2].

Epidemiology

Fowl cholera in broilers occurs worldwide, with prevalence rates varying by region and production system [11, 28]. A longitudinal study in Banda Aceh and Aceh Besar reported prevalence rates of 8.2%, 11.2%, and 14.0% over three consecutive years, indicating an upward trend [11]. The disease is more common in free-range and organic broiler operations due to increased exposure to carrier birds, wildlife, and environmental contamination [4, 12, 35]. Outbreaks can be acute, with mortality reaching 100% in slow-growing broiler flocks, as documented in a case where a flock of 25,000 birds experienced complete mortality by 14 weeks of age [2]. Risk factors include poor biosecurity, presence of rodents or wild birds, and stress from overcrowding or temperature fluctuations [4, 10, 34]. Carrier birds, including recovered chickens and other avian species, serve as reservoirs for P. multocida, perpetuating cycles of infection [10, 28]. Molecular epidemiological studies have demonstrated that outbreak strains can persist on farms and spread between flocks through contaminated equipment or personnel [12, 6]. In Indonesia, P. multocida serotype A was isolated from broilers with lameness and respiratory distress, and the isolate was pathogenic to mice [7]. Similarly, outbreaks in Korea have been characterized by chronic fowl cholera in broiler breeders, with lameness and swollen joints as prominent signs [13, 14]. The epidemiology of fowl cholera in broilers is closely linked to the presence of other bacterial infections, as mixed infections with Escherichia coli can exacerbate disease severity [15, 35].

Clinical Signs

The clinical presentation of fowl cholera in broilers varies with the form of the disease. In acute cases, birds may die suddenly without premonitory signs, often with high morbidity and mortality within 24 to 48 hours [1, 2]. Affected broilers show depression, anorexia, fever, increased thirst, and mucoid or bloody diarrhea [1, 11, 16]. Respiratory signs such as dyspnea, coughing, and rales are common due to pulmonary congestion and edema [11, 3]. Cyanosis of the comb and wattles may be observed [2]. In peracute outbreaks, mortality can reach 100% before clinical signs are recognized, as reported in slow-growing broilers [2]. Subacute and chronic forms are characterized by localized infections including arthritis, tenosynovitis, and swollen joints, leading to lameness and reluctance to move [11, 16, 13]. Wattle edema and facial cellulitis are also described in chronic cases [34]. In broiler breeders that survive acute infection, chronic fowl cholera may present as purulent conjunctivitis, sinusitis, and abscesses in the wattles and footpads [13, 14]. The clinical signs of fowl cholera can resemble those of other bacterial diseases such as avian colibacillosis and salmonellosis, necessitating laboratory confirmation [29, 35]. Experimental challenge studies in broilers have shown that oral inoculation with P. multocida (10^8 CFU/mL) reproduces clinical signs including depression, ruffled feathers, and mortality within 72 hours [17]. Probiotic supplementation has been shown to attenuate these clinical signs significantly [17].

Pathology

Gross lesions in acute fowl cholera are primarily those of a septicemia. Petechial and ecchymotic hemorrhages are found on the heart, liver, spleen, and serosal surfaces [1, 11, 2]. The liver is often enlarged, friable, and covered with multiple pinpoint foci of necrosis, giving a mottled appearance [11, 16]. The spleen is similarly mottled and enlarged [2]. Lungs are congested, edematous, and may show consolidation [11, 2]. Ascites and hydropericardium are common in acute cases [11, 34]. In chronic forms, fibrinous pericarditis, airsacculitis, and arthritis with purulent exudate in joint spaces are observed [11, 13]. Histopathological examination reveals multifocal coagulative necrosis in the liver, with infiltration of heterophils and macrophages [11, 9]. Vasculitis with fibrinoid necrosis of vessel walls is a hallmark finding [11]. Pulmonary lesions include congestion, edema, and proliferation of bacterial colonies within alveolar capillaries [2]. Splenic necrosis with depletion of lymphoid follicles is frequently noted [2]. In the trachea, submucosal edema and heterophilic infiltration are present [11]. Immunohistochemical staining confirms the presence of P. multocida capsular type A antigen within necrotic foci and vascular lesions [9]. The severity of lesions correlates with the pathogenicity index of the infecting strain [8].

Diagnostics

A definitive diagnosis of fowl cholera requires isolation and identification of P. multocida from affected tissues [7, 28]. Samples of liver, spleen, heart blood, or bone marrow are collected aseptically and cultured on blood agar or selective media such as MacConkey agar, on which P. multocida does not grow [7]. Colonies are identified by Gram stain (Gram-negative coccobacilli), positive catalase and oxidase tests, and biochemical profiles [7, 28]. Molecular confirmation is achieved through polymerase chain reaction (PCR) targeting the P. multocida-specific gene, and capsular typing is performed using multiplex PCR for serogroups A, B, D, E, and F [6, 9]. LPS serotyping using the Heddleston scheme can be done with antisera for somatic antigens [5]. Quantitative real-time PCR (qPCR) provides rapid detection and quantification of P. multocida in clinical samples [2]. Enzyme-linked immunosorbent assay (ELISA) is used to detect antibodies against P. multocida in serum, which is valuable for vaccine response monitoring [26, 27, 32]. Histopathological examination of hematoxylin and eosin (H&E) stained sections reveals characteristic lesions as described above [11, 9]. Ancillary techniques such as restriction enzyme analysis (REA) and MLST provide genotyping data for epidemiological investigations [2, 6]. A diagnostic decision tree for fowl cholera in broilers is presented in Figure 1.

flowchart TD
    A[Broiler flock with elevated mortality, respiratory signs, or lameness], > B{Postmortem examination}
    B, > C[Gross lesions: petechiae, liver necrosis, splenomegaly, pulmonary edema]
    C, > D[Collect samples: liver, spleen, heart blood, bone marrow]
    D, > E{Culture on blood agar}
    E, > F[Gram-negative coccobacilli, oxidase positive]
    F, > G[Identify: P. multocida]
    G, > H[Confirm with PCR]
    H, > I[Serotyping: capsular PCR or Heddleston serology]
    I, > J[Genotyping: MLST, REA for molecular epidemiology]
    C, > K[Histopathology: H&E staining]
    K, > L[Findings: multifocal hepatic necrosis, vasculitis, bacterial colonies]
    L, > H
    G, > M[Differential diagnoses: colibacillosis, salmonellosis, coryza]
    M, > N[Rule out other pathogens via culture or PCR]
    N, > O[Final diagnosis: Fowl cholera]

Treatment

Antimicrobial therapy is a mainstay for treating fowl cholera outbreaks in broilers. Historically, sulfonamides such as sulfaethoxypyridazine have shown efficacy against P. multocida infections in chickens and turkeys [18]. Doxycycline administered in drinking water at therapeutic doses effectively reduces mortality in experimentally infected broilers [19]. Comparative studies of oral treatment regimens in broilers and turkeys indicate that tetracyclines, fluoroquinolones, and potentiated sulfonamides are among the most effective classes [20]. However, antimicrobial susceptibility varies by region and isolate. In Indonesia, a serotype A isolate was sensitive to ampicillin, doxycycline, erythromycin, gentamicin, and sulfamethoxazole-trimethoprim, but resistant to tetracycline, kanamycin, and oxytetracycline [7]. The emergence of antimicrobial resistance underscores the need for culture and sensitivity testing before treatment selection [20, 19]. Treatment should be administered for 5 to 7 days, and dead birds should be removed promptly to reduce environmental contamination [21]. Supportive care including improved ventilation and nutrition may aid recovery [21]. However, treatment does not eliminate carrier status, and recovered birds may remain sources of infection [28].

Control and Prevention

Control of fowl cholera in broilers relies on stringent biosecurity, vaccination, and management practices. Biosecurity measures include preventing contact with wild birds, rodents, and domestic animals, as well as implementing all-in/all-out production, cleaning and disinfection of facilities, and controlling visitor access [4, 21, 35]. In free-range systems, these measures are more challenging, and the risk of introduction from the environment is higher [4, 12]. Vaccination is widely used in broiler breeders but less commonly in fast-growing broilers due to the short production cycle [30, 33]. Both inactivated (killed) and live attenuated vaccines are available. Inactivated vaccines are typically bacterins or whole culture preparations adjuvanted with oil or aluminum hydroxide [5, 22, 23]. The potency of inactivated vaccines is influenced by factors such as immunogen dose (10^11 CFU/mL being optimal), the chemical nature of the adjuvant, and the removal of LPS and media components [22]. Live vaccines, often of serotype 3,4, have been associated with vaccine-related fowl cholera outbreaks in broiler breeders when reversion to virulence occurs or when titer is too high [5]. Differentiating vaccine-related from naturally occurring disease requires molecular characterization of isolates [5]. In broilers, vaccination at 1 to 6 weeks of age induces variable immune responses as measured by ELISA, with older birds developing stronger antibody titers [30]. Multiple serotypes may need to be included in vaccines for broad protection [23, 32].

Alternative control strategies include the use of probiotics and prebiotics. A novel multi-strain probiotic containing Lactobacillus plantarum, L. fermentum, Pediococcus acidilactici, Enterococcus faecium, and Saccharomyces cerevisiae (10^8 CFU/kg feed) significantly reduced mortality, intestinal P. multocida load, and clinical signs in challenged broilers [17]. Probiotic treatment also upregulated anti-inflammatory genes (HIF1A, TSG-6, PTGER2) in intestinal mucosa and improved hematobiochemical parameters [17]. Mannan oligosaccharides have been shown to ameliorate the pathology of fowl cholera in broilers [24]. Plant extracts such as Petiveria alliacea, Cestrum lanatum, Coutarea hexandra, and Jatropha curcas have been evaluated for their preventive effects, although their efficacy is limited compared to conventional vaccines [31]. In broiler operations with recurrent fowl cholera, a comprehensive control program combining vaccination, biosecurity, and antimicrobial therapy is recommended [21]. Table 1 summarizes the key control measures.

Table 1. Summary of Control Strategies for Fowl Cholera in Broilers

Conclusion

Fowl cholera in broilers remains a significant threat to poultry production worldwide, with the potential for catastrophic losses in susceptible flocks [1, 2]. The causative agent, Pasteurella multocida, exhibits considerable genetic and serotypic diversity, complicating control efforts [2, 6, 9]. Clinical signs range from peracute death to chronic lameness, and diagnosis requires integrated bacteriological, molecular, and pathological approaches [11, 7, 28]. Effective control relies on a combination of biosecurity, vaccination tailored to circulating serotypes, and judicious antimicrobial use guided by susceptibility testing [20, 19, 21]. Emerging strategies such as probiotic supplementation offer promising adjuncts to reduce disease severity and antibiotic reliance [17]. Ongoing surveillance and molecular epidemiology are essential to monitor strain evolution and inform prevention programs [4, 12, 6]. The increasing incidence of fowl cholera in free-range and organic broiler systems demands heightened vigilance and adaptive management [35].

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