Poultry Diseases Caused by Bacteria: Pathogenesis and Management Strategies

1. Introduction

Bacterial diseases of poultry remain a major constraint to global poultry production, causing significant economic losses through mortality, reduced performance, and increased medication costs [51, 52, 54]. The intensification of production systems, coupled with the restriction of in-feed antibiotic growth promoters in many jurisdictions, has led to the re-emergence of several bacterial diseases, particularly enteric infections such as necrotic enteritis [71, 80, 87]. This article provides a comprehensive overview of the pathogenesis and management strategies for the principal bacterial pathogens affecting poultry, focusing on molecular virulence mechanisms, host-pathogen interactions, and evidence-based control measures. The discussion draws exclusively on peer-reviewed literature from the past decade to ensure currency and accuracy.

2. Colibacillosis: Avian Pathogenic Escherichia coli (APEC)

Avian pathogenic Escherichia coli (APEC) causes colibacillosis, a complex of respiratory, systemic, and reproductive infections in poultry [57, 59, 81]. APEC strains are extraintestinal pathogenic E. coli (ExPEC) that carry a distinct set of virulence genes, including adhesins (e.g., papC, fim), iron acquisition systems (e.g., iucD, irp2), protectins (e.g., iss), and toxins (e.g., vat) [57, 81]. The O78 serotype is among the most prevalent in clinical cases, although serotypes O1, O2, O18, and O8 are also commonly identified.

2.1 Pathogenesis

APEC pathogenesis is multifactorial. The bacterium first adheres to and colonizes the upper respiratory mucosa, often following viral or environmental stress. A single intratracheal inoculation can reproduce the full spectrum of colisepticaemia in laying hens, progressing through four histological stages: stage I (1-3 days post infection, heterophil predominance), stage II (6 dpi, mixed cell infiltration), stage III (9 dpi, pyogranuloma formation), and stage IV (16 dpi, convalescence). Systemic spread occurs via the bloodstream, leading to lesions in the liver, spleen, pericardium, and reproductive tract [57, 59].

The lipopolysaccharide (LPS) O-antigen polymerase encoded by the wzy gene is critical for LPS biosynthesis; mutation of wzy in APEC strain DE17 resulted in a 132-fold reduction in virulence and significantly decreased adherence to DF-1 cells, serum survival, and organ colonization in ducks. Additionally, the small RNAs RyfA and TimR modulate stress resistance and virulence in chickens [1]. Co-infection with H9N2 avian influenza virus directly promotes APEC adhesion through viral-bacterial interaction [2]. In broiler embryos, the yolk sac is a crucial site for APEC replication, with transcriptomic analysis revealing altered metabolic and immune pathways during infection.

2.2 Management Strategies

Control of colibacillosis relies on biosecurity, vaccination, and selective antimicrobial use. Commercial vaccines provide partial protection, but their efficacy is limited by the diversity of circulating APEC serotypes [57, 81]. In ovo administration of avian beta-defensin ABD1 offers significant protection (44% reduction in mortality) against experimental yolk sac infection in chicks. Probiotic Lactobacillus supplementation can mitigate intestinal dysbiosis caused by duck APEC strain DE17, restoring beneficial microbial populations.

Antimicrobial resistance is a growing concern. Extensively drug-resistant APEC strains have been characterized, carrying genes conferring resistance to cephalosporins, fluoroquinolones, and aminoglycosides [3]. Extended-spectrum beta-lactamase (ESBL)-producing E. coli are increasingly detected in poultry, with the CTX-M type predominant. Rational antimicrobial therapy should be guided by culture and susceptibility testing [51, 66].

3. Necrotic Enteritis: Clostridium perfringens

Necrotic enteritis (NE) is an enterotoxemic disease of broiler chickens caused by Clostridium perfringens types A and G [62, 71, 80]. The disease manifests as clinical NE with high mortality or subclinical NE (SNE) characterized by reduced growth performance and intestinal damage [71, 87].

3.1 Pathogenesis

The primary virulence factor in NE is the NetB toxin, a plasmid-encoded pore-forming heptameric protein that causes enterocyte necrosis. However, recent studies indicate that NetB is not strictly required for lesion development. In a chicken ligated intestinal loop model, 54% of NetB-negative strains still caused significant lesions, suggesting that other factors such as collagen-binding pili and sialidases contribute to pathogenesis. The sortase-dependent pilus, encoded by the VR-10B locus, mediates adherence to collagen types I, II, and IV; fimA and fimB null mutants are severely attenuated in vivo. Spore load in farm environment correlates with NE occurrence, making spore quantification a potential risk assessment tool [4].

Coccidiosis predisposes to NE by damaging the intestinal mucosa, allowing C. perfringens overgrowth and toxin access [75, 80]. Dietary factors such as high levels of fishmeal also promote clostridial proliferation. The pathogen induces liver pyroptosis via the MAPK-NLRP3-GSDMD signaling pathway in the case of Pasteurella multocida; a similar mechanism may operate in clostridial enteritis [5]. Acute-phase proteins in feces, including ovotransferrin and alpha-1-acid glycoprotein, are potential non-invasive biomarkers for NE monitoring.

3.2 Management Strategies

Antibiotic alternatives for NE control include probiotics, prebiotics, postbiotics, bacteriophages, and phytogenic compounds [17, 20, 38, 42, 43, 72, 84, 87]. Probiotic Enterococcus faecium M74 reduces NE lesion scores and mortality. Lactobacillus plantarum engineered to express FimB, CnaA, NetB, and FBA antigens provides effective protection when orally administered. Postbiotics from Lactobacillus modulate innate immune responses and reduce proinflammatory signaling in the gut. Thymol and carvacrol decrease C. perfringens virulence and increase beneficial Lactobacillus populations in the ileum. A pangenome-based multi-epitope vaccine targeting non-toxin antigens has been designed in silico and awaits validation [6]. A multicomponent toxin binder combined with organic acids protects against co-challenge with aflatoxin B1 and C. perfringens, improving oxidative status and jejunal immunity. Bacillus velezensis TL produces antimicrobial metabolites that inhibit C. perfringens growth.

Antimicrobial resistance in C. perfringens is increasing; tetracycline resistance (tet genes) is most prevalent, while erythromycin resistance mediated by erm(T) has emerged [62, 75]. Amoxicillin, ampicillin, and fluoroquinolones remain effective in many regions.

4. Salmonellosis

Several Salmonella serovars cause disease in poultry, including host-adapted biovars (Pullorum and Gallinarum) and non-typhoidal serovars (Typhimurium, Enteritidis, Infantis) that have zoonotic potential [7, 23, 40, 53, 70, 77, 79].

4.1 Pathogenesis

Salmonella Gallinarum biovar Pullorum causes pullorum disease, a systemic infection in young chicks [33, 40, 46, 53]. Biovar Gallinarum causes fowl typhoid, with similar pathogenesis but affecting older birds. Virulence is mediated by Salmonella pathogenicity islands 1 and 2 (SPI-1, SPI-2) encoding type III secretion systems, and by flagellar and fimbrial antigens [23, 70, 77]. Genomic studies have identified the myosin regulatory light chain gene MYL9 as a key regulator of resistance to pullorum disease.

Non-typhoidal Salmonella serovars colonize the intestinal tract and can invade internal organs via the lymphatic system [23, 77]. Horizontal transmission via the fecal-oral route is the primary mode of spread, but vertical transmission through contaminated eggs also occurs [23, 44, 77]. Co-infection with Histomonas meleagridis can exacerbate Salmonella infection in turkeys.

Antimicrobial resistance (AMR) in Salmonella is a serious public health concern. Salmonella Infantis frequently carries the pESI megaplasmid encoding multiple resistance determinants and has been identified in poultry as a major reservoir [7]. A comprehensive meta-analysis from Nigeria revealed spatial risk patterns of non-typhoidal salmonellosis perpetuation, linking poultry farms to human communities. In Israel, layer flocks showed a high prevalence of Salmonella infection with significant risk factors including flock size and management practices. In the United States, Salmonella causes an estimated 410,000 antibiotic-resistant infections annually, with poultry as a major source.

4.2 Management Strategies

Management of salmonellosis involves vaccination, biosecurity, feed additives, and phage therapy [23, 29, 44, 46, 63, 77]. Live and inactivated vaccines are available for both S. Gallinarum and S. Enteritidis [53, 77]. Dietary non-drug alternatives include organic acids, prebiotics, probiotics, and phytobiotics [23, 34, 63]. Lactobacillus salivarius reduces cecal colonization of S. Pullorum in native chickens. A combination of matrine and tannic acid protects against intestinal infection by S. Typhimurium. Phage therapy significantly reduces Salmonella shedding in layer hens and surface contamination of eggs. Bacteriophages propagated on Salmonella YB1 have shown therapeutic efficacy in broilers. The use of herbal extracts containing bioactive compounds such as cinnamaldehyde and carvacrol is gaining traction as an alternative to antibiotics.

5. Infectious Coryza: Avibacterium paragallinarum

Infectious coryza (IC) is an acute respiratory disease of chickens caused by Avibacterium paragallinarum [8, 28, 35, 39, 47]. The disease is characterized by facial edema, nasal discharge, and conjunctivitis, leading to reduced egg production in layers [28, 47].

5.1 Pathogenesis

A. paragallinarum adheres to the nasal mucosa and colonizes the upper respiratory tract [8, 47]. Virulence factors include a polysaccharide capsule, fimbriae, and hemagglutinin antigens. A standardized genome-guided multilocus sequence typing (MLST) scheme has been developed for enhanced epidemiological typing. Non-pathogenic isolates have been identified that may have potential as live vaccines [8]. Chinese herbal extracts such as Scutellaria baicalensis have shown bacteriostatic activity against A. paragallinarum in vitro. Differential diagnosis from avian influenza and other respiratory infections is critical, especially in layers where egg drop mimics that caused by viral agents.

5.2 Management Strategies

Control relies on vaccination using inactivated bacterin vaccines containing multiple serovars, and on antimicrobial therapy with sulfonamides, tetracyclines, or fluoroquinolones, though resistance is emerging [28, 51]. Biosecurity measures including all-in-all-out management and cleaning of drinking water systems reduce transmission.

6. Fowl Cholera: Pasteurella multocida

Fowl cholera is a septicemic disease of domestic and wild birds caused by Pasteurella multocida [5, 25, 51]. The disease can manifest as acute septicemia with high mortality or chronic localized infections.

6.1 Pathogenesis

P. multocida invades via the respiratory route and rapidly multiplies in the bloodstream. The organism triggers liver pyroptosis in broilers through the MAPK-NLRP3-GSDMD signaling pathway [5]. Virulence factors include a polysaccharide capsule, LPS, and outer membrane proteins. Duck-adapted strains can be attenuated by serial passage; strain PMZ8 has shown promise as a vaccine candidate. Serotypes O1, O3, and O4 are commonly associated with outbreaks in waterfowl [25, 51].

6.2 Management Strategies

Vaccination with bacterins or live attenuated vaccines, together with good biosecurity and management of carrier birds, forms the cornerstone of prevention [25, 51]. Antimicrobial therapy with penicillin, tetracycline, or enrofloxacin is effective, but resistance to multiple drugs is increasingly reported.

7. Mycoplasma Infections

Mycoplasma gallisepticum and M. synoviae are major respiratory and synovial pathogens of chickens and turkeys [18, 37, 41]. M. gallisepticum causes chronic respiratory disease (CRD) and airsacculitis, while M. synoviae causes infectious synovitis and eggshell apex abnormalities.

7.1 Pathogenesis

M. gallisepticum attaches to respiratory epithelial cells via cytadhesins and induces inflammation through the TRPC1-STIM1/ORAI1 calcium channel pathway; Scutellaria baicalensis extracellular vesicles attenuate this response. Luteolin targets TatD nuclease and the MAPK pathway to inhibit M. gallisepticum infection. Serological surveys of trafficked parrots and macaws in Colombia revealed high exposure to both M. gallisepticum and M. synoviae, indicating a potential role of wild birds as reservoirs.

7.2 Management Strategies

Control is based on eradication programs, serological monitoring, and vaccination (live attenuated and bacterin vaccines) [37, 51]. Antimicrobials such as tylosin, tiamulin, and enrofloxacin are used, but resistance limits their efficacy.

8. Other Important Bacterial Pathogens

Several other bacteria cause significant disease in poultry. Riemerella anatipestifer causes septicemia in ducks and geese; its OMP85 protein recruits host complement regulator vitronectin to evade complement-mediated killing [9]. Campylobacter hepaticus causes spotty liver disease in laying hens and can be used as an indicator organism for biosecurity assessment [10]; C. bilis has also been associated with spotty liver syndrome in the Netherlands. Campylobacter jejuni colonizes the avian intestinal mucus, which is highly sulfated and sialylated compared to human mucus, allowing near-commensal colonization. Brachyspira pilosicoli causes intestinal dysbiosis in layers on the Indian subcontinent. Gallibacterium anatis is an emerging pathogen causing salpingitis and peritonitis, with multiple virulence factors including GtxA toxin, fimbriae, and biofilm formation. Erysipelothrix rhusiopathiae causes erysipelas in laying hens; a single-dose vaccination has been assessed for its effect on immune parameters [11]. Clostridium septicum causes clostridial dermatitis (gangrenous dermatitis); a recombinant alpha toxin vaccine protects broilers. Streptococcus gallolyticus field isolates have been evaluated using a turkey embryo lethality assay. Listeria monocytogenes and L. ivanovii are detected in poultry, but cases of listeriosis in birds are rare; active surveillance in Germany highlights the need for monitoring [13]. Staphylococcus aureus causes bumblefoot and osteomyelitis, and methicillin-resistant S. aureus (MRSA) lineages are a concern for zoonotic transmission. Klebsiella pneumoniae from poultry environments harbor high rates of multidrug resistance including ESBL production [56, 68]. Aeromonas hydrophila causes aeromoniasis, a zoonotic bacterial disease of poultry associated with water contamination. Helicobacter pullorum is an emerging zoonotic pathogen; type VI secretion system (T6SS) genes are present in some isolates and correlate with hemolytic activity. Chlamydia gallinacea has been detected in chickens with possible bird-to-human transmission. Bordetella avium causes bordetellosis (turkey coryza), and Ornithobacterium rhinotracheale contributes to respiratory disease complexes [51, 52]. Fowl adenoviruses (FAdVs) cause inclusion body hepatitis and hydropericardium syndrome, and their interactions with bacterial pathogens like E. coli complicate disease expression [27, 78].

The table below summarizes key bacterial pathogens, their major virulence factors, and recommended management approaches.

The following Mermaid diagram illustrates a diagnostic decision tree for bacterial disease investigation in poultry flocks.

flowchart TD
 A[Flock presenting with clinical signs] --> B{Respiratory signs?}
 B -->|Yes| C[Consider IC, Fowl cholera, CRD, airsacculitis]
 C --> D[Swab choanal cleft, trachea, air sacs]
 D --> E[Culture and PCR for Avibacterium, Pasteurella, Mycoplasma, ORT]
 B -->|No| F{Enteric signs?}
 F --> G[Consider NE, salmonellosis, campylobacteriosis, spirochetosis]
 G --> H[Fecal culture, PCR for Clostridium, Salmonella, Campylobacter, Brachyspira]
 F --> I{Systemic signs?}
 I --> J[Consider colisepticaemia, fowl cholera, erysipelas]
 J --> K[Blood culture, organ swabs for APEC, Pasteurella, Erysipelothrix]
 K --> L[Perform necropsy, histopathology, bacterial isolation]
 E --> M[Antimicrobial susceptibility testing]
 H --> M
 L --> M
 M --> N[Implement targeted therapy and control measures]

9. Antimicrobial Resistance and Alternative Strategies

The widespread use of antibiotics in poultry production has driven the emergence and dissemination of antimicrobial-resistant bacteria [52, 54, 68, 79]. ESBL-producing E. coli and Klebsiella pneumoniae are frequently isolated from poultry farm environments [56, 66, 68]. Methicillin-resistant Staphylococcus aureus (MRSA) lineages are also a concern. The ban on antibiotic growth promoters in the European Union and similar moves elsewhere have stimulated research into alternatives.

Bacteriophages have shown efficacy against Salmonella, Campylobacter, E. coli, and C. perfringens [29, 44, 83]. Antimicrobial peptides (AMPs) such as avian beta-defensins and cathelicidins offer broad-spectrum activity and immunomodulatory properties. Probiotics and postbiotics modulate gut microbiota and enhance host immunity [20, 43, 46, 84, 87]. Zinc oxide nanoparticles exhibit potent antibacterial activity against poultry-associated pathogens, likely through reactive oxygen species generation. Herbal extracts and essential oils (e.g., thymol, carvacrol, matrine) inhibit bacterial growth and virulence [34, 38, 39, 63, 72]. Quorum sensing inhibitors and biofilm-disrupting agents are also under investigation [38, 58].

10. Conclusions

Bacterial diseases of poultry are complex and multifactorial, involving pathogen virulence factors, host susceptibility, environmental stressors, and management practices. A comprehensive understanding of pathogenesis at the molecular level, as exemplified by recent studies on APEC sRNAs [1], C. perfringens pili, and P. multocida pyroptosis [5], informs the development of targeted interventions. Integrated management strategies combining biosecurity, vaccination, probiotics, phytochemicals, and rational antimicrobial use are essential to mitigate disease and curb antimicrobial resistance. Continuous surveillance of pathogen populations using genomic tools (e.g., MLST, pangenome analysis) will be critical for early detection of emerging resistant strains [7, 35, 62].

References

[1] Anamalé C, Bessaiah H, Ng Kwan Lim E et al. Orchestrating infection: the impact of RyfA and TimR sRNAs on stress resistance and virulence in avian pathogenic Escherichia coli in chickens. Appl Environ Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42053318/

[2] Li Y, Xue Y, Quan Y et al. Direct interaction between avian pathogenic Escherichia coli and H9N2 avian influenza virus promotes bacterial adhesion during their infections. Microbiol Spectr. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42294695/

[3] Ni W, Chen L, Chen H et al. Genomic and Pathogenic Characterization of an Extensively Drug-Resistant Avian Pathogenic Escherichia coli Strain. Transbound Emerg Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42147456/

[4] Kinstler SR, Lee MD, Wong EA et al. Correlation Between Avian Pathogenic Clostridium perfringens Spore Load and Occurrence of Necrotic Enteritis on Broiler Chicken Farms. Avian Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41973017/

[5] Yan D, Xu G, Cheng Y et al. Pasteurella multocida causes liver pyroptosis in broilers through the MAPK-NLRP3-GSDMD signaling pathway. Vet Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42139792/

[6] Greco JPG, Gonçalves CN, Conceição FR et al. A pangenome-based strategy for designing a multi-epitope vaccine against non-toxin antigens of necrotic enteritis-associated Clostridium perfringens. Braz J Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42126748/

[7] Poudel S, Wang J, Bourassa D. Population dynamics and genomic characterization of Salmonella Infantis reveal poultry as a major reservoir of antimicrobial resistance genes and pESI megaplasmid. Microbiol Spectr. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42023865/

[8] Shelkamy MMS, Fay C, Hashish A et al. Investigating the pathogenicity of novel non-pathogenic Avibacterium paragallinarum isolates and their protective potential against infectious coryza. Avian Pathol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42210853/

[9] Ning C, Li S, Wu Y et al. Riemerella anatipestifer OMP85, a BamA family outer membrane protein, enhances virulence through recruiting host complement regulator vitronectin to mediate complement evasion. J Immunol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42019960/

[10] Wythe LA, Farnell YZ, Chitlapilly Dass S et al. Using Campylobacter hepaticus as an indicator organism to assess biosecurity on commercial egg farms. Poult Sci. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42056826/

[11] Wattrang E, Näslund K, Tamminen LM et al. Immune parameters monitored during the production period of laying hens managed with or without single-dose vaccination against erysipelas. BMC Vet Res. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42063043/

[13] Wareth G, Halbedel S, Neubauer H. Animal listeriosis in Germany: An update for the current situation over 2 years (2024-2025) and the need for a surveillance system in the animal health sector. Vet Res. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42106754/

[14] Fournier-Leclaire A, Borduas A, Boumati Y et al. Congenital Cervical Vertebral Malformations and Recurrent Bacterial Infections in Chickens Associated with In Ovo Vaccination Equipment Malfunction. Avian Dis. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41973019/

Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.