Common Poultry Diseases: A Veterinary Overview of Bacterial and Viral Pathogens

In poultry production systems worldwide, infectious diseases caused by bacterial and viral pathogens represent a major constraint to animal welfare and economic sustainability. Respiratory syndromes, enteric infections, and systemic septicemias account for the majority of morbidity and mortality in commercial flocks [1]. Pathogen interactions, such as those between avian pathogenic Escherichia coli (APEC) and respiratory viruses, further complicate disease dynamics [2]. This review provides a structured classification of the principal bacterial and viral agents, with emphasis on mechanisms of pathogenesis, available diagnostic modalities, and integrated control approaches.

Bacterial Pathogens of Poultry

Avian Pathogenic Escherichia coli (APEC)

Colibacillosis, caused by APEC, is among the most prevalent bacterial diseases of poultry. APEC strains harbor a distinct set of virulence-associated genes encoding adhesins, iron acquisition systems, and toxins [3]. A systematic review and meta-analysis has confirmed that specific serogroups, particularly O78, O1, and O2, dominate clinical isolates globally [3]. APEC is a primary cause of first-week mortality in broiler chicks, with yolk sac infection and omphalitis frequently linked to poor hatchery hygiene [4]. The emergence of extended-spectrum beta-lactamase (ESBL) producing Enterobacteriaceae in poultry flocks is a growing concern, as these resistance phenotypes limit therapeutic options [5]. Direct interaction between APEC and H9N2 avian influenza virus has been demonstrated to promote bacterial adhesion to respiratory epithelium, suggesting that viral co-infection enhances colibacillosis severity [2]. For further details on pathogenesis and food safety, readers are referred to the article on Escherichia coli in Chickens and Poultry Products.

Salmonella and Campylobacter spp.

Salmonella enterica serovars, including Salmonella Enteritidis and Salmonella Typhimurium, are major zoonotic agents carried asymptomatically by poultry. Campylobacter jejuni and Campylobacter coli colonize the avian intestinal tract at high densities, with horizontal transmission within flocks facilitated by contaminated water and litter [6]. Novel intervention strategies targeting Campylobacter in the poultry chain include bacteriophage therapy, competitive exclusion cultures, and organic acid supplementation [6]. The article on Salmonella in Chickens provides expanded coverage.

Streptococcus spp.

Streptococcosis in commercial and noncommercial avian species presents as septicemia, polyserositis, or arthritis. A retrospective study of 95 cases in California identified Streptococcus gallolyticus and Streptococcus zooepidemicus as the most common isolates, often associated with underlying immunosuppression or concurrent viral infections [7]. The article on Streptococcus zooepidemicus Bacterial Infection in Poultry offers further clinical context.

Other Notable Bacterial Agents

Pasteurella multocida causes fowl cholera, an acute septicemic disease in turkeys and chickens. Respiratory syndromes involving Pasteurella spp. are reviewed in the context of multifactorial etiology [1]. Avibacterium paragallinarum, the agent of infectious coryza, is discussed in the dedicated article Infectious Coryza in Poultry and Ducks. Clostridium perfringens type A is responsible for necrotic enteritis in broilers, a condition described in detail in Necrotic Enteritis in Broiler Chickens. Additional bacterial pathogens, including Mycoplasma gallisepticum and Mycoplasma synoviae, are major contributors to chronic respiratory disease and are covered in Mycoplasma synoviae.

Viral Pathogens of Poultry

Newcastle Disease Virus (NDV)

Newcastle disease, caused by virulent strains of avian paramyxovirus 1, remains a notifiable disease with global economic impact. The phosphoprotein (P protein) T25M substitution has been linked to quasispecies dynamics and virulence modulation, underscoring the role of minor viral variants in pathogenicity [8]. NDV exploits the host phospholipid flippase complex ATP11c-CDC50A to facilitate viral entry, a molecular interaction that represents a potential target for antiviral intervention [9]. Maternal antibody transfer against NDV is influenced by breeder flock age, affecting early protection in broiler chicks [10]. The interaction between NDV and other immunosuppressive viruses complicates vaccination strategies.

Avian Influenza Virus (AIV)

Low pathogenicity and highly pathogenic avian influenza (HPAI) viruses continue to circulate in wild birds and poultry. H9N2 subtype has become endemic in many regions, with environmental sampling identified as a sensitive and practical alternative to individual bird sampling for surveillance in vaccinated turkey flocks [11]. Cross-reactive human antibody responses to H9N2 have been documented, emphasizing the zoonotic potential of this subtype [12]. The emergence of HPAI H5 clade 2.3.4.4b reassortants in Europe, with Germany acting as a key transit hub, highlights the role of migratory flyways in viral spread [13]. Co-infection of AIV with APEC leads to enhanced bacterial adhesion, as noted above [2]. The article Highly Pathogenic Avian Influenza (H5N1) in Poultry and Wild Birds provides additional surveillance details.

Fowl Adenovirus (FAdV)

Fowl adenovirus serotype 4 (FAdV-4) is the etiological agent of inclusion body hepatitis and hydropericardium syndrome. Residue 188 of the hexon protein governs FAdV-4 pathogenicity by activating the PINK1/Parkin-mediated mitophagy pathway, a discovery that links capsid structure to mitochondrial dysfunction and cell death [14].

Duck Plague Virus (DPV)

Duck plague (duck viral enteritis) is caused by an alphaherpesvirus. A newly identified gene, SORF3, specific to DPV has been characterized for its role in pathogenicity. Deletion of SORF3 attenuates viral replication and reduces virulence in ducks, providing insights into herpesvirus pathogenesis [15].

Circoviruses and Bornaviruses

Circovirus infections in avian species include beak and feather disease virus (BFDV) in psittacines and circovirus in Old World vultures. Endogenous circoviral elements have been detected in vulture genomes, leading to false-positive BFDV detection by conventional PCR [16]. In Croatian populations of Eurasian griffon vultures (Gyps fulvus), circovirus prevalence has been documented using molecular screening [17]. Parrot bornavirus 4 (PaBV-4), the causative agent of proventricular dilatation disease, has been molecularly characterized in captive psittacine birds in India, expanding the known geographic range of this virus [18].

Infectious Bursal Disease Virus (IBDV)

IBDV causes immunosuppression in young chickens. Antibody transfer from breeder hens to progeny depends on breeder age and vaccination schedule, as demonstrated in Ross 308 flocks [10].

Other Notable Viral Pathogens

Marek's disease virus (serotype 1 MDV), infectious laryngotracheitis virus (gallid alphaherpesvirus 1), and avian reoviruses are well recognized but fall outside the scope of papers included in this review. The role of artificial intelligence in monitoring broiler vocalizations for early disease detection has been explored, though vocal repertoire changes showed limited sensitivity for specific pathogen identification [19].

Diagnostic Approaches and Surveillance

Accurate diagnosis of poultry diseases requires integration of clinical, pathological, and laboratory methods. A diagnostic decision tree for respiratory disease outbreaks is presented in Figure 1.

graph TD
    A[Respiratory signs in flock] --> B{Acute mortality >1%?}
    B -->|Yes| C[Collect tracheal and cloacal swabs, serum]
    B -->|No| D["Observe for 24 hours; if worsens, sample"]
    C --> E[Conduct real-time RT-PCR for AIV and NDV]
    E --> F{Positive for AIV or NDV?}
    F -->|Yes| G["Report to veterinary authority; implement quarantine"]
    F -->|No| H[Bacterial culture and PCR for APEC, ORT, MG]
    H --> I[Antimicrobial susceptibility testing]
    I --> J[Targeted therapy and biosecurity adjustments]
    D --> E

Serological monitoring using commercial ELISA kits is widely employed to assess vaccine response and maternal antibody transfer [10]. Environmental sampling, such as boot swabs and dust samples, offers a sensitive, non-invasive approach for AIV surveillance in vaccinated flocks [11]. For bacterial pathogens, culture on selective media followed by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry or sequencing of 16S rRNA genes is standard. Detection of viral nucleic acids via PCR or real-time RT-PCR remains the cornerstone of rapid diagnosis. Identification of conserved linear B-cell epitopes, as demonstrated for porcine circovirus type 4 capsid protein, can inform the development of diagnostic immunoassays [20]. In zoonotic contexts, serological cross-reactivity between animal and human strains must be considered [12].

Control and Prevention Strategies

Biosecurity measures, including all-in/all-out management, disinfection of transport vehicles, and control of wildlife access, form the foundation of disease prevention [1, 4]. Vaccination programs for NDV, IBDV, and AIV are tailored to regional risk profiles and breeder flock serostatus [10]. Antibiotic use must be guided by culture and sensitivity results to curb the spread of ESBL producers [5]. Alternatives to antibiotics, such as probiotics, organic acids, and bacteriophages, are under investigation for reduction of Campylobacter and Salmonella carriage [6]. Nanoparticle-based vaccines targeting viral glycoproteins, as developed for pseudorabies virus, represent a promising platform for future poultry vaccines [21]. For parasitic co-infections, integrated management with anticoccidials and anthelmintics is recommended; refer to Ectoparasites of Poultry and Respiratory and Intestinal Nematodes of Poultry for additional details.

Future Perspectives

The growing application of computational biology and bioinformatics promises to enhance pathogen surveillance through genomic epidemiology. Flux balance analysis and network theory can model host-pathogen metabolic interactions. Automated acoustic monitoring, while currently limited in sensitivity, may be refined for early outbreak detection [19]. Continued research into pathogen interactions, such as the APEC-AIV synergy, will inform more effective vaccination and management strategies [2].

Tables

Table 1. Major Bacterial Pathogens of Poultry

Table 2. Major Viral Pathogens of Poultry

References

[1] Liu H, Pan S, Wang C, et al. Review of respiratory syndromes in poultry: pathogens, prevention, and control measures. Vet Res. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40382667/

[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] Jamali H, Akrami F, Bouakkaz S, et al. Prevalence of specific serogroups, antibiotic resistance and virulence factors of avian pathogenic Escherichia coli (APEC) isolated from clinical cases: A systematic review and meta-analysis. Microb Pathog. 2024. URL: https://pubmed.ncbi.nlm.nih.gov/39117015/

[4] Swelum AA, Elbestawy AR, El-Saadony MT, et al. Ways to minimize bacterial infections, with special reference to Escherichia coli, to cope with the first-week mortality in chicks: an updated overview. Poult Sci. 2021. URL: https://pubmed.ncbi.nlm.nih.gov/33752065/

[5] Saliu EM, Vahjen W, Zentek J. Types and prevalence of extended-spectrum beta-lactamase producing Enterobacteriaceae in poultry. Anim Health Res Rev. 2017. URL: https://pubmed.ncbi.nlm.nih.gov/28641596/

[6] Soro AB, Whyte P, Bolton DJ, et al. Strategies and novel technologies to control Campylobacter in the poultry chain: A review. Compr Rev Food Sci Food Saf. 2020. URL: https://pubmed.ncbi.nlm.nih.gov/33337085/

[7] Crispo M, Shivaprasad HL, Cooper GL, et al. Streptococcosis in Commercial and Noncommercial Avian Species in California: 95 Cases (2000-2017). Avian Dis. 2018. URL: https://pubmed.ncbi.nlm.nih.gov/29944398/

[8] Daguia-Wenam P-T, Lu K, Zhou X, et al. The P protein T25M substitution is involved in the quasispecies and virulence of Newcastle disease virus. Microbiol Spectr. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41627030/

[9] Zhang D, Hou Y, Qiu X, et al. Newcastle disease virus exploits the phospholipid flippase ATP11c-CDC50A complex to promote viral infection. J Biol Chem. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40812423/

[10] Wegner M, Gesek M. The influence of Ross 308 breeder flock age on serum Newcastle Disease Virus and Infectious Bursal Disease Virus antibody levels and their transfer to offspring. Pol J Vet Sci. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42299112/

[11] Kalvelage L, Casteel M, Hartmann M, et al. Environmental samples, a sensitive and practical alternative to individual bird sampling in the surveillance of H9N2 vaccinated turkey flocks. J Gen Virol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42262844/

[12] Singh G, Bhavsar D, Ferreri LM, et al. Cross-reactive human antibody responses to H9N2 influenza virus, New York, United States, 2025. Euro Surveill. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42170746/

[13] Ahrens AK, Grund C, Beer M, et al. Germany as a key transit hub for the emergence and spread of high pathogenicity avian influenza H5 clade 2.3.4.4b reassortants in Europe. Front Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42293551/

[14] Wang B, Qiao Q, Yang P, et al. Residue 188 of the Hexon protein governs FAdV-4 pathogenicity by activating PINK1/Parkin-mediated mitophagy. Vet Res. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42288922/

[15] Wang Z, Cao H, Wang M, et al. Characterizing a newly identified avian herpesvirus-specific gene SORF3 in DPV and its roles in potential pathogenicity. J Virol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41459945/

[16] Palacios-Martínez I, Morinha F, Blanco G. Not an infection: Endogenous circoviral elements underlie BFDV detections in Old World vultures. PLoS One. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42296149/

[17] Lozica L, Faraguna S, Lukač M, et al. Circovirus infection in Croatian population of Eurasian griffon vultures (Gyps fulvus). Sci Rep. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41408401/

[18] Deka P, Das S, Hazarika R, et al. Molecular detection and characterization of Parrot Bornavirus 4 (PaBV-4) in captive psittacine birds in India. Sci Rep. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42297940/

[19] Soster P, Grzywalski T, Carvalho CL, et al. AI-based monitoring of broiler vocal repertoire dynamics reveals robust developmental and diurnal patterns but limited disease sensitivity. Poult Sci. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/41933532/

[20] Xu L, Chen H, Ji L, et al. One novel conserved linear B-cell epitope identified in the capsid protein of porcine circovirus type 4. Vet Microbiol. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42214286/

[21] Sun X, Liu R, Yang H, et al. A nanoparticle vaccine targeting glycoprotein D induces immune protection against pseudorabies virus. Vaccine. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42140091/ *** 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.

[22] Faraji K, Hadi P, Seyedeh Parastoo Y, et al. Assessment of Oxidative Stress Biomarkers in Felines Infected with Calicivirus. Arch Razi Inst. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/41179633/

[23] Capucci L, Di Giovanni V, Schiavitto M, et al. Variation of rabbit serum IgA in relation to RHDV vaccination and infection: implications for serological diagnosis and epidemiological investigations. Vet Res. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42265812/

[24] Epaminondas D, Mazeri S, Lyraki M, et al. Epidemiological and Clinical Insights from 68 Veterinarian-Reported Cases of Feline Infectious Peritonitis During the Documented FIP Epizootic in Cyprus. Pathogens. 2026. URL: https://pubmed.ncbi.nlm.nih.gov/42198625/