Avian Colibacillosis: Etiology, Diagnosis, and Vaccine Development

Introduction

Avian colibacillosis is a bacterial disease of poultry caused by avian pathogenic Escherichia coli (APEC) and is responsible for substantial economic losses worldwide due to high morbidity, mortality, and slaughterhouse condemnations [1, 2, 3]. The disease manifests as a range of clinical syndromes including acute septicemia, fibrinous polyserositis, airsacculitis, pericarditis, perihepatitis, and omphalitis in chicks [2, 3]. APEC strains belong to the extraintestinal pathogenic E. coli (ExPEC) group and share virulence traits with human uropathogenic and neonatal meningitis-associated E. coli [4, 27, 32]. The increasing prevalence of multidrug-resistant APEC, including extended-spectrum beta-lactamase (ESBL) producers and colistin-resistant isolates, complicates therapeutic management and poses a zoonotic risk [5, 6, 7, 35]. This article provides an exhaustive reference on the etiology, epidemiology, clinical presentation, diagnostic approaches, treatment options, and vaccine development for avian colibacillosis.

Etiology

Avian Pathogenic Escherichia coli (APEC)

APEC strains are a subset of ExPEC that carry a combination of virulence-associated genes (VAGs) enabling colonization, invasion, and survival within the avian host [4, 8, 28]. Common VAGs include those encoding adhesins (type 1 fimbriae, fimH), iron acquisition systems (iroN, aerJ), protectins (iss, ompT), and toxins (hlyF, vat) [8, 9, 28]. The ColV plasmid is frequently associated with APEC virulence, carrying many of these genes [8, 10]. Serogroups O1, O2, O78, O25, O86, and O8 are among the most prevalent in colibacillosis cases, although serogroup distribution varies by geographic region and over time [4, 8, 6, 10]. For example, surveillance in Georgia, USA, identified O78 and O25 as dominant serogroups, with a shift in prevalence observed between years [4, 8]. In Algeria, O2 and O1 were most frequent among ESBL-producing APEC isolates [6]. Whole-genome sequencing has revealed diverse sequence types (STs), including ST117, ST23, ST131, and ST48, which often carry multiple resistance genes [6, 7, 11, 10, 30]. Phylogenetic group analysis places most APEC isolates in groups D, B2, G, or A, with group G emerging as a significant lineage in recent studies [8, 6].

Pathogenicity Mechanisms

APEC strains cause disease through a combination of adhesion, invasion, evasion of host defenses, and production of toxins. The bacteria initially colonize the respiratory tract, often following viral infections (e.g., infectious bronchitis virus, Newcastle disease virus), then enter the bloodstream and disseminate to internal organs [2, 12]. Experimental models using nebulized APEC aerosol challenge have successfully reproduced colibacillosis in young chicks, confirming the respiratory portal of entry [12]. Once systemic, APEC induces a fibrinoheterophilic inflammatory response, leading to the characteristic polyserositis [3]. Biofilm formation is another critical trait: APEC strains can form biofilms on mucosal surfaces and abiotic surfaces, contributing to persistence and resistance to antimicrobials [13, 28]. Co-infection with Enterococcus faecalis has been shown to enhance APEC growth under iron-restricted conditions and increase virulence in embryo lethality assays, suggesting polymicrobial interactions exacerbate colibacillosis [33].

Epidemiology and Transmission

Prevalence and Risk Factors

Avian colibacillosis is reported globally, with prevalence rates varying by region. A retrospective study in Nigeria found that 13.1% of poultry cases presented to a veterinary teaching hospital over a decade were diagnosed with colibacillosis [1]. In Mozambique, outbreaks in broiler farms caused mortality within 3–5 days, often with 100% morbidity in affected flocks [3]. In Pakistan, the prevalence of colistin-resistant E. coli from broiler farms was 24.78%, and preceding respiratory viral infection was identified as a significant risk factor (OR = 4.808) [5]. Other risk factors include high stocking density, poor biosecurity, contaminated feed and water, and stress from transport or temperature fluctuations [5, 2].

Transmission Routes and the Role of Chicken Feces

Transmission occurs via aerogenous, alimentary, and transovarial routes [2]. APEC is shed in high numbers in the feces, and contaminated litter, feed, and water serve as major sources of horizontal transmission [2, 14]. The phrase chicken e coli poop highlights the importance of fecal contamination: studies have shown that fecal samples can yield a 100% prevalence of APEC in affected farms, making feces a significant reservoir and continuous source of environmental contamination [14]. Young chicks are particularly susceptible during the first two weeks of life, and vertical transmission from infected breeder flocks via eggs has been documented [7, 10]. In Japan, CTX-M-55-type ESBL-producing APEC ST23 strains were repeatedly isolated from farms that received chicks from common hatcheries, indicating vertical spread [7]. Similarly, in Finland, two major colibacillosis outbreaks were linked to clonal APEC lineages (ST117 and ST23) that were traced to parent and grandparent flocks through vertical transmission [10].

Clinical Signs and Pathology

Clinical Presentation

The clinical course of colibacillosis can be acute, subacute, or chronic [2]. In acute septicemic form, birds show depression, anorexia, ruffled feathers, dyspnea, watery diarrhea, and sudden death [2, 3, 26]. Peracute cases may die without premonitory signs. In chronic or localized forms, signs include lameness (due to femoral head necrosis or arthritis), respiratory distress, and reduced egg production [2, 31]. Young birds (1–14 days old) and layers at the onset of lay are most vulnerable [2]. Mortality rates can reach 30% in untreated broiler flocks [26].

Gross and Histopathological Lesions

Postmortem examination typically reveals fibrinous polyserositis: pericarditis (thickened, opaque pericardium with fibrin), perihepatitis (Glisson's capsule covered by fibrin), and airsacculitis (thickened, cloudy air sacs) [3, 26]. Hepatomegaly with white pinpoint necrotic foci and splenomegaly with mottled appearance are common [3]. Other findings include catarrhal hemorrhagic enteritis, omphalitis in chicks, and egg peritonitis in layers [2, 35]. Histologically, a fibrinoheterophilic inflammatory infiltrate is observed in affected serosal surfaces and parenchymal organs, often with coccobacillary bacterial aggregates [3]. Necrotic foci are found in the liver, spleen, and intestines [3]. Immunohistochemical staining confirms E. coli antigen within lesions [3].

Diagnosis

Bacteriological and Serological Methods

Diagnosis begins with clinical and gross pathological evaluation [2]. Isolation of E. coli from affected organs (liver, heart blood, bone marrow, air sacs) is essential for confirmation [2, 3, 28]. Samples are cultured on MacConkey agar and eosin methylene blue agar; typical colonies are lactose-fermenting with a metallic sheen on EMB [2, 28]. Identification is done by Gram staining, biochemical tests (IMViC), and, in some laboratories, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) [6, 2]. Serogrouping by slide agglutination using antisera against common O groups (O1, O2, O78) provides rapid serotype identification [8, 6, 2].

Molecular and Genomic Tools

Polymerase chain reaction (PCR) is widely used for detection of VAGs and resistance genes. Multiplex PCR assays targeting the most common APEC serogroups, such as the Klao9-SeroPCR, can serotype 62% of isolates rapidly [8]. Real-time PCR and conventional PCR also detect ESBL genes (e.g., blaCTX-M, blaSHV) and mcr genes for colistin resistance [5, 6, 7]. Whole-genome sequencing (WGS) and multilocus sequence typing (MLST) provide fine-scale epidemiological resolution, revealing clonal lineages and resistance determinants [11, 10, 30]. For outbreak investigations, WGS can identify predominant clones such as ST117 O78:H4 or ST23 O78:H4 [10].

Antibiogram

Antimicrobial susceptibility testing by disk diffusion or broth microdilution is critical for guiding treatment due to widespread resistance [5, 35]. Studies report high resistance to ampicillin (up to 100%), trimethoprim-sulfamethoxazole, cefotaxime, and fluoroquinolones [5, 35]. Multidrug resistance (resistance to three or more classes) is common, with MAR indices exceeding 0.2 [35]. Colistin resistance, mediated by mcr-1 and mcr-2 genes, is an emerging threat [5].

Treatment

Antimicrobial Therapy and Resistance

Conventional treatment relies on antibiotics, but resistance severely limits options [5, 2, 26, 35]. In broilers, cefquinome (a fourth-generation cephalosporin) has been shown to reduce mortality and improve clinical signs, with a withdrawal time of 3 days in muscle and 7 days in liver and kidney [26]. However, ESBL, fluoroquinolone, and colistin resistance are prevalent [5, 6, 7]. The misuse of antibiotics also disrupts the gut microbiota, creating a dysbiotic environment that favors APEC overgrowth [29].

Alternative and Phytotherapeutic Approaches

Given the rise of multidrug-resistant APEC, alternative strategies are under investigation. Plant extracts such as Thymus vulgaris (thyme) have demonstrated antibacterial and antibiofilm activity, with synergistic effects when combined with ampicillin [15]. Urtica dioica and Chamerion angustifolium extracts inhibit biofilm formation in APEC strains [13]. Quercetin-loaded caffeic acid-grafted chitosan micelles (CA-g-CS/QR) disrupt bacterial membranes and reduce intestinal Escherichia spp. counts in broilers [16]. Other natural compounds such as matrine combined with berberine hydrochloride [17], Schisandrin A [18], and herb pair extracts from Ilex rotunda and Cyperus rotundus [19] have shown therapeutic efficacy in experimental colibacillosis, improving survival, reducing inflammation, and modulating gut microbiota. Probiotics and postbiotics, including solid-state fermentation products of Lactobacillus plantarum, Candida utilis, and Bacillus coagulans, enhance growth performance and reduce APEC burden [20]. Saponin-based postbiotics have also been evaluated in combination with vaccination [31]. Phytogenic compounds and plant growth-promoting rhizobacteria combined with zinc oxide nanoparticles present further avenues for control [21].

Control and Prevention

Biosecurity and Management

Biosecurity measures are fundamental: all-in/all-out production, effective cleaning and disinfection, control of rodents and wild birds, and provision of clean feed and water [2]. Risk factor analyses have shown that daily removal of dead or diseased birds is protective (OR = 0.308) against colistin-resistant strains [5]. Vaccination is a key component of a comprehensive control program.

Vaccine Development

Overview of Vaccine Strategies

Vaccination against colibacillosis aims to reduce clinical disease and shedding. Inactivated (bacterin) vaccines are commercially available but often serotype-specific and provide limited cross-protection [22]. Advances in vaccine research have led to several novel platforms.

Inactivated and Subunit Vaccines

A bivalent oil-adjuvanted vaccine targeting necrotic enteritis and colibacillosis has been developed, combining antigens from Clostridium perfringens and APEC [23]. The phrase e coli chicken vaccine increasingly refers to novel subunit candidates. Membrane vesicles (MVs) derived from an APEC strain with low lipopolysaccharide expression (ΔmsbB) induced robust antibody responses and cross-protection against multiple serotypes (O1, O7, O45, O78, O101) in chickens [22]. The nitrogen cavitation technique increased MV yield, facilitating industrial scalability [22].

Bacterial Ghosts and Nanovaccines

Bacterial ghosts (BGs) are empty cell envelopes of inactivated bacteria preserving surface antigens. A BG vaccine of E. coli O78:K80 administered via injection or respiratory route significantly reduced air sac lesions and induced IFNγ, IgA, and IgY responses in broilers [34]. An in ovo nanovaccine based on cationic maltodextrin nanoparticles loaded with three inactivated APEC strains has been tested under commercial hatchery conditions, demonstrating safety, no adverse effects on hatchability or growth, and persistence of antibody responses until slaughter without interference with live viral vaccines [24]. Trivalent in ovo nanovaccines are compatible with routine hatchery vaccination programs [24].

Compatibility and Future Directions

Co-administration of postbiotic feed additives with vaccination has been explored; one study found that a saponin-containing postbiotic did not enhance vaccine protection but did not impair it either [31]. Future vaccine development may exploit conserved antigens (e.g., outer membrane proteins, flagellin) and multi-epitope approaches to achieve broad serotype coverage. Genomic epidemiology will guide selection of relevant strains for autogenous or commercial vaccines [10].

flowchart TD
    A[Clinical suspicion of colibacillosis], > B[Postmortem examination and sample collection<br/>Liver, heart blood, bone marrow, air sacs, feces]
    B, > C[Culture on MacConkey/EMB agar<br/>Lactose-fermenting colonies]
    C, > D[Biochemical identification or MALDI-TOF MS]
    D, > E[Antimicrobial susceptibility test<br/>Disk diffusion or broth microdilution]
    E, > F{ESBL or colistin resistance detected?}
    F, >|Yes| G[PCR for resistance genes<br/>blaCTX-M, mcr-1/2]
    F, >|No| H[Serogrouping via slide agglutination or multiplex PCR]
    G, > H
    H, > I[Virulence gene profiling<br/>fimH, iroN, ompT, iss, hlyF]
    I, > J[Epidemiological typing<br/>MLST, WGS]
    J, > K[Implement control measures:<br/>Biosecurity, vaccination, targeted therapy]

Conclusion

Avian colibacillosis remains a major challenge for the poultry industry, driven by diverse APEC serogroups, increasing antimicrobial resistance, and complex transmission dynamics including the role of contaminated feces. Diagnosis requires integrated bacteriological, serological, and molecular tools, with WGS providing critical epidemiological insights. Treatment is complicated by multidrug resistance, but phytochemicals, probiotics, and nanoparticle-based formulations offer promising alternatives. Vaccine development has progressed from simple bacterins to sophisticated platforms such as membrane vesicles, bacterial ghosts, and in ovo nanovaccines, which can provide broader protection. Continued surveillance, prudent antimicrobial use, and adoption of effective vaccination strategies are essential for sustainable control.

References

[1] Orakpoghenor, O. (2025). Avian colibacillosis: A review of cases (2013–2023) diagnosed at the Avian Clinic of the Veterinary Teaching Hospital, Ahmadu Bello University Zaria, Kaduna State, Nigeria. Collective Journal of Veterinary Science. URL: https://www.semanticscholar.org/paper/52dc577b06862c093579f84d3743c53d9d995ec8

[2] Gerasimova, A. O., Novikova, O. B., & Savicheva, A. A. (2023). Avian colibacillosis – current aspects. Veterinary Science Today. URL: https://www.semanticscholar.org/paper/6709bc72aa3c2bf1242190d84494e86dd72e6bbd

[3] Taunde, P., Bianchi, M., Mathai, V. M., et al. (2021). Pathological, microbiological and immunohistochemical characterization of avian colibacillosis in broiler chickens of Mozambique. Pesquisa Veterinária Brasileira. URL: https://www.semanticscholar.org/paper/e61531c55f4d181f9a49eac82b440d7e3a18d390

[4] Runcharoon, K., Favro, M. E., & Logue, C. M. (2025). The pathogenicity traits of avian pathogenic E. coli (APEC) O25-ST131 associated with avian colibacillosis in Georgia poultry and their genotypic and phenotypic overlap with other ExPEC. Journal of Applied Microbiology. URL: https://www.semanticscholar.org/paper/526a4ae9b6ee6a697fdcc53cee7f9efae221c4a7

[5] Saeed, M. A., Asif, H., Ehtisham-Ul-Haque, S., et al. (2025). Detection and risk factor analysis of avian colibacillosis associated with colistin-resistant Escherichia coli and Klebsiella pneumoniae. Frontiers in Veterinary Science. URL: https://www.semanticscholar.org/paper/d9ead34a8ad1a755b82df1ea43df304bbdbaeacf

[6] Chenouf, N., Messaï, C. R., Carvalho, I., et al. (2025). Serogrouping and molecular characterization of ESBL-producing avian pathogenic Escherichia coli from broilers and turkeys with colibacillosis in Algeria. Antibiotics. URL: https://www.semanticscholar.org/paper/334f8a1df3a813b02ff66399e4639a47d79e1ee1

[7] Misumi, W., Magome, A., Okuhama, E., et al. (2023). CTX-M-55-type ESBL-producing fluoroquinolone-resistant Escherichia coli sequence type 23 repeatedly caused avian colibacillosis in Kagoshima Prefecture, Japan. Journal of Global Antimicrobial Resistance. URL: https://www.semanticscholar.org/paper/c0bb4f10fc57fc5fa479936d11341b736693abe7

[8] Runcharoon, K., Favro, M. E., & Logue, C. M. (2024). Longitudinal analysis of avian pathogenic Escherichia coli (APEC) serogroups and pathotypes from avian colibacillosis in Georgia: A continued investigation – year 2 analysis. Poultry Science. URL: https://www.semanticscholar.org/paper/5556a87e222933c469eaf19e835c587795ab2ea4

[9] Joseph, J., Magee, C., Jia, L., et al. (2024). Phenotypic virulence characterization of avian pathogenic Escherichia coli (APEC) isolates from broiler breeders with colibacillosis in Mississippi. Journal of Applied Microbiology. URL: https://www.semanticscholar.org/paper/5f9078cb6de45f3eb5f8f479c0d8bc99a150e746

[10] Biström, M., Vennerström, P., Lienemann, T., et al. (2025). Genomic epidemiology of avian pathogenic Escherichia coli in two broiler colibacillosis outbreaks in Finland. Veterinary Microbiology. URL: https://www.semanticscholar.org/paper/a0b7e4950f94115d7afba4ec0f37eb5994ac0d51

[11] Zhang, J., Cherbuin, J. D. R., Khan, M., et al. (2025). Complete genome sequences of three avian pathogenic Escherichia coli strains isolated from colibacillosis-affected poultry in Pakistan. Microbiology Resource Announcements. URL: https://www.semanticscholar.org/paper/865717cc9a8fc208d59153d9ab4b335525a8e15

[12] Saliha, U., Tivendale, K. A., Noormohammadi, A. H., et al. (2022). Nebulization as a more efficient method than atomizer for experimental reproduction of avian colibacillosis in young chickens. Avian Pathology. URL: https://www.semanticscholar.org/paper/4b08f6b22863cf8b9ee9894ecb5caf273e188f32

[13] Sutormina, L., Samoilova, Z., Smirnova, G., et al. (2025). Effect of medicinal plant extracts on biofilm formation in Escherichia coli strains causing avian colibacillosis. Journal of Innovative Solutions for Eco-Environmental Sustainability. URL: https://www.semanticscholar.org/paper/9d06a513ccfaa4467e9bdfa4ce516a5f6fb0e704

[14] Sani, H., Iliyasu, M., Alhaji, S., et al. (2025). Virulence phenotype of avian pathogenic Escherichia coli (APEC) and prevalence of colibacillosis among poultry birds in Bauchi Metropolis, Nigeria. International Journal of TROPICAL DISEASE & Health. URL: https://www.semanticscholar.org/paper/0ca4252ec4c16aeada1fdd887b6bf8ea19d73a47

[15] Saci, S., Nabti, E., Sebbane, H., et al. (2025). Exploring Thymus vulgaris extract as a phytotherapeutic agent: A multifaceted approach to tackle avian colibacillosis and drug resistance. Poultry Science. URL: https://www.semanticscholar.org/paper/e162b0ccfc25ce13e4dfd050be9b31163ab48c8d

[16] Ren, X., Yuan, S., Ren, J., et al. (2024). Effect of caffeic acid grafted chitosan loaded quercetin lyophilized powder formulation on avian colibacillosis and tissue distribution. Frontiers in Veterinary Science. URL: https://www.semanticscholar.org/paper/4741487f4ffbdeacf6b5af4dbc55528b1058cb83

[17] Meng, J., Wang, W., Ding, J., et al. (2024). The synergy effect of matrine and berberine hydrochloride on treating colibacillosis caused by an avian highly pathogenic multidrug-resistant Escherichia coli. Poultry Science. URL: https://www.semanticscholar.org/paper/91e809783baa48b5e0c48353ae279f695e3afec9

[18] Bao, J., Zhang, Y., Zhang, L., et al. (2021). Therapeutic effect of Schisandrin A on avian colibacillosis through gut-liver axis. Poultry Science. URL: https://www.s

[19] Liu, H., Yang, A., Wei, X., et al. (2026). Preventive effects and underlying mechanisms of Ilex rotunda Thunb.-Cyperus rotundus L. herb pair extract on avian colibacillosis in chickens. Poultry Science. URL: https://www.semanticscholar.org/paper/964db70021440e62ddd0904c16f38d4629440d91

[20] Li, F., Lv, B., Zuo, J., et al. (2024). Effect of solid-state fermentation products of Lactobacillus plantarum, Candida utilis, and Bacillus coagulans on growth performance of broilers and prevention of avian colibacillosis. Veterinary Sciences. URL: https://www.semanticscholar.org/paper/8baf7bca6863d2e2451b03d272adf1de0ccdfc22

[21] Masood, K., Yasmin, H., Batool, S., et al. (2021). A strategy for mitigating avian colibacillosis disease using plant growth promoting rhizobacteria and green synthesized zinc oxide nanoparticles. Saudi Journal of Biological Sciences. URL: https://www.semanticscholar.org/paper/7713900427f97f63e728760ad420fb70b3e23ed3

[22] Zhu, D., Zhang, Y., Wang, Z., et al. (2024). Exploiting membrane vesicles derived from avian pathogenic Escherichia coli as a cross-protective subunit vaccine candidate against avian colibacillosis. Poultry Science. URL: https://www.semanticscholar.org/paper/10a81e6ab315944c7e7ac5b02fb303a64ecc9111

[23] Indrawati, A., Safika, S., Nugroho, C., et al. (2025). Development and evaluation of a bivalent oil-adjuvanted vaccine against necrotic enteritis and avian colibacillosis in poultry. Advances in Animal and Veterinary Sciences. URL: https://www.semanticscholar.org/paper/ae70af3a78c87f31edd8e066da29ef06de35015f

[24] Scuotto, A., Ogonczyk-Makowska, D., Magnez, R., et al. (2026). Safety, immunogenicity, and vaccine compatibility of a trivalent inactivated in ovo nanovaccine against avian colibacillosis in broilers under commercial hatchery conditions. Animals. URL: https://www.semanticscholar.org/paper/b0de0f51d7f846defbfa4872a31d883634d2acdf

[25] Kamil, M., Abu-Bakar, L., Reduan, M. H. F., et al. (2023). The use of medicinal plants in avian colibacillosis management: A review. Veterinary Integrative Sciences. URL: https://www.semanticscholar.org/paper/9f36e63c7f99602025d53aaeda187fe213f6fc6b