Understanding avian pathogenic Escherichia coli

Avian Pathogenic E. coli, or simply APEC, is an extra-intestinal subgroup of pathogenic Escherichia coli that can cause disease in susceptible poultry, most commonly leading to clinical colibacillosis. Because colibacillosis is generally a secondary disease – and E. coli and APEC are extremely diverse – it has been challenging to identify and differentiate between APEC strains.

For decades, commensal E. coli inhabitants of the poultry gut have been largely differentiated from pathogenic variants like APEC using classical serotyping, specific virulence gene markers and the associated disease pathology. Today, however, these basic concepts are increasingly challenged by advances in molecular research and field observations.

Evolving APEC pathotype

Historically, clinical isolates from diseased birds were often dominated by serogroup O78 or O2, which was reported in textbooks for years. Early in the 21st century, plasmids known as ‘ColV’ and ‘ColBM’ were identified among specific APEC-containing virulence genes. These have been subsequently referred to as the APEC plasmids and are recognised as a differentiating feature of isolates from various serogroups causing colibacillosis.

Advances such as genome sequencing and bioinformatics, coupled with changing etiology, have begun to reveal a more complex disease process and variability among isolates. New serogroups are continually emerging and the presence of an APEC plasmid alone is now considered insufficient to predict virulence, as these plasmids are now commonly found in commensal E. coli from a healthy avian gastrointestinal tract.

The use of high-resolution genomics has revealed that APEC likely becomes successful through a mix-and-match combination of certain serogroup or sequence type chromosomal backgrounds combined with APEC plasmid variants. Clearly, there are dominant mix-and-match combinations which are responsible for the vast majority of colibacillosis cases in poultry, and there is substantial strain overlap across different bird types raised for poultry production.

Non-specific management of APEC risk

Like most disease-causing microbial agents, intervening in the transmission cycle is essential to preventing exposure. An emphasis on biosecurity and general management practices are the primary defences against APEC intrusion into a flock. APEC may enter a system via multiple routes and, aside from vertical transmission from breeder stock, used litter may be a leading exposure source. However, exposure alone does not indicate a strong probability of an outbreak, as APEC is very likely to be present in most healthy flocks.

While some APEC are capable of eliciting disease alone, APEC risk is highest when a population becomes susceptible as a result of stress from environmental or primary infectious sources, increasingly compromising flock and individual bird resilience. While APEC is transmitted via the faecal-oral route, respiratory transmission also occurs and is the primary route of disease.

Environmental stressors, such as increasing ammonia levels or dust, increase respiratory stress and exacerbate disease susceptibility. Other stress stimuli, such as heat stress, nutrient deficiencies or other disease challenges, also increase susceptibility to APEC, emphasising the focus on biosecurity and good management practices.

Specific management of APEC risk

The most common and targeted preventative tool to reduce APEC risk is vaccination. A commercially-available live-attenuated vaccine is available for serogroup O78 and offers some degree of cross protection for other serogroups. However, APEC vaccination is often customised for an operation by the development and administration of autogenous vaccines. In general, vaccination approaches confer protection against problematic APEC strains that may be predominant and endemic to an operation or geographical region.

However, other serogroups or strains may emerge for which the vaccine does not offer protection. While there is ongoing research on novel vaccine technologies that would offer broad, heterologous protection, most are far from gaining regulatory approval or commercial availability. Vaccination is likely to benefit from the combined application of other technologies, such as antibiotic-alternative feed additives that promote health through modulation of the immune system and gut microbiome, thereby potentially helping to reduce susceptibility to APEC. The use of such technologies could, for example, support vaccination efforts and improve general resilience over production life, consistent with their daily use.

Feed additive technologies

As mentioned, feed additives that promote gut health, microbiome and immune system modulation may generally support improved systemic health and therefore resilience against environmental stressors and infectious challenges, such as APEC. In recent years, postbiotics derived from proprietary ex vivo Saccharomyces cerevisiae fermentations, often described as Saccharomyces cerevisiae fermentation products (SCFPs), have increasingly been evaluated in various animal species.

In poultry, these products have been shown to play a role in supporting improved functional health, performance and disease resilience. Environmental stressors increase susceptibility to APEC infections because of compromised immunity. Thus improved stress responses modulated by effective feed additives can aid in offsetting adverse impacts on production and increased susceptibility to infectious diseases.

A common environmental source in poultry production is heat stress. To give an example, SCFP postbiotic administration was evaluated in a broiler heat stress model in which the SCFP postbiotic significantly lowered corticosterone, heterophil/lymphocyte ratios and physical asymmetry scores. In another study, 2 levels of SCFP postbiotic inclusion were compared to a control diet in broilers challenged with Eimeria tenella, demonstrating that SCFP postbiotic support improved average daily gain and significantly increased CD3+, CD4+, and CD8+ T-lymphocyte counts, as well as other immunity markers.

From these studies we may conclude that SCFP postbiotic supplementation helps improve immune function and growth performance under coccidia challenge, a primary disease stressor.

SCFP postbiotics have repeatedly been associated with reductions in the colonisation potential of various Salmonella serovars, as well as Campylobacter spp. In view of the taxonomical similarities shared by Salmonella and E. coli, as well as the overall impact of the additive on improving performance and health responses under various challenges, it is reasonable to consider SCFP postbiotic a candidate feed additive solution for managing APEC risk. Thereby supporting existing biosecurity, management and vaccination strategies.

A study in the proceedings of the 2022 American Association of Avian Pathologists (AAAP) conference reported on the evaluation of SCFP postbiotic in an APEC O78 direct challenge model. The study divided 120 chickens into 8 experimental groups of which 4 were fed a control diet and the remainder were fed the same diet supplemented with the SCFP postbiotic feed additive. At 14 days of age, the chickens were challenged via an oral or intratracheal route with APEC O78 and subsequently necropsied at 21 days of age with colibacillosis lesion scoring and tissue sampling for APEC enumeration. The authors of the study reported “consistently lower lesion scores” for those APEC-challenged birds fed the SCFP postbiotic product, suggesting a potential protective effect was imparted.

Prospective solutions

Phytogenics, or plant extract-based feed additives, as well as organic acids may also hold promise as prospective solutions for managing APEC risk. A variety of plant extracts, like SCFP postbiotics, have demonstrated varying effectiveness in modulating immunity and microbiome dynamics under various stress or infectious challenge models.

A recent study, for example, reported on the combination of thyme and carvacrol essential oils in combination with hexanoic, benzoic and butyric acids in an APEC O78 broiler challenge model. The comprehensive study reported the combined treatment was associated with lower gross lesion scores and E. coli colonisation, improved feed conversion and modulation of a variety of immune markers and microbiome taxa. Collectively, these effects contributed to mild alleviation of disease severity induced by the APEC O78 challenge.

In another study, researchers explored garlic and ginger extract supplementation in broiler chicks challenged with a multi-drug resistant APEC O78 isolate after in vivo and ex vivo research demonstrated favourable microbial growth inhibition and enhanced immunomodulatory outcomes. Birds receiving the dietary extracts for 3 weeks prior to challenge were reported to have significantly reduced mortality and tissue colonisation loads when compared to the challenged controls.

The addition of oregano essential oil to drinking water was comparatively evaluated with difloxacin and controls as a therapeutic intervention to an APEC O27 intra-tracheal challenge. Birds were evaluated and sampled in intervals up to 21 days post infection with detailed hematological, biochemical and histological analyses. Oregano essential oil supported livability comparatively to the antibiotic treatment and, similarly, provided indications of reduced severity of the infection comparable to the antibiotic, leading researchers to conclude that it has practical use as an antibiotic alternative, and a hepato and nephro-protective therapeutic.

Other research groups have recently reported in vitro effects of phytogenic extracts, such as cinnamon essential oil and combinations of multiple compounds, against multiple serotypes and strains of APEC isolates, perhaps strengthening the idea that such feed-additive products may confer added protection alongside vaccination alone and risk management practices.

In conclusion, there is unlikely to be a single solution to all known or emergent APEC strains. However, growing evidence suggests that feed additive technologies, particularly postbiotics and phytogenics, may offer benefits in promoting resilience against APEC infection and, subsequently, help reduce the severity of infection if it occurs.

Notably, many of these products are likely to be complementary to co-administer together with APEC vaccines and also aid in managing other biosecurity-related concerns, such as foodborne pathogens (Salmonella or Campylobacter), or other microbial agents of concern for poultry health and welfare. However, feed additives vary greatly in function, application and mechanisms which also need to be considered. Ultimately, vaccination, biosecurity and good farm management practices will remain key preventative measures against APEC for the foreseeable future. However, the right feed additive solutions are likely to provide additional support and should be carefully considered as part of the nutritional strategy.