Sources of Salmonella colonization in chickens have been extensively discussed in Part 1 of this article.1 Part 2 will emphasize on-farm (or pre-harvest) controls for minimizing the prevalence and/or concentrations of Salmonella in broilers. A number of sources exist for Salmonella colonization of the broilers:

• Poultry house external environment
• Poultry feed
• Hatchery
• Chicks
• Poultry house internal environment
• Water
• Bird droppings and litter.

An effective and significant risk reduction strategy is to minimize the potential for colonization from each of these sources. This article discusses practical Salmonella control measures that can be implemented by the poultry industry at the pre-harvest stage or on the farm.

Controlling Salmonella Introduction into the Flock

Reducing Salmonella prevalence in poultry meat and broiler carcasses requires reducing and controlling the introduction of the microorganism into the poultry flock. Once Salmonella introduction and subsequent colonization occurs even in one bird in a flock, the rapid spread of the microorganism to other birds through horizontal transmission can occur within a short period of time. Salmonella colonization results in the excretion of the Salmonella cells in the feces within a few days in high concentrations, often up to 9 log CFU/g. Even if the fecal droppings dry over time, the ability of Salmonella to survive low-moisture conditions for extended periods of time and the coprophagic tendencies of the birds can serve to spread and further colonize other birds in the flock. There is a critical need to reduce and/or minimize the risk of Salmonella introduction into the flock from the identified sources of this microorganism. However, eliminating the risk of Salmonella introduction from the identified sources of the microorganism on the scale of the entire poultry industry may not be economically feasible. As such, there is a need to identify other measures that can modulate the spread and colonization of the microorganism.

Poultry house surroundings have been identified as a source for Salmonella and, potentially, other foodborne pathogens such as Campylobacter. Spread of avian influenza in farm birds through wild birds is an excellent example of the role the external environment can play. Other factors—such as control of on-farm personnel and their hygiene in moving between poultry houses; visitors, especially those that have been in contact with other birds on other farms; and upkeep of poultry house surroundings—are important for control of Salmonella spread and colonization to chickens. Proper biosecurity training onsite, as well as the implementation and maintenance of biosecurity measures, are important to minimize the spread of Salmonella across flocks.

Biosecurity refers to practices designed to prevent the introduction and spread of disease-causing microorganisms on farms. While these measures are meant to mitigate the spread of microorganisms that cause poultry diseases, they are also effective in minimizing the spread of Salmonella and Campylobacter. This can be accomplished by maintaining the farm, equipment used to manage the operations within and outside the poultry house, boots, gowns, birds within the house, and vehicles that frequent the farm in a manner that minimizes the spread of microorganisms such as bacteria and viruses. Care should also be taken to minimize the presence and spread of rodents and pests across areas within the farm.

Biosecurity is a critical aspect of minimizing the risk of Salmonella introduction and colonization of birds in poultry houses. Primary biosecurity measures for poultry farms are described thoroughly in a University of Georgia Extension Bulletin.2 These measures include the following actions:

• Keep visitors to the farms to a minimum
• Maintain proper hygiene of personnel entering each house
• Keep all animals out of poultry houses
• Practice sound rodent and pest control programs
• Avoid contact with non-commercial poultry or wild birds
• Inspect flocks daily
• Keep areas around the poultry houses and feed bins clean.
• Implementation of all of these measures is critical for minimizing the risk of Salmonella introduction into the flock.

In addition to the above measures to reduce and/or eliminate the sources of Salmonella, additional mitigation strategies that can be implemented are broadly categorized as: (1) poultry house management, (2) dietary strategies, and (3) immunity modulating strategies.

Poultry House Management

Eliminating sources of Salmonella in the exterior environment of the poultry house, as currently structured or constructed in the U.S., may be impossible. However, reasonable measures to minimize the risk of introduction of the microorganism to the interior of the poultry house, and eventually the birds, should be implemented. These measures include proper hygiene of personnel entering the poultry house—e.g., cleaning and sanitizing of the boots and providing boot dips prior to entering the poultry house, as well as sanitation of any equipment introduced into the poultry house.

In addition to these measures, proper litter management is critical as the birds are continually exposed to litter throughout the growth period, and the litter can be a source of Salmonella from previous flocks. The rising costs of poultry bedding material and/or the difficulty of procuring bedding material has resulted in U.S. poultry producers reusing the same litter for multiple flocks, with a top dressing of fresh litter between flocks. This practice allows Salmonella persistence in the litter if it is not properly managed or treated. While the practice has the benefit of contributing beneficial bacteria to the subsequent flocks, Salmonella can survive in the litter if present in the prior flock, thus resulting in horizontal transmission to subsequent flocks.

In a recent study, approximately 46 percent of the litter from poultry farms was positive for Salmonella, with populations between less than 1 log CFU/g and greater than 5 log CFU/g.3 While the down time between the flocks can vary, it should not be less than 14 days to allow for pathogen (necrotic enteritis causing Clostridium perfringens and coccidia pathogen Eimeria spp.) destruction and infection to the new flock. Proper windrowing of the litter between flocks can be a means to reduce these poultry pathogens, as well as populations of Salmonella, if not eliminate it from the poultry litter. Reduction in Salmonella population can occur in the litter based on the pH, water activity, and temperature. Reduction in Salmonella population in litter may not be adequate, however, as even low numbers can result in colonization of the birds and subsequent spread through fecal droppings.

Research is underway to identify temperature and humidity conditions that would significantly reduce the populations of C. perfringens, Eimeria spp., and Salmonella. Elimination of Salmonella in poultry litter should be the goal of windrow composting practices adopted by poultry farmers. However, effective composting is reliant upon achieving adequate temperatures. Thus, windrow composting must be done as close to flock removal as possible to allow for adequate time to be effective.

Dietary Strategies

Supplementing poultry feed with antibiotics has been practiced for the past five decades, and use of antibiotics at subtherapeutic levels has been a primary practice of the industry to minimize the risk of pathogenic and non-pathogenic enteric microorganisms. Antibiotic usage, although at sub-therapeutic levels, has resulted in improvement to animal gut health and, consequently, reduction in the risk of certain pathogens.

However, recent changes in consumer awareness and regulatory pressure on the potential risk of development of antibiotic-resistant human pathogens have resulted in industry adopting an antibiotic-free (ABF) production system. At present, approximately 46 percent of all poultry meat produced in the U.S. is produced under the ABF system, which has resulted in poultry integrators searching for alternatives to antibiotics to provide the same protection to bird gut health and associated issues. Several strategies have been evaluated, including feeding the birds organic acids, probiotics and prebiotics, botanicals, bacteriocins, bacteriophages, novel compounds, and other feed additive combinations to provide the same level of protection in terms of gut health. Each of these strategies is discussed below.

Organic Acids. Supplementation of poultry feed or drinking water with organic acids has gained prominence with the switch to the ABF production system. The organic acids evaluated include short-chain fatty acids (e.g., formic, acetic, propionic, and butyric acids) and medium-chain fatty acids (e.g., capric, caproic, caprylic, and lauric acids) to acidify the feed, as well as the bird's gut environment, and to reduce the risk of high Salmonella populations in the chicken gut contents, which may contaminate the carcasses during processing. While the main mechanism of action of these organic acids is through acidification of the chicken gut contents, research has indicated minimal drop in chicken gut pH, while providing the protective bacteriostatic activity through controlling Salmonella populations.

Combinations of organic acids show more promise, with buffered formic acid and sodium formate providing reductions in Salmonella prevalence in birds with increasing doses (up to 0.9 percent4). In some cases, the organic acids were administered through drinking water closer to the harvest age of the birds. While research has shown that organic acid supplementation can reduce the risk of Salmonella prevalence and/or concentrations in the chicken gut system, the results are not consistent across studies. Other strategies for reducing Salmonella contamination in the feed and/or feedstuffs includes the use of formaldehyde or other aldehyde-based products.

Probiotics and Prebiotics. The concept of probiotics in human health has been the focus of research for decades, and several probiotics are commercially available. The human health benefits from such probiotics have been numerous, with the primary benefits being the maintenance of gut health and the balance of bacteria within the various segments of the gastrointestinal tract (GIT).

Probiotics are either single or mixed cultures of live microorganisms that provide beneficial effects in maintaining host intestinal microbial balance.5 Research on the use of probiotics in poultry is extensive, with the most common being lactic acid bacteria including several species within the genera Lactobacillus, Lactococcus, and Bifidobacterium, and others including Bacillus, Enterococcus, Escherichia coli, some molds, and yeasts. The most common mode of action attributed to probiotics is the competitive exclusion (CE), where the beneficial microorganisms occupy the common niche in the chicken gut, hence excluding the attachment and subsequent colonization with foodborne pathogens such as Salmonella.

Prebiotics are non-digestible feed ingredients that pass the upper GIT and enhance the activity of the beneficial microorganisms (probiotics) in the colon. The prebiotics characteristically are not absorbed in the upper GIT, are resistant to the acidic pH, stimulate the growth and metabolic activity of beneficial microorganisms, and modulate the host defense system. The common probiotics used in poultry include fructo-oligosaccharides (FOS), inulin, mannan-oligosaccharides (MOS), and xylo-oligosaccharides (XOS). Several of these are components of yeast cells—the cell walls and fermentation products.

The prebiotics and probiotics seem to work in tandem, providing the beneficial effects of maintaining gut health, with beneficial effects in reducing Salmonella colonization and shedding; reducing C. perfringens, E. coli, and Eimeria populations; and reduced mortality.

Botanicals. Synonymously referred to as phytogenic feed additives, phytonutrients, phytobiotics, phytochemicals, or plant-based feed additives, botanicals are a wide range of compounds derived from plants with a variety of biological activities. These can be classified as (1) phenolic compounds (apigenin, quercetin, curcumin, and resveratrol), (2) organosulfur compounds (allicin and other compounds), (3) terpenes (eugenol, thymol, carvacrol, and artemisinin), and (4) aldehydes (cinnamaldehyde and vanillin).7 Research indicates that the essential oils derived from botanicals such as thyme, basil, and oregano are effective in controlling foodborne pathogen colonization and shedding. The majority of these oils are considered as Generally Recognized as Safe (GRAS) and, therefore, suitable for supplementation of poultry feed. These agents modulate gut health and often improve microbial community; consequently, they reduce colonization of foodborne pathogens. The efficacy of these oils depends on several factors such as the inclusion level and feed composition.

Bacteriocins. These are peptides produced by certain bacteria and possess either wide- or narrow-spectrum antimicrobial properties. Often, bacteriocins are produced by lactic acid bacteria (LAB) such as Lactobacillus, Lactococcus, Pediococcus, and other bacterial genera. Often, some of the antimicrobial properties of the probiotics (LAB) are attributed to the bacteriocins produced by these bacteria in addition to other compounds such as lactic acid, hydrogen peroxide, etc. While use of bacteriocins to control foodborne pathogens in swine has been evaluated, studies focused on poultry use are limited.

Bacteriophages. These bacterial viruses often have limited or narrow host specificity and, therefore, can be used to control specific microorganisms such as foodborne pathogens Salmonella and Campylobacter. The use of bacteriophages to control disease-causing bacteria in poultry dates back to 1919, when d'Herelle used them as a therapy for chicken typhus.6 The majority of the bacteriophages used for control of foodborne pathogens are lytic phages, which infect the bacterial cell, replicate, and then destroy the bacterial cell. While some in vivo studies have shown their efficacy in reducing Salmonella fecal colonization by > 4.2 log CFU within 24 hours, widespread use in poultry production is limited. Research shows that use of phage cocktails prior to colonization of the birds with Salmonella, the administration of numerous doses of bacteriophage cocktails, and the need for a proliferation threshold (i.e., minimum level of bacteria concentration required to sustain a growing bacteriophage population) are necessary for their efficacy.

Immunity Modulating Strategies (Vaccines)

The poultry sector has relied on vaccines to control the Salmonella-related poultry diseases pullorum disease and fowl typhoid. These diseases have been practically eradicated in advanced countries through properly implemented vaccination programs, surveillance, and proper biosecurity measures. The vaccines harness the immune systems of the hosts to reduce Salmonella numbers upon infection, rather than controlling the disease. As immunizing young chicks within the first week is not effective since their immune systems are not fully developed, it is ideal to vaccinate the breeders to provide protective immunity to the progeny. This approach is used extensively in the layer industry to control Salmonella Enteritidis in the layer flocks, and consequently mitigate the Salmonella Enteritidis risk in the shell eggs.

All three Salmonella vaccine types—live-attenuated, inactivated, and sub-unit vaccines—are used in the broiler industry. Although the majority of them are tailored for intramuscular or sub-cutaneous delivery, oral immunization is probably the best delivery method in terms of ease of immunization of the birds, especially at the commercial broiler grow-out level. Most poultry integrators utilization vaccination against specific or multiple Salmonella serotypes or groups at the breeder level to avoid them at the hatchery and broiler grow-out and, consequently, at the processing level and in poultry meat.

While in ovo vaccination, vaccinating the eggs to provide immunity to the chick after hatch is popular for poultry viral pathogens (such as those that cause Marek's disease, infectious bursal disease, and fowl pox), vaccines for Salmonella have not been developed and evaluated for this type of delivery. This delivery mechanism may be valuable in controlling Salmonella in poultry as it presents the easier delivery of vaccine to the chicks, with the ability to immunize greater than 70,000 eggs within a short period. Experimental advances in vaccine approaches are encouraging for their increased efficacy and coverage.

Summary and Conclusions

In Part 1, we discussed the potential sources of Salmonella introduction into the poultry flocks and practical control measures to address each of those sources to reduce the risk of Salmonella in broiler flocks. It is necessary to control the sources of Salmonella introduction as much as possible, as relying only on subsequent measures will not be adequate to reduce the risk, especially in the post-antibiotic period.

With such control measures implemented to reduce Salmonella introduction through the identified sources, implementation of good biosecurity measures, along with a mixture of properly designed interventions outlined in this article, will further reduce the risk of Salmonella through reduction in concentrations and/or prevalence of Salmonella in and on the birds presented at slaughter. This is critical to address the challenges of further reducing Salmonella prevalence in poultry meat and, consequently, Salmonella-related foodborne illness from chicken meat.

Part 3 of this article, to be published in the December/January issue, will focus on post-harvest controls during processing to minimize the risk of Salmonella from poultry products.

References

1. Thippareddi, Harshavardhan, and Manpreet Singh. "A Critical Look at Reducing the Risk of Salmonella in Poultry Products." Food Safety Magazine August/September 2022. https://digitaledition.food-safety.com/august-september-2022/feature-spotlight/.
2. Fairchild, Brian D., and Dan L. Cunningham. "Biosecurity basics for poultry growers." University of Georgia Extension Bulletin no. 1306 (2020). https://extension.uga.edu/publications/detail.html?number=B1306&title=Biosecurity Basics for Poultry Growers.
3. Dunn, L. L., V. Sharma, T. K. Chapin, L. M. Friedrich, C. C. Larson, C. Rodrigues, M. Jay-Russell, K. R. Schneider, and M. D. Danyluk. "The prevalence and concentration of Salmonella enterica in poultry litter in the southern United States." PLOS One 17 (2022).
4. Adhikari, P., S. Yadav, D. E. Cosby, N. A. Cox, J. A. Jendza, and W. K. Kim. "Effect of organic acid mixture on growth performance and Salmonella Typhimurium colonization in broiler chickens." Poultry Science 99 (2020): 2645–2649.
5. Fuller, R. "Probiotics in man and animals." Journal of Applied Bacteriology 66 (1989): 365–378.
6. Summers W.C. "Bacteriophage Therapy." Annual Review of Microbiology 55 (2001): 437–451.
7. Rossi, B., A. Toschi, A. Piva, and E. Grilli. "Single components of botanicals and nature-identical compounds as a non-antibiotic strategy to ameliorate health status and improve performance in poultry and pigs." Nutrition Research Reviews 33, no. 2 (December 2020): 218–234. https://pubmed.ncbi.nlm.nih.gov/32100670/.