As the complexity of global food production continues to grow, the “One Health” concept that human health and animal health are interdependent becomes all the more relevant to food safety.

Zoonotic infectious diseases transmitted in food and water are a significant cause of illness in both developing and developed countries. Some pathogens that cause infection live in the intestinal tracts of animals and can spread to humans through contamination of the environment or of products—such as eggs during laying.

According to an April 2019 report from the Foodborne Diseases Active Surveillance Network of the U.S. Centers for Disease Control and Prevention’s Emerging Infections Program,[1] the incidence of infections with Enteritidis, the most common Salmonella serotype, has not declined in over 10 years. Enteritidis is adapted to live in poultry, and eggs and chicken meat are important sources of infection. In contrast, a decline in serotype Typhimurium was observed during the same period, coinciding with declines in human illnesses related to this serotype. Changes in poultry production practices, including vaccination against Typhimurium, might have resulted in these declines. In the United Kingdom, vaccination of both broiler and layer chickens against Enteritidis, along with improved hygiene, was followed by a marked decrease in human Enteritidis infections.

Eggshell Contamination Can Cause Bacterial Transmission
Eggs are laid from the cloaca, the single posterior opening for a bird’s digestive, urinary, and reproductive tracts that is used to expel feces and lay eggs. This means that eggshells are naturally contaminated.  

The cuticle coating the eggshell surface is the egg’s first line of defense against microbial infection. While nature provides a very strong barrier to contamination getting through the shell, contamination of the navel or yolk sac can occur in 1–5 percent of baby chicks. This can cause baby chick mortality and transmission of Salmonella on the eggshells from the parents to infect the progeny.  

To reduce baby chick mortality as well as bacterial transmission, producers can ensure nest cleaning and sanitation and increase the frequency of egg collection so that eggs don’t sit in the nest or on belts, collecting more contamination. 

Intestinal Health Is a Significant Concern in Poultry Production
The gastrointestinal tract has many important functions, including digestion and absorption of nutrients and energy, and immunological protection against infectious and noninfectious disease. The disruption of intestinal health can affect one or more systemic functions and subsequently impact the health and welfare of birds as well as humans.  

The intestinal health of poultry has broad implications for animal welfare, food safety, and environmental impact, as well as for production efficiency.

Extra nutrient density in the back of the intestine of the chicken, in the cecal pouches, causes proliferation of bacteria in the lower gut, such as pathogenic Clostridium perfringens, coccidia, Escherichia coli, or Salmonella spp.

Uncontrolled, Clostridium in the ceca can overgrow and produce toxins. This bacteria and the toxin produced are then drawn forward by normal retroperistalsis, causing necrotic enteritis, an acute Clostridium infection characterized by severe necroses of intestinal mucosa, which can be fatal, or dysbacteriosis, a microflora imbalance that results in disruption of nutrient absorption and poor feed efficiency (Figure 1).[2] Since feed efficiency is the largest driver of cost of production, poor feed efficiency means significantly higher cost of production for affected flocks.

Coccidiosis Causes Widespread Damage and Bacterial Proliferation
One of the most common and economically important diseases of chickens worldwide, coccidiosis is ubiquitous in poultry houses and has a major economic impact on the global poultry industry. This intestinal parasitic infection causes heavy damage to intestinal cells and the lower gut, and cell debris (protein) travels down to the ceca. The damaged cells don’t absorb nutrients efficiently, so extra undigested feed also finds its way to the lower gut.

Coccidiosis is typically controlled by anticoccidial products, such as ionophores or chemicals. The ionophores are considered antibiotics in the U.S., although they are not commonly used in human medicine. In Europe, ionophores are classified as a non-antibiotic feed additive. These products have long-term sustainable efficacy against coccidiosis as well as significant anti-Clostridium activity, helping avoid bacterial overgrowth in the lower gut. Chemical anticoccidials are highly effective but unsustainable, as resistance develops rapidly; additionally, they do not provide anti-Clostridium activity.

Farm management has a significant effect on coccidiosis control. For example, lower stocking density reduces the stress level on the flock and can reduce factors such as litter moisture that help drive the coccidiosis reproduction cycle.

Longer downtime between flocks allows the ammonia that is naturally in reused litter to kill off coccidiosis. Although coccidiosis populations naturally decline with these management tools, they don’t go away.

Vaccines induce immunity against coccidiosis, playing a large role in regaining control of the infection. Coccidiosis control with live vaccines has become a significant strategy in the U.S. but is not as widely used in Europe. Vaccines are also highly sensitive to anticoccidials and can be used to repopulate poultry houses with more sensitive strains to prolong the efficacy of chemical anticoccidials. However, live vaccines cause some intestinal damage while they are inducing immunity, producing cellular debris and reduced nutrient absorption at a critical time from 2 to 3 weeks of age.  

Several natural products have an anticoccidial effect strong enough to reduce coccidiosis vaccine reaction, which may help modify the intestinal microflora.

Strategies to directly influence the growth of Clostridium in the lower intestine, which can rapidly overwhelm flocks even when coccidiosis is under control, can also be used.

Farm Management Strategies Are Critical
When birds are out of feed for a short time, they tend to gorge themselves afterward. When they “binge”-eat, they don’t digest the feed as completely as they would when they are eating in a normal steady state. The undigested nutrients find their way to the lower gut, causing proliferation of bacteria.

Farm management is critical to avoiding feed competition. Reduced stocking density can avoid feed competition, as can attention to feed form (pellets, crumbles), to ensure that birds can consume as much as they want, as quickly as they want. 

Opening space to the flock between the first and second week of age is important to avoid high densities with excessive competition for feed, water, and physical space that causes stress.

Accidental feed outage can be caused by hot weather (birds back off feed at peak heat) or delays in feed delivery. Lighting programs may also impact feed intake and digestion. Four to six hours of total darkness per day have been found to improve feed conversion ratios, probably due to slower feed intake and better digestion.

To avoid undigested protein in the lower gut and ensure that appropriate foregut digestion occurs most of the time, attention to feed particle size, good water quality, quality control of dietary fat type and levels, and addition of exogenous enzymes can help maintain good digestion.

Ration formulation from highly digestible protein sources is recommended. While least-cost ingredients can reduce live production cost, these may also be much less digestible, and some may contain rancid fats or mycotoxins that can damage cells, resulting in cell debris that reaches the lower gut. The first 3 weeks of age are critical when the birds are on a live coccidiosis vaccine, which is actively growing in the intestine, peaking at 14 to 21 days. The cell debris from the vaccination reaction is already finding its way to the back of the intestine.

Dietary fat type and level can affect digestion, and it is important to maintain a minimum of 1 percent fat in the diet to stimulate gastric function and slow release to the small intestine.

Protease enzymes help more effectively break down the protein to avoid excess protein escaping to the lower gut, thus reducing microbial proliferation by reducing the indigestible components of feed.

Probiotics or direct-fed microbials are combinations of live bacteria or yeast intended for competitive exclusion or antagonism of the pathogenic bacteria. They may increase digestive enzyme activity, and they may produce antibacterial substances as part of their antagonistic activity. The U.S. allows only defined cultures, while the EU has registered some undefined cultures, which seem to have better competitive exclusion ability. The main two categories of probiotics are the Bacillus spp. and lactic acid-producing species, such as Lactobacillus acidophilus, from active yogurt cultures.

Prebiotics are products made from dead yeasts, yeast extracts, or purified oligosaccharides. These products bind several major pathogens while reducing inflammation and modulating the intestinal immune system. They provide a good source for “good” bacteria to allow them to create their own competitive exclusion of bad bacteria. While prebiotics have a direct beneficial effect against Salmonella and E. coli, they rely more upon the competitive exclusion effect of healthy “good” bacteria to prevent the overgrowth of Clostridium.

Each type of product has demonstrated efficacy either independently or in combination use, and evidence suggests that the combined administration of prebiotics and probiotics—symbiotics—may be an effective strategy.

Salmonella spp. also grow in the ceca. Things that disrupt the ceca, like coccidiosis or overgrowth of Clostridium, may also favor Salmonella overgrowth. Unlike with Clostridium, however, there are vaccines for Salmonella infection.

Farm management strategy to improve overall intestinal health also reduces conditions that promote Salmonella growth. This is where the prebiotics are also effective. Products based on Saccharomyces cerevisiae (baker’s yeast) have been widely accepted because they have demonstrated measurable practical efficacy to reduce Salmonella (Figures 2 and 3).

Conclusion
In summary, due to increasing demands for animal welfare and food and environmental safety, as well as production efficiency, intestinal health is a significant issue in poultry production. Improvements in housing and changes in feeding, management, and health programs can help manage the numerous factors that add up to intestinal health and increase poultry safety.  

Thanks to Linnea Newman, D.V.M., of Merck Animal Health for contributing her expertise for this article.

References
1. www.cdc.gov/mmwr/volumes/68/wr/mm6816a2.htm?s_cid=mm6816a2_x.
2. Moore, R.J. 2016. “Necrotic Enteritis-Predisposing Factors in Broiler Chickens.” Avian Pathol 45:3:275–281.