Reducing the risk of Salmonella food poisoning in an age of antimicrobial resistance

Salmonella is commonly found in the intestines of healthy birds and mammals, including swine and ruminants. The most common non-motile serotypes in poultry are S. Pullorum and S. Gallinarum, which are species-specific and only occur in birds. These two serotypes target chicks and poults respectively, and have been responsible for severe financial losses to poultry producers in the past. The serotypes that are motile in poultry, referred to as paratyphoid salmonellae, cause foodborne disease (zoonosis) in humans. Paratyphoid Salmonella infections are very common, and are characterized in poultry by asymptomatic colonization of the gastrointestinal tract. This contaminates the eggs and broiler carcasses, raising concerns for public health.

Significance for Public Health

Poultry has been identified as the main reservoir of Salmonella that can ultimately infect humans. Although there are other zoonotic bacteria, some of which have recently had a high profile in the media (such as Campylobacter jejuni), Salmonella remains one of the main sources of foodborne illness worldwide. Some historical data demonstrate the significance ofSalmonella for public health: 51% of human food poisoning cases in the United States between 1973 and 1987 were attributed to Salmonella (Bean and Griffin, 1990). Nearly 84% of human foodborne disease in Scotland between 1980 and 1989 were caused by Salmonella (Oboegbulem et al., 1993). Surveillance studies carried out in 1995 revealed that three serotypes (Enteritidis, Typhimurium and Typhi) accounted for 76% of all isolates (Herikstad et al., 2002). The Laboratory-based Enteric Disease Surveillance (LEDS) system collects reports of infections in the USA, and the last surveillance report (in 2016) reported 2.4% fewer cases than in 2015. However, the incidence of infections caused by serotypes Infantis, Muenchen, Montevideo, and Braenderup have increased (see Figure 1 and Table 1, showing data taken from) www.cdc.gov/nationalsurveillance/pdfs/2016-Salmonella-report-508.pdf). The highest incidence of infection in the USA and Europe was recorded in infants. A major outbreak of Salmonella was reported in the USA in 2018, when over 200 million eggs were recalled. The outbreak was traced to the second-largest egg producer in the USA, in North Carolina. The federal Food and Drug Administration (FDA) reported that eggs from the infected farm were likely involved in 22 cases of Salmonella, and this case was subsequently linked to Salmonella Braenderup.

Table 1. Percentage change among the 20 Salmonella serotypes most frequently reported to LEDS, comparing 2006, 2011, and 2016
Table 1. Percentage change among the 20 Salmonella serotypes most frequently reported to LEDS, comparing 2006, 2011, and 2016
Figure 1. Incidence rate of culture-confirmed human Salmonella infection reported to LEDS (all serotypes and individual serotypes with ≥ 1,000 infections reported in 2016), by year, United States, 1970–2016
Figure 1. Incidence rate of culture-confirmed human Salmonella infection reported to LEDS (all serotypes and individual serotypes with ≥ 1,000 infections reported in 2016), by year, United States, 1970–2016

The European Surveillance System (TESSy) reports annual data for Salmonella in the European Union and has revealed that the number of Salmonella cases in the EU decreased between 2004 and 2014. Before 2004, more than 200,000 human cases were reported each year in the 15 Member States, but this dropped to fewer than 100,000 cases in the 28 Member States in 2014. However, Salmonella outbreaks since 2014 have contributed to a change in this trend in both humans and poultry. This demonstrates that national Salmonella control plans in every EU country must continue their risk management activity. The latest data available in April 2018 reported more salmonellosis cases in 2015 than in 2014. The highest notification rates were in the Czech Republic and Slovakia (see https://ecdc.europa.eu/sites/portal/files/documents/AER_for_2015-salmonellosis.pdf).

Table 2. Distribution of confirmed salmonellosis cases per 100,000 population, EU/EEA, 2011–2015
Table 2. Distribution of confirmed salmonellosis cases per 100,000 population, EU/EEA, 2011–2015

Significance for Animal Health

Motile paratyphoid Salmonella, which are considered zoonotic, can be introduced into poultry farms from various sources and can be spread within and between flocks. Outcomes depend on whether the bacteria affect newly hatched chicks or mature birds. The most common effect in chicks of up to seven days old starts with the colonization of the gastrointestinal tract, followed by persistent shedding of Salmonella in the feces. The bacteria then multiply in the liver and spleen (Barrow et al., 1987), finally leading to bacteremia, which causes high levels of mortality. The situation is different in mature birds, as paratyphoid Salmonella  is not associated with morbidity or mortality in this age group. Timoney et al. (1989) demonstrated that, although S. Enteritidis disseminates extensively throughout the internal organs, including the ovary and oviduct, the birds remained clinically healthy, suffering mild diarrhea only. This enables Salmonella–contaminated eggs and meat to be produced inadvertedly, which is of a particular concern for public health.

Salmonella can be introduced into poultry farms from many different sources, but the most common are contaminated feed, animals and insects. Feed containing animal proteins and mash feed, have been identified as common sources of Salmonella (Cox et al., 1983). Biological vectors, such as cockroaches (Kopanic et al., 1994) and mice (Henzler and Opitz, 1992), can carry Salmonella internally and externally, spreading the bacteria throughout the poultry house. Paratyphoid Salmonella can also be transmitted vertically, and eggs from breeding flocks can be contaminated both internally and externally. Salmonella can penetrate the eggshell during or after oviposition, or it can be deposited inside the egg before oviposition (i.e. transovarian contamination), transmitting the infection to the developing embryo (Keller et al., 1995). Consequently, paratyphoid Salmonella can be found in hatcheries, and Bailey et al. (1994) demonstrated that Salmonella is released from hatching debris and fluff into the air, circulating in the hatchery air system. Once paratyphoid Salmonella has been introduced into the farm, horizontal contamination can occur: the bacteria can be transmitted by direct contact between birds, ingestion of contaminated feces, litter, or water, and contaminated staff and equipment (Nakamura et al., 1994). This diversity of sources that can introduce Salmonella into poultry flock makes establishing any kind of plan to prevent and control paratyphoid infections very difficult. In recent years, many countries have developed and implemented various testing and monitoring programs.

National

Many countries worldwide identify primary production (e.g., poultry farms) as an important point for Salmonella surveillance and control, so legislation has been passed to implement measures to detect and control Salmonella at all stages of production, including feed production. This legislation requires that monitoring programs are put in place. Different countries have adopted different programs, such as the National Poultry Improvement Plan (NPIP) in the USA, and the Lion Code in the UK. In many cases, national plans exceed the legal requirements and are intended to provide consumers with eggs of the highest quality and safety, while maintaining the highest possible welfare standards and environmental controls. These plans are designed for the different stages of poultry production, from the hatchery to the breeder farm, from the laying farm to the packing center and feed mill. National plans also describe specific sampling and testing programs that are specific to Salmonella, and hygiene measures must also be in place.

National control programs have generally been a success, dramatically reducing the incidence of Salmonella outbreaks in humans. However, as mentioned before, Salmonella cases in poultry have increased since 2014. In 2018, the European Union reported data on Salmonella outbreaks in broilers and layers during 2016 (Figures 2 and 3). Salmonella Infantis was the most reported serovar in broiler flocks, and Salmonella Enteritidis in laying flocks (source: The European Union summary report on antimicrobial resistance in zoonotic and indicator bacteria from humans, animals and food in 2016).

Figure 2. Breakdown of Salmonella serovars in broilers flocks, EU MSs, 2016 (N = 1,707)
Figure 2. Breakdown of Salmonella serovars in broilers flocks, EU MSs, 2016 (N = 1,707)
Figure 3. Breakdown of Salmonella serovars in laying hen flocks, EU MSs, 2016 (N = 1,194)
Figure 3. Breakdown of Salmonella serovars in laying hen flocks, EU MSs, 2016 (N = 1,194)

Reducing Salmonella Challenges in the Poultry Industry Despite Antibiotic Resistance

Antibiotics have been used effectively in poultry for many years, both as therapeutic and prophylactic agents, to comply with the national control plan. The extensive use and misuse of antimicrobials has dramatically increased the emergence and spread of resistant bacteria (Sengupta et al., 2013). The European Food Safety Authority and European Centre for Disease Prevention and Control reported that levels of resistance were generally higher in Salmonella spp. isolated from broilers than from laying flocks. This suggests that laying hens in Europe are probably treated with antibiotics less frequently than broilers. At the beginning of 2018, a summary report on antimicrobial resistance in poultry in Europe was published, highlighting moderate to high levels of resistance in Salmonella to ampicillin, sulfamethoxazole, tetracycline and ciprofloxacin in the fluoroquinolone class of drugs (source: The European Union summary report on antimicrobial resistance in zoonotic and indicator bacteria from humans, animals and food in 2016. See https://www.efsa.europa.eu/en/efsajournal/pub/5182

Many European member states also detected colistin-resistant Salmonella in poultry. Third generation cephalosporins and most classes of fluoroquinolones are critically important to treat life-threatening salmonellosis in humans. The level of resistance to these two important classes of antibiotics is described in the same report as low to very low in Europe (https://www.efsa.europa.eu/en/efsajournal/pub/5182).

Antibiotic-resistant Salmonella  isolated at farm level may spread to humans through direct contact or contaminated meat (Dolejska et al., 2013). Since the ban on antibiotic growth promoters in many developed countries, poultry producers are looking for alternatives to control the bacteria during production. Organic acid feed additives are one alternative to antibiotics (Adil et al., 2010), as are feed additives containing cinnamaldehyde (Demir et al., 2005). Biotronic® Top3 is a feed additive that has been formulated to control Gram-negative bacteria in poultry production systems. It contains various ingredients, such as organic acids, cinnamaldehyde, and BIOMIN® Permeabilizing Complex™ on a sequential release medium (carrier). Permeabilizing substances, like the Permeabilizing ComplexTM, which increase the permeability of the outer membranes of Gram-negative bacteria, can increase the passage of the organic acids and cinnamaldehyde across the Salmonella membrane. The product also contains a carrier to transport the acids, cinnamaldehyde and Permeabilizing ComplexTM and to release them sequentially along the gastrointestinal tract. 
 

Effect of Permeabilizing Complex™, organic acids and cinnamaldehyde (Biotronic® Top3) on Salmonella Enteritidis colonization in broilers

Several studies have demonstrated that dietary supplementation with Biotronic® Top3 reduces Salmonella Enteritidis counts in the cecum.

One trial was carried out at IZSLER Brescia, Italy, where 60 day-old specific-pathogen-free White Leghorn chicks were randomly assigned to three groups of 20 chicks each, and placed in poultry isolators. From the day of hatching, each group was treated according to the following study design (Table 3)

Table 3. Study design
Table 3. Study design

The results of this study demonstrated that S. Enteritidis in the cecum was significantly (P < 0.05) reduced on days 5 and 10 post infection, as can be seen in Figure 4.

Figure 4. Salmonella Enteritidis counts (log CFU/g) in the cecal contents of broilers on days 5 and 10 post infection.
Figure 4. Salmonella Enteritidis counts (log CFU/g) in the cecal contents of broilers on days 5 and 10 post infection.

Conclusions

Supplementing broiler diets with Biotronic® Top3 helped prevent Salmonella Enteritidis the broiler cecum. Salmonella counts in the cecum were lower in broilers in both Biotronic® Top3 groups. Biotronic® Top3 can be used as a management tool to effectively prevent and control Salmonella infections. Preventing Salmonella from colonizing the bird will reduce the bacteria being shed into the environment, and possibly reduce the prevalence and incidence of problems caused by Salmonella in poultry production systems. It is important to highlight the fact that a holistic approach that includes effective management, a satisfactory vaccination plan and adequate nutrition is required to achieve these results on commercial farms.