Treatment and prevention of Salmonella in poultry
Salmonella is an enteric pathogen that can infect almost all animals including humans. Salmonellosis in poultry is caused by Gram-negative bacteria from the genus Salmonella. There are only two species in this genus, enterica and bongori (Lin-Hui and Cheng-Hsun, 2007), but almost 2,700 serotypes (serovars), of which around 10% have been isolated from birds.
In general, most serotypes of Salmonella can infect several animal species (Gast, 2008), such as Salmonella Typhimurium and Salmonella Enteritidis.
Food safety and Salmonella in poultry
Food safety is a key topic when it comes to commercial poultry production. The main concern related to Salmonella is that poultry meat and eggs are the most common sources of human infection (food poisoning). Birds can be infected and show no signs of disease.
The complete eradication of Salmonella from poultry production is an incredibly difficult goal. The need for the combination of proper management, biosecurity, proper vaccination protocols and together with many other aspects can help to take the first steps in the right direction. The use of feed additives can be a helpful tool to prevent disease outbreaks by ensuring a healthy gut and deliver good performance levels.
Prevalence of Salmonella in commercial poultry
The most common serotypes of Salmonella in commercial chickens, turkey and ducks worldwide are:
- S. Gallinarum
- S. Typhimurium
- S. Enteritidis
- S. Heidelberg
- S. Montevideo
- S. Infantis
- S. Mbandaka
- S. Kentucky
- S. Javiana
- S. Newport
In 2018, the European Union reported data on Salmonella outbreaks in broilers and layers during 2016. Salmonella Infantis was the most reported serovar in broiler flocks, and Salmonella Enteritidis in laying flocks (EFSA 2016).
The prevalence can vary among countries and between regions of the same country.
Complexity and control
In addition to the large number of serotypes, the genus Salmonella presents a large variability among the serotypes. Some are more adapted to the intestine and do not go beyond the gut, others can get into the blood stream and have the ability to colonize liver and spleen. Some survive longer in the environment, others do not. Most of the animal species can be infected with Salmonella, therefore cross infection is very common among birds.
These and other general features of Salmonella make its control difficult. It requires a lot of knowledge and investments. We have to establish a program and not just a single procedure. Salmonellosis is not the most devastating poultry disease, but it is one of the most difficult diseases (agent) to control. The main reason is the large variety of serotypes and the very complex epidemiology of this microorganism.
To explore alternatives to control Salmonella in poultry, we have to divide them into two groups: Typhoid and Paratyphoids.
All other serotypes
Table 3.Salmonella groups and serotypes
The typhoid group includes two members: Salmonella gallinarum and Salmonella pullorum. The paratyphoid group contains all other serotypes of Salmonella.
For the control of typhoid infection in poultry we have to focus on good biosecurity, all-in-all out management of the flock and eventually the use of vaccine (if available). In case of outbreaks, the eradication procedure is costly, but at the end it is more efficient and results in better economics. When done properly and associated with biosecurity, it works very well. Nowadays, in a global market, raising birds free of typhoid Salmonella is essential for broiler producers that want to remain competitive.
The difficulties for the control of paratyphoid Salmonella are greater. There is no single procedure that guarantees a positive flock to become negative. Also, a negative flock can become infected due to a variety of contamination vectors. Rigorous biosecurity can minimize the chances for Salmonella, but cannot guarantee absolute control. It is important to remember that not all Salmonella are the same: some respond to a certain product or treatment strategy better than others. We must be aware of which one is working better with the serotype that we are dealing with in order to get the best results.
Typhoid group: Salmonella gallinarum and Salmonella pullorum
This group is represented by only two serotypes. Salmonella gallinarum and Salmonella pullorum, the causative agents of fowl typhoid and Pullorum disease, respectively, are specific to poultry and found mainly in chickens and turkeys.
Among the 2,700 serotypes, only these two can cause a high mortality rate in birds. They can be transmitted both horizontally within a flock and vertically from generation to generation. Once the flock is infected the survivors will remain carriers forever (Shivaprasad and Barrow, 2008). Because of these characteristics, the commercial poultry meat industry worldwide uses eradication as a standard control procedure.
A company or a producer that has positive breeders or broiler flocks will have a hard time competing economically with other companies or producers that are free of typhoid Salmonella. Considering that, in case of an outbreak, eradication becomes the rule. The use of antibiotics can be a strategy to reduce mortality in breeders, layers and broilers, but the flock remains positive and becomes a source of infection for other flocks. It is important to consider that the eradication procedure works well to control outbreaks of Salmonella gallinarum/Salmonella pullorum, but needs to be followed by good biosecurity procedures.
Biosecurity and sources of contamination
Because Salmonella gallinarum/Salmonella pullorum are found mainly in chickens and turkeys, avoiding contact with these birds outside of the farm is the key to prevention.
Good biosecurity is key in preventing the infection from getting into the farms. In our experience, humans as carriers are the main source of typhoid infection, and backyard chickens are the most important reservoir of these bacteria. Most of the time, employees are the ones that have contact with an infected chicken and then introduce the infection into a clean flock. A comprehensive biosecurity program will cover all potential sources of poultry farm contamination.
A vaccine called 9R, used for typhoid infection, is available worldwide. It is a rough strain of Salmonella Gallinarum (Shivaprasad and Barrow, 2008), but in most countries, it is not allowed in broilers, because it interferes in the serology monitoring of chicken meat. If used, it will protect against both Salmonella gallinarum and Salmonella pullorum.
Salmonella gallinarum in egg production
In case of layer chickens (eggs), the frequency of typhoid infection, mainly caused by Salmonella gallinarum, is a lot higher worldwide when compared to that of broiler flocks. The main reason is the lack of good biosecurity. Most of the layer farms have multiple ages, which do not allow all-in all-out management, compromising biosecurity.
Once the infection is installed, it becomes impossible to eradicate, unless the whole farm is cleaned. For that reason, most of the layer flocks are vaccinates with 9R. The vaccine avoids high mortality and reduces the egg production, but the infection can still occur.
This group is represented by all other serotypes of Salmonella except for the two in the typhoid group. As a general rule, paratyphoid types do not cause mortality in poultry and do not interfere with performance.
The main reason to establish a control program is to reduce or avoid human infection by consuming contaminated meat and eggs. The control strategies are a lot more complex than for the typhoid group.
The main factors that add complexity in Salmonella typhoid control strategies:
- a variety of animals including mammals can be a source of cross-infection in birds
- the bacteria are widespread in nature and able to survive weeks or months
- Once a flock is infected the amount of Salmonella can be reduced but not completely eliminated
- A single flock can be infected with more than one serotype
- Limited benefit of vaccine use because there is minimal or no protection among different serotypes (vaccine for S. enteritidis does not protect against S. typhimurium)
- Treatment with antibiotics can reduce the number of excreted bacteria but does not eliminate them completely
- Infected birds are asymptomatic and they do not present signs of the disease
As a result, effective control cannot be based on one or two procedures. Rather, the whole chain must be involved: breeders, hatchery, grow-out, feed and processing plant. It is important to point out that the port of entry for paratyphoid Salmonella in a broiler flock is the same for breeders.
Salmonella control program
Therefore, the challenge in establishing a Salmonella control program is to consider its very complex epidemiology and the entire production chain involved.
Setting up a monitoring program and serotyping the isolates are essential to any control program. Once we know which serotype is circulating and where the source is, then we can set up a control program.
The program has to start with the knowledge of the final product, generally at the processing plant. If Salmonella is present, its serotype needs to be identified. Once the serotype is known, we have to go back to the chain (breeders, hatchery, grow-out and feed), get the isolates and serotype them to find the source.
If the same serotype is found in the breeders, then our focus for control should be in the breeders. If we do not find Salmonella in the day-old chick, but instead find it in the feed, our emphasis for the control should be in the feed and not in the breeders.
Sometimes we can identify more than one source of infection; in this case all of them need to be considered for the control program. The main sources of infection and products/procedures available for the control program in the chain of poultry production are shown below.
Breeders and paratyphoid Salmonella control
In case breeders are positive for paratyphoid Salmonella, it is important to identify the source of infection: from the grandparents or acquired on the farm. Once infected, birds will remain infected, and in this case, the work has to be done at the growth promoter (GP) levels. If the infection was acquired on farm, then we have to reinforce biosecurity, rodent control, cleaning and disinfection, downtime, other animal contact, visitors, repair crew and vaccination team. Any equipment introduced into the farm has the potential of carrying Salmonella. In special circumstances, vaccines can be used in breeders. Probiotics can be used during the first days or after the medication/stress periods. Products in the feed can also be used (see feed mill section).
Feed mills and paratyphoid Salmonella control
Feed can be an important source of infection for paratyphoid Salmonella (less so for Typhoid Salmonella). The feed mill environment, feed ingredients and the delivered feed must be monitored for Salmonella, because they are a potential source (Jones, 2011). The feed pelletizing process destroys Salmonella, but contamination can occur during cooling and transportation of the feed.
Animal by-products are common sources of Salmonella in a feed mill, they also have to be monitored and treated if necessary. Soybean can also be a source of infection, corn in a lesser degree.
The feed can be used to deliver products against Salmonella for breeders, broilers and layers such as: antimicrobials, probiotics, organic acids, MOS, essential oils and others. Not all products are the same, but most of them can make a contribution in reducing Salmonella infection and should be used accordingly.
Hatchery and Salmonella control
Proper management, cleaning and disinfection of the hatchery all contribute to limiting the incidence and spread of Salmonella. Cross contamination can occur, mainly when there is a mixture of birds from positive and negative flocks.
If we keep eggs from a positive flock, incubate and hatch them separately, most of the time they will not contaminate chicks from negative flocks. For typhoid Salmonella, chicks hatched from a positive breeder flock will be positive. Therefore, a well-managed hatchery can avoid cross contamination, but will not eliminate Salmonella from a positive flock. Probiotics, antimicrobials and eventually vaccines can be delivered in the hatchery and help with overall Salmonella control.
Broiler grow-out and Salmonella control
For typhoid Salmonella not much intervention can be done at grow-out for a positive flock other than treatment with antibiotics. Because of the short life of the broiler, infection by Salmonella gallinarum/Salmonella pullorum almost always comes from the breeder and not from the field.
For paratyphoid Salmonella, the infection can come from the breeder, but can also occur during the rearing period. Various possible sources of infection are previous flock, delivered feed, rodents and wild animals, backyard chickens, neighbors, other animals in the farm, poor cleaning and disinfection, bird’s disposal and humans such as employees/visitors (Vatche, 2011).
Considering that the port of entry is diverse, it is necessary to monitor them to understand where the main sources are found. In addition, a short downtime (less than 2 weeks) and increased bird density has a lot of influence in the presence and persistence of paratyphoid Salmonella. As for breeders, good biosecurity procedure plays an important role in avoiding the entry of Salmonella. If the fasting before slaughter and the transport time are too long, Salmonella proliferation is also favored. Antibiotics are not very efficient in controlling Salmonella infection in grow-out. They can reduce the infection, but as soon as they are removed, the infection can return. Several other products such as: probiotics, organic acids, essential oils, herbs extract, acids, MOS and vaccines can be used to reduce/control Salmonella but they have to be part of a holistic program that includes biosecurity. Because of the complex epidemiology, if they are used alone the best benefit cannot be achieved.
Processing plant and paratyphoid Salmonella control
The processing plant can have an important role in the control of Salmonella. This is true for countries that allow the use of chemicals like chlorine during the processing and in the chiller. Levels of 5, 10 or 20 ppm can contribute a lot in reducing contamination. There are other chemicals that can be used and are efficient as well. There are countries that allow only a very limited use of chemicals during the processing, which are not effective in the controlling Salmonella contamination.
In this case, the focus for control has to be done before the broiler arrives for slaughter. Good hygiene, cleaning and disinfection contributes a lot in the control of Salmonella, therefore they cannot be neglected. There is a link between the processing plant and grow-out, which is the transport system, mainly the coops or cages. A lack of good disinfection of the cages can distribute the bacteria from a positive to a negative flock in the field. This system needs constant attention.
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 EU report on antimicrobial resistance in poultry in Europe was published, highlighting moderate to high levels of resistance in Salmonella to several antimicrobials.
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.
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).
Efficacy of organic acids against Salmonella
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).
Until very recently, the use of organic acids or single chain fatty acids (SCFA) mainly focused on their efficacy outside of the gastrointestinal tract. An increasing number of studies have been published focusing on the use of SCFA as supporters of gut health and as preventive tools to avoid an uncontrolled proliferation of pathogenic bacteria.
The exact mode of action of SCFA as gut performance promoters are not yet clear. However, it has been demonstrated that organic acids have a direct antimicrobial activity against pathogens such as E. coli and Salmonella, and they might contribute to gut health indirectly by improving digestibility. This ensures a proper feed digestion—meaning that less non-digested feed reaches the lower part of the intestine where it could feed opportunistic bacteria, leading to pathogen proliferation.
Generally, the addition of Biotronic® Top3 to the diet of broilers reduces the total number of E. coli and Salmonella while creating a favorable environment for the proliferation of beneficial bacteria (Figure 4).
Prevention against Salmonella Enteritidis gut colonization: Biotronic® Top3
In a trial performed in cooperation with the Istituto Zooprofilattico Sperimentale della Lombardia e dell’Emilia Romagna (ISZLER – Italy), Biotronic® Top3 was evaluated as a tool to prevent gut colonization by Salmonella Enteritidis in experimentally infected broilers. The group partitioning is listed in Table 6.
Animals were fed a control diet without the addition of any additive or the same diet supplemented with 1 or 2 kg/ton of Biotronic® Top3. a,b Means with different superscripts differ significantly (P<0.05)
Diets were fed from day 1 to day 25
At 15 days of age all the specific pathogen free animals were eye-drop infected with 1x105 CFU Salmonella enteritidis, a field strain isolated in Italy. At 5 days post infection, 10 animals per group were sacrificed and cecum was subjected to bacteriological analysis for the recovery of Salmonella enteritidis.
Results are shown in Figure 8, and clearly indicate that in both treatment groups the counts of S. enteritidis were significantly reduced on both day 5 and day 10 post infection. Converting log reduction in percentages both trial groups had between 50% and 70% lower Salmonella counts compared to the control group.