Signs, causes, treatment and prevention of Salmonella infection in chickens and other avian species
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), while birds can be infected and show no signs of disease.
The complete eradication of Salmonella from poultry production is an incredibly difficult goal. Applying a combination of strategies including proper management, biosecurity, vaccination protocols, nutritional feed additives, and research are steps in the right direction.
Prevalence of Salmonella in commercial poultry
The most common serotypes of Salmonella in commercial chickens, turkey and ducks worldwide are:
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.
In addition to the large number of serotypes, the genus Salmonella exhibits extreme diversity among the serotypes in terms of their characteristics and pathogenicity. Some are more adapted to the intestine and do not go beyond the gut, while others can get into the blood stream and have the ability to colonize the liver and spleen. Some survive longer in the environment, while others are sensitive. Salmonella as a genus are able to infect multiple species, therefore cross infection of serotypes is very common.
These and other general features of Salmonella make its control difficult. Eradication programs require extensive knowledge and investment. A comprehensive program must be established, not just a single procedure.
To explore alternatives to control Salmonella in poultry, we have to divide them into two groups: typhoid and paratyphoid.
Table 3. Salmonella groups and serotypes
All other serotypes
The typhoid group includes two members: Salmonella Gallinarum and Salmonella Pullorum. The paratyphoid group contains all other serotypes of Salmonella.
Nowadays, in a global market, raising birds free of typhoid Salmonella is essential for broiler producers that want to remain competitive. For the control of typhoid infection in poultry, we have to focus on good biosecurity, all-in all- out management of the flock, and potentially the use of vaccines (if available). In case of outbreaks, the eradication procedure is costly, but, in the end, it is more efficient and results in better economics. When done properly and associated with biosecurity, it works very well.
The difficulties for the control of paratyphoid Salmonella are greater. There is no single procedure that guarantees a positive flock to become negative. Additionally, a negative flock can become infected due to a variety of contamination vectors. Rigorous biosecurity can minimize the chances for paratyphoid 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 rates 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 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 laying 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 established, it becomes impossible to eradicate, unless the whole farm is cleaned. For that reason, most of the layer flocks are vaccinated with 9R. The vaccine avoids high mortality and reduces 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 paratyphoid Salmonella 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 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.
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. 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, obtaining isolates and serotyping along the production chain (breeders, hatchery, grow-out, and feed) is needed to find the source of contamination. This will dictate what control program is required and at what stage intervention is needed. For example, 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 multiple sources may be identified; 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 described below.
Breeders and paratyphoid Salmonella control
When breeders are positive for paratyphoid Salmonella, it is important to identify the source of infection: from the grandparents or acquired on the farm. 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 certain circumstances, vaccines may be used in breeders. 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 to a lesser degree. The feed can be used to deliver products that reduce Salmonella infection in poultry such as antimicrobials, probiotics, organic acids, MOS, phytogenic feed additives, and others. Not all products have the same mechanism of action, but most of them have been shown to contribute to reducing Salmonella risk and should be used acco
Hatchery andSalmonellacontrol Salmonella is transmitted vertically, thus breeder flocks positive for typhoid Salmonella will result in positive chicks. Proper management, cleaning, and disinfection of the hatchery all contribute to limiting lateral spread of Salmonella from positive to negative flocks
To reduce the transmission of Salmonella from positive flock to negative ones, eggs from positive flocks must be incubated and hatched separately. Therefore, a well-managed hatchery can avoid cross contamination, but will not eliminate Salmonella from incoming eggs from a positive breeder flock. Probiotics, antimicrobials, and potentially 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 the previous flock, delivered feed, rodents and wild animals, backyard chickens, neighbors, other animals in the farm, poor cleaning and disinfection, bird disposal, and humans such as employees/visitors.
Considering that the port of entry is diverse, it is necessary to have a comprehensive monitoring program to understand where the source of contamination is. Additionally, a short downtime (less than 2 weeks) and increased bird density has a lot of influence in the presence and persistence of paratyphoid Salmonella. Salmonella can proliferate if fasting before slaughter and transport time are too long. 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, phytogenic feed additives, organic 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, single pronged strategies are often ineffective.
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 processing and in the chiller. Levels of 5, 10, or 20 ppm can significantly reduce 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.