The Current Challenge for Salmonella Control in the Global Poultry Industry
Salmonella is an enteric pathogen that can infect almost all animals including humans. It belongs to the Salmonella genus and most of the time it is not associated with clinical diseases. There are only two species in this genus, enterica and bongori (Lin-Hui and Cheng-Hsun, 2007), but a very large number of serotypes (serovars), almost 2,700. Salmonella is found worldwide and mainly in the intensive animal production. Some serotypes are well adapted to a single species, such as: Typhi to humans, Choleraesuis to pigs, Dublin to cattle, Abortusequi to equine, Gallinarum and Pullorum to chickens, and so on. There are serotypes that can infect a large number of animal species, such as: Typhimurium and Enteritidis. In general, most of them can infect several animal species (Gast, 2008). Around 10% of the 2,700 serotypes have been isolated from birds, but a few of them are the most common in commercial birds worldwide: Gallinarum, Typhimurium, Enteritidis, Heidelberg, Montevideo, Infantis, Mbandaka, Kentucky, Javiana and Newport. The prevalence can vary among countries and between regions of the same country.
The main concern related to Salmonella is that poultry meat and eggs are the most common sources of human infection. Birds can be infected and show no signs of disease. 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. The main objective of this paper is to show that it is possible to control Salmonella, but it requires a lot of knowledge and investments. We have to establish a program and not just a single procedure. Salmonelosis 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.
Salmonella in Poultry and its Control Challenges
To explore alternatives to control Salmonella in poultry, we have to divide them into two groups: Typhoid and Paratyphoids.
This group is represented by only two serotypes, Gallinarum (SG) and Pullorum (SP). What makes them special is that they do not infect mammals naturally, but they are found adapted in birds, mainly chickens and turkeys. Among the 2,700 serotypes, only these two can cause a high mortality rate in birds. They can be transmitted not only horizontally but also vertically, and 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 SG/SP, but needs to be followed by good biosecurity procedures. A 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. Because SG/SP are found mainly in chickens and turkeys, avoiding contact with these birds outside of the farm is the key to prevention. 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 SG and SP.
In case of layer chickens (eggs), the frequency of typhoid infection, mainly caused by SG, is a lot higher worldwide when compared to meat type of chicken. 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 others serotypes of Salmonella. As a general rule, they do not cause mortality in poultry and do not interfere in 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 reasons are related to some of the characteristics of this Salmonella group, such as: they infect a variety of animals including mammals that can be a source of cross-infection in birds; they are resistant in the environment (survive weeks/months); they are widespread in nature; 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; the benefit of vaccine use is limited because there is minimal or no protection among different serotypes (vaccine for Salmonella Enteritidis does not protect against Salmonella Typhimurium); treatment with antibiotics can reduce the number of excreted bacteria but does not eliminate them completely, the flock remains infected; birds infected are asymptomatic and they do not present signs of the disease (Gast, 2008). These are some of the features that make the control of paratyphoid Salmonella in poultry very difficult. The control cannot be based on one or two procedures, 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, hatchery, grow-out, feed and processing plant.
Therefore, the challenge in establishing a Salmonella control program is to consider its very complex epidemiology and the wide chain involved in the bird production. 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 don’t find Salmonella in the day-old chick, but we find it in the feed, our emphasis for the control should be in the feed and not in the breeders. Therefore, a monitoring program and serotyping the isolates are the basics in establishing a control program. Once we know which serotype is circulating and where the source is, then we can set up a control program. 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.
In case breeders are positive for paratyphoid Salmonella, it has to be identified if they got infected from the grandparents or if the infection was acquired in the farm. Once infected, it will remain infected, and in this case the work has to be done at the GP levels. If the infection was acquired 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, vaccine 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).
Typhoid Salmonella are poorly or not transmitted by feed, but the paratyphoid Salmonella feed can be an important source of infection. 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 byproducts 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 like: antibiotics, probiotics, organic acids, MOS, essential oil and others. Not all products are the same, but most of them have a contribution in reducing Salmonella infection and should be used accordingly.
If a proper management is done, cleaning and disinfection of the hatchery is not a good multiplier 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, antibiotics and eventually vaccines can be delivered in the hatchery and help with the overall 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, almost 100% of the time the infection by SG/SP 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 - employee/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, a 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 program. Because of the complex epidemiology, if they are used alone the best benefit cannot be achieved.
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/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.
The take home massage
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 a meat poultry company that wants to compete. 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 entries as mentioned above. Very rigorous biosecurity associated with several procedures described before can minimize the chances for Salmonella, but cannot guarantee the 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.