Tuesday, October 25, 2011

Salmonellosis


Salmonella pathogens are some of the most common causes of foodborne outbreaks in developed regions of the world, and are also the cause of significant morbidity and mortality in quite different settings in the developing world. This time at Infection Landscapes, we will explore the disease salmonellosis as it occurs in both geographic contexts.

The Pathogen. Salmonellosis is caused by bacteria species in the genus Salmonella. Most infections in endothermic animals, such as mammals, are caused by the species Salmonella enterica, subspecies enterica. However, there are several subspecies within S. enterica and some of these other subspecies cause disease in ectothermic animals, such as reptiles, while still others are free living in the environment. Each subspecies has many serotypes (or serovars), which are defined by their unique antigen composition. S. enterica enterica (also referred to as subspecies I) is comprised of many serovars that cause disease in cattle, poultry, swine, and humans. The naming convention typically uses the genus and the serovar only. Some of the most important serovars to infect humans and domestic animals are Salmonella Enteritidis, S. Typhimurium, S. Montevideo, S. Typhi (humans only), S. Paratyphi (humans only), and many others.


Salmonella enterica is a gram-negative bacillus and a facultative anaerobe. It has a flagellum for motility, and fimbriae and adhesins for attachment to host cells. The O and H antigens are present on this organism, antigens you will recall from previous enteric organisms covered at Infection Landscapes. The O antigen is part of the lipopolysaccharide (LPS) molecule and the H antigen is the flagellum.

In general, pathogenic Salmonellae employ their fimbriae and adhesins to attach to the host cells of the small intestine. Subsequently, the Salmonella bacteria utilize a type III secretion system to inject proteins into the host cell, altering the host cell's structure and facilitating the uptake of the bacterium into the host cell, where it can then replicate.

The important virulence factors vary across the different serovars, which leads to host-specific infectivity, and ultimately varied pathogenicity and virulence across different susceptible hosts. Here is a nice animated video by the Institute of Microbiology ETH Zurich demonstrating the infectivity and pathogenicity of Salmonella Typhimurium:



The Disease. As mentioned above, the pathogenicity and virulence of Salmonella infections are complex and typically host-dependent. Some Salmonella serovars are pathogenic only in their natural reservoir hosts, causing little or no disease in other hosts (e.g. S. Typhi, S. Pullorum, and S. Gallinarum). Some serovars, on the other hand, are not pathogenic in their natural reservoir host, but can cause pathogenic infections in animals outside the natural reservoir. S. Enteritidis is a good example of this, where the serovar causes little disease in poultry, but is one of the most important sources of foodborne salmonellosis in humans. Finally, there are some recognized serovars that are pathogenic to their natural reservoir hosts, but develop even more virulent pathogenesis when they infect other hosts. S. Cholerae-suis and S. Dublin can cause severe clinical presentations in humans, with disseminated pathogenesis and high mortality.

Salmonellosis comprises 3 major disease forms in humans: enteric fever, non-typhoid invasive disease, and gastroenteritis.

Enteric fever is caused by S. Typhi and is more commonly known as typhoid fever. This is an important global disease in itself and so will be covered independently in a separate post next time.

Non-typhoid invasive salmonellosis is typically a pyemic infection, meaning abscess-forming and disseminated. The abscesses are often found in the bones and joints, meninges, serosa, and visceral organs. Salmonella Dublin, S. Virchow, and S. Cholerae-suis are the classic non-typhoid serovars that are associated with invasive disease. Nevertheless, serovars that are not usually invasive in humans can cause invasive disease among those who are not immunocompetent, or among those who receive an unusually high infectious dose of the pathogen. Invasive disease is a serious condition and requires directed antibiotic treatment.

Most foodborne salmonellosis results in gastroenteritis with a spectrum of diarrhea from watery to frank dysentery. Nausea, vomiting, and abdominal cramps are also common. This infection is most often self-limited, though distinctly uncomfortable and tiring. The typical disease course notwithstanding, more severe clinical presentation can occur in extremes of age, including children less than 5 years and adults greater than 65 years. There are many serovars that cause sporadic cases of gastroenteritis salmonellosis, however, S. Enteritidis is particularly important in causing foodborne outbreaks in developed countries.

The Epidemiology and the Landscape. There are approximately 3 billion incident cases of salmonellosis each year including all the serovars capable of infecting humans. There is substantial morbidity and mortality associated with typhoid fever, with approximately 22 million incident cases and 200, 000 deaths each year. However, as mentioned above, this posting is focusing on non-typhoid salmonellosis, so we will return to typhoid fever in the next post.

The epidemiology of non-typhoid salmonellosis is geographically distinct between the developed and developing regions of the world.

In the developed world, outbreaks have been driven by the agricultural industry as well as by the improper handling and cooking of food products. In this setting, salmonellosis in humans is essentially a foodborne disease. The Salmonella pathogens are maintained in animal reservoirs, and infection in humans follows ingestion of contaminated animal products or ingestion of produce contaminated with infected animal feces. S. Enteritidis and S. Typhimurium are the most important disease-causing serovars in these regions of the world. Poultry is the reservoir for S. Enteritidis, which has caused the greatest number of cases of gastroenteritis salmonellosis in the United States and Europe.


Because the organism can be transmitted vertically from a hen to her eggs, eggs have thus also been an important source of foodborne infection in humans.


S. Typhimurium has also become an important source of foodborne outbreaks in both Europe and North America. Originally identified in sea gulls in Britain, this serovar subsequently became epidemic in cattle and poultry, and is now a relevant pathogen in beef, chicken and pork products. It has also been identified in infections among those who work closely with the animal sources in this industry, though infection by way of animal contact is far less common than the eating of contaminated foods. Antibiotic resistance has become especially problematic with S. Typhimurium.



It is important to keep in mind that Salmonella contamination is not limited to the animal products themselves, but can also include any product that comes into contact with animal waste containing pathogen laden feces. The pathways of agricultural product movement from farm to markets is such that cross-contamination between animal waste and other non-animal products, especially fruits and vegetables, is readily facilitated. As such, produce often becomes an even more important vehicle for human infection than infected animal products. This is amplified when the flow of food products crosses international boundaries (which is commonplace for modern produce sales), where different governments apply different public health policies in the protection and movement of foods.


There are approximately 150,000 incident cases of salmonellosis in the US each year, and roughly the same number of cases per year in the EU. The annual deaths range from tens to thousands. Somewhere between 15% and 20% of all chickens are infected with Salmonella organisms that are pathogenic to humans.

In the developing world, non-typhoid pathogenic Salmonellae are commonly associated with nosocomial outbreaks, rather than foodborne outbreaks. Indeed, person to person transmission is the norm in this setting, where there is typically a complete absence of a food animal reservoir. Also, the bacteria are often more virulent. In South Asia and the Middle East, S. Typhimurium has been particularly prominent, with the case-fatality as high as 30% in some outbreaks. The strain in this geographic region is considerably more virulent than that found in northern Europe and North America. S. Wien has been more prominent in countries in northern Africa and the Mediterranean, while also appearing in India. The more virulent strains in the geographic regions of poor and middle income are also associated with extensive resistance to antibiotic treatment. This is largely due to the widespread overuse of antibiotics especially in the context of unregulated access to these drugs in the community.

Unfortunately, we do not currently have good surveillance data to estimate the burden of disease attributable to non-typhoid salmonellosis in the developing world.

Control and Prevention. Public health strategies for salmonellosis need to be directed by the unique circumstances found in different parts of the world where Salmonellae comprise significant human pathogens.

In the developed world, as with pathogenic E. coli, salmonellosis control and prevention must focus on the agricultural industry. First, the application and maintenance of rigorous production standards must be applied to the processing of animal products, with special attention to poultry, beef and pork. The use of antibiotics as growth stimulators in these animals is also an important component of the development of antibiotic resistance in organisms that are pathogenic to both humans and animals. Second, rigorous environmental controls are required to prevent cross-contamination between domestic livestock and produce, which is probably a larger source of foodborne outbreaks than, or at least as significant as, the contaminated animal products themselves. Unfortunately, since large-scale agribusiness is now a global phenomenon, produce is distributed far and wide throughout the world. As such, the application of uniform public health standards is often not possible and so a significant proportion of produce reaches local markets contaminated with Salmonella pathogens.

In addition to sources of infection in the agricultural industry, it is also important to focus on sources of infection in the home. First, thorough cooking of all animal products, including eggs, should be standard practice. Second, when preparing animal products, it is critical to segregate food preparation areas and utensils, and to wash all food preparation areas and utensils with soap and water after use to avoid cross-contamination. Thirdall produce must always be thoroughly washed. This includes fruits or vegetables with skins that are removed prior to eating. Remember this, cantaloupe are a regular source of Salmonella pathogens. The pathogens reside on the surface, but the act of cutting the melon can easily introduce the bacteria to its flesh and infect us upon eating, even though the rind is not eaten. 

In the developing world, most specifically, a new approach to regulating the use of antibiotics is critical to reducing the spread of antibiotic resistance in pathogenic Salmonella as well as other pathogenic organisms. This is especially important for enteric pathogens as these can easily end up in the the water supply since poor sanitation infrastructure is often characteristic of the landscape in such regions.

More generally, since person to person transmission is far more common for Salmonella infections in the developing world, the importance of consistent hand washing after using the toilet and before preparing and eating meals should be emphasized in the home, but also especially in hospital settings since virulent salmonellosis is nosocomially important in these regions.


34 comments:

  1. Like many of the infectious organisms we talk about, the routes of transmission in the developed and developing world differ and therefore call for different prevention strategies. In the developed world we see that the majority of salmonella infections occur via food processing and food preparation practices; likewise, in the developing world, the lack of water and sanitation infrastructure, coupled with increased antibiotic resistance, characterize the salmonella landscape in these areas. How do we, as public health professionals, ensure that we address both of these problem areas? We often hear of efforts to bring clean water into the areas that lack it most; however, we often hear of outbreaks of salmonella in the developed world as individuals become lax in the handling and preparation of food. We need to make sure that we address both of these areas as much as we can in limiting the spread of salmonella and other infectious organisms that are spread via these routes.

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  2. Salmonella Enteritidis causes little to no disease in its natural host, for example, poultry and is one of the most important causes of foode borne outbreaks in humans. This means preventing this disease in the natural host would be very difficult. Agricultural industries must maintain a rigorous standard of cleanliness imposed by the federal and state agencies (i.e. FDA) and constantly check for any symptoms and signs of infection and manage/treat as appropriate. On the other hand, we humans must maintain proper hand hygiene and boil our foods thoroughly before consumption and remember to avoid cross contamination of surfaces that are later used for fruits and vegetables that are washed but not boiled prior to consumption.

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  3. In the developing world, clean water and sanitation go hand-in-hand, seeming to have the biggest impact on salmonella outbreaks. Enteric pathogens can easily end up in the water supply, raising concern about the spread of antibiotic resistance in pathogenic Salmonella as well as other organisms. It is interesting that Salmonella causes nosocomial outbreaks in the developing world, rather than the food-borne outbreaks we encounter in the developed world. Since person to person transmission is most common in the developing world and bacteria are known to be more virulent, the need for different approaches to control Salmonella outbreaks between the developed and developing landscapes, could not be more obvious.

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  4. It’s surprising to me how handling techniques are still a major factor in salmonella contamination of food items in developed countries. According to the USDA website, the food and safety inspection service put out the pathogen reduction final rule, a rule that came out in 1996 for animal slaughtering establishments used to measure the effectiveness of avoiding contamination of food products. As part of the rule, random samples are taken back to food and safety laboratories to test for contamination. I wonder how effective putting this rule in place has been in reducing salmonella contamination.

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    1. I used to work for the NYC DOHMH for a few years as a health inspector and in my time doing that, I can see why foodborne illness outbreaks occur. Even with tighter USDA regulations, it still would have no effect on salmonella contamination. In food service establishments, the complete and utter disregard for food handling leading to food contamination is two-fold.
      Firstly, there is a barrier of indifference; since those who cook do not necessarily have to eat it, they don't care how the food is cooked or stored so this can increase the risk of illness even though it came relatively "fresh" from a slaughterhouse. Secondly, there is a huge cultural barrier particularly in the U.S.; restaurants are not big money producers so they tend to hire at the lowest possible pay. This in turn creates very high job turnover and attracts more first or second-generation immigrants to do the work. The proper handling of food is not taught to them nor is it a job requirement so they will either be unfamiliar with cooking certain types of food that require extensive heat treatment and are more likely to get people sick or cooking certain foods in a certain way that may not be relatively good practice.

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  5. We see again, through Salmonella pathogen, how important hand hygiene and safe food handling is in the develop world. It is interesting that Salmonella in the natural reservoir does not cause virulence, but causes significant virulence in the affected host. A more important factor is that most of the non typhoid Salmonella pathogen are associated with nosocomial outbreaks, even though there are still so many foodborne outbreaks. This is so important to consider in the effort to control outbreaks, because not only do we need to focus on hand hygiene and safe food handling to control foodborne outbreaks, but there needs to be a significant effort in place to control for infection in the healthcare setting, as well as better control on antibiotics use, which would also benefit in the control of other pathogens. If there was a proper surveillance system in place, I wonder how many more infections could be prevented along with the other before mentioned techniques?

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  6. As Stella mentioned, poultry has to be properly washed and cooked to get rid of salmonella infection. A chicken liver outbreak took place in five states just recently because the frozen meat products were only partially cooked before packaging. The product has been recalled but the situation sheds light on how difficult it can be to avoid contaminated food when products like these appear ready to eat and a consumer wouldn’t think otherwise.

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  7. Chris - You bring up a very interesting point with the need to treat Salmonella outbreak in the developed world differently than in the developing world. However, you also ask, "How do we, as public health professionals, ensure that we address both of these problem areas?"
    I think that we need to treat Salmonella in the developing world almost as a different disease than Salmonella in the developed world. To try and address both issues at once is a waste of resources. What we should do as public health professionals is work on getting clean water to the developing world which will decrease all gut infection including Salmonella. In addition we need to come up with a better system to prevent Salmonella outbreaks in the developed world. Maybe there is a need for more stringent food handling laws, or we need to better educate people on the importance of thoroughly cooking foods that are high risk for Salmonella.

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  8. Dov - You make a great point. So much of public health is making the most of the limited resources that we have. We can only do so much with the resources that we have, which makes prioritizing a critical first step before any actions can be taken. At times, we may feel as if we should be putting developing countries first since there is a greater need. At the same time, for public health officials working for the government in America, the epidemiologic aspects of Salmonella in developed countries are also critical as they have a responsibility to care for the people they are serving. Unfortunately, they also do not have enough resources to visit all these different agricultural sites to ensure adherence to the regulations that are in place. This aspect also needs to be stressed in protecting citizens within the United States.

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  9. Salmonella Enteritidis is an example of pathogen that does not harm its natural host, but can cause severe illness in humans. It can also cross contaminate for ex poultry to produce. It would make sense to cut off this pathogen at the source, the live stock. Of course, this is easier said then done, and there are many barriers such as antibiotic resistance and the huge task that such a plan would require. That being said, it leaves us with the rules and regulations that are put into place by federal agencies, the individual laws governing customs, and most important our own hygiene practices. The first two precautions are directed by authoritative figures to limit the number of this pathogen that can ultimately end up in our food. Hygiene on the other hand, is the part that we (the consumers) have control over and can effectively combat the remaining pathogens that had escaped the other two measures.

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  10. Salmonella is an infection that is constantly in the news for US and European food-borne outbreaks, but what caught my attention most in this article was the information about its challenges in the developing world (reading other student comments, I can see that I am not the only one who found this interesting). One comment that stuck out to me was that antibiotic-resistant salmonella was "largely due to widespread overuse of antibiotics especially in the context of unregulated access to these drugs in the community." It got me thinking about the overall quality of medical care and regulation available to the populations of these developing regions. Who is giving out these antibiotics to local residents? How can they afford them? And what regulation is currently in place?

    With antibiotic-resistance becoming more and more of a health issue, should we be concerned about a potential superbug taking form in a developing country and slowly spreading around the world?

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    1. This comment also stuck out to me! Antibiotic resistance is an interesting topic because it reminds us of the importance of developing and practicing the best treatments for individuals while keeping population health in mind as well. And this is relevant in developing and developed countries alike. Studies in the U.S. show significant proportions of Salmonella serotypes are resistant, contributing to thousands of additional illnesses per year and making the treatment of those with more severe cases of Salmonella infections even more difficult.

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    2. (Source for above reply: http://www.foodsafetynews.com/2015/06/cdc-antibiotic-resistance-increasing-in-certain-salmonella-serotypes/)

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    3. Similar to everyone else, I too was surprised to read about the differences in transmission tendencies between developed and developing nations. I was slightly confused on whether the antibiotic resistance was coming from sprays used on agriculture or humans taking medications, but since salmonella relies greatly on nosocomial transmission, I assume it has more to do with human practices. In addition to finding better treatments for salmonella, we also need to ensure adherence to treatment regimens, as incomplete compliance helpes lead to antibiotic resistance as well. Having public health officials in the field to ensure that medicines are being taken as directed can go a long way in halting the increase in antibiotic resistance we are seeing.

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  11. I had no idea that salmonella infections manifested in so many disease forms. I always only associated it with gastrointestinal illness! This changes the way that I look at the disease, knowing that the disease can be so severe. I think that food handling is a very big problem and there is no simple solution. The amount of workers that are unaware of the dangers the improper food handling is very alarming and we need to find a way to get the appropriate messages across effectively.

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  12. Sheaba Daniel

    I think that the challenge of preventing salmonellosis is that many food products can carry the bacteria and the food safety chain is quite long (farmers, retailers, food service workers, and consumers) and food handling steps ( washing hands and surfaces, cooking to appropriate temperature,, and avoiding cross contamination) get compromised along the way.

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  13. Racquel BreretonJuly 9, 2014 at 3:30 PM

    Salmonella outbreaks are quite common and often peak during the summer months. Most recently is a salmonella outbreak linked to a poultry plant http://www.nbcnews.com/health/health-news/foster-farms-salmonella-outbreak-expands-n148466. Typical means of intervention include recalling tainted items. In this case it was boneless chicken, However, this is challenging as contaminated items may still remain in the homes of consumers long after confirmation of source and recall. Such is in this case as cases are still increasing over a five month span.
    It was also interesting to learn the dynamics behind salmonella virulence. I wasn't aware before that different serovars with diverse virulence existed, which determined the severity of the disease. I knew that gastroenteritis caused by Salmonella was self-limiting and assumed that severe cases of Salmonella were linked to other species i.e. typhimurium rather than just variations of serotypes.

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    1. Salmonella outbreaks are quite common, unfortunately. I just read that the former Peanut Corporation of America owner was sentenced to 28 years in prison in Albany, Georgia on 9/21/2015 after a jury convicted him of knowingly shipping out salmonella-contaminated peanut butter and hiding the evidence. He was convicted on crimes related to a salmonella outbreak that sickened 714 people in 46 states and may have contributed to nine deaths, according to the CDC. The outbreak prompted one of the largest food recalls in US history.

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  14. Alexandr PinkhasovJuly 15, 2014 at 11:45 AM

    There was a really interesting point presented in this disease description which also underlines a potential public health concern. There are roughly 150,000 incident cases, and several 1000 deaths each year attributed to this pathogen. While the disease burden may not be as great the mode of transmission through cross contamination from animals to humans are concerning. There are already many regulations in place focused on sanitation, however there should be a greater amount of effort put into regulation. Another concerning issue that was underlined was that 15% to 20% of all chickens are infected with Salmonella that are pathogenic to humans. I found this pretty alarming because in the past 2 decades we have seen a dramatic shift in meat consumption in the United States. Now 45% of all meat consumed in the United States in poultry, meaning that nearly 20% of that 40% are infected with Salmonella that are pathogenic to humans. Two more points that stood out to me, was the possibility of vegetable cross contamination through feces, and vertical transmission from hens to eggs. This stood out to me because it underlines the broad range of transmission/contamination that there is.

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    1. This is very alarming the fact that there has been a shift in meat consumption geared towards the consumption of poultry. I personally do not consume any other meat besides poultry when I go to a restaurant, but knowing these statistics might actually change some of my choices. There was also recent news warning individuals to not wash poultry before eating because of that activates a certain bacteria in the meat.

      I have also heard that turtles also are able to transmit salmonella, and if this is the case then there should be more education geared toward children with turtles as pets about the importance of washing their hands after playing with them.

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    2. The problem with washing chicken prior to cooking is not that it activates the bacteria, but rather that it unnecessarily spreads the bacteria around the kitchen and potentially contaminates surfaces and other foods. Washing is unnecessary because cooking will kill all the pathogenic bacteria (provided, of course, that the meat is cooked at the appropriate temperature and duration).

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  15. Throughout reading this article, the thing that stuck out most in my mind was that 15-20% of chickens are infected with Salmonella that is harmful to humans. Because the meat industry adds so many hormones and cleaning agents to the food, I wonder how effective it actually is at decreasing the risk of humans contracting Salmonella. When we were younger, I'm sure we all were told not to eat raw cookie dough because you would get Salmonella poisoning. However, no one ever talked about Salmonella being a risk in the other meat we were consuming. I feel as though society may have the assumption that you can only get Salmonella from raw eggs, but never think of other animal products and produce. I believe that public health efforts in regards to educating individuals how to properly clean and cook meat at the proper temperature is very important to eliminate any risk of being infected with Salmonella. I think that this education would be more precedent in the developing world where raw meat is consumed more frequently due to lack of resources to properly cook and clean it.

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  18. Salmonella is one of the commonest causes of food poisoning, in 2012 the CDC identified that about 116 people in 20 states and the District of Columbia have been sickened by the outbreak, common symptoms were diarrhea, fever, and abdominal cramps. No deaths have been reported.
    Most of the illnesses were caused by consumption of SUSHI.
    It is interesting that most of the measure to avoid salmonella infection are very simple, avoid consuming raw meats, eggs or fish, not to drink unpasteurized milk and wash vegetables and fruits before ingestion and frequent hand washing. However in places where there is no enough water supply the disease is prevalent, vaccination of chickens, along with other intervention measures, is an important strategy that is currently being used to reduce the levels of Salmonella in poultry flocks, which will ultimately lead to lower rates of human Salmonella infections
    Even though most cases of salmonella resolves spontaneously in sickle cell disease patient they can have severe systemic salmonellosis due to lack of immunological defense system against salmonella and might lead to osteomyelitis of the bone in his patients population.

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  19. The biggest takeaway I found from this is that second method of contagion in regards to agriculture is actually produce. Moreover, that it would take “rigorous environmental controls” to prevent it from occurring. That is to say, that it would relatively easy for cross contamination to occur between domestic livestock and produce. While it makes sense, that it would be due to cross-contamination, I never knew the dangers of cross-contamination until taking the food handling course for the NYC DOHMH. In fact, I might not remember the course referencing salmonella except in regards to handling poultry. Likewise, I was raised to always wash my produce, but it would be really effective if the public were aware of the why or what we are washing off. Some think it is merely to wash away pesticides. We can’t forget the pathogens! Especially on cantaloupes!

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  20. The US is currently experiencing a multi-state outbreak of Salmonella Poona infections linked to imported cucumbers. According to the CDC, to date, 418 people across 31 states have been infected, 91 of whom have been hospitalized. More than half of the patients are children. Cucumbers grown in Mexico have been implicated in the outbreak. The post points out that "large-scale agribusiness is a global phenomenon" which results in produce distribution throughout the world. The problem is that each country or region has different public health policies in place for the protection and transportation of foods. There are no uniform public health standards for food movement on a global level. As a result, a significant amount of produce reaches local markets contaminated with Salmonella, which is what happened with the cucumbers responsible for the current Salmonella outbreak. To reduce the incidence of Salmonella outbreaks, stringent environmental controls on a global level are needed to prevent cross-contamination between livestock and produce.

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  21. The transmission of Salmonella differs between developed and developing countries. This underlines the importance of understanding the social landscape and its effects on disease dynamics. In the developed world, Salmonella is transmitted via food contamination. However, in the developing world, Salmonella is nosocomial. I find this to be very interesting. I assumed that those in developing countries have more people living an agricultural lifestyle, and therefore, there would be more contact with animal waste. However, Salmonella cases in developing countries are acquired more due to exposure while under medical care. I'm curious to find out why. Perhaps those in developing countries are more exposed to strains of Salmonella that are transmitted via food? And thus, they have more built immunity to such strains?

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  22. The nosocomial spread of Salmonella in developing countries is interesting. Unfortunately, public hospitals are under-funded and as a result, may not have the proper decontamination resources needed. However, it is unacceptable if the spread of Salmonella is due to improper hand washing techniques by healthcare workers. Healthcare workers have the education to know how crucial it is to wash your hands. Those who live in agricultural societies may not be informed of the importance of consistent hand washing; that was the case for my family living in rural India. Public health professionals should initiate interventions targeting hospitals.

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  23. I find it interesting that there approximately 150,000 incident cases of salmonellosis in the US each year, which is a very large number, but there has not been more done in terms of prevention. In the UK, they started to vaccinate chickens against salmonella in 1997, and in just the next year their number of incident cases decreased 96%. It seems that such a large decrease is very substantial. It appears that this is a simple and straight forward public health initiative that would be able to prevent a lot of disease incidence. I wonder why the FDA has not made this a requirement.

    http://www.nytimes.com/2010/08/25/business/25vaccine.html?_r=0

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    1. Great points! Based on the results in the UK, vaccinating chickens against salmonella does seem like it would be a straight-forward yet effective public health initiative. I was curious as to whether cost played a role in why this was not implemented but according to the New York Times it would cost less than a penny per a dozen eggs. (Not sure why they used eggs as a reference rather than chickens). Interestingly, the FDA claims they decided not to mandate salmonella vaccination for chickens because the evidence to show the vaccine efficacy in preventing humans from getting sick was not strong enough.

      As a side note, as mentioned in the post, consumption of contaminated produce may be an even more important driver of transmission than animal products. For instance, according to the CDC since September 2015 there have been a number of Salmonella outbreaks associated with produce - alfalfa sprouts, packaged salads, cucumbers, and pistachios - consumption in the US.

      Sources:
      http://www.nytimes.com/2010/08/25/business/25vaccine.html?_r=0
      http://www.cdc.gov/outbreaks/index.html

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  25. The video by ETH Zurich states that once in the intestines Salmonella must compete with the gut microflora. I wonder if the infectivity of Salmonella is affected by a more robust microflora; meaning someone with a more robust microflora would be better able to defend against a Salmonella infection and thus an infection would not be pathogenic (or may be less virulent). This could be a potentially interesting investigation: Salmonella exposure and differences in infectivity, pathogencity, and/or virulence associated with variations in gut microflora.

    A general question: As we have talked about in class, the reservoir is where the pathogen lives naturally. Is it possible for a pathogen to have a reservoir that is also its host? This was mentioned in the article but a pathogen that makes its reservoir sick seems to be kind of counterintuitive.

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