Transmission of Salmonella Bacteria
In the past two decades, consumption of produce, especially sprouts, tomatoes, fruits, leafy greens, nuts, and nut butters, has been associated with Salmonella illnesses. The surface of fruits and vegetables may be contaminated by human or animal feces. Changes in food consumption and production, as well as the rapid growth of international trade in agricultural products, have facilitated the transmission of Salmonella associated with fresh fruits and vegetables.
In the United States, Salmonella is the second most commonly isolated bacterial pathogen when laboratory diagnosis of diarrhea is sought. However, passive laboratory surveillance, which uses voluntary reporting by health care providers and facilities, captures only a fraction of illnesses that actually occur. Furthermore, only a small proportion of illnesses are confirmed by laboratory testing and reported to public health agencies. Thus, researchers rely on quantitative statistical modeling to estimate the incidence of foodborne illness. These estimates are used to direct policy and interventions.
In 2011, Scallan and colleagues published results of a comprehensive study of estimates of the burden of foodborne illness in the United States. Using surveys, study results, and statistical methods, their estimates have provided the most accurate picture yet of which foodborne pathogens (bacteria, viruses, and parasites) cause the most illnesses.
Scallan et al. estimate that, each year in the United States, 31 pathogens cause 9.4 million foodborne illnesses. Illnesses caused by consumption of contaminated food result in 55,961 hospitalizations and 1,351 deaths annually.
For each nontyphoidal Salmonella infection reported, Scallan et al. estimate that there are 29.3 infections that are not diagnosed and/or reported. Thus, foodborne nontyphoidal Salmonella spp. account for 1.0 million cases or 11% of the total burden of foodborne illness each year. Nontyphoidal Salmonella was estimated to result in 19,336 hospitalizations each year, making it the leading cause of hospitalizations attributed to contaminated food eaten each year in the United States. Foodborne nontyphoidal Salmonella was estimated to result in 378 deaths each year.
The CDC Foodborne Disease Outbreak Surveillance System (FDOSS) collects data on foodborne disease outbreaks, using a web based platform, the National Outbreak Reporting System or NORS. Salmonella was the most commonly reported agent for all bacteria associated outbreaks reported to NORS in 2017. That year, there were 122 Salmonella outbreaks (113 laboratory-confirmed and nine suspected). Salmonella caused the most outbreak-associated hospitalizations (472 hospitalizations). Eight outbreak-associated deaths were reported. Twenty of the 32 multistate outbreaks with the first identified illness occurring in 2017 were caused by Salmonella. The most frequent serotypes were S. Newport (three outbreaks) and S. Braenderup (two outbreaks). There were an additional 10 multistate Salmonella outbreaks investigated in 2017 but not included in the tally for 2017, as the first illnesses occurred in 2016.
The food sources in the 2017 multistate Salmonella outbreaks were coconut (three outbreaks), papaya (three outbreaks), chicken, eggs, kratom powder, melon, raw sushi, and sprouts. Leafy greens, mango, Mexican-style cheese, papaya, romaine lettuce, and watermelon were suspected sources. A food was not identified for two outbreaks caused by Salmonella. Implicated foods in 10 outbreaks investigated in 2017—but with illnesses beginning prior to 2017—include chicken, ground beef, and papaya.
Foodborne illness outbreak data from previous years support these findings. During 2009-2015, FDOSS received reports of 5,760 outbreaks. Among 2,953 outbreaks with a single confirmed etiology, Salmonella was the second most common cause of outbreaks. For the seven-year period, there were 896 Salmonella outbreaks and 23,662 associated illnesses. The largest of the 177 multistate outbreaks was caused by Salmonella serotype Enteritidis associated with contaminated shell eggs. An estimated 1,939 persons were infected in 10 states beginning in 2010. A multistate outbreak of Salmonella Poona infections attributed to cucumbers in 2015 had the second highest number of illnesses (907 illnesses in 40 states). This outbreak also had the most outbreak-associated hospitalizations (204, or 22% of cases).
Human Salmonella infections associated with indirect or direct contact with animals have emerged as an important public health problem. Salmonella is found in the intestinal tract of many animals including reptiles, amphibians, and live poultry such as chicks, chickens, ducklings, ducks, geese, and turkeys. Although the majority of Salmonella infections are foodborne, about 3% are acquired through indirect and direct contact with animals, making it an important public health problem. Indirect transmission occurs through contact with things in areas where animals live and roam, or consumption of food/drink prepared in contaminated environments. In recent years, numerous outbreaks of Salmonella infections in humans have been linked to contact with live poultry in both household and public settings. Live poultry infected with Salmonella appear healthy, but can intermittently shed bacteria. An increase in backyard flock ownership in the United States has likely contributed to the rise in live poultry-associated salmonellosis. Large outbreaks occur almost annually. From 1996 to 2012, 45 outbreaks of human Salmonella infections linked to live poultry were documented. These outbreaks resulted in more than 1,581 illnesses, 221 hospitalizations, and five deaths. Because of unreported infections, many more illnesses likely occurred in these outbreaks. Multiple Salmonella serotypes were identified, and specific outbreak strains were repeatedly linked to individual mail-order hatcheries over multiple years. From 2013 to 2019, multiple outbreaks were investigated with illnesses occurring in every state.
Owners and caretakers of backyard poultry should always wash their hands thoroughly with soap and water right after touching live poultry or anything in the area where they live and roam. Adults should supervise hand washing for young children. CDC provides guidelines for safely keeping backyard poultry.
Other animals such as reptiles, amphibians, and household pets have also been associated with transmission of Salmonella to humans. Proper handwashing will help reduce the risk of getting sick from bacteria that pets can carry. More information can be found here.
Direct person-to-person contact, nosocomial transmission, waterborne transmission, and contaminated drugs and solutions are important but less common modes of transmission for human Salmonella infections.
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