There are two main species of Salmonella: Salmonella enterica (S. enterica) and Salmonella bongori (S. bongori). S. bongori primarily colonizes cold-blooded reptiles, whereas S. enterica can infect both humans and mammals. These species are further classified into subgroups known as serotypes or serovars, based on morphology, structure, mode of reproduction, and other criteria. To date, more than 2,600 serotypes of Salmonella have been identified, distinguished by the types of animals they inhabit or the clinical symptoms they cause. Notably, fewer than 100 of these serotypes are responsible for most Salmonella infections in humans.
Salmonella is widely distributed in nature and commonly found in the intestinal tracts of both wild and domesticated animals, as well as in humans. Salmonella infections can occur when a person ingests contaminated fecal matter from an infected individual or animal.
Specific serotypes of Salmonella enterica, such as Typhi, Sendai, and Paratyphi A, B, or C, are found exclusively in humans. Collectively known as typhoidal Salmonella, these serotypes are responsible for enteric fever (typhoid and paratyphoid fever, depending on the serotype involved). Most cases of enteric fever result from consuming food or water contaminated with human feces. In the United States, the majority of diagnosed typhoidal Salmonella infections occur in individuals who have traveled to areas where these diseases are prevalent.
Three types of vaccines against S. Typhi are commercially available, but there is no licensed vaccine for S. Paratyphi A. Travelers outside of the United States should check whether a typhoid fever vaccine is recommended.
Most Salmonella infections stem from consuming contaminated food. One study indicated that 87% of all confirmed Salmonella cases are foodborne. Foods of animal origin, such as meat, poultry, eggs, and dairy products, are frequently contaminated with Salmonella. Consuming raw or inadequately cooked food, or food cross-contaminated with uncooked items, can lead to infection. According to a comprehensive report by the U.S. Department of Agriculture’s Economic Research Service Salmonella contamination occurs in a wide range of animal and plant products. Poultry products and eggs are often contaminated with S. Enteritidis, while beef products are commonly contaminated with S. Typhimurium. Other potential sources of Salmonella include raw milk, other dairy products, and pork.
In the past two decades, the consumption of produce—especially sprouts, tomatoes, fruits, leafy greens, nuts, and nut butters—has been associated with Salmonella illnesses. The surfaces of fruits and vegetables can become contaminated, leading to potential health risks.