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How Common Are Salmonella Infections?
The most common infection caused by Salmonella is salmonellosis (also termed salmonella gastroenteritis). Over 1.4 million cases per year reportedly occur in the U.S. Other countries that are industrialized have similar occurrence rates in their populations. However, many cases go unreported, so some experts suggest that the actual number in the U.S. could be over 20 million cases per year. About 500-1,000 cases per year may result in death. In contrast, developing countries have a much higher rate of salmonellosis, but accurate estimates of its prevalence are lacking. Nonetheless, S. spp are considered by some researchers to cause the majority of food-borne infections in the U.S.
Typhoid fever occurs infrequently in industrialized countries; only about 400 cases per year are reported in the U.S., and the majority of these occur in people returning from a visit to a developing country. The U.S. Centers for Disease Control and Prevention (CDC) estimate that 21 million cases occur worldwide per year with about 200,000 deaths. Paratyphoid fever, like typhoid fever, occurs infrequently in industrialized countries. About 100-400 cases per year occur in the U.S., with a majority originating in people who travel to a developing country. Fortunately, paratyphoid fever is not as severe an infection as typhoid. Deaths result in less than 1% of diagnosed patients. Both typhoid and paratyphoid fevers have been termed "enteric fever," but this term is not specific and a few authors use the term for any Salmonella infection.
How Do Salmonella Bacteria Cause Disease(s)?
The majority of Salmonella infections are due to ingestion of food or water, although direct contact with animals has become more common as a source of the organisms to cause infection. In people with normal gastrointestinal tracts and immune systems, researchers have estimated that about 1 million to 1 billion S. spp need to be ingested to cause infection, because normal human stomach acid can kill large numbers of these bacteria. If some bacteria reach the intestine, the organisms can attach to intestinal cells where S. spp toxins (cytotoxin and enterotoxin) can damage and kill cells. The intestinal cell damage results in the inability of the body to normally retain and adsorb fluids, so diarrhea results. In some people, the diarrhea can cause serious dehydration. However, the majority of S. spp-caused infections are then eventually eliminated by the person's immune defenses. Some S. spp are not eliminated; these bacteria survive the initial immune response by living inside cells (macrophages) of the immune system. The bacteria can sometimes spread to the blood (bacteremia). Some S. spp (for example, S. typhi) can also enter the gallbladder and remain there. The patient can recover from the disease but still sheds bacteria through the gallbladder secretions (bile) into the feces. This person thus becomes a carrier of S. spp and potentially can infect many others, especially if the person lives in unsanitary conditions or works in the food-processing industry.
Medically Reviewed by a Doctor on 5/25/2016
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