Salmonella (cont.)
How is Salmonella transmitted to humans?
Salmonellosis is spread to people by ingestion of Salmonella bacteria that contaminate food. Salmonella is worldwide and can contaminate almost any food type, but recent outbreaks of the disease involve raw eggs, raw meat (ground beef and other poorly cooked meats), egg products, fresh vegetables, cereal, pistachio nuts, and contaminated water. Contamination can come from animal or human feces that contact the food during its processing or harvesting. New data about types of food contamination (food poisoning by Salmonella spp) is available from the U.S. Centers for Disease Control and Prevention (CDC) or the FDA. See, for example, the second last Web citation below. Potential direct sources of Salmonella are pets such as turtles, dogs, cats, most farm animals, and humans that are infected or are carriers of the organisms.
Although typhoid and paratyphoid fevers can be transmitted by the same
methods as salmonellosis listed above, the most frequent manner of transmission
is by the feces of infected people contaminating the water or food source of
uninfected people.
How do Salmonella spp cause disease in people?
Usually, Salmonella spp are ingested. It is commonly accepted that at between 1 million to 1 billion bacteria are needed to cause infection although some investigators suggest some people may be infected by far fewer bacteria. Nevertheless, most data suggest food, water, or other sources of contamination contain large amounts of bacteria. Although human stomach acid can reduce and sometimes eliminate Salmonella spp, occasionally some bacteria get through to the intestine and then attach and penetrate the cells. Toxins produced by the bacteria can damage and kill the cells that line the intestines, which results in intestinal fluid loss (diarrhea).
Some Salmonella can survive in cells of the immune system and can reach the
bloodstream, causing blood infection (bacteremia). Other Salmonella spp can enter
the gallbladder, leaving the affected patient a chronic carrier of the
organisms. Salmonella can then be shed with the bile from the gallbladder into
the feces and then may infect other people. Perhaps the most famous such
carrier, the first to be identified in 1907, was a cook named Mary Mallon, also
known as "Typhoid Mary." She was suspected of infecting hundreds of individuals.
Next: How do you diagnose Salmonella infections? »
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