The three types of Salmonella infections ( enterocolitis, enteric fever, and septicemia ) have different pathogenic features.

(1) Enterocolitis is characterized by an invasion of the epithelial and subepithelial tissue of the small and large intestines. Strains that do not invade do not cause disease. The organisms penetrate both through and between the mucosal cells into the lamina propria, with resulting inflammation and diarrhea. Neutrophils limit the infection to the gut and the adjacent mesenteric lymph nodes; bacteremia is infrequent in enterocolitis. In contrast to Shigella enterocolitis, in which the infectious dose is very small (on the order of 100 organisms), the dose of Salmonella required is much higher, at least 100,000 organisms. Various properties of salmonellae and shigellae are compared in Table 1. Gastric acid is an important host defense; gastrectomy or use of antacids lowers the infectious dose significantly.

Table 1. Comparison of Important Features of Salmonella and Shigella

(2) In typhoid and other enteric fevers, infection begins in the small intestine, but few gastrointestinal symptoms occur. The organisms enter, multiply in the mononuclear phagocytes of Peyer's patches, and then spread to the phagocytes of the liver, gallbladder, and spleen. This leads to bacteremia, which is associated with the onset of fever and other symptoms, probably caused by endotoxin. Survival and growth of the organism within phagosomes in phagocytic cells are a striking feature of this disease, as is the predilection for invasion of the gallbladder, which can result in establishment of the carrier state and excretion of the bacteria in the feces for long periods.

(3) Septicemia accounts for only about 5% to 10% of Salmonella infections and occurs in one of two settings: a patient with an underlying chronic disease, such as sickle cell anemia or cancer, or a child with enterocolitis. The septic course is more indolent than that seen with many other gram-negative rods. Bacteremia results in the seeding of many organs, with osteomyelitis, pneumonia, and meningitis as the most common sequelae. Osteomyelitis in a child with sickle cell anemia is an important example of this type of salmonella infection. Previously damaged tissues, such as infarcts and aneurysms, especially aortic aneurysms, are the most frequent sites of metastatic abscesses. Salmonella are also an important cause of vascular graft infections.

The epidemiology of Salmonella infections is related to the ingestion of food and water contaminated by human and animal wastes. Salmonella typhi, the cause of typhoid fever, is transmitted only by humans, but all other species have a significant animal as well as human reservoir. Human sources are either persons who temporarily excrete the organism during or shortly after an attack of enterocolitis or chronic carriers who excrete the organism for years. The most frequent animal source is poultry and eggs, but meat products that are inadequately cooked have been implicated as well. Dogs and other pets, including turtles, snakes, lizards, and iguanas, are additional sources.