Members of the genus Salmonella can cause a variety of diseases, including gastroenteritis and enteric (typhoid) fever. Although Salmonella classification has undergone numerous revisions, currently, all strains affecting humans are grouped in a single species, Salmonella enteritidis, which has approximately 2,500 different serotypes, or serovars, including the clinically significant serotypes Typhimurium and Typhi. Most strains of Salmonella are Lac^– and produce acid and gas during fermentation of glucose. They also produce H₂S from sulfur-containing amino acids.

A. Epidemiology

Salmonella are widely distributed in nature. Serovar Typhi is an exclusively human pathogen, whereas other strains are associated with animals and foods (for example, eggs and poultry). Fecal–oral transmission occurs and Salmonella serovar Typhi may involve chronic carriers. Pet turtles have also been implicated as sources of infection. Young children and older adults are particularly susceptible to Salmonella infection. Individuals in crowded institutions may also be involved in Salmonella epidemics.

 B. Pathogenesis

 Salmonella invade epithelial cells of the small intestine. Disease may remain localized or become systemic, sometimes with disseminated foci. The organisms are facultative, intracellular parasites that survive in phagocytic cells (Figure 1). Salmonella infection can cause both intestinal and extraintestinal diseases.

Fig1. Mechanism of Salmonella infection causing enteric fever.

C. Clinical significance

Salmonella infection can cause both intestinal and extraintestinal diseases.

1. Gastroenteritis: This localized disease (also called salmonellosis) is caused primarily by serovars Enteriditis and Typhimurium. Salmonellosis is characterized by nausea, vomiting, and diarrhea (usually nonbloody), which develop generally within 48 hours of ingesting contaminated food or water. Fever and abdominal cramping are common. In uncompromised patients, disease is generally self-limiting (48 to 72 hours), although convalescent carriage of organisms may persist for a month or more. More than 95 percent of cases of Salmonella infection are foodborne, and salmonellosis accounts for approximately 30 percent of deaths resulting from foodborne illnesses in the United States.

2. Enteric or typhoid fever: This is a severe, life-threatening systemic illness, characterized by fever and, frequently, abdominal symptoms. It is caused primarily by serovar Typhi. Nonspecific symptoms may include chills, sweats, headache, anorexia, weakness, sore throat, cough, myalgia, and either diarrhea or constipation. About 30 percent of patients have a faint and evanescent (transient) maculopapular rash on the trunk (rose spots). The incubation period varies from 5 to 21 days. Untreated, mortality is approximately 15 percent. Among survivors, the symptoms generally resolve in 3 to 4 weeks. Timely and appropriate antibiotic therapy reduces mortality to less than 1 percent and speeds resolution of fever. Complications can include intestinal hemorrhage and/or perforation and, rarely, focal infections and endo carditis. A small percentage of patients become chronic carriers. [Note: Infected gallbladders are the main source of chronic carriage.] Typhoid fever remains a global health problem. In the United States, however, typhoid fever has become less prevalent and is now primarily a disease of travelers and immigrants.

3. Other sites of Salmonella infection: Sustained bacteremia is often associated with vascular Salmonella infections that occur when bacteria seed atherosclerotic plaques. Salmonella can also cause abdominal infections (often of the hepatobiliary tract and spleen); osteomyelitis; septic arthritis; and, rarely, infections of other tissues or organs. Chronic carriage of non-typhoidal serovars may develop, although this is rare.

D. Laboratory identification

 In patients with diarrhea, Salmonella can typically be isolated from stools on MacConkey agar or selective media (Figure 2). For patients with enteric fever, appropriate specimens include blood, bone marrow, urine, stool, and tissue from typical rose spots (if they are present).

Fig2. Summary of Salmonella disease. 1 Indicates first-line drugs.

E. Treatment and prevention

For gastroenteritis in uncompromised hosts, antibiotic therapy is often not needed and may prolong the convalescent carrier state. For enteric fever, appropriate antibiotics include β-lactams and fluoroquinolones (see Figure 2). Prevention of Salmonella infection is accomplished by proper sewage disposal, correct handling of food, and good personal hygiene. Two different vaccines are available to prevent typhoid fever: One vaccine is delivered orally and consists of live attenuated Salmonella serovar Typhi. The other vaccine consists of the Vi capsular polysaccharide and is delivered parenterally. Vaccination is recommended for people who travel from developed countries to endemic areas including Asia, Africa, and Latin America.

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