Author(s): Ian Plumb, Patricia (Patti) Fields, Beau Bruce

Infectious Agent

Salmonella are gram-negative, rod-shaped bacilli. More than 2,500 Salmonella serotypes have been described. Salmonella serotypes can be categorized as typhoidal, which cause typhoid and paratyphoid fever, and nontyphoidal serotypes, which cause other human illness, typically acute diarrhea. See the Sec. 5, Part 1, Ch. 24, Typhoid & Paratyphoid Fever, for illness caused by Salmonella serotypes Typhi, Paratyphi A, tartrate negative Paratyphi B, and Paratyphi C.

Transmission

Animal reservoirs include both domestic and wild animals, including food animals, amphibians, and reptiles. Human infection can result from direct contact with infected animals or their environments. Transmission usually occurs from eating contaminated foods (e.g., dairy, eggs, meat, raw produce); drinking contaminated water; or from contact with people who have a diarrheal illness. The risk for infection after exposure is increased by taking antibiotics or antacid medication.

Epidemiology

Nontyphoidal Salmonella is one of the leading bacterial causes of diarrhea, causing ≈150 million illnesses and ≈60,000 deaths globally each year. Salmonella infection is diagnosed in ≈5 per 1,000 travelers who return with diarrhea. Among travelers returning to the United States, the rate of confirmed infection per 100,000 air travelers is estimated to be 26 after travel to Africa; 6–9 after travel to the Caribbean, Central America, or Asia; and 2–3 after travel to South America, Europe, or Oceania.

Clinical Presentation

Nontyphoidal Salmonella infection usually presents with an acute diarrheal illness. The incubation period of salmonellosis is typically 12–96 hours, but it can be ≥7 days. Illness manifests commonly with acute diarrhea, abdominal cramps, and fever, and usually resolves without treatment after 1–7 days.

Approximately 5% of people develop bacteremia or focal invasive infection (e.g., osteomyelitis, meningitis, endovascular infection, septic arthritis). Rates of invasive disease are generally higher among infants, older adults, and people who are immunocompromised, including those with HIV. People with atherosclerosis, hemoglobinopathies, or malignant neoplasms also have increased risk for extraintestinal infection. Infection with antibiotic-resistant organisms has been associated with a greater risk for bloodstream infection and hospitalization.

Diagnosis

Culture provides confirmation of nontyphoidal Salmonella infection. Approximately 90% of isolates are obtained from routine stool culture; isolates also can be obtained from other sites of infection (e.g., abscesses, blood, cerebrospinal fluid, urine). Although clinical laboratories increasingly use culture-independent diagnostic tests to detect Salmonella infection, isolates are necessary for antimicrobial susceptibility testing and for characterization during public health investigations. Reflex bacterial culture is recommended, if possible, on the same specimen, for positive culture-independent specimens. Serologic testing is unreliable and not advised.

Salmonellosis is a nationally notifiable disease. Most states mandate that Salmonella isolates or clinical material be submitted to the local or state public health laboratory. Clinical laboratory staff should be aware of disease reporting and mandatory isolate submission regulations for their state; they can contact their local public health department with questions.

Treatment

Indications for Antibiotic Therapy

Most patients can be treated with supportive care alone, including oral rehydration therapy. Antibiotic therapy is not recommended for most patients with uncomplicated salmonellosis caused by nontyphoidal Salmonella; it does not shorten the duration of illness and can prolong bacterial shedding.

Consider antibiotic therapy for patients with suspected invasive disease (e.g., patients with severe diarrhea, high fever, manifestations of extraintestinal infection) and for patients at increased risk for invasive disease (e.g., infants, older adults, people who are immunocompromised, patients with known atherosclerosis). For these populations, treat infections empirically until susceptibility results are available.

Bacteremic patients generally require ≥7 days of antimicrobial drug therapy and an investigation for possible sites of infection. Longer therapy, specialist consultation, and surgical intervention might be required for extraintestinal infections. Immunocompromised patients are at risk for recurrent invasive disease and require therapy of longer duration.

Choice of Empiric Antimicrobial Drug Therapy

Salmonella resistance to older antimicrobial agents (ampicillin, chloramphenicol, and trimethoprim-sulfamethoxazole) has been recognized for many years; none of these should be considered first-line empiric agents in returning travelers (see Sec. 2, Ch. 6, Travelers’ Diarrhea). Resistance to antimicrobial agents varies by Salmonella serotype and geographic region.

Fluoroquinolones

Fluoroquinolones are considered first-line treatment in adult travelers. Resistance to fluoroquinolones among Salmonella strains has been rising globally, however; among travelers returning to the United States with a diagnosis of nontyphoidal salmonellosis during 2004–2014, decreased susceptibility to fluoroquinolones was present in 41% of isolates from travelers to Asia.

Ceftriaxone

Ceftriaxone can be used to treat children or adults with invasive disease. Although ceftriaxone resistance is rare, it has increasingly been detected among bloodstream isolates in sub-Saharan Africa. Azithromycin can be used for children and is an alternative agent for adults. Decreased susceptibility to azithromycin is rare, but has been documented in multiple settings globally. Clinical laboratories do not commonly test for resistance to azithromycin, however, because susceptibility breakpoints have not been established.

Prevention

No vaccine against nontyphoidal Salmonella infection is available. Travelers should follow preventive measures, such as eating food that is adequately cooked and drinking from safe sources (see Sec. 2, Ch. 8, Food & Water Precautions), and by frequently washing hands, especially after contact with animals or their environments. In general, travelers should avoid uncooked vegetables, but travelers can gain some protection by washing raw produce properly.

People with diarrheal illness should avoid preparing food for others. After their symptoms have resolved, people who had diarrheal illness should continue to practice safe food preparation and carefully wash hands regularly because they can shed bacteria for weeks afterward.

CDC website: Salmonella