Chapter 4 Travel-Related Infectious Diseases
Jessica M. Healy, Beau B. Bruce
Salmonella enterica subspecies enterica is a gram-negative, rod-shaped bacillus. More than 2,500 Salmonella serotypes have been identified, but only a small proportion are commonly associated with human illness. Nontyphoidal salmonellosis refers to illnesses caused by all serotypes of Salmonella except for Typhi, Paratyphi A, Paratyphi B (tartrate negative), and Paratyphi C.
Usually through the consumption of food or water contaminated with animal feces. Transmission can also occur through direct contact with infected animals or their environment and directly between humans.
Nontyphoidal salmonellae are one of the leading causes of bacterial diarrhea worldwide; they are estimated to cause approximately 153 million cases of gastroenteritis and 57,000 deaths globally each year. The risk of Salmonella infection among travelers returning to the United States varies by region of the world visited; the highest risk is among those who visited Africa (incidence of 25.8 cases per 100,000 air travelers), Latin America and the Caribbean (7.1 cases per 100,000), and Asia (5.8 cases per 100,000). A systematic review of travelers’ diarrhea studies found that Salmonella (including typhoidal serotypes) was detected in <5% of patients who had traveled to Latin America, the Caribbean, and South Asia and in 5%–15% of patients who had traveled to Africa or Southeast Asia. Salmonella infection and carriage has been reported among internationally adopted children.
Gastroenteritis is the most common clinical presentation of nontyphoidal Salmonella infection. The incubation period is typically 6–72 hours; although atypical, illness has been documented even 16 days after exposure. Illness is commonly manifested as acute diarrhea, abdominal pain, fever, and vomiting. The illness usually lasts 4–7 days, and most people recover without treatment.
Approximately 8% of people develop bacteremia or focal infection (such as meningitis, osteomyelitis, or septic arthritis). Serotypes more frequently associated with invasive infection include Dublin, Choleraesuis, and Typhimurium variant ST313 (currently only found in sub- Saharan Africa and Brazil). Rates of invasive infections and death are generally higher among infants, older adults, and people with immunosuppressive conditions (including HIV), hemoglobinopathies, and malignant neoplasms. Infection with antibiotic-resistant organisms has been associated with a higher risk of bloodstream infection and hospitalization.
Culturing organisms continues to be the mainstay of clinical diagnostic testing for nontyphoidal Salmonella infection. Approximately 90% of isolates are obtained from routine stool culture, but isolates can also be obtained from other sites of infection if present, including blood, urine, abscesses, and cerebrospinal fluid. Although culture-independent diagnostic tests are used increasingly by clinical laboratories to diagnose Salmonella infection, isolates are necessary for serotyping and antimicrobial susceptibility testing. Serologic testing to detect infection with Salmonella is not advised.
Most states mandate that Salmonella isolates or clinical material be submitted to the local or state public health laboratory. To understand submission requirements in a particular state, clinical laboratories are advised to review the disease reporting and mandatory isolate submission regulations of that state and to contact their local public health department with any questions. Salmonellosis is a nationally notifiable disease.
Current recommendations are to treat most patients with uncomplicated Salmonella infection with oral rehydration therapy but not with antimicrobial agents, as treatment can prolong bacterial shedding. Antimicrobial therapy should be considered for patients who are severely ill (those with severe diarrhea, high fever, or manifestations of extraintestinal infection) and for people at increased risk of invasive disease (infants, older adults, and the debilitated or immunosuppressed). When antimicrobial therapy is indicated, empiric treatment is usually required until susceptibility data are available. Resistance to antimicrobial agents varies by serotype and geographic region.
Fluoroquinolones are considered first-line treatment in adult travelers. However, resistance to fluoroquinolones among Salmonella strains is rising globally. In a study of international travelers diagnosed with S. enterica serotype Enteritidis infection in the United States, 24% of isolates showed decreased susceptibility to fluoroquinolones compared with only 3% of isolates from patients with no history of international travel. Azithromycin can be used for children and is an alternative agent for adults returning from Latin America or Asia, where fluoroquinolone resistance in this organism may exceed 10%. Azithromycin resistance has been documented in multiple settings globally but is not commonly reported.
Invasive strains of nontyphoidal Salmonella, such as Typhimurium variant ST313, emerging in areas of sub-Saharan Africa, have shown resistance to chloramphenicol, ampicillin, trimethoprim-sulfamethoxazole, and cephalosporins. Resistance to older antimicrobial agents (chloramphenicol, ampicillin, and trimethoprim-sulfamethoxazole) has been present for many years among nontyphoidal Salmonella serotypes; these should not be considered first-line empiric agents in returning travelers (see Chapter 2, Travelers’ Diarrhea).
No vaccine is available against nontyphoidal Salmonella infection. Preventive measures include food and water precautions (see Chapter 2, Food & Water Precautions), such as avoiding foods and drinks at high risk for contamination, and frequent handwashing, especially after contact with animals or their environment. Although rare, patients should be informed of the potential for continued bacterial shedding after symptoms have resolved.
CDC website: www.cdc.gov/salmonella
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- Association of Public Health Laboratories. State legal requirements for submission of isolates and other clinical materials by clinical laboratories: a review of state approaches 2015 [cited 2019 Jan 10]. Available from: www.aphl.org/aboutAPHL/publications/Documents/StateRequirements_Appendix_v6.pdf.
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