Shigellosis is an acute infection of the intestine caused by bacteria in the genus Shigella . There are 4 species of Shigella: S. dysenteriae, S. flexneri, S. boydii , and S. sonnei (also referred to as group A, B, C, and D, respectively). Several distinct serotypes are recognized within the first 3 species.
Shigella , which is host-adapted to humans and nonhuman primates, is transmitted via the fecal–oral route, including through direct person-to-person or sexual contact or indirectly through contaminated food, water, or fomites. Because as few as 10 organisms can cause infection, shigellosis is easily transmitted. It can be acquired even during short-term travel to settings with Western-style amenities.
In the United States, S. sonnei infection is usually transmitted through interpersonal contact, particularly among young children and their caregivers; however, outbreaks have also been linked to contaminated foods, infected food handlers, contaminated drinking water, swimming in contaminated water, and sexual activity.
Worldwide, Shigella is estimated to cause 80–165 million cases of disease and 600,000 deaths annually; of these, 20–119 million illnesses and 6,900–30,000 deaths are attributed to foodborne transmission. Shigella spp. are endemic in temperate and tropical climates. Transmission of Shigella spp. is most likely when hygiene and sanitation are insufficient. Shigellosis is caused predominantly by S. sonnei in industrialized countries, whereas S. flexneri prevails in the developing world. Infections caused by S. boydii and S. dysenteriae are less common globally but can make up a substantial proportion of Shigella spp. isolated in sub-Saharan Africa and South Asia.
Shigella spp. are detected in the stools of 5%–18% of patients with travelers’ diarrhea, and studies in Australia and Canada found that 40%–50% of locally diagnosed shigellosis cases were associated with international travel. In a study of travel-associated enteric infections diagnosed after return to the United States, Shigella was the third most common bacterial pathogen isolated by clinical laboratories (of note, these laboratories did not test for enterotoxigenic Escherichia coli , a common cause of travelers’ diarrhea). Many infections caused by S. dysenteriae (56%) and S. boydii (44%) were travel-associated, but infections caused by S. flexneri and S. sonnei were less often associated with travel (24% and 12%, respectively). In this study, the risk of infection caused by Shigella spp. was highest for people traveling to Africa, followed by Central America, South America, and Asia.
In 2014–2015, a large outbreak of S. sonnei occurred in the United States after travelers returned from India, Haiti, the Dominican Republic, and other countries with ciprofloxacin-resistant shigellosis. In 2015–2016, an outbreak of azithromycin-resistant S. sonnei infections occurred among men who have sex with men (MSM) and homeless people in Oregon. Outbreaks of infections caused by multidrug-resistant Shigella , including isolates resistant to azithromycin or ciprofloxacin, have been reported in Australia, Europe, Taiwan, Canada, and the United States among MSM. Infections caused by Shiga toxin–producing S. flexneri and S. dysenteriae have been reported repeatedly among travelers to Haiti and the Dominican Republic.
Illness typically begins 1–2 days after exposure with symptoms lasting 5–7 days. Disease severity varies according to species: serotype S. dysenteriae serotype 1 (Sd1) is the agent of epidemic dysentery, whereas S. sonnei commonly causes milder, nondysenteric diarrheal illness. However, Shigella of any species can cause severe illness among people with compromised immune systems. Shigellosis is characterized by watery, bloody, or mucoid diarrhea; fever; stomach cramps; and nausea. Occasionally, patients experience vomiting, seizures (young children), or postinfectious manifestations, including reactive arthritis, glomerulonephritis, and intestinal perforation. Hemolytic uremic syndrome can occur after infection with Shiga toxin–producing strains, particularly Sd1.
Rapid diagnostic tests for shigellosis are increasingly available in the United States; however, culture of a stool specimen or rectal swab should be conducted, including when a rapid test is positive, to obtain an isolate for species determination and antimicrobial susceptibility testing to guide treatment. Fecal specimens should be processed rapidly to optimize the chance of a positive culture. Shigellosis is a nationally notifiable disease in the United States.
Shigellosis can be mild and usually resolves within 5–7 days with supportive care alone; however, antimicrobial treatment given early in the course of illness can shorten the duration of symptoms and of carriage (asymptomatic shedding of the organism in the stool). Consider antimicrobial treatment for patients with severe disease or those with immune compromise.
When antimicrobial treatment is indicated for shigellosis associated with travel outside the United States, a fluoroquinolone, azithromycin, or ceftriaxone may be used empirically until antimicrobial susceptibility data are available, but clinicians should be aware that multidrug resistance among Shigella spp. is common. Shigellae acquired internationally are usually resistant to ampicillin and trimethoprim-sulfamethoxazole, and resistance to fluoroquinolones, azithromycin, and third- and fourth-generation cephalosporins occurs, particularly in South and East Asia. Additional information on antimicrobial resistance among US shigellosis cases is available at www.cdc.gov/narms/reports/index.html. Additional discussion of symptomatic management can be found in Chapter 2, Travelers’ Diarrhea.
No vaccines are available for Shigella . The best defense against shigellosis is thorough, frequent handwashing, strict adherence to standard food and water safety precautions (see Chapter 2, Food & Water Precautions), and minimizing fecal–oral exposures during sexual activity. When soap and water are not available, alcohol-based hand sanitizers can be a useful adjunct to washing hands. General recommendations to prevent diarrhea while traveling are addressed in Chapter 2, Travelers’ Diarrhea.
CDC website: www.cdc.gov/shigella
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Louise K. Francois Watkins, Grace D. Appiah