Shigellosis is an acute infection of the intestine caused by bacteria in the genus Shigella. There are 4 species of Shigella: Shigelladysenteriae, 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.
Transmission occurs via the fecal-oral route, through direct person-to-person contact, or indirectly through contaminated food, water, or fomites. Since as few as 10 organisms can cause infection, shigellosis is easily transmitted and can be acquired during short-term travel. Only humans and higher primates carry Shigella. In the United States, S. sonnei infection is usually transmitted through interpersonal contact, particularly among young children and their caregivers. Foodborne outbreaks have been linked to contaminated foods commonly consumed raw, as well as infected food handlers. Outbreaks have also been traced to contaminated drinking water, swimming in contaminated water, and sexual contact between men.
Worldwide, Shigella is estimated to cause 80–165 million cases of disease and 600,000 deaths annually. Shigella spp. are endemic in temperate and tropical climates. Transmission of Shigella spp. is most likely when hygiene and sanitation are insufficient. Shigellosis is predominantly caused 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. 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. Outbreaks of infections caused by multidrug-resistant Shigella, including isolates resistant to azithromycin or ciprofloxacin, have been reported in Australia, Europe, and North America among men who have sex with men.
Illness typically begins 12–96 hours after exposure. The symptoms of shigellosis range from mild to severe and typically last 4–7 days. Disease severity varies according to species; serotype S. dysenteriae serotype 1 (Sd1) is the agent of epidemic dysentery, while S. sonnei commonly causes milder 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 arthritis. Hemolytic uremic syndrome can occur after infection with Sd1.
Rapid diagnostic tests for shigellosis are used in some US laboratories; however, shigellosis should be confirmed through culture of a stool specimen or rectal swab. Shigella isolates may then be speciated and serotyped and their antimicrobial susceptibilities determined to help guide treatment. Fecal specimens should be processed rapidly because Shigella often does not survive for long outside the body.
Although antimicrobial treatment, when given early in the course of illness, can slightly shorten the duration of symptoms and of carriage, shigellosis can be mild and typically resolves within 4–7 days without treatment. When treatment is required for shigellosis associated with travel outside the United States, a fluoroquinolone or ceftriaxone may be used empirically until antimicrobial susceptibility data are available. However, clinicians should be aware that rates of multidrug resistance among Shigella spp. are high globally, including resistance to fluoroquinolones, azithromycin, and third- and fourth-generation cephalosporins, particularly in South and East Asia.
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 contact.
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