Shigellosis is an acute infection of the intestine caused by bacteria in the genus Shigella. There are 4 species of Shigella: Shigella 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.
Transmission occurs via the fecal-oral route, through direct person-to-person contact, or indirectly through contaminated food, water, or fomites. As few as 10 organisms can cause infection. Only humans and higher primates carry Shigella. In the United States, S. sonnei infection is usually transmitted through interpersonal contact, particularly among young children in day care settings. 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 are uncommon. S. dysenteriae is even more uncommon, but makes up ≥25% of all 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 FoodNet 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 do not test for enterotoxigenic Escherichia coli, a common cause of travelers’ diarrhea). Most infections caused by S. dysenteriae were travel-associated (56%). Many infections caused by S. boydii (44%) were acquired while traveling, but infections caused by S. flexneri and S. sonnei were less often associated with travel (24% and 12%, respectively). Risk of infection caused by Shigella spp. is highest for people traveling to Africa, followed by Central America, South America, and Asia.
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 is a common cause of milder diarrheal illness. The disease 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.
Shigellosis is confirmed through culture of a stool specimen or rectal swab. Samples should be processed rapidly because Shigella cannot survive for long outside the body. Shigella isolates may then be speciated and serotyped and their antimicrobial susceptibilities determined to help guide treatment.
In healthy people, shigellosis will typically resolve within 4–7 days, even without treatment. Antimicrobial treatment, when given early in the course of illness, can slightly shorten the duration of symptoms and of carriage. The possibility of resistance should be considered for patients in whom treatment is indicated. For shigellosis associated with travel outside the United States, a fluoroquinolone (for adults and, if infection is acquired in regions with high rates of multidrug resistance, children) or ceftriaxone (for children) may be used empirically until antimicrobial susceptibility data are available. However, resistance to fluoroquinolones and third- and fourth-generation cephalosporins has been reported, particularly among Shigella isolates acquired in South and East Asia. Multidrug-resistant strains are especially common among travelers; empiric treatment should be tailored to region of travel.
No vaccines are available for Shigella. The best defense against shigellosis is thorough, frequent handwashing and strict adherence to standard food and water safety precautions (see Chapter 2, Food & Water Precautions).
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