Shigellosis

Infectious Agent

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

Because only humans and higher primates carry Shigella , transmission occurs via the fecal-oral route, including through direct person-to-person or sexual contact or indirectly through contaminated food, water, or fomites. Since 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, an estimated 30% of US shigellosis is foodborne. Foodborne outbreaks have been linked to contaminated foods commonly consumed raw and to infected food handlers. Outbreaks have also been traced to contaminated drinking water, swimming in contaminated water, and sexual contact between men.

Epidemiology

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, and recent 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 ciprofloxacin-resistant S. sonnei infections occurred in the United States after travelers to India, Haiti, the Dominican Republic, and other countries returned with shigellosis. 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 men who have sex with men. Infections caused by Shiga toxin–producing S. flexneri and S. dysenteriae have been reported repeatedly among travelers to Haiti and the Dominican Republic.

Clinical Presentation

Illness typically begins 0.5–4 days after exposure. The symptoms of shigellosis 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, 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 arthritis. Hemolytic uremic syndrome can occur after infection with Shiga toxin–producing strains.

Diagnosis

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. Shigellosis is a nationally notifiable disease.

Treatment

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., including resistance to fluoroquinolones, azithromycin, and third- and fourth-generation cephalosporins, are high outside the United States, particularly in South and East Asia.

Prevention

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. Alcohol-based hand sanitizers may be a useful adjunct to washing hands with soap and water or when soap and water are not available.

CDC website: www.cdc.gov/shigella

Bibliography

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Author

Anna Bowen