Shigellosis

Definition

Shigellosis results from inflammation of the large intestine due to infection with bacteria of the genus Shigella . Clinical presentation is either with acute watery diarrhea or bloody diarrhea, known as dysentery, and is commonly accompanied by fever. Shigellosis is a major global health problem compounded by increasing levels of antimicrobial resistance. It is the most common cause of diarrheal deaths for which no vaccine is currently available.

Epidemiology

Shigella is a human-restricted pathogen. Transmission most commonly occurs via the fecal-oral route, predominantly from direct person-to-person contact or via fomites. Infection is more common among populations in which overcrowding is common and without access to clean water and proper sanitation. A low number of bacilli, as little as 10 organisms for S. dysenteriae and 180 for S. flexneri and S. sonnei, is required for infection.

Transmission also occurs through contaminated food and water. Shigella was the third most common pathogen associated with food-borne gastroenteritis in the United States in 2016. Transmission by infected asymptomatic food-handlers presents a particular risk. In settings in which open defecation is common, transmission can be facilitated through house flies. Shigellosis is also increasingly recognized as a sexually transmitted disease, particularly among men who have sex with men.

Shigellosis is predominantly a disease of low- and middle-income countries. Worldwide, Shigella is the second most common cause of diarrheal death, following rotavirus (which is also the most common cause of moderate to severe diarrhea; Chapter 350 ), and it is the most common bacterial cause. Shigella is the main pathogen associated with dysentery (attributable fraction 64%), and the second most attributable pathogen for watery diarrhea (attributable fraction 13%). The Global Burden of Disease 2019 report estimated about 150,000 worldwide deaths attributable to Shigella , with about 95,000 occurring among children under 5 years of age. Disability-adjusted life years lost to shigellosis are much higher and are estimated at about 10.5 million overall, about 8.5 million of which are lost in children under age 5 years. By serotyping, 66% of Shigella isolates are S. flexneri, predominantly S. flexneri 2a, whereas 24% are S. sonnei, 5% are S. dysenteriae, and 5% are S. boydii .

Children under 5 years and adults who live in poor and crowded circumstances are at high risk of shigellosis. Daycare centers present an environment where transmission of shigellosis is common. At the other end of the age spectrum, the elderly are also at increased risk of shigellosis, especially elderly individuals living in residential care facilities.

The incidence of shigellosis is much lower in high-income countries compared with low- to middle-income countries. Nevertheless, shigellosis is the second most common cause of acute gastroenteritis outbreaks in the United States after norovirus ( Chapter 350 ). Shigellosis is common in the military, and Shigella has been estimated to be responsible for 2 to 9% of cases of travelers’ diarrhea ( Chapter 265 ).

S. dysenteriae type 1 was responsible for four pandemics with high attack rates and case fatality rates between 1968 and the 1990s. For unknown reasons, S. dysenteriae type 1 has largely disappeared in the 21st century, but has the potential to reemerge, particularly in settings of natural disasters with displaced peoples and poor sanitation and hygiene.

Although incidence levels have remained high, deaths due to shigellosis have decreased, likely because of improved management with antibiotics, oral rehydration solution, supplementation of zinc and vitamin A, and less preinfection baseline malnutrition. The change in profile of Shigella species, with the disappearance of S. dysenteriae type 1 and increased proportion of mild shigellosis due to S. sonnei in developed countries, also may be responsible.