Chapter 1

Shigella Food Poisoning

What is Shigella and how does it cause food poisoning?

Shigella is a species of enteric bacteria that causes disease in humans and other primates. [16, 20] The disease caused by the ingestion of Shigella bacteria is referred to as shigellosis, which is most typically associated with diarrhea and other gastrointestinal symptoms. [11, 16] “Shigella infection is the third most common cause of bacterial gastroenteritis in the United States, after Salmonella infection and Campylobacter infection and ahead of E. coli O157 infection.” [19]

The global burden of shigellosis has been estimated at 165 million cases per year, of which 163 million are in developing countries. [23] More than one million deaths occur in the developing world yearly due to Shigella infection. [23, 29] By one estimate, Shigella infections are responsible for 300,000 illnesses and 600 deaths per year in the United States. [25] By another estimate, each year 450,000 Americans are infected with Shigella, causing 6,200 hospitalizations and 70 deaths. [27]

In general, Shigella is one of the most communicable and severe forms of the bacterial-induced diarrheas. [18] No group of individuals is immune to shigellosis, but certain individuals are at increased risk. [16] Small children acquire Shigella at the highest rate, and [24, 28] persons infected with HIV experience shigellosis much more commonly than other individuals. [4]

Shigella is easily spread person-to-person because of its relatively tiny (compared to other bacteria) infectious dose. [16, 23] Infection can occur after ingestion of fewer than 100 bacteria. [1, 16, 17] Another reason Shigella so easily cause infection is because the bacteria thrive in the human intestine and are commonly spread both by person-to-person contact and through the contamination of food. [11, 22, 32]

The Discovery and Naming of Shigella

The several types of Shigella bacteria have been named after the lead workers who discovered each one. [11, 16, 20] The first bacterium to be discovered, Shigella dysentariae, was named after Kiyoshi Shiga, a Japanese scientist who discovered it in 1896 while investigating a large epidemic of dysentery in Japan. [22, 37] The bacterium was also referred to more generally as the dysentery bacillus (the term “bacillus” referring to a genus of Gram-positive, rod-shaped bacteria of which Shigella is a member). [37]

In a summary published annually, the CDC provides an overview of the classification of various types (species) of Shigella bacteria, as follows:

There are 4 major subgroups of Shigella, designated A, B, C and D, and 44 recognized serotypes. Subgroups A, B, C and D have historically been treated as species: subgroup A for Shigella dysenteriae; subgroup B for Shigella flexneri; subgroup C for Shigella boydii and subgroup D for Shigella sonnei. These subgroups and serotypes are differentiated from one another by their biochemical traits (ability to ferment D-mannitol) and antigenic properties. The most recently recognized serotype belongs to subgroup C (S. boydii). [12]

S. sonnei, also known as Group D Shigella, accounts for over two-thirds of shigellosis in the United States. Shigella flexneri, or group B Shigella, accounts for almost all the rest. [11, 19] More specifically, according to one recent study, “From 1989 to 2002, S. flexneri accounted for 18.4% of Shigella isolates submitted to CDC. [4] From 1973 to 1999, only 49 S. flexneri-associated outbreaks of foodborne disease were reported.” [32] In contrast, in developing countries, S. flexneri is the most predominant cause of shigellosis, but S. dysenteriae type 1 is still a frequent cause of epidemic throughout the developing world. [1, 16, 23, 37]