Shigella Background

Shigella are Gram-negative, non-motile rods which cause shigellosis or bacillary dysentery. Shigella is considered among the oldest human-specific pathogens for which evolutionary origins are known. It is thought that it evolved between 35000 and 170 000 years ago, which, coincidentally, is the time period when its sole natural host (i.e., Homo sapiens) originated and expanded. The genus Shigella comprises Gram-negative, nonspore forming, nonmotile prokaryotic rods. The Shigella genus is part of the larger Enterobacteriaceae family and, in fact, many researchers have suggested that Shigella and Escherichia should be considered one genus. However, due to their clinical and epidemiological differences, they remain as separate genera. Shigella can be classified into four major serological groups. Group A, Shigella dysenteriae, Group B, Shigella flexneri, Group C, Shigella boydii, and Group D, Shigella sonnei, which includes only one serotype. Shigella sonnei accounts for most cases of dysentery in the developed world (CDC).

Shigella is very contagious. People get infected with shigella when they come in contact with and swallow small amounts of bacteria from the stool of a person who is infected with shigella. The transfer of shigellae by flies breeding on faeces has been established as a very important transmission route during some outbreaks. Shigella bacteria can also be passed in infected food or by drinking or swimming in unsafe water. Children under age 5 are most likely to get shigella infection, but it can occur at any age. A mild case usually clears up on its own within a week. When treatment is needed, antibiotics are usually prescribed. Shigella species cause an estimated 160 million episodes of diarrhea worldwide and over a million deaths.

References

• WHO. Shigella factsheet
• CDC. Shigella – Shigellosis factsheet