Shigella
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Shigella is a bacterium that belongs to a small group of pathogens (including E. coli O157:H7 and Cryptosporidium) that can infect the gut after the ingestion of relatively few organisms, and can cause sudden and severe diarrhea (gastroenteritis) in humans. When ingested, Shigella bacteria penetrate the lining of the intestine, causing swelling and possibly causing sores to develop (Mayo Clinic, 2007, April 14).
Volunteer experiments have demonstrated that shigellosis – the illness caused by the ingestion of Shigella bacteria, which is also known as “bacillary dysentery” – can occur after ingestion of fewer than 200 bacteria (DuPont, et. al. 1989), making Shigella one of the most communicable and severe forms of the bacterial-induced diarrheas (Gomez, et.al. 2002).
Shigella is named after Kiyoshi Shiga, a Japanese scientist who discovered Shigella dysenteriae type 1 in 1896 during a large epidemic of dysentery in Japan (Keusch & Acheson, 1996). Since that time, several types of Shigella bacteria have been discovered – S. dysenteriae, S. flexneri, S. boydii, and S. sonnei – all named after the lead workers who discovered them (CDC, 2005, October 13).
Shigella thrives in the human intestine and is commonly spread both through food and by person-to-person contact. About 25,000 laboratory-confirmed cases of shigellosis are reported each year in the U.S. (Mead, et al., 1999); however, many cases go undiagnosed and/or unreported and the Centers for Disease Control and Prevention (CDC) estimates that 450,000 total cases of shigellosis occur in the United States every year (Baer, et al., 1999; CDC, 2005, October 13).
No group of individuals is immune to shigellosis, but certain individuals are at increased risk. Small children acquire Shigella at the highest rate. Persons infected with HIV experience shigellosis much more commonly than other individuals, but this may largely be due to an increased risk among men having sex with men (Baer, et al., 1999).
In developing countries, S. flexneri is the most predominant cause of shigellosis, but S. dysinteriae type 1 is the most frequent cause of epidemic and endemic disease. In developed countries such as the United States, S. sonnei is the predominant cause of Shigellosis; S. sonnei is involved in over 75% of cases reported annually in the US (Keusch & Acheson, 1996).
How is Shigella transmitted?
The ultimate source of Shigella bacteria is the infected excrement of a previously infected individual. A new case of bacillary dysentery occurs after the organism is ingested and approximately 20 percent of cases of shigellosis are transmitted via contaminated food or water (Mead, et.al. 1999). Food can become contaminated due to unsanitary practices followed by food workers, but may also become contaminated during harvest or processing. Contamination of drinking water by Shigella is a problem that generally only occurs in the developing world (Barzilay, Weinberg & Eley, 1999).