Shigellosis is an illness caused by infection by bacteria from the genus Shigella (more specifically, S. sonnei, S. boydii, S. flexneri, S. dysenteriae). Shigella are Gram-negative, nonmotile, nonsporeforming rod-shaped bacteria. Shigellosis accounts for less than 10% of the reported outbreaks of foodborne illness in the USA. Shigella rarely occurs in animals; principally a disease of humans except other primates such as monkeys and chimpanzees. The organism is frequently found in water polluted with human feces.

Abdominal pain; cramps; diarrhea; fever; vomiting; blood, pus, or mucus in stools; tenesmus. Onset time: 12 to 50 hours.

Infections are associated with mucosal ulceration, rectal bleeding, drastic dehydration; fatality may be as high as 10-15% with some strains. Reiter's disease, reactive arthritis, and hemolytic uremic syndrome are possible sequelae that have been reported in the aftermath of shigellosis.

Method of infection
As few as 10 cells depending on age and condition of host can be enough to cause an infection. The Shigella spp. are highly infectious agents that are transmitted by the fecal-oral route.

The disease is caused when virulent Shigella organisms attach to, and penetrate, epithelial cells of the intestinal mucosa. After invasion, they multiply intracellularly, and spread to contiguous epitheleal cells resulting in tissue destruction. Some strains produce enterotoxin and Shiga toxin (very much like the verotoxin of E. coli O157:H7).

Shigella can be transmitted through food. Food known to do so includes salads (potato, tuna, shrimp, macaroni, and chicken), raw vegetables, milk and dairy products, and poultry. Contamination of these foods is usually through the fecal-oral route. Fecally contaminated water and unsanitary handling by food handlers are the most common causes of contamination.

An estimated 300,000 cases of shigellosis occur annually in the United States. Infants, the elderly, and the infirm are susceptible to the severest symptoms of disease, but all humans are susceptible to some degree. Shigellosis is a very common malady suffered by individuals with acquired immune deficiency syndrome (AIDS) and AIDS-related complex, as well as non-AIDS homosexual men.

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What You Should Know
Shigellosis, although commonly regarded as waterborne, is also a foodborne disease restricted primarily to higher primates, including humans. It is usually spread among humans by food handlers with poor personal hygiene. Foods most often incriminated in the transmission have been potato salad, shellfish, raw vegetables, and Mexican dishes.

The genus Shigella consists of four species: S. dysenteriae (subgroup A), S. flexneri (subgroup B), S. boydii (subgroup C), and S. sonnei (subgroup D). Shigella organisms may be very difficult to distinguish biochemically from Escherichia coli. Brenner (1) considers Shigella organisms and E. coli to be a single species, based on DNA homology. Nonetheless, Shigella species are Gram-negative, facultatively anaerobic, nonsporulating, nonmotile rods in the family Enterobacteriaceae. They do not decarboxylate lysine or ferment lactose within 2 days. They utilize glucose and other carbohydrates, producing acid but not gas. However, because of their affinity to E. coli, frequent exceptions may be encountered, e.g., some biotypes produce gas from glucose and mannitol. Neither citrate nor malonate is used as the sole carbon source for growth, and the organisms are inhibited by potassium cyanide.