Shigella spp.
Shigella are Gram-negative, nonmotile, nonsporeforming rod-shaped bacteria. The illness caused by Shigella (shigellosis) accounts for less than 10% of the reported outbreaks of foodborne illness in this country. 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.
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Nature of Acute Disease:
Shigellosis (bacillary dysentery).
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Nature of Disease:
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Symptoms — Abdominal pain; cramps; diarrhea; fever; vomiting; blood, pus, or mucus in stools; tenesmus.
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Onset time — 12 to 50 hours.
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Infective dose — As few as 10 cells depending on age and condition of host.
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).
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Diagnosis of Human Illness:
Serological identification of culture isolated from stool.
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Associated Foods:
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.
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Relative Frequency of Disease:
An estimated 300,000 cases of shigellosis occur annually in the U.S. The number attributable to food is unknown, but given the low infectious dose, it is probably substantial.
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Course of Disease and Complications:
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.
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Target Populations:
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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Food Analysis:
Organisms are difficult to demonstrate in foods because methods are not developed or are insensitive. A genetic probe to the virulence plasmid has been developed by FDA and is currently under field test. However, the isolation procedures are still poor.
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Selected Outbreaks: For more information on recent outbreaks see the Morbidity and Mortality Weekly Reports from CDC.
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Education and Background Resources:
Literature references can be found at the links below.
Loci index for genome Shigella spp.
Available from the GenBank Taxonomy database, which contains the names of all organisms that are represented in the genetic databases with at least one nucleotide or protein sequence.
ShigellosisFAQ’s from the CDC.
