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Shigellosis
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.
Symptoms
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
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| 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.
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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.
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