Shigellosis is an intestinal infection caused by a genus of bacteria known as Shigella that causes diarrheal disease of the colon or large intestines. The Shigella bacteria are highly infectious; 10 to 200 organisms are enough to cause infection. Shigellosis rarely occurs in animals; principally a disease of humans except other primates such as monkeys and chimpanzees. In 2005, statistics show that about 140 million people worldwide contract shigellosis and 600,000 cases die from the disease, mostly children under 5 years old living in concentrated urban centers of developing countries without adequate sanitation systems.
There are several different kinds of Shigella bacteria: S. dysenteriae (group A), S. flexneri (group B), S. boydii (group C), and S. sonnei (group D). Shigella sonnei (group D Shigella), accounts for over two-thirds of the shigellosis in the United States. A second type, Shigella flexneri, (group B Shigella), accounts for almost all of the rest in the US. Other types of Shigella are rare in the United States, though they continue to be important causes of disease in the developing world. One type found in the developing world, Shigella dysenteriae type 1 (group A), causes deadly epidemics with case fatality rates of 5-15%. Shigella dysenteriae was the original bacterium isolated by Dr. Kiyoshi Shiga.
The disease is caused when virulent Shigella bacteria attach to, and penetrate, epithelial cells of the intestinal mucosa. After invasion, they multiply intracellularly, and spread to contiguous epithelial cells resulting in tissue destruction. Some strains produce enterotoxin and Shiga toxin (very much like the verotoxin of E. coli O157:H7).
History of Shigellosis
The intestinal amebiasis, a similar disease to shigellosis caused by the bacteria Entamoeba histolytica was first described by Fedor Losch in 1875. Scientists in American, Europe, and Japan suspected a bacterial agent to be also responsible for shigellosis, a nonamebic intestinal disorder. The disease was feared for its high mortality rate among children.
The bacterium Shigella dysenteriae was discovered by Japanese bacteriologist Kiyoshi Shiga in 1897. At the time, Dr. Shiga was working as a research assistant in the Institute for Infectious Disease under the direction of Dr. Shibasaburo Kitasato, the famous Japanese scientists who cultivated Clostridium tetani and studied plague bacteria in Hong Kong. Dr. Kitasato directed Shiga's attention to an investigation of a sekiri (dysentery) outbreak. The Japanese word sekiri means "red diarrhea," a more accurate description of the disease than the generic term dysentery used to describe a variety of diarrheal diseases. The 1897, more than 90,000 cases of sekiri occurred with a mortality rate of more than 20%.
Dr. Shiga studied 36 patients at the Institute for Infectious Disease and isolated the bacteria from the intestinal tissue of a dysentery patient. When the bacteria was cultivated and fed to dogs, it caused disease. Dr. Shiga discovered that the bacterium, which he named Bacillus dysenterie, excreted toxins, later named Shiga toxins, that caused shigellosis. In 1930, it was the Bergey's Manual of Determinative Bacteriology that renamed the bacteria Shigella. In the 1950s, the four species of Shigella (group A, B, C, and D) were assembled together taxonomically. Following the discovery of the causal agent of shigellosis, Dr, Shiga worked on a vaccine for the disease, and even testing the first killed-cell vaccine on himself. A hundred years after Dr. Shiga's discovery, a vaccine has yet to be developed.
History of Shigellosis as a Biological Weapons Agent
Shigella bacteria is an attractive potential biological weapons agent due to its antibiotics resistance, its infectiousness, and the lack of a vaccine against the disease.
During World War II, the Japanese biological weapons program Unit 731 in Manchuria experimented with shigellosis on prisoners of war. In 1949, when a Soviet military tribunal tried Japanese scientists for their wartime biological experimentations, the Japanese accused the Soviets of experimentation with a variety of biological agents including Shigella.
Reports suggested that Iraq's biological weapons program experimented and developed with shigellosis as a possible offensive biological agent.
Shigella has also been used as a terrorist weapon. In 1997, a laboratory worker of a large medical center stole samples of Shigella bacteria and contaminated muffins and doughnuts in the staff room. His co-workers suffered shigellosis from contaminated snacks. The attacked caused cases of gastroenteritis and a few hospitalizations but no fatalities.
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