by Kate Finnegan
There are four Shigella species., S. dysenteriae, S. flexneri, S. boydii and S. sonnei (3). Shigella sonnei is the most common infectious form in North America and the developed world (4).
Shigella are present in the stools of infected people, and most infections occur from contaminated fingers coming into contact with the mouth (1). The bacteria can also come from food contaminated due to poor hygienic practices in farming and harvesting produce (1). Flies are also known vectors of Shigella from feces to food (3).
Humans are the primary reservoir (3), though infections have been reported in monkeys and chimpanzees (2). The feces of infected individuals (1) and waters contaminated by infected feces (2) are also reservoirs of the pathogenic agent.
Shigella are gram-negative rods that are non-motile and non-sporeforming (2). The bacteria is primarily a human disease, but has been found in some primates (2). Shigella are facultative anaerobes, similar to enterics such as E. coli (3).
Key Tests for Identification
Neutrophils in the stool are an easily identifiable sign of Shigella infection (3). In the medical laboratory, identification of individual Shigella species is often accomplished through slide agglutination using absorbed rabbit antisera (3). Isolation with MacConkey’s, Hektoen Enteric Agar or Salmonella-Shigella Agar are used as differential/selective media (3). In order to differentiate with E. coli species, PCR and ELISA testing must be performed on plasmid genes (3).
Signs and Symptoms of Shigellosis
Those infected with the disease typically exhibit diarrhea, which is often bloody and filled with mucosa, stomach cramping and fever within several days of exposure (1, 3). Other symptoms include nausea, vomiting and abdominal tenderness (3). In children under two years of age, the high fever may lead to convulsions or seizures (1). Some infected individuals will have no symptoms of the disease, but are still capable of spreading it (1).
Shigella bacteria were discovered in 1897 by a Japanese Scientist named Kiyoshi Shiga (1, 6). An outbreak of over 90,000 cases was occurring in Japan, with a mortality rate of over 20% (6). He originally named the disease Bacillus dysenterie, but it was renamed by Bergey’s Manual of Determinative Bacteriology in 1930 in order to pay homage to Dr. Shiga (6). Due to its infectiousness and antibiotic resistance, Shigella has been studied for use as a biological weapon (6). During World War II, the Japanese experimented with Shigella as a biological weapon on prisoners of war, and, more recently, reports suggest that Iraqi scientists may have experimented with the use of Shigella for similar purposes (6).
Shigella can be passed from person to person through the fecal-oral route (1). An infective dose can be as few as 10 bacteria, which attach to and penetrate intestinal cell walls of the small intestines (2). Scientists have recently discovered two new toxins produced by Shigella that may promote the diarrhea characteristic of the disease (4). The Shiga toxin enables the bacteria to penetrate the epithelial lining of the intestines, leading to a breakdown of the lining and hemorrhage (7). Shigella also have adhesins that promote binding to epithelial cell surfaces and invasion plasmid antigens that allow the bacteria to enter target cells, thus increasing its virulence (7).
Most infections can resolve without antibiotic treatment, however, ampicillin, trimethoprim/sulfamethoxazole or ciprofloxacin are all used when necessary (1). Anti-diarrheal medications can actually make the infection worse (1). Hydration is an important concern of a patient infected with Shigella. Oral rehydration is necessary to prevent severe dehydration, electrolyte imbalances and metabolic acidosis caused by fluid loss (3).
There is currently no vaccine for Shigellosis prevention (1), but there is current research that appears promising (3). The most effective method for prevention is frequent and vigorous handwashing with warm, soapy water (1) and insuring clean drinking water sources and proper sewage disposal in developing nations (3).
Every year, nearly 14,000 cases of Shigellosis are reported in the United States, but it is suspected that nearly 20 times that number of cases actually exist (1). Despite this, cases of Shigellosis in the United States make up only 10% of cases of foodborne illness (2). Fatality may be as high as 10-15% with some strains of Shigella (2). Worldwide, it is estimated that 164.7 million cases of Shigellosis occur annually, of which 163.2 million were in underdeveloped nations (5). Over 1.1 million of these infections resulted in death, with the vast majority of cases in children under the age of 5 (5).
1. Centers for Disease Control and Prevention. “Shigellosis”. 03/27/08. URL: http://www.cdc.gov/nczved/dfbmd/disease_listing/shigellosis_gi.html accessed on 05/04/08.
2. United States Food & Drug Administration. “Shigella spp.” 12/28/07. URL: http://vm.cfsan.fda.gov/~mow/chap19.html accessed on 05/04/08.
3. Hale, T. and G. Keusch. “Shigella”. (no date). URL: http://gsbs.utmb.edu/microbook/ch022.htm accessed on 05/04/08.
4. National Institute of Allergy and Infectious Disease NIH. “Shigellosis”. 02/27/07. URL: http://www3.niaid.nih.gov/healthscience/healthtopics/shigellosis/Shigellosis.htm accessed on 05/05/08.
5. Kotloff, KL et al. Global burden of Shigella infections: implications for vaccine development and implementation of control strategies. Bulletin of the World Health Organization. 1999; 77(8): 651 – 66. URL: http://www.ncbi.nlm.nih.gov/pubmed/10516787 accessed on 05/05/08.
6. GlobalSecurity.org. “Weapons of Mass Destruction (WMD): Shigellosis”. 10/23/07. URL: http://www.globalsecurity.org/wmd/intro/bio-shigellosis.htm accessed on 05/05/08.
7. Todar, K. “Shigella and Shigellosis”. Todar’s Online Textbook of Bacteriology. 2008. URL: http://www.textbookofbacteriology.net/Shigella.html accessed on 05/05/08.