Shigella dysenteriae


Shigella dysenteriae

Description, Causes and Risk Factors:

A species causing severe necrotizing dysentery in humans induced by a virulent Shiga toxin found only in feces of symptomatic individuals; the type species of the genus Shigella.

Shigella dysenteriae is a Gram-negative, non-spore forming Bacilli that survives as a facultative anaerobe. It is part of the family Enterobacteriaceae. When testing for it in the laboratory, you can help identify it by the fact that it is non-motile, and lactose and lysine (-). This organism, unlike some enterics, does not produce gas when breaking down carbohydrates.

Virulence genes of this organism are coded on both chromosomes and plasmids. It is able to obtain iron from host cells by use of siderophores, allowing its growth. The severe irritation that is responsible for the bloody diarrhea is caused partly by lipopolysaccharide in its outer membrane, as well as a Shiga toxin (an A-B toxin), produced by this organism, which acts as a cytotoxin (causing cell necrosis and ulceration, etc). There are 2 immunological groups that are non-cross reactive, Stx1 and Stx2. These are both coded on the chromosome by insertion of a bacteriophage. This toxin can also act as an enterotoxin and a neurotoxin. The Shiga toxin causes cell death by preventing protein synthesis by cleaving a specific adenine residue from the 28s rRna in the 60s subunit.

Very few organisms are required to cause disease. This is due to the fact that this organism tends to be very acid tolerant, therefore gastric acids will have little effect on destroying the invading "bug". As little as 10 cells are required for disease to occur in 10% of healthy adults.

S. dysenteriae spread via the fecal-oral route, through ingestion of food or water contaminated with infected stool, or through touching contaminated materials, such as dirty diapers, and then touching the mouth. Even after symptoms of diarrhea subside, the stool continues to carry the bacteria for up to two weeks.

The epidemiology of this disease has been seen Worldwide. The occurrence of S. dysenteriae in the US annually is approximately 300,000 cases. In developing countries, that number tends to be greater, approximately 500,000 or more with substantial deaths caused by it (approx. 20,000). This was the case during an epidemic from 1969-1973 in central America. The greatest risk of infection occurs in children ages 1-4 and elderly adults. This has also become a complication in some HIV patients. Custodial workers are also at high risk of infection.

Prognosis for those infected is good. This organism can be successfully treated with antibiotics. Fluids can be given to those individuals suffering from dehydration. In less severe cases, 1-2 weeks allows the organism to pass through the system with the aid of the body's defenses. The body's own natural defenses include normal flora, secretory IgA, and phagocytosis. Antibiotic resistance is on the rise, hence antibiotics for severe cases.

Prevention of the disease is simple. Since it is predominantly spread by the fecal-oral route, thoroughly washing hands after using the restroom and changing diapers is effective. Proper food handling is also an important method of prevention, as well as maintaining clean water supplies, which is highly effective. There is currently no vaccine, however, there does appear to be a degree of immunity if one has recovered from infection. It is unlikely that it is antibody mediated, more so than desensitization of the mucosa to the organism.

Symptoms:

Symptoms vary, but include cramping, nausea, vomiting, fever, pain in the rectum and watery or bloody diarrhea (often causing dehydration) within two days of infection. Infection with S. dysenteriae provides immunity to this strain of Shigella, but people can be infected with other strains. Children younger than two years of age are most at risk, because they may develop seizures from the high fever associated with S. dysenteriae. Some diarrhea may persist for weeks or months after infection.

Diagnosis:

Blood culture should be obtained in children who appear toxic, very young, severely ill, malnourished, or immunocompromised because of their increased risk of bacteremia.

Isolation of Shigella from feces or rectal swab specimen is diagnostic but lacks specificity. Routine microscopy may reveal sheets of leukocytes on methylene-blue stained stool smear, which is a sensitive test for colitis but not specific for Shigella species.In approximately 70% of patients with S. dysenteriae, fecal blood or leukocytes (confirming colitis) are detectable in the stool.

Enzyme immunoassay: An enzyme immunoassay for Stx is used to detect S. dysenteriae type 1 in the stool.

Rapid techniques: With rapid techniques, gene probes or polymerase chain reaction (PCR) primers are directed toward virulence genes (invasion plasmid locus).

Other testing modalities, such as fluorescent antibody test and enzyme-linked DNA probes, are available in Research laboratories.

Treatment:

S. dysenteriae is increasingly resistant to antibiotics, so doctors often choose to provide supportive treatment, such as fluids, for mild infections and allow the infection to clear without antibiotics. If antibiotics are used, tests (culture and sensitivities) can help to determine which antibiotic is effective. Commonly used drugs include BactrimTM, Novo-AmpicillinSM and ciprofloxacin. Antidiarrheal medications are not effective and may make the infection worse, as they prevent the bacteria from clearing the system.

No vaccine is available to prevent S. dysenteriae infection, so prevention efforts focus on basic Hygiene & sanitation. Hand washing is especially important. Anyone changing soiled diapers should dispose of the diaper in a closed container, as flies can spread the bacteria. Cleanse the diaper-changing area with cleaning agents and sanitary wipes and wash the hands thoroughly.

NOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.

DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.

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