Bioterrorism

Profiles for Health Care Workers (Fact Sheets) - "B" Agents

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Shigella

Causative Agent:
Shigella are gram-negative, nonmotile, nonsporeforming, rod-shaped bacteria that are comprised of four species or serogroups which are further divided into serotypes and subtypes. Shigellosis 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 epithelial cells resulting in tissue destruction. Some strains produce enterotoxin and Shiga toxin.

Routes of Exposure:
Fecal-oral transmission through direct and indirect person-to-person contact is the main route of exposure. Ingesting contaminated foods and beverages can also spread infection.

Infective Dose & Infectivity:
Shigella bacteria are highly infectious. The ingestion of very few organisms (10-100) is sufficient to cause infection. Though all people are believed to be susceptible to some degree, infants, the elderly, and the infirm are most likely to experience severe symptoms of disease.

Incubation Period:
The incubation is usually between 1 and 3 days, but can range from 12 to 96 hours for most strains. Some strains have incubation periods of up to one week.

Clinical Effects:
The illness is characterized by diarrhea accompanied by fever, nausea, toxemia, vomiting, cramps, and tenesmus. Though cases may also present with watery diarrhea, typical stools contain blood, mucus, or pus, which is the result of mucosal ulcerations and confluent colonic crypt microabscesses caused by the invasive organisms. Bacteremia is uncommon. Mild and asymptomatic infections can occur. Illness is usually self-limited, lasting an average of 4-7 days. Severe complications can include toxic megacolon, the hemolytic uremic syndrome, and Reiter syndrome. Convulsions, which could be the result of rapid temperature elevation or metabolic alterations, may occur in young children.

Lethality:
Although the mortality rate for some strains of Shigella may be as high as 10-20%, it is generally quite low. Two-thirds of the cases, and most of the deaths are in children under 10 years old.

Transmissibility:
Shigella infection is caused by fecal-oral transmission. Individuals primarily responsible for transmission are those who do not practice proper hand washing techniques, especially after defecating. Infection may be spread to others directly through physical contact or indirectly through contaminated food and beverages. Unsanitary food handling is the most common cause of contamination. Flies can also transfer organisms from latrines to uncovered food items.

Primary Contamination & Methods of Dissemination:
In a terrorist attack, Shigella would most likely be disseminated through the intentional contamination of food or water supplies.

Secondary Contamination & Persistence of organism:
Secondary transmission can result from exposure to the stool of infected individuals. Diarrheal fluids are highly infectious. In households, secondary attack rates can be as high as 40%. Following illness, stool typically remains infectious for 4 weeks, though the bacteria can persist for months or longer in asymptomatic carriers. Antimicrobial treatment can reduce the period of infectivity to a few days.

Decontamination & Isolation:

  • Patients – No decontamination necessary. Patients can be treated with standard precautions, with contact precautions for diapered or incontinent patients. Hand washing is of particular importance.
  • Equipment & other objects – 0.5% hypochlorite solution (one part household bleach and nine parts water), EPA approved disinfectants, and/or soap and water can be used for environmental decontamination.

Laboratory Testing:
Diagnosis is made by isolation of Shigella from feces or rectal swabs. Prompt laboratory processing of specimens and use of appropriate media increase the likelihood of Shigella isolation. Infection is usually associated with the presence of copious numbers of fecal leukocytes detected by microscopic examination of stool mucus stained with methylene blue or gram stain. .

Therapeutic Treatment: Fluid and electrolyte replacement is important when diarrhea is watery or there are signs of dehydration. Antibacterial therapy shortens the duration and severity of illness and the duration of Shigella excretion
.

Multidrug resistance is common; the choice of empiric antibiotics is best determined by local susceptibility patterns. Usually effective antibiotics include fluoroquinolones, third generation cephalosporins, and trimethoprim-sulfamethoxazole. Antimotility agents such as loperamide are not approved for children under 2 years old. Their use is generally discouraged in bacterial infections as these drugs may prolong the illness. Nevertheless, if they are administered in an attempt to alleviate the severe cramps that often accompany shigellosis, they should never be given without concomitant antimicrobial therapy.

Prophylactic Treatment: Prophylactic administration of antibiotics is not recommended.

Differential Diagnosis: Salmonella, E. coli O157:H7, Campylobacter, Yersinia enterocolitca, and bacterial food poisoning may show similar signs and symptoms.

References:

  • Chin J. Control of Communicable Diseases Manual, Seventeenth Edition, American Public Health Association; 2000.
  • Center for Food Safety and Applied Nutrition. Foodborne Pathogenic Microorganisms and Natural Toxins Handbook, U.S. Food and Drug Administration

Find the PDF version of this Fact Sheet in the Zebra Manual.