Bad Bug Book – Foodborne Pathogenic Microorganisms and Natural Toxins – Second Edition

shigella1.jpeg.jpg1. Organism

Shigella are Gram-negative, non-motile, non- sporeforming, rod-shaped bacteria. Shigella species, which include Shigella sonnei, S. boydii, S. flexneri, and

S. dysenteriae, are highly infectious agents. Some strains produce enterotoxins and Shiga toxin. The latter is very similar to the toxins produced by E. coli O157:H7.

Humans are the only host of Shigella, but it has also been isolated from higher primates. The organism is frequently found in water polluted with human feces.

In terms of survival, shigellae are very sensitive to environmental conditions and die rapidly. They are heat sensitive and do not survive pasteurization and cooking temperatures. In terms of growth, shigellae are not particularly fastidious in their requirements and, in most cases, the organisms are routinely cultivated in the laboratory, on artificial media. However, as noted in subsequent sections, the relative difficulty of cultivating this organism is dependent, in part, on the amount of time within which stool or food samples are collected and processed.

Shigella species are tolerant to low pH and are
able to transit the harsh environment of the
stomach. These pathogens are able to survive
and, in some cases, grow in foods with low pH,
such as some fruits and vegetables. They are
able to survive on produce commodities
packaged under vacuum or modified
atmosphere and can also survive in water, with a slight decrease in numbers.

2. Disease

The illness caused by Shigella is shigellosis (also called bacillary dysentery), in which diarrhea may range from watery stool to severe, life-threatening dysentery. All Shigella spp. can cause acute, bloody diarrhea. Shigella spp. can spread rapidly through a population, particularly in crowded and unsanitary conditions.

S. dysenteriae type 1 causes the most severe disease and is the only serotype that produces the Shiga toxin, which may be partially responsible for cases in which hemolytic uremic syndrome (HUS) develops. S. sonnei produces the mildest form of shigellosis; usually watery diarrhea.

S. flexneri and S. boydii infections can be either mild or severe.
In developed countries, S. sonnei is the Shigella species most often isolated, whereas S. flexneri predominates in developing countries.

• Mortality: In otherwise healthy people, the disease usually is self-limiting, although some strains are associated with fatality rates as high as 10-15%. (See Illness / complications section, below.)

• Infective dose: As few as 10 to 200 cells can cause disease, depending on the age and condition of the host.

• Onset: Eight to 50 hours.

• Illness / complications: In otherwise healthy people, the disease usually consists of self- limiting diarrhea (often bloody), fever, and stomach cramps. Severe cases, which tend to occur primarily in immunocompromised or elderly people and young children, are associated with mucosal ulceration, rectal bleeding, and potentially drastic dehydration. Potential sequelae of shigellosis include reactive arthritis and hemolytic uremic syndrome.

• Symptoms: May include abdominal pain; cramps; diarrhea; fever; vomiting; blood, pus, or mucus in stools; tenesmus (straining during bowel movements).

• Duration: Uncomplicated cases usually resolve in 5 to 7 days. Most of the time, the illness is self-limiting. In some circumstances, antibiotics are given; usually trimethoprim-sulfamethoxazole, ceftriaxone, or ciprofloxacin.

• Route of entry: The fecal-oral route is the primary means of human-to-human transmission of Shigella. With regard to foods, contamination is often due to an infected food handler with poor personal hygiene.

• Pathway: The disease is caused when Shigella cells attach to, and penetrate, colonic epithelial cells of the intestinal mucosa. After invasion, they multiply intracellularly and spread to contiguous epithelial cells, resulting in tissue destruction. As noted, some strains produce enterotoxin and Shiga toxin similar to those produced by E. coli O157:H7.

3. Frequency

A recent Centers for Disease Control and Prevention (CDC) report on foodborne illnesses acquired annually in the United States revealed that about 15,000 laboratory-confirmed isolates are reported each year, with estimates of actual occurrence ranging from 24,511 to 374,789 cases (average of 131,243). About 31% of these are estimated to be foodborne. Estimates of foodborne illness episodes (mean) caused by 31 pathogens placed Shigella as the sixth most frequent cause (after norovirus, Salmonella species, Clostridium perfringens, Campylobacter, and Staphylococcus aureus, in that order).

Episodes of shigellosis appear to follow seasonal variations. In developed countries, the highest incidences generally occur during the warmer months of the year.

4. Sources

Most cases of shigellosis are caused by ingestion of fecally contaminated food or water. In the case of food, the major factor for contamination often is poor personal hygiene among food handlers. From infected carriers, this pathogen can spread by several routes, including food, fingers, feces, flies, and fomites.

Shigella is commonly transmitted by foods consumed raw; for example, lettuce, or as non- processed ingredients, such as those in a five-layer bean dip. Salads (potato, tuna, shrimp, macaroni, and chicken), milk and dairy products, and poultry also are among the foods that have been associated with shigellosis.

5. Diagnosis

Diagnosis is by serological or molecular identification of cultures isolated from stool. Shigella may be more difficult to cultivate if stool samples are not processed within a few hours.

6. Target Populations

All people are susceptible to shigellosis, to some degree, but children 1 to 4 years old, the elderly, and the immunocompromised are most at risk. Shigellosis is very common among people with AIDS and AIDS-related complex.

7. Food Analysis

Shigellae remain a challenge to isolate from foods. A molecular-based method (PCR) that targets a multi-copy virulence gene has been developed and implemented by FDA. Improvements in the bacterial isolation method continue and should be available in the near future.

The window for collecting and processing Shigella from foods, for cultivation, may be days (rather than hours, as is the case with stool), depending on the food matrix and storage conditions; e.g., temperature. Shigella species can be outgrown by the resident bacterial populations found in foods, which may reflect the usual low numbers of the organism present in foods and, in some foods, a very large number of non-Shigella bacteria. Another factor that reduces the chance of isolating Shigella from foods may be the physiological state of the pathogen at the time of analysis. Environmental conditions could affect its ability to either grow or survive in any food matrix.

8. Examples of Outbreaks

The CDC’s Morbidity and Mortality Weekly Reports provide information about Shigella outbreaks.

9. Other Resources

• Loci index for genome Shigella spp.

• GenBank Taxonomy database

• More information about Shigella and shigellosis can be found on the CDC website.


Continue Reading Bad Bug Book – Foodborne Pathogenic Microorganisms and Natural Toxins – Second Edition – Shigella species

According to press reports, the Escambia County Health Department has reminded the public that people can reduce the spread of gastrointestinal illnesses by practicing simple hygiene. “We like to be proactive about this, and we’ve started seeing cases of Shigella, or shigellosis, which is a bacterial illness that can produce nausea, vomiting, diarrhea and

After a person has recovered from shigellosis, Shigella bacteria remain active for a week or two. It is even possible for a person to be infected with Shigella without exhibiting symptoms.  Individuals who exhibit no symptoms are known as asymptomatic, but can pass the illness on to others. 

Small children acquire Shigella at the highest

According to press reports, two Maplewood Elementary School children have been infected with shigellosis, the illness caused by Shigella infection.  The Marion County Health Department contacted Maplewood officials on Wednesday, informing them that two students had contracted the illness; the school district launched an immediate review and response.

School district spokesman Kevin Christian said the bacteria

A new study published in the Centers for Disease Control and Prevention’s (CDC) journal, Emerging Infectious Diseases, found that the rate of outbreaks caused by unpasteurized milk (raw milk) and unpasteurized milk products was 150 times greater than outbreaks linked to pasteurized milk. In addition, the study revealed that states where raw milk sales

Shigella bacteria cause human illness when they are ingested, and can lead to Shigella infection, or shigellosis, through various modes of transmission, including through food and water sources, animal-to-human contact, and person-to-person contact in daycares and other settings.

Improper sanitation and cross-contamination can be contributing factors to Shigella outbreaks associated with restaurant food.

The introduction

Forty-five people in Brooklyn’s ultra-Orthodox communities in Williamsburg and Borough Park have been infected by shigella since August, according to city health officials.

Each year, New York City has between 300 and 400 cases of the disease, which frequently crops up in day care centers and other places where children congregate. The majority of those affected

Shigella is a highly contagious and virulent bug that is commonly the culprit in outbreaks at schools and other institutional settings.  Shigella outbreaks are frequently caused by the fecal-oral route, whether the route of ultimate transmission be food or contact with surfaces contaminated by the bacteria.  This means that bacteria from the stool of an