Shigella Blog - Surveillance & analysis on Shigella news & outbreaks

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

1. 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.

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 rate.

Prevent the spread of Shigella from an infected person to others with frequent and careful handwashing with soap.

If a child in diapers has shigellosis, everyone who changes the child’s diapers should be sure the diapers are disposed of properly in a closed-lid garbage can, and should wash his or her hands and the child’s hands carefully with soap and warm water immediately after changing the diapers. After use, the diaper changing area should be wiped down with a disinfectant such as diluted household bleach or bactericidal wipes.  When possible, young children with a Shigella infection who are still in diapers should not be in contact with uninfected children.

Basic food safety precautions and disinfection of drinking water prevents outbreaks of shigellosis from food and water. However, people with shigellosis should not prepare food or drinks for others until they have been shown to no longer be carrying Shigella bacteria, or if they have had no diarrhea for at least 2 days. 

At swimming beaches, having enough bathrooms and handwashing stations with soap near the swimming area helps keep the water from becoming contaminated.  Daycare centers should not provide water play areas.

When traveling in developing countries, only treated or boiled water, and eat only cooked hot foods or fruits you peel yourself.

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 are legal had more than twice the rate of outbreaks as states where it was illegal.

The 13-year study, involved a review of dairy product outbreaks from 1993 to 2006 in all 50 states. The authors compared the amount of milk produced in the United States during the study period (about 2.7 trillion pounds) to the amount that CDC estimates was likely consumed raw (1 percent or 27 billion pounds) to determine the 150 times higher rate for outbreaks caused by raw milk products. Raw milk products include cheese and yogurt.

The study included 121 dairy–related disease outbreaks, which caused 4,413 illnesses, 239 hospitalizations and three deaths. In 60 percent of the outbreaks (73 outbreaks) state health officials determined raw milk products were the cause. Nearly all of the hospitalizations (200 of 239) were in those sickened in the raw milk outbreaks. These dairy-related outbreaks occurred in 30 states, and 75 percent (55 outbreaks) of the raw milk outbreaks occurred in the 21 states where it was legal to sell raw milk products at the time. The study also reported that seven states changed their laws during the study period.

For a consumer, it is impossible to tell if raw milk is safe to drink by simply looking at, smelling, or tasting it. Even under ideal conditions of cleanliness, the process of collecting milk introduces some bacteria. Unless the milk is pasteurized, these bacteria can multiply and grow in the milk and cause illness in those who consume it. Pasteurization involves heating milk to kill disease-causing bacteria.

“This study shows an association between state laws and the number of outbreaks and illnesses from raw milk products,” said Robert Tauxe, M.D., M.P.H., deputy director of CDC’s Division of Foodborne, Waterborne and Environmental Diseases (DFWED). “Restricting the sale of raw milk products is likely to reduce the number of outbreaks and can help keep people healthier. The states that allow sale of raw milk will probably continue to see outbreaks in the future.”

The study also found that the raw milk product outbreaks led to much more severe illnesses, and disproportionately affected people under age 20. In the raw milk outbreaks with known age breakdowns, 60 percent of patients were younger than age 20, compared to 23 percent in outbreaks from pasteurized products. Because of their underdeveloped immune systems, children are more likely than adults to get seriously ill from the bacteria in raw milk.

“While some people think that raw milk has more health benefits than pasteurized milk, this study shows that raw milk has great risks, especially for children, who experience more severe illnesses if they get sick,” said study co-author Barbara Mahon, M.D., M.P.H., deputy chief of CDC’s DFWED Enteric Diseases Epidemiology Branch. “Parents who have lived through the experience of watching their child fight for their life after drinking raw milk now say that it’s just not worth the risk.”

Additional information on evidence-based scientific studies covering the benefits and risks of raw milk consumption can be found HERE (pdf).

Shigellosis cases continue to rise in Louisville, with 18 laboratory-confirmed cases last week.

That’s the highest number in any week so far during the current outbreak, which has seen 84 confirmed cases in 13 weeks.

Shigellosis is a highly-contagious diarrhea illness caused by Shigella bacteria. Symptoms include watery or loose stools for several days, and in severe cases abrupt onset of fever, nausea, abdominal cramping and vomiting.

For the past five years, Louisville has had an average of 62 confirmed cases annually.

To prevent the illness, health officials are urging people to wash their hands carefully with soap and water, including under the fingernails. This is especially important after handling items such as diapers, after each bowel movement, after helping children use the toilet and before preparing or eating food, health officials said.

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 in this latest outbreak were also children.

No one has been hospitalized, city health department officials said.  The health department has asked doctors not to use antibiotics to treat shigella because drug-resistant strains of the bacteria may be starting to spread.