Common Foodborne Pathogens - VIII

Shigella spp. Risk Profile

Shigella infection (shigellosis) is an intestinal infection caused by a family of bacteria known as shigella. Shigella is very contagious and the main sign of shigella infection is diarrhea with blood, which is caused by Shigella sonnei, S. boydii, S. flexneri, and S. dysenteriae. Shigellae are Gram-negative, non-motile, non-spore-forming, rod-shaped bacteria. Some strains produce enterotoxins and Shiga toxin, where 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 and it is frequently found in water polluted with human feces. 

Shigellae are very sensitive to environmental conditions die rapidly and heat-sensitive, hence do not survive the pasteurization and cooking temperatures. Shigella species are tolerant to low pH and are able to transit the harsh environment of the stomach. These pathogens can grow in low pH foods, such as some fruits and vegetables, or survive on produce commodities packaged under vacuum or modified atmosphere and can also survive in water, with a slight decrease in numbers.
 
Growth Factors

Temperature:

            Minimum – 6.1°C     Maximum – 47.1°C 

pH:

Minimum – 4.8         Maximum – 9.3        

Water Activity (aW):

Minimum – 0.96       Maximum – -            

Water Phase Salt:

Maximum – 5.2%    

 
Disease

The infection caused by Shigella is shigellosis and also called bacillary dysentery, where diarrhea may range from watery stool to severe, life-threatening dysentery. All Shigella spp. can cause acute, bloody diarrhea, and can spread rapidly through a population under unsanitary conditions with mass populations. 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. On the other hand, S. sonnei produces the mildest form of shigellosis, which is usually watery diarrhea; S. flexneri and S. boydii infections can be either mild or severe.
 

Mortality

The disease usually is self-limiting in healthy people, although some strains are associated with fatality rates as high as 10-15%.

 

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.

 

Complications

The disease usually consists of self-limiting diarrhea in healthy people and the symptom includes fever and stomach cramps, but 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, such as trimethoprim-sulfamethoxazole, ceftriaxone, or ciprofloxacin.

 

Route of entry

The fecal-oral route is the primary means of human-to-human transmission of Shigella. However, the contamination of foods 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 and they multiply intracellularly while spreading into contiguous epithelial cells, resulting in tissue destruction.

 
Frequency

According to the US Centers for Disease Control and Prevention (CDC), there are about 15,000 domestically acquired foodborne infections of laboratory-confirmed isolates are reported each year, with estimates of actual occurrence ranging from 24,511 to 374,789 cases (average of 131,243) and about 31% of them estimated to be foodborne. Nonetheless, most of the foodborne infections are caused by 31 different pathogens, where Shigella is ranked as the sixth most frequent cause after norovirus, Salmonella species, Clostridium perfringens, Campylobacter, and Staphylococcus aureus. Shigellosis appears to follow seasonal variations, and the highest incidences generally occur during the warmer months of the year in developed countries.
  
Diagnosis
Diagnosis is by serological or molecular identification of cultures isolated from the stool, but Shigella may be more difficult to cultivate if stool samples are not processed within a few hours. However, cultivating Shigella spp is relatively difficult and depends on the amount of time within which stool or food samples are collected and processed. 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.
 
Target Populations
Everyone in the community is susceptible to shigellosis, but children 1 to 4 years old, the elderly, and the immunocompromised are most at risk to some degree. Further, Shigellosis is very common among people with AIDS and AIDS-related complex.
 
Food Analysis

A molecular-based method (PCR) that targets a multi-copy virulence gene has been developed and implemented by the FDA. Improvements in the bacterial isolation method continue as Shigellae remain a challenge to isolate from foods. The window for collecting and processing Shigella from foods, for cultivation, maybe days rather than hours, as is the case with stool, depending on the food matrix and storage conditions, such as 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, or a very large number of non-Shigella bacteria in some foods. The physiological state of the pathogen at the time of analysis may be another factor that reduces the chance of isolating Shigella from foods. Environmental conditions could affect its ability to either grow or survive in any food matrix.

 
Reference:
FDA Bad Bug Book, Foodborne Pathogenic Microorganisms and Natural Toxins. Second Edition. 2013
Preventive Controls for Human Foods. 2016