Shigellosis

Shigella is a member of the Enterobacteriaceae family and is classified into four serological subgroups:

• S. dysenteriae;
• S. flexneri;
• S. boydii;
• S. sonnei;

Although many authors have treated these subgroups of Shigella as distinct taxonomic species, Escherichia coli and Shigella are genetically very similar (80-90%), such that almost all strains of Shigella could be considered a biotype of E. coli.

Because of their substantial genetic similarity, distinguishing Shigella from E. coli is often a challenge for the clinical microbiology laboratory.

This disease is highly infectious-just 10 to 100 organisms to cause the disease. Outbreaks occur most often in the summer months through fecal-oral transmission, specifically following consumption of water or food contaminated with feces from infected individuals.
After 1-4 days the infection, which is non-systemic and enterically invasive, becomes acute leading to destruction of the colonic epithelium. The damage is irregular and dramatic and leads to the main clinical symptom: diarrhea containing blood and sometimes mucus, sometimes accompanied by abdominal cramps and fever.
Children under 5 years of age are more likely to contract Shigella infection, however people of any age are at risk. Staying in group quarters or participating in group activities contributes to the spread of bacteria. Shigella outbreaks are most common in daycare centers, children's swimming pools, nursing homes, prisons and military barracks.
People who live or travel in areas without sanitation are more likely to become infected with Shigella.

Shigella remains a major cause of childhood morbidity and mortality. The Global Enteric Multicenter Study (GEMS) recently provided a solid update on the incidence of Shigella among severe forms of diarrhea and showed convincingly that in the areas surveyed (sub-Saharan Africa and Asia), Shigella appeared among the top pathogens identified. Many studies have reported how Shigella species are geographically stratified according to the level of economic development in a given country. S. flexneri is the main infectious species in developing regions, while rates of S. sonnei increase in more economically advanced regions. S. boydii is most commonly restricted to Bangladesh and Southeast Asia and rarely occurs outside these regions.S. dysenteriae type 1 (Sd1) occurs sporadically in epidemic settings. Prominent examples occurred in refugee camps during the civil war in Rwanda between November 1993 and February 1995, where more than 180,000 cases occurred with a significant mortality rate. It has been reported that the worldwide incidence of shigellosis is about 165 million cases; the mortality level has decreased substantially only in the last three decades.

Symptoms of Shigella infection include fever, malaise, watery diarrhea, abdominal cramps, and muscle aches. The incubation period is 1 to 4 days, and the illness often resolves in 5 to 7 days. After 2-3 days, the volume and frequency of diarrhea may decrease and then be replaced by blood and mucus in the stool. Some individuals may have no symptoms but may still transmit the bacteria to others.
Inflammation of the colon is observed in shigellosis, but this alone is not specific enough for diagnosis.
Other symptoms include acute renal failure, thrombocytopenia, microangiopathic hemolytic anemia; the mortality rate is 35 percent.
Post-reactive arthritis is a complication of Shigella infection, occurring in 2% of cases and characterized by joint pain, painful urination, and eye irritation, with chronic arthritis lasting months to years.

The clinical diagnosis of shigellosis is complex because it does not differ from other intestinal infections in which bloody diarrhea is present.
Microscopic examination of stool is one of the most effective screening tests for invasive bacterial diarrhea. However, definitive diagnosis from Shigella is made only by isolating the organism from the stool through serotyping.

Antimicrobial therapy has been shown to reduce the duration, transmission and severity of symptoms. First-line treatment for all patients with shigellosis should be via fluoroquinolone (such as ciprofloxacin, azithromycin, ceftriaxone).
Fluid loss due to infection is treated orally.
Antidiarrheal treatments should be excluded as they may prolong the duration of the disease.
The decision of which treatment to pursue is based on the severity of symptoms and aims to reduce bacterial spread so as to avoid the development of more resistant strains.
Indeed, some subtypes of Shigella have high resistance to drug treatments (particularly tetracyclines); however, this varies widely by geographic region.

Hand hygiene is the cornerstone of prevention, particularly before food handling. To prevent the spread of Shigella, it is important to follow certain instructions such as:

• Wash hands frequently with soap and water for at least 20 seconds;
• Help young children wash their hands thoroughly;
• Dispose of soiled diapers properly;
• Disinfect diaper changing areas after use;
• Do not prepare food for others if you have diarrhea;
• Keep children with diarrhea home from child care, playgroups or school;
• Avoid ingesting water from untreated ponds, lakes or pools;
• Avoid sexual activity with anyone with diarrhea or who has recently recovered from diarrhea;
• Not to swim until fully recovered.

Appropriate isolation procedures should be adopted with respect to patients and carriers.

Vaccine
The decrease in the incidence of shigellosis with increasing age suggests that natural immunity develops. This implies that vaccines may be effective, as they elicit such a natural response.There is currently no licensed Shigella vaccine, but there are multiple vaccines in preclinical or clinical trials, including live attenuated, whole-cell killed, conjugated, and subunit vaccines