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Shigellosis

Shigellosis is an infection of the intestines caused by Shigella bacteria. It is also called bacillary dysentery, and Marlow syndrome

Shigella seen in a stool sample
Shigella seen in a stool sample

Signs and Symptoms

Shigellosis is an intestinal infection characterised by symptoms that typically begin one to two days after exposure to the Shigella bacteria. These symptoms include diarrhoea, fever, and abdominal pain, often accompanied by a sensation of needing to pass stools even when the bowels are empty. The diarrhoea can sometimes be bloody, and the duration of symptoms is usually five to seven days.

However, it may take several months for bowel habits to return completely to normal. Complications from the infection can include reactive arthritis, sepsis, seizures, and hemolytic uraemic syndrome. The most common neurological symptom is seizures.

Cause

Bacteria

Shigellosis is caused by infection with Shigella bacteria. There are four main types of Shigella:

  • Shigella flexneri
  • Shigella boydii
  • Shigella dysenteriae
  • Shigella sonnei (serotype)

The prevalence of each strain varies globally. For instance, S. sonnei is most common in the United States, while S. dysenteriae and S. boydii are rare there.

Transmission

Shigella is transmitted through the faecal-oral route, often via contaminated food, water, or hands. It can spread rapidly among children, especially in daycare settings, and among travellers. Long-term carriers are uncommon. The bacteria can infect both humans and other primates.

Mechanism

Upon ingestion, Shigella bacteria pass through the gastrointestinal tract to the small intestine, where they multiply before reaching the large intestine. In the large intestine, they cause cell injury through direct invasion of epithelial cells and production of enterotoxins. Unlike other bacteria, Shigella is not destroyed by gastric acid, making it highly infectious even in small doses (10 to 200 cells).

Diagnosis

Diagnosis of shigellosis is confirmed through stool culture. Shigella species are typically non-motile and do not ferment lactose, except for S. sonnei, which can. They usually do not produce gas from carbohydrates and are biochemically inert. In laboratory tests, Shigella species are urea hydrolysis negative and produce bluish-green colonies on Hektoen enteric agar.

Prevention

Preventive measures include proper handwashing, especially before handling food, and thoroughly cooking all food. Improved sanitation and personal and food hygiene are primary methods of prevention. Keeping infected children out of daycare for 24 hours after symptoms disappear can also help reduce transmission.

Vaccine

Currently, there is no licenced vaccine for Shigella. Several vaccine candidates are in development, including live attenuated, conjugate, ribosomal, and proteosome vaccines. Given that Shigella is resistant to many antibiotics, vaccination is considered an important strategy to reduce morbidity and mortality.

Treatment

Treatment mainly involves replacing fluids and salts lost due to diarrhoea. Oral rehydration is usually sufficient, but intravenous fluids may be required in severe cases. Antidiarrheal drugs like diphenoxylate or loperamide are not recommended as they may prolong the infection.

Antibiotics

Antibiotics are reserved for severe cases or certain populations with mild symptoms, such as the elderly, immunocompromised individuals, food service industry workers, and childcare workers. Common antibiotics include trimethoprim-sulfamethoxazole and ciprofloxacin. However, many Shigella strains are resistant to these antibiotics, and the disease often resolves within four to eight days without them.

Epidemiology

Shigellosis is estimated to cause 34,000 deaths annually in children under five and 40,000 deaths in people over five. In the United States, there are about 500,000 cases each year. The disease is more common and severe in developing countries, with fatality rates in epidemics reaching 5-15%. Orthodox Jewish communities are known to be a risk group, with Shigella sonnei being cyclically epidemic in these populations.


Self-assessment MCQs (single best answer)

Which of the following is NOT a common symptom of shigellosis?



What is the primary mode of transmission for Shigella bacteria?



Which Shigella species is most commonly found in the United States?



What is the primary method for diagnosing shigellosis?



Which of the following is NOT a complication associated with shigellosis?



Which age group is most at risk for severe outcomes from shigellosis?



What is the recommended treatment for mild cases of shigellosis?



Why are antidiarrheal drugs like loperamide not recommended for treating shigellosis?



What is a significant preventive measure for reducing the spread of shigellosis?



Why is the development of a Shigella vaccine considered important?



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