Shigellosis (Shigella Infection)

What is shigellosis?

Shigellosis is an intestinal infection caused by the Shigella species of bacteria. It is an acute infection which lasts for about 7 days in adults but may persist for longer and be more severe in infants and children. Death is more likely in children younger than 5 years. Abdominal pain, vomiting, diarrhea and a fever are the main symptoms similar to other forms of gastroenteritis and enteritis. Bloody diarrhea is a common distinguishing feature in shigellosis compared to similar bacterial infections like salmonellosis.

Shigellosis Incidence

It is estimated that some 165 million cases of shigellosis occur worldwide every year. Most of these are in developing countries with only about 450,000 cases reported annually in the United States. Shigellosis mainly affects children between the children between 6 months and 5 years. It is the main cause of diarrheal death in young children and accounts for about 10% of all cases of diarrhea in infants.

Shigellosis Pathophysiology

The Shigella bacteria primarily infect the epithelial cells lining intestine. It enters the gut through the mouth when consuming contaminated food and water. The bacteria are resistant to stomach acid and therefore not destroyed within the stomach as are most bacteria that pass down the esophagus. The bacteria enter the cells and multiply within it, destroying the epithelial cells and penetrating slightly deeper into the intestinal wall. Inflammation causes swelling, increased blood flow and excessive mucus secretion in the intestine. The left colon is the most adversely affected in shigellosis. Some species of the Shigella bacteria produce a Shiga toxin that is the main cause of diarrhea.

Shigellosis Spread

The Shigella bacteria are spread through the feco-oral route. This means that fecal particles containing the bacteria gain entry through the mouth. Proper hygiene after using the toilet and using only clean drinking water can greatly contribute to the prevention of shigellosis.

Water

Contaminated water is a major source of infection. It is more likely to occur when sourcing water from unsafe sources, like rivers and lakes that are contaminated, and in certain regions without properly treated drinking water. It is therefore advisable that travelers to developing countries and endemic regions only consume bottled water that is packed by reputable manufacturers.

Food

Contamination of food is mainly due to washing items and cooking with contaminated water. Heat can kill the Shigella bacteria and shigellosis can be prevented with proper cooking of food. Contamination also occurs with unhygienic practices especially when kitchen staff and food handlers touch items without proper washing of the hands.

Direct Contact

The most common method of spread of the Shigella bacteria is from person-to-person. This may involve direct contact with contaminated hands, infected body fluids or inanimate contaminated objects (fomites). Mothers and nannies of infants may also be a greater risk should they not wash their hands after changing diapers.

Shigellosis Incubation Period

The incubation period varies between 1 to 4 days. This is the period of time from infection until symptoms arise. There are usually no symptoms despite the bacteria multiplying rapidly within the body. Sometimes there may be very mild and vague symptoms for a short period which is often ignored. The major symptoms usually become evident thereafter.  The incubation period for shigellosis can be as short as 12 hours or as long as 7 days. This can often prove confusing in identifying the exact cause or event that may have transmitted the bacteria.

Shigellosis Causes

There are several different species of Shigella bacteria.

  • S. sonnei is the most common cause of shigellosis in the United States.
  • S. flexneri is the most common cause in developing nations.
  • S. dysenteriae is a less common species but is responsible for more severe infections and epidemic shigellosis particularly in Africa and Central America.

S. sonnei and S. flexneri jointly account for 90% of shigellosis cases. The least common cause is S. boydii.

Shigellosis Symptoms

There are four main symptoms of shigellosis :

  • Diarrhea
  • Abdominal pain
  • Fever
  • Malaise

Other symptoms which may always be present or as prominent includes :

  • Vomiting
  • Tenesmus (urging to pass stool)
  • Abdominal tenderness
  • Overactive bowel sounds
  • Dehydration

Shigella Diarrhea

Shigellosis diarrhea is known to be bloody. However, bloody diarrhea may not be evident immediately once the loose stool begins. Usually there is watery diarrhea with no signs of blood at the outset. Initially during this watery diarrhea phase there is abdominal cramps. Later on, bloody diarrhea develops with abdominal pain. The bloody diarrhea is often accompanied by mucus. Although the symptoms resolve after a few days, diarrhea can be on and off for several weeks thereafter.

Shigellosis Diagnosis

Shigellosis can be diagnosed by a stool test which verifies the presence of the bacteria. Other blood tests may be conducted to verify the patient’s general status of health and the development of complications such as dehydration or hemolytic-uremic syndrome. In most cases, there is no need to test for shigellosis as the condition resolves after a few days and only supportive measures are necessary to prevent dehydration and other complications.

Shigellosis Complications

A host of complications can arise with shigellosis. In most cases these complications never arise. However, patients with a weakened immune system (like in HIV/AIDS), infants and the elderly are at the greater risk of complications. Eventually some of these complications can lead to death. It is important to note that prompt medical treatment is the key to preventing complications and death. The main, and most severe complications of shigellosis, includes :

  • Dehydration
  • Convulsions
  • Hemolytic-uremic syndrome
  • Reiter’s syndrome (post-infectious arthritis)

Shigellosis Treatment

Most cases of shigellosis need no medical treatment. Supportive measures are sufficient to prevent complications and the infection resolves spontaneously within a few days. Severe infections or persisting shigellosis requires antibiotics. The best approach for shigellosis is prevention and a few simple measures can effectively prevent the infection altogether.

Supportive Treatment

Proper hydration requires the use of oral rehydration solutions (ORS). Water alone is not sufficient as it lacks sufficient electrolytes to counteract the loss with diarrhea, perspiration (fever) and sometimes vomiting. Although an ORS can be made at home, ideally a commercially available ORS should be used.

These solutions have the optimum concentration of electrolytes which ensures that it adequately replenishes the lost electrolytes without aggravating the diarrhea (osmotic diarrhea). Patients who are severely dehydrated or are vomiting and not able to keep down electrolytes will require fluid and electrolyte administration via an intravenous (IV) drip. Strict bed rest is also important.

Antibiotics

Antibiotics are usually not necessary for shigellosis unless the infection is severe and not showing any indication of resolving spontaneously. The preferred antibiotics include :

  • Ampicillin
  • Ceftriaxone
  • Trimethoprim-sulfamethoxazole
  • Ciprofloxacin

Medication to Avoid

Certain medication that are traditionally used to treat diarrhea should be avoided in shigellosis. This includes loperamide and diphenoxylate with atropine. These drugs are commonly used and easily available anti-diarrheal agents but hold a risk of worsening the infection and contributing to complications. Anti-diarrheal agents stop diarrhea but by doing so the bacteria are retained in the bowels thereby allowing it to continue to multiply and worsen the infection.

Shigellosis Prevention

  • Always source drinking and cooking water from reputable sources. Opt for bottled water when traveling, especially to developing countries and endemic regions.
  • Only eat meals in reputable establishments that are frequented by international travelers and are part of well known chain stores.
  • Avoid fresh fruit and raw vegetables.
  • Ensure that meat is thoroughly cooked especially when eating in high risk areas.
  • Wash hands frequently with antiseptic hand washes, especially after using the toilet.
  • Dispose of soiled diapers through appropriate means to avoid further contact with it.
  • Reduce interaction with others when infected. Children should not attend day care or school as they may spread the infection.

References :

http://www.cdc.gov/nczved/divisions/dfbmd/diseases/shigellosis/

http://emedicine.medscape.com/article/182767-overview

http://www.merckmanuals.com/professional/infectious_diseases/gram-negative_bacilli/shigellosis.html

http://www.mayoclinic.com/health/shigella/DS00719

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