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Colorized scanning electron micrograph of Escherichia coli

Image: Colorized scanning electron micrograph of Escherichia coli. By: NIAID. License: CC BY 2.0


Definition

Amoebic dysentery is a disease that is caused by Entamoeba histolytica and presents with bloody mucous diarrhea (dysentery) that lasts less than 2 weeks. Other common causes of dysentery include campylobacter, shigella, and salmonella.

Epidemiology

The identification of the etiology of dysentery is important, as the clinical course of the disease is usually more severe than that of non-bloody diarrhea. Therefore, the number of cases of dysentery is usually classified according to the causative organism.

SalmonellaNIAID

Image: Color-enhanced scanning electron micrograph showing Salmonella typhimurium (in red) invading cultured human cells.

Salmonella is the most common cause of dysentery, with an estimated 1 million cases per year (see image). The second most common cause is campylobacter, which is responsible for approximately 845,000 new cases per year in the United States. Shigella is associated with more severe illness but is responsible for a significantly lower number of cases (131,000) compared with other common causes.

These figures include all cases affected by the given organism and not only true bloody-diarrhea cases. Approximately 36% of patients infected with shigella will develop dysentery. However, 65% of patients who present with salmonella are at risk of developing dysentery.

Description

Escherichia coli O157:H7, which produces shiga toxin, is significantly associated with dysentery, as approximately 85% of infected patients will develop bloody diarrhea.

Dysentery is more common in children, but the condition can also occur in adults.

Etiology

The most commonly identified organisms of dysentery are shigella, salmonella, campylobacter, and E. histolytica. Fortunately, most laboratories can identify all of these causative organisms from a single stool culture.

While bacterial causes of dysentery are common, E. histolytica should be excluded in these patients. Other less common organisms include aeromonas, plesiomonas, and Yersinia enterocolitica. Y. enterocolitica is an important trivial etiology. as approximately 65% of patients go on to develop bloody diarrhea.

Complications

In addition to acute dehydration, more specific complications of dysentery are common and should be identified early and prevented if possible.

Patients with dysentery are more likely to require hospitalization compared with people with non-bloody diarrhea. Hospitalization puts the patient at risk of acquiring hospital-based infections, which are caused by multi-resistant organisms.

Shigella-related dysentery, especially in children, can be associated with significant mortality. Patients who are malnourished are at a significantly higher risk of developing severe dysentery and possibly dying from diarrheal illness.

Shigella is also associated with ileus, toxic mega-colon, and intestinal obstruction in children. Patients can also develop seizures and headaches, and become confused or lethargic. Urinary tract infections are also common.

Microscopic Image of campylobacter jejuni

Image: Campylobacter jejuni, which triggers about 30% of cases of Guillain-Barré syndrome. License: Public domain.

Non-typhoid salmonella and campylobacter are invasive organisms that can cause bacteremia, especially in the immunocompromised.

Shiga toxin–producing E. coli and Shigella spp can cause hemolytic-uremic syndrome. This condition is characterized by acute hemolysis leading to anemia and thrombocytopenia and renal failure. Patients may present with dyspnea, bleeding tendencies, and uremic features.

Campylobacter-associated dysentery may be associated with Guillain-Barré syndrome (see image). One-third of patients develop neurological disturbances. Reactive arthritis is also commonly associated with salmonella and campylobacter dysentery.

Clinical Presentation

The most important clinical presentation of dysentery is the passage of grossly bloody stools. Patients are also usually ill and have a fever.

Immunocompromised patients may develop bloody diarrhea without significant systemic illness, or severe invasive disease. Patients with hemolytic-uremic syndrome develop acute renal failure and pallor, and may become short of breath.

Patients who come from the developing world are more likely to have amoebic dysentery, rather than bacterial dysentery. Children who develop bloody diarrhea may be severely dehydrated.

Diagnostic Work-up

Once a patient presents to the emergency department with bloody stools, it is important to identify the causative organism rather than simply starting empirical therapy. The choice of investigations depends largely on the immunologic state of the patient.

Immune-competent patients should undergo stool analysis and culture. A stool culture can identify shigella, campylobacter, salmonella, E. coli, and E. histolytica.

Immunocompromised patients are at risk of developing cytomegalovirus dysentery in addition to Clostridium difficile–related dysentery. Testing for cytomegalovirus and for C. difficile toxins is therefore indicated.

Fecal leukocytes are common in dysentery. Patients with invasive pathogens may also develop leukocytosis.

Patients with severe disease who appear toxic may have developed complications such as toxic mega-colon. In these patients, abdominal computerized tomography is useful, as it can visualize the colon and exclude the condition.

Finally, patients who are suspected to have hemolytic-uremic syndrome should undergo renal function testing and a peripheral blood smear in addition to a complete blood count. These tests can help identify this severe complication.

Treatment

Azithromycin

Image: Chemical structure of azithromycin. By: Edgar181. License: Public domain.

Patients who present with very high fever and severe dysentery should be put on empirical antibiotic therapy until the stool culture results confirm the diagnosis.

Patients with suspected shigella, salmonella, or campylobacter infection should receive azithromycin, as this antibiotic is effective for treating all of these organisms. Adults with suspected salmonella infection should be started on ciprofloxacin, rather than azithromycin.

Patients with C. difficile dysentery should receive oral vancomycin. These patients are usually immunocompromised or have a recent history of hospital admission.

Children and adults with recent travel history to the developing world are at risk of amoebic rather than bacterial dysentery. These patients should receive tinidazole or metronidazole. E. histolytica can be easily identified with stool analysis; therefore, specific treatment with metronidazole or tinidazole is usually possible early in the disease.

Finally, patients with a confirmed diagnosis of shiga toxin–producing E. coli should receive azithromycin or rifaximin. While the organism is sensitive to other antibiotics, other antibiotics are thought to be responsible for the increased production of shiga toxin by the bacteria; hence, there is an increased risk of developing hemolytic-uremic syndrome.

Additionally, current diagnostic approaches, though helpful and easy to perform, are costly in the developing world, where diarrheal illness is more common. Therefore, the identification of the causative organism and specific treatments are usually difficult to obtain in areas where dysentery is endemic.

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