Definition
Shigellosis is an infection of the intestinal tract by a group of bacteria called Shigella.
Description
Shigellosis is a well-known cause of traveler's diarrhea and illness throughout the world. The major symptoms of shigellosis are diarrhea, abdominal cramps, fever, and severe fluid loss (dehydration). The bacteria causing shigellosis is named after Shiga, a Japanese researcher, who discovered the organism in 1897. Four different groups of Shigella can affect humans; of these, S. dysenteriae generally produces the most severe attacks, and S. sonnei the mildest. Shigella are extremely infectious bacteria, and ingestion of just 10 organisms is enough to cause dysentery. The most serious form of the disease is called dysentery, which is characterized by severe watery (and often blood- and mucus-streaked) diarrhea, abdominal cramping, rectal pain, and fever. Shigella is only one of several organisms that can cause dysentery, but the term bacillary dysentery is usually another name for shigellosis.
Demographics
Shigella accounts for 10 to 20 percent of all cases of diarrhea worldwide, and in any given year infects over 140 million persons and kills 600,000, mostly children and the elderly.
Most deaths are in less-developed or developing countries, but even in the United States, shigellosis can be a dangerous and potentially deadly disease. Poor hygiene, overcrowding, and improper storage of food are leading causes of infection. Shigellosis is often passed within families when young children are not yet toilet-trained and hand washing is poorly done. The following statistics show the marked difference in the frequency of cases between developed and less-developed countries: in the United States, about 30,000 individuals are hit by the disease each year or about 10 cases per 100,000 population. By contrast, infection in some areas of South America is 1,000 times more frequent. Shigellosis is most common in children below the age of five years and occurs less often in adults over 20.
Causes and Symptoms
Shigella share several of the characteristics of a group of bacteria that inhabit the intestinal tract. E. coli, another cause of food-borne illness, can be mistaken for Shigella both by physicians and the laboratory. Careful testing is needed to assure proper diagnosis and treatment.
Shigella are very resistant to the acid produced by the stomach, and this allows them to easily pass through the gastrointestinal tract and infect the colon (large intestine). The result is a colitis that produces multiple ulcers, which can bleed. Shigella also produce a number of toxins (Shiga toxin and others) that increase the amount of fluid secretion by the intestinal tract. This fluid secretion is a major cause of the diarrhea symptoms.
Shigella infection spreads through food or water contaminated by human waste. Sources of transmission are as follows:
- contaminated milk, ice cream, vegetables, and other foods which often cause epidemics
- household contacts (40% of adults and 20% of children develop infection from such a source)
- poor hygiene and overcrowded living conditions
- daycare centers
- sexual practices which lead to oral-anal contact, directly or indirectly
Symptoms can be limited to mild diarrhea or develop into dysentery. Dehydration results from the large fluid losses due to diarrhea, vomiting, and fever. Inability to eat or drink worsens the situation.
In developed countries, most infections are of the less severe type and are often due to S. sonnei. The period between infection and symptoms (incubation period) varies from one to seven days. Shigellosis can last from a few days to several weeks, with the average duration seven days.
Complications
Areas outside the intestine can be involved, including the following:
One of the most serious complications of this disease is HUS, which involves the kidney. The main findings are kidney failure and damage to red blood cells. As many as 15 percent of patients die from this complication, and half the survivors develop chronic kidney failure, requiring dialysis.
Another life-threatening condition is toxic megacolon. Severe inflammation causes the colon to dilate or stretch, and the thin colon wall may eventually tear. Certain medications (particularly those that diminish intestinal contractions) may increase this risk, but this interaction is unclear. Clues to this diagnosis include sudden decrease in diarrhea, swelling of the abdomen, and worsening abdominal pain.
Diagnosis
Shigellosis is one of the many causes of acute diarrhea. Culture (growing the bacteria in the laboratory) of freshly obtained diarrhea fluid is the only way to be certain of the diagnosis. But even this is not always positive, especially if the patient is already on antibiotics. Shigella are identified by their appearance under the microscope, along with various chemical tests. These studies take several days.
Treatment
The first aim of treatment is to keep up nutrition and avoid dehydration. Ideally, a physician should be consulted before starting any treatment. Antibiotics may not be necessary, except for the more severe infections. Many cases resolve before the diagnosis is established by culture. Medications that control diarrhea by slowing intestinal contractions can cause problems and should be avoided by patients with bloody diarrhea or fever, especially if antibiotics have not been started.
Rehydration
The World Health Organization (WHO) has developed guidelines for a standard solution taken by mouth and prepared from ingredients readily available at home. This oral rehydration solution (ORS) includes salt, baking powder, sugar, orange juice, and water. Commercial preparations, such as Pedialyte, are also available. In many patients with mild symptoms, this is the only treatment needed. Severe dehydration usually requires intravenous fluid replacement.
Antibiotics
Patients who have very mild cases of shigellosis may improve without any antibiotic therapy; therefore, these drugs are indicated only for treatment of moderate or severe disease, as found in the tropics. Choice of antibiotic is based on the type of bacteria found in the geographical area and on laboratory results. Recommended antibiotics include ampicillin, sulfa derivatives such as trimethoprim-sulfamethoxazole (TMP-SMX) sold as Bactrim, or fluoroquinolones, such as Ciprofloxacin.
Prognosis
Many patients with mild infections need no specific treatment and recover completely. In those with severe infections, antibiotics decrease the length of symptoms and the number of days bacteria appear in the feces. In rare cases, an individual may fail to clear the bacteria from the intestinal tract; the result is a persistent carrier state. This may be more frequent in patients with acquired immune deficiency syndrome (AIDS). Antibiotics are about 90 percent effective in eliminating these chronic infections.
In patients who have suffered particularly severe attacks, some degree of cramping and diarrhea can last for several weeks. This is usually due to damage to the intestinal tract, which requires some time to heal. Since antibiotics can also produce a form of colitis, which may cause persistent or recurrent symptoms.
Prevention
Shigellosis is an extremely contagious disease; good hand washing techniques (especially after toileting young children or changing diapers) and proper precautions in food handling help in avoiding the spread of infection. Children in daycare centers need to be reminded about hand washing during an outbreak to minimize contagion. Shigellosis in schools or daycare settings almost always disappears when holiday breaks occur, which severs the chain of transmission.
Traveler's Diarrhea (TD)
Shigella accounts for about 10 percent of diarrhea illness in travelers to Mexico, South America, and the tropics. Most cases of TD are more of a nuisance than a life-threatening disease.
In some cases, though, aside from ruining vacation plans, these infections can interrupt business conference schedules and, in the worst instances, lead to a life-threatening illness. Therefore, researchers have tried to find a safe, yet effective, way of preventing TD. Of course the best prevention is to follow closely the rules outlined by the WHO and other groups regarding eating fresh fruits, vegetables, and other foods.
One safe and effective method of preventing TD is the use of large doses of Pepto Bismol. Tablets are easier for use during travel; usage must start a few days before departure. Patients should be aware that bismuth turns bowel movements black.
Antibiotics have also proven highly effective in preventing TD. They can also produce significant side effects. Therefore, a physician should be consulted before use. Like Pepto Bismol, antibiotics need to be started before beginning travel.
Parental Concerns
Parents of children suffering from shigellosis need to follow closely their healthcare provider's directions for preventing dehydration. Excellent hand washing is crucial to prevent the spread of the infection throughout family members. This is particularly important while helping to toilet train a child or while changing diapers.
Resources
Books
Cleary, Thomas G. "Shigella." In Nelson Textbook of Pediatrics. Edited by Richard E. Behrman et al. Philadelphia: Saunders, 2004.
Web Sites
Kroser, Joyann A. "Shigellosis." eMedicine, May 17, 2002. Available online at www.emedicine.com/med/topic2112.htm (accessed December 30, 2004).
"Shigellosis." New York State Info for Consumers. Available online at www.health.state.ny.us/nysdoh/Communicable_diseases/en/shig.htm (accessed December 30, 2004).
[Article by: David Kaminstein, MD Rosalyn Carson-DeWitt, MD]
