Shigellosis Health Article

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:

• nervous system (irritation of the meninges or meningitis, encephalitis, and seizures)
• kidneys (producing hemolytic uremic syndrome or HUS which leads to kidney failure)
• joints (leading to an unusual form of arthritis called Reiter's syndrome)
• skin (rash)

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.