Shigellosis

Shigellosis is an infection of the intestinal tract caused by a group of bacteria called Shigella. The major symptoms are diarrhea, abdominal cramps, fever, and severe fluid loss (dehydration). Four different species of the genus Shigella can affect humans; of these, S. dysenteriae generally produces the most severe attacks, and S. sonnei the mildest. The other two species of Shigella include S. boydii and S. flexneri. Shigella is also sometimes called Marlow syndrome.

The bacteria are named in honor of Kiyoshi Shiga (1871–1957), a Japanese physician and microbiological researcher, who discovered the organism in 1897. In the 2010s, Shigellosis is a well-known cause of traveler's diarrhea and illness throughout the world.

The organisms making up Shigella are extremely infectious bacteria and ingestion of just 10 organisms is enough to cause severe diarrhea and dehydration. Shigeliosis accounts for 10% to 20% of all cases of diarrhea worldwide. In any given year, it is estimated to infect from 90 million to over 140 million persons and to kill at least 100,000 people but sometimes upwards to 600,000. Children and the elderly in developing countries are most at risk. The most serious form of the disease is called dysentery, which is characterized by severe watery (and often blood- and mucous-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.

Most deaths are in less-developed or developing countries, but even in the United States, shigellosis can be a dangerous and potentially deadly disease. The common way to transmit shigellosis is person-to-person contact, primarily hand-to-mouth such as shaking hands with someone and then putting a finger into one's mouth. Being a contagious disease, shigellosis can be spread by air, blood, blood transfusions, coughing, fecal-oral, mother-to-fetus, needles, saliva, sexual contact, and surface contact. Poor hygiene, overcrowding, and improper storage of food are leading causes of infection. The following statistics show the marked difference in the frequency of cases between developed and less-developed countries; in the United States, the disease harms upwards to 30,000 individuals each year, or about 10 cases per 100,000 of the population. On the other hand, infection in some areas of South America is 1,000 times more frequent. Shigellosis is most common in children below the age of 5 years and occurs less often in adults over 20 years.

One major outbreak from shigellosis occurred in Zaire in 1994. According to the World Health Organization (WHO), between 500,000 and 800,000 Rwandan refugees had escaped into the northern Kivu region of Zaire. However, they contracted the infection and within one month about 20,000 of them had died from dysentery caused by S. dysenteriae type 1.

Shigellosis is almost never found in animals, except for humans and other primates like monkeys and chimpanzees. The organism is frequently found in water polluted with human feces, which usually is found in poor and undeveloped countries of the world. Children are at higher risk than other groups of humans because they are less likely to perform common hygiene practices, such as washing of the hands after a bowel movement.

The species Shigella sonnei accounts for over two-thirds of the shigellosis in the United States. Shigella flexneri makes up for almost all of the rest. The other species of Shigella are rare in the United States; however, they occur much more frequently in the developing world. In fact, the species S. dysenteriae type 1 causes deadly epidemics in many developing regions and nations.

Between 10,000 and 30,000 cases of shigellosis are reported annually in the United States. Because mild cases of the disease are not diagnosed as shigellosis, the real number of infections has been estimated to be as high as twenty times greater. Shigellosis is more common in the months of summer than in winter ones. In the United States, young children from the age of two to four years are most likely to contract shigellosis. The spread of the disease is often reported in child-care facilities and in families with several children. In developing countries, especially the poorest ones, the presence of shigellosis is much more common and is usually present in various degrees in most villages and other community settings.

Shigella shares several of the characteristics of a group of bacteria that inhabit the intestinal tract. Escherichia coli (E coli), another cause of food-borne illness, can be mistaken for Shigella both by physicians and laboratory personnel. Careful testing is needed to assure proper diagnosis and treatment.

Shigella organisms 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 produces a number of toxins (such as Shiga toxin) that increase the amount of fluid secretion by the intestinal tract. This fluid secretion is a major cause of diarrhea symptoms.

Shigella infection spreads through food or water contaminated by human waste. Sources of transmission are:

Symptoms can be limited to only mild diarrhea or can progress to full-blown 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 an average of seven days.

Areas outside the intestine can be involved, including:

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% of patients die from this complication, and half of the survivors develop chronic kidney failure, which requires 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.

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. However, even this is not always positive, especially if the patient is already on antibiotics. Shigella are identified by a combination of their appearance under the microscope and various chemical tests. These studies take several days but quicker means to recognize the bacteria and its toxins are being developed.

The first aims of treatment are 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.

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. For many patients with mild symptoms, this is the only treatment needed. Severe dehydration usually requires intravenous fluid replacement.

In the early and mid-1990s, researchers began to realize that not all cases of bacterial dysentery needed antibiotic treatment. 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. Recommendations include ampicillin, sulfa derivatives such as Trimethoprim-Sulfamethoxazole (TMP-SMX) sold as Bactrim, or fluoroquinolones (such as Ciprofloxacin, which is not approved by the U.S. Food and Drug Administration for use in children due to risk of permanent musculoskeletal system injury, except in cases when safe or effective alternatives are not available).

Cases of shigellosis should be reported at the state level. The states then report to the national level, to the U.S. Centers for Disease Control and Prevention (CDC). Public health laboratories throughout the United States survey and analyze foodborne infections. Public health laboratories contribute to the surveillance and analysis of foodborne infections, such as shigelosis. A national network of public health and food regulatory agency laboratories exist throughout the United States. For additional information on this network, go to PulseNet ( http://www.cdc.gov/pulsenet/

Prognosis

Prevention