Shigellosis, also known as bacillary dysentery, is the infectious disease caused by the Shigella bacteria. Most who are infected with Shigella develop diarrhea, fever, and stomach cramps starting a day or two after exposure. The diarrhea is often bloody. Shigellosis usually resolves in 5 to 7 days. In some persons, especially young children and the elderly, the diarrhea can be so severe that the patient needs to be hospitalized. A severe infection with high fever may also be associated with seizures in children less than 2 years old. Some persons who are infected may have no symptoms at all, but may be carriers, and still pass the Shigella bacteria to others.
There are several different kinds of Shigella bacteria: Shigella sonnei, also known as “Group D”, accounts for much of the infection in the United States and is the strain most often linked to infection from food (shigellosis accounts for less than 10% of the reported outbreaks of foodborne illness in the United States.) A second type, Shigella flexneri, or “group B”, accounts for almost all of the rest of the infections in the U.S., and recent studies have determined a high percentage of infection by this strain is transmitted sexually. Other types of Shigella, such as Shigella dysenteriae type 1, are most often found in water contaminated with human feces. Such strains are rare in the U.S., although they continue to be important causes of death and disease in the developing world.
Shigellosis is more common in summer than winter. Children, especially toddlers aged 2 to 4, are the most likely to get shigellosis. Many cases are related to the spread of illness in child-care settings, and many more are the result of the spread of the illness in families with small children.
Increasingly, multidrug-resistant shigellosis cases are appearing in the US, often linked to travelers from outside the US.[1] These include Shigella sonnei that is resistant to ciprofloxacin, a main line antibiotic used to treat infections in adults. Shigella is already resistant to ampicillin.
Where does Shigella come from?
The organism is frequently found in water polluted with human feces. Salads (potato, tuna, shrimp, macaroni, and chicken), raw vegetables, dairy products, and poultry are foods commonly listed as sources of Shigella. Contamination of these foods is usually through the fecal-oral route. Therefore, water polluted with human feces and unsanitary handling by food handlers is the most common causes of contamination.
An estimated 500,000 cases of shigellosis occur annually in the U.S. and Shigella infections cause an estimated 600,000 deaths per year worldwide.[2] The number of infections attributable to contaminated food is unknown, but as it takes very little Shigella to cause infection, it’s probably substantial. Most of these deaths occur in developing countries with poor hygiene and unsafe water supplies.
How is Shigella detected?
Determining that Shigella is the cause of the illness depends on laboratory tests that identify the bacteria in the stools of an infected person. These tests are sometimes not routinely performed unless the laboratory is instructed specifically to look for the organism. The laboratory can also do special tests to tell which type of Shigella the person has and which antibiotics, if any, would be best to treat it. Organisms like Shigella are difficult to isolate in foods because standard methods are insensitive to such toxins. Recently, a method to identify foodborne pathogens, like Shigella, by specifying segments of their DNA has been developed by FDA and is currently under field test. However, the isolation procedures are still inadequate.
What are the typical symptoms of Shigella?
Symptoms include abdominal pain, cramps, diarrhea, fever, vomiting, blood, pus, or mucus in stools, and painful rectal spasms. These symptoms normally manifest one to two days after ingestion. The disease is caused when virulent Shigella organisms attach to, and penetrate, the wall of the intestinal tract. After invasion, they multiply, and spread to other smooth surface cells within the body, resulting in tissue destruction. Some strains produce enterotoxin and Shiga toxin, very much like the verotoxin of E. coli O157:H7.
Once someone has had shigellosis, they are not likely to get infected with that specific type again for at least several years. However, they can still get infected with other types of Shigella. Infants, the elderly, and those with weakened immune systems are susceptible to the severest symptoms of disease, but all humans are susceptible to some degree. Shigellosis is a very common illness suffered by those with acquired immune deficiency syndrome (AIDS) and other immuno-compromised individuals.
[1] See CDC “Multidrug-resistant Shigellosis Spreading in the United States.” April 2, 2015. Web April 23, 2015. http://www.cdc.gov/media/releases/2015/p0402-multidrug-resistant-shigellosis.html.
[2] Id.: “Shigella causes an estimated 500,000 cases of diarrhea in the United States every year. It spreads easily and rapidly from person to person and through contaminated food and recreational water. It can cause watery or bloody diarrhea, abdominal pain, fever, and malaise.”