About Shigella

Shigella is a family of bacteria that can cause sudden and severe diarrhea (gastroenteritis) in humans. Shigellosis – the illness caused by the ingestion of Shigella bacteria – is also known as bacillary dysentery. It can occur after ingestion of fewer than 100 bacteria, making Shigella one of the most communicable and severe forms of the bacterial-induced diarrheas.

Shigella thrives in the human intestine and is commonly spread both through food and by person-to-person contact. Most Shigella infections are passed through the fecal-oral route. This happens when basic hygiene and handwashing habits are inadequate and can happen during certain types of sexual activity. Transmission is particularly likely to occur among toddlers who are not fully toilet-trained. Family members and playmates of such children are at high risk of becoming infected.

Food may become contaminated by infected food handlers who don’t wash their hands with soap after using the bathroom. Vegetables can become contaminated if they are harvested from a field with sewage in it. Flies can breed in infected feces and then contaminate food.

Water may become contaminated with Shigella bacteria if sewage goes into it or if someone with shigellosis swims in or plays with the water (especially in splash tables, untreated wading pools, or shallow play fountains used by daycare centers). Shigella infections can then be acquired by drinking, swimming in, or playing with the contaminated water.

The number of shigellosis cases reported annually to the Centers for Disease Control and Prevention (CDC) has varied over the past several years, from more than 17,000 during 1978–2003, to an all-time low of 14,000 in 2004, to almost 20,000 in 2007. Many cases go undiagnosed and/or unreported, however. The CDC estimates that 450,000 total cases of shigellosis occur in the U.S. every year.

Shigella is the third most common pathogen transmitted through food. During 2006, a total of 1,270 foodborne-related outbreaks from 48 states in the U.S. were reported. Although Shigella was responsible for only 10 (1%) of those outbreaks, 183 confirmed cases of shigellosis were reported. This contrasts with an average of 659 cases annually in the previous five years. Shigella has also responsible for a substantial portion of foodborne outbreaks on cruise ships.

Symptoms of Shigella infection

Most people who are infected with Shigella develop diarrhea, fever, and stomach cramps after they are exposed to the bacteria. Symptoms may start within 12 to 96 hours after exposure, usually 1 to 3 days. Diarrhea is bloody 25 to 50 percent of the time and most often contains mucus. Rectal spasms, medically termed “tenesmus,” are common. The diarrhea may range from mild to very severe diarrhea. Shigellosis usually resolves in 5 to 7 days.

A severe infection with high fever may be associated with seizures in children less than two years old. Some persons who are infected may have no symptoms at all, but may still pass the Shigella bacteria to others. Persons with shigellosis in the U.S. rarely require hospitalization, although the hospitalization rate has been estimated to be in excess of 50,000 per year. The hospitalization rate tends to be highest among older individuals.

Detection and treatment of Shigella infection

Shigella infections are diagnosed with a laboratory test (stool culture) on a person’s stool specimen. Prompt processing of specimens and use of appropriate culture media increases the likelihood of isolating the bacteria. The laboratory can also do special tests to tell which species of Shigella the person has and which antibiotics would be best to treat it; antibiotic sensitivity tests are important since Shigella is often resistant to multiple antibiotics. More advanced methods, such as plasmid profiling and chromosomal fingerprinting using pulsed-field gel electrophoresis (PFGE) are two molecular techniques that can help to characterize Shigella isolates in food and human samples. These tests can assist in determining whether cases are isolated or associated with outbreaks.

Although shigellosis is usually a self-limited illness, antibiotics can shorten the course, and in the most serious cases, might be life saving. The antibiotics commonly used for treatment are ampicillin, trimethoprim/sulfamethoxazole (also known as Bactrim or Septra), ceftriaxone (Rocephin), or, among adults, ciprofloxacin. Some Shigella bacteria have become resistant to one or more antibiotics. This means some antibiotics might not be effective for treatment. Using antibiotics to treat shigellosis can sometimes make the germs more resistant. Therefore, when many persons in a community are affected by shigellosis, antibiotics are sometimes used to treat only the most severe cases.

Antidiarrheal agents such as loperamide (Imodium) or diphenoxylate with atropine (Lomotil) can make the illness worse and should be avoided. The best way to determine which antibiotic is effective is to obtain a stool culture and antibiotic sensitivity tests.

Persons with diarrhea caused by S. sonnei in particular usually recover completely, although it may be several months before their bowel habits are entirely normal. About 2% of persons who are infected with S. flexneri later develop pains in their joints, irritation of the eyes, and painful urination. This is called post-infectious arthritis (see the Marler Clark sponsored site on Reiter’s Syndrome for more information). Other complications of shigellosis include severe dehydration, seizures in small children, rectal bleeding, invasion of the blood stream by the bacteria (bacteremia or sepsis), proctitis, rectal prolapse, toxic megacolon, and hemolytic uremic syndrome (HUS). 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.

In the U.S., it is estimated that about 700 persons die yearly from shigellosis. Young children and the elderly are at greatest risk of death from a Shigella infection. More than one million deaths occur in the developing world yearly due to infections with Shigella; the victims are mostly children.

How can a Shigella infection be prevented?

Shigella bacteria remain active during the illness and for a week or two after an infected individual recovers. It is possible for a person to carry Shigella without developing symptoms, but then pass the illness to others. The spread of Shigella from an infected person to other persons can be avoided by frequent and careful handwashing with soap. Frequent and careful handwashing is important among all age groups. Handwashing among children should be frequent and supervised by an adult in daycare centers and homes with children who have not been fully toilet trained.

If a child in diapers has shigellosis, everyone who changes the child’s diapers should be sure the diapers are disposed of properly in a closed-lid garbage can, and should wash his or her hands and the child’s hands carefully with soap and warm water immediately after changing the diapers. After use, the diaper changing area should be wiped down with a disinfectant such as diluted household bleach, Lysol, or bactericidal wipes. When possible, young children with a Shigella infection who are still in diapers should not be in contact with uninfected children.

Basic food safety precautions and disinfection of drinking water prevents shigellosis from food and water. However, people with shigellosis should not prepare food or drinks for others until they have been shown to no longer be carrying the Shigella bacteria, or if they have had no diarrhea for at least 2 days. At swimming beaches, having enough bathrooms and handwashing stations with soap near the swimming area helps keep the water from becoming contaminated. Daycare centers should not provide water play areas.

Simple precautions taken while traveling to the developing world can prevent shigellosis. Drink only treated or boiled water, and eat only cooked hot foods or fruits you peel yourself. The same precautions prevent other types of traveler’s diarrhea.

At swimming pools, maintaining a chlorine level of at least 0.5 PPM will kill Shigella. At swimming beaches, children not yet toilet trained should be excluded from public swimming areas; stay clear if this rule is broken. Children with diarrhea should never be taken to public swimming areas.