Shigella is a group of bacteria known to cause an intestinal infection called shigellosis, which can lead to severe diarrheal disease. These gram-negative bacteria are highly infectious and pose a public health concern globally. This article explores the mechanisms by which Shigella causes illness, its common symptoms, how it spreads, and the methods for identifying, treating, and preventing infection.
How Shigella Causes Illness
Shigella bacteria are transmitted primarily through the fecal-oral route, entering the human body via contaminated food or water. These bacteria are remarkably resilient, with as few as 10 to 100 microorganisms being sufficient to cause disease. Their ability to survive the acidic environment of the stomach is attributed to effective acid resistance systems, allowing them to reach the large intestine where they establish infection.
Once in the large intestine, Shigella invades the cells lining the colon, targeting specialized immune cells called M cells. After crossing the M-cell barrier, the bacteria encounter macrophages, immune cells that engulf invaders. Shigella can induce an apoptosis-like cell death in these macrophages, allowing the bacteria to evade degradation and be released on the basolateral side of the epithelial cells.
Subsequently, Shigella invades the epithelial cells directly from their basolateral side, initiating intracellular replication and spread. The bacteria use a needle-like structure called a Type III Secretion System (T3SS) to inject effector proteins into the host cells. These proteins manipulate the host cell’s cytoskeleton by inducing actin polymerization, which allows the bacteria to move freely within the cytoplasm and spread from one cell to another without exiting the host cell, thus avoiding the immune system.
This invasion and subsequent intracellular multiplication lead to widespread inflammation and destruction of the intestinal cells, contributing to the characteristic symptoms of shigellosis. The damage to the intestinal lining can result in the formation of ulcers. Some strains, notably Shigella dysenteriae type 1, produce Shiga toxin. This toxin further exacerbates the disease by damaging host cells and small blood vessels, which can lead to more severe outcomes, including bloody diarrhea.
Common Symptoms and How it Spreads
Shigellosis causes a range of gastrointestinal symptoms that can vary in severity. Common signs include diarrhea, which often progresses from watery to containing blood or mucus. Individuals experience stomach cramps, nausea, and vomiting, along with a fever. Another symptom is tenesmus, a persistent feeling of needing to pass stool even when the bowels are empty.
Symptoms usually appear within one to two days, though they can range from one to seven days after exposure. Symptoms last between five to seven days. Infected individuals may not show any symptoms, yet they can still shed the bacteria and transmit the infection.
Shigella is highly contagious and spreads through the fecal-oral route. This can occur through direct person-to-person contact, especially in settings like childcare centers or schools. Transmission also happens through the consumption of food or water contaminated with infected feces.
Identifying, Treating, and Preventing Shigella Infection
Diagnosing shigellosis involves laboratory testing of stool samples. Healthcare professionals collect a stool specimen for analysis, using bacterial culture or Polymerase Chain Reaction (PCR) to detect its genetic material. These tests help confirm the presence of Shigella and guide appropriate treatment.
Treatment for shigellosis focuses on managing symptoms and preventing complications. Rehydration is a primary concern, as diarrhea can lead to significant fluid loss. Oral rehydration solutions are effective in replenishing lost fluids and electrolytes. For severe dehydration, intravenous hydration in a hospital setting may be necessary.
Antibiotics may be prescribed for severe cases, infants, older adults, or individuals with weakened immune systems, and to reduce the risk of spreading the disease. While antibiotics can shorten the duration and severity of the illness, there is a growing concern regarding antibiotic resistance in Shigella strains, which can make treatment more challenging. Mild cases resolve on their own within a week without the need for antibiotics.
Preventing Shigella infection relies on practicing good hygiene and safe food handling. Thorough handwashing with soap and water is important after using the bathroom, changing diapers, and before preparing or eating food. Ensuring access to safe drinking water and properly cooking and storing food are also important measures. Avoiding swimming in contaminated water and refraining from preparing food for others while ill can further reduce transmission.