6334 – Shigella infections

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Definition

Shigella infection, also known as shigellosis, is an intestinal illness caused by the bacteria Shigella. It's highly contagious and is typically spread through the fecal-oral route, meaning it's transmitted when someone ingests the bacteria from contaminated sources, often through unwashed hands or contaminated food and water. Symptoms, which can include diarrhea, fever, stomach cramps, and nausea, usually appear within one to two days after infection and can last for up to a week. 

Etiology

Shigella infection, also known as shigellosis, is caused by the Gram-negative bacterium belonging to the genus Shigella. There are four main species: S. dysenteriae, S. flexneri, S. boydii, and S. sonnei, all of which are capable of causing illness in humans. 

Key aspects of Shigella etiology

• Fecal-oral transmission: The primary mode of transmission is through the fecal-oral route, meaning the bacteria are spread through the stool of an infected person to the mouth of another. This can happen directly through person-to-person contact, or indirectly via contaminated food or water. For instance, contaminated food might look and smell normal, but can become a vehicle for spread if handled by infected food handlers with poor hygiene. Similarly, water can be contaminated if sewage runs into it or if someone with shigellosis swims in it.
• Low infectious dose: Only a very small number of Shigella organisms (as few as 10-100) are needed to cause infection. This low infectious dose makes it highly communicable and contributes to its spread.
• Intestinal invasion and inflammation: Shigella species primarily infect the large intestine and invade the colonic epithelium. This invasion, along with an inflammatory response, leads to the characteristic symptoms of shigellosis, such as bloody diarrhea, abdominal pain, and fever.
• Virulence factors and toxins: Shigella employs various virulence factors to establish and cause illness. These include a type III secretion system (T3SS) and its effector proteins, which are responsible for the most severe symptoms of shigellosis. S. dysenteriae serotype 1 is particularly noteworthy because it produces Shiga toxin, a potent cytotoxin that can cause severe complications like hemolytic uremic syndrome (HUS).

Signs & Symptoms

People with shigellosis usually experience mild or severe diarrhea (loose stool/poop), sometimes with fever, stomach pain, and traces of blood or mucous in the stool. These symptoms usually last 5-7 days. Some infected people may not show any symptoms.

Tests

Diagnosing Shigella infection typically involves laboratory tests to identify the bacteria in a patient's stool. 

Here are the main tests used for Shigella infection:

1. Stool Culture: This is the traditionally accepted method for identifying Shigella bacteria. A stool sample is collected and cultured in a laboratory to grow and identify the specific Shigella species causing the infection.
2. Antimicrobial Susceptibility Testing (AST): If Shigella is identified through stool culture, AST is recommended, especially if antibiotic treatment is being considered. This test helps determine which antibiotics are most effective against the particular Shigella strain, as resistance to common antibiotics is increasing.
3. Culture-Independent Diagnostic Tests (CIDTs) / PCR Testing: These tests are becoming more prevalent and offer advantages like faster turnaround times compared to traditional stool culture.
   • PCR (Polymerase Chain Reaction) tests can detect Shigella DNA in a stool sample, even if the bacteria are not viable for culture.
   • An example is the invasion plasmid antigen H (ipaH) gene sequence, found in all Shigella species, which can be targeted for PCR detection.
   • While PCR testing is more sensitive and quicker, it may not differentiate between Shigella and enteroinvasive Escherichia coli (EIEC) as both carry the ipaH gene. However, EIEC is uncommon in many regions, including Asia, so most ipaH detection in stool from these areas is likely Shigella. 

Treatment

Most cases of Shigella infection resolve on their own within 5 to 7 days, primarily requiring supportive care focused on fluid and electrolyte replacement to combat dehydration caused by diarrhea. 

However, for severe cases, certain individuals, or those at risk of complications, antibiotics may be necessary. According to the Centers for Disease Control and Prevention (CDC), these include patients who work in public settings (like food handlers or childcare providers), individuals with weakened immune systems (including those with HIV infection), or patients experiencing severe disease that might require hospitalization. 

Antibiotic treatment

• Choice of Antibiotic: The selection of the appropriate antibiotic should be guided by laboratory tests to determine the specific strain of Shigella and its susceptibility to different medications.
• Recommended Antibiotics: Fluoroquinolones (such as ciprofloxacin), azithromycin, and third-generation cephalosporins (like ceftriaxone) are commonly recommended antibiotics.
• Antibiotic Resistance: It is crucial to note that some strains of Shigella have developed resistance to certain antibiotics. Therefore, your doctor may need to perform sensitivity testing to identify the most effective antibiotic for your specific infection.
• Importance of Adherence: If antibiotics are prescribed, it's vital to take them exactly as directed and complete the entire course of medication, even if you start feeling better. 

Residuals

Long-term complications or residuals after shigella infection (shigellosis) are uncommon, but can occur. Here are some potential residuals:

• Post-Infectious Arthritis (Reactive Arthritis): This can affect about 2-3% of people, especially those infected with Shigella flexneri. Symptoms include joint pain and swelling (often in ankles, knees, feet, hips), eye irritation (conjunctivitis), and painful urination. Reactive arthritis can become chronic and challenging to treat.
• Post-infectious Irritable Bowel Syndrome (PI-IBS): Some individuals who experience bacterial gastroenteritis, including shigellosis, may develop IBS symptoms afterward. While studies have shown an increased risk of IBS in the years following a shigella infection, one study indicated that this risk may become similar to that of uninfected controls after 10 years. Risk factors for PI-IBS may include younger age, previous history of functional bowel disorder, and longer duration of diarrhea during the initial infection.
• Hemolytic-Uremic Syndrome (HUS): This is a rare, but serious complication primarily associated with Shiga toxin-producing strains, most commonly S. dysenteriae type 1. HUS involves hemolytic anemia, thrombocytopenia, and renal insufficiency (kidney damage).
• Bloodstream Infections (Bacteremia): Shigella infection can, in some cases, lead to bacteria entering the bloodstream. This is more common in individuals with weakened immune systems, such as those with HIV, cancer, or severe malnutrition.
• Neurological Problems: In rare cases, especially in young children, shigellosis can lead to seizures and, in very rare instances, a severe form of encephalopathy called Ekiri syndrome.
• Impact on Child Growth: Studies have shown that Shigella infection, especially early in life, can be associated with reduced linear growth (stunting) in children in low-resource settings, according to PLOS. 

It is important to note that most people who contract shigellosis recover completely. However, seeking prompt medical attention, particularly if experiencing severe symptoms, is important to minimize the risk of complications.