Salmonella Infections

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Basics

Description

Salmonella isolates are broadly divided into the following: (i) nontyphoidal serotypes, with illness ranging from uncomplicated gastroenteritis to meningitis; and (ii) typhoidal serotypes, responsible for typhoid and paratyphoid fever (collectively called enteric fever).

Epidemiology

  • Salmonella is a leading cause of foodborne infection in the United States and worldwide.
  • Reservoirs
    • Nontyphoidal Salmonella serotypes are commonly found in agricultural products, particularly cattle, poultry, and eggs. Less common sources include produce, dairy products, and processed foods.
    • Humans may asymptomatically shed the bacteria for weeks, even months.
    • Reptiles are another well-recognized reservoir for infection. Small turtles remain a source of potential Salmonella infection despite a 1975 federal ban on their sale.
    • Typhoidal serotypes are found only in humans with acute or chronic infection.
  • Transmission
    • Spread occurs via the fecal–oral route. Food and water contamination is the most common mechanism of exposure, followed by direct contact with contaminated surfaces and live animals.
    • Salmonella generally requires a high bacterial inoculum to cause infection.
  • Age/season: Salmonella infections are most common in children <4 years of age and during the summer months.

Risk Factors

  • Young infants (especially those <3 months of age), children with sickle cell disease (SCD), HIV, malignancy, and other immunocompromised conditions are at higher risk for extraintestinal complications from nontyphoidal Salmonella gastroenteritis.
  • Travel to underdeveloped countries

General Prevention

  • Hand hygiene, particularly when handling foods at risk for Salmonella contamination
  • Proper cleaning of food preparation surfaces, particularly when handling foods at risk for Salmonella contamination
  • Foods that frequently harbor Salmonella, such as meat, poultry, and eggs, require thorough cooking.
  • Children <5 years old, and all those with high-risk conditions, should avoid contact with reptiles (e.g., lizards, snakes, turtles).
  • Vaccines for typhoid fever have an efficacy of 50–80% and are recommended for the following: (i) travel to endemic areas and (ii) close contacts of carriers. Available vaccines include the following:
    • Ty21a, live vaccine for children ≥6 years of age; given PO every other day for 4 doses
    • ViCPS, inactivated vaccine for children ≥2 years of age; given as a single IM dose
    • Neither vaccine can be relied on to protect against Salmonella serotype paratyphi A and B.

Etiology

Salmonella is classified into two species: Salmonella enterica and Salmonella bongori. Species are further divided into 1 of over 2,500 serotypes.

  • Common nontyphoidal serotypes include the following: S. enterica serotype enteritidis, S. enterica serotype typhimurium, S. enterica serotype Newport, and S. enterica serotype Heidelberg.
  • Typhoidal serotypes include the following: S. enterica serotype typhi and S. enterica serotype paratyphi A, B, and C.

Commonly Associated Conditions

Following infection of the intestinal epithelium, Salmonella strains present with a variety of clinical manifestations.

  • Acute gastroenteritis is the most common illness involving nontyphoidal serotypes:
    • Diarrhea is often watery but can be inflammatory with varying amounts of mucus and/or blood.
    • The incubation period is 12 to 48 hours, and illness usually resolves within 3 to 5 days. Asymptomatic shedding is common with a mean duration of 5 weeks—longer in infants. A small percentage of children can have asymptomatic shedding for up to 1 year.
  • Transient bacteremia (nontyphoidal)
    • Bacteremia occurs in up to 5% of infected immunocompetent children and in 10% or more of high-risk patients. Young infants are generally at higher risk for bacteremia.
    • The most common serotypes associated with bacteremia include Salmonella enteritidis, Salmonella Heidelberg, and Salmonella typhimurium.
    • Bacteremia can result in localized extraintestinal infection.
  • Localized extraintestinal infection (nontyphoidal)
    • Local infections occur in 3–5% of otherwise healthy bacteremic children and in up to 30% of high-risk bacteremic patients.
    • Infections include meningitis, septic arthritis, osteomyelitis, and pneumonia.
    • Infants <3 months of age are at higher risk for complications of bacteremia including meningitis.
  • Enteric fever (typhoid and paratyphoid fever)
    • The most important serotypes are Salmonella typhi, followed by the less frequent and milder paratyphi A, B, and C strains.
    • Incubation is usually 7 to 10 days but can be 3 to 60 days.
    • The clinical course is often insidious with progression of disease over 3 to 4 weeks.
      • Weeks 1 to 2: Fever, headache, myalgia, abdominal pain, and listlessness are common. Diarrhea occurs in less than half of patients, and constipation is common.
      • Weeks 2 to 3: Fever increases, and rose spots (maculopapular rash) may appear. Splenomegaly and respiratory symptoms may develop.
      • Weeks 3 to 4: Fever gradually improves, however, serious complications, such as intestinal perforation, may develop at this time.

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