Salmonella Infections

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.