A Clinical guide to pediatric infectious disease
Salmonella has more than 2,000 serotypes. Although one can attempt to memorize all of them, a more practical method is to divide salmonella into two basic categories: invasive (enteric fever) and noninvasive (nontyphoidal). These categories are different in transmission, presentation, and management.
Invasive salmonella refers mainly to Salmonella typhi, although other salmonella serotypes can cause invasive disease. S. typhi is found only in humans and is spread person to person. These serotypes have certain antigens that allow them to become invasive from the gastrointestinal tract, causing a prolonged bacteremic illness. This bacteremic illness is referred to as enteric fever or typhoid fever.
Infected patients often have fever, leukopenia, hepatosplenomegaly, and abdominal distention. Diarrhea can occur, but because the bacteria is invasive and does not reside long in the gastrointestinal tract, constipation can also be noted. “Rose spots” represent embolic salmonella to the skin and are rare in children. A major issue in typhoid fever is early consideration. Typhoid fever should be considered in any child with a fever and recent travel to an endemic area. This is particularly true if the child has hepatosplenomegaly, leukopenia, and negative malarial smears. Typhoid fever is frequently misdiagnosed as malaria because the endemic regions are similar and both may present with high, spiking fever.
Diagnosis of enteric fever rests on isolation of the organism from blood culture. Blood cultures are positive in a large percentage of patients. Stool cultures can also be positive and may be diagnostic in the correct clinical setting. Febrile agglutinins
(Widal's test) have previously been used, with elevation of O and H titers greater than 1:160 being diagnostic. Currently, these tests are not recommended owing to high rates of false-positive and false-negative results.
Treatment of Invasive Disease
Patients with invasive salmonella disease should always be treated. An increasing percentage of isolates are resistant to ampicillin and trimethoprim-sulfamethoxazole (Bactrim), traditionally the front-line antibiotics for treatment of this organism. Treatment is usually initiated with a third-generation cephalosporin such as cefotaxime or ceftriaxone. Second-generation cephalosporins and gentamicin are not considered efficacious, although in vitro assays may show sensitivity. Fluoroquinolones such as ciprofloxacin are frequently used for treatment of invasive salmonella disease, particularly in developing countries.
About 3% of patients infected with typhoid fever develop chronic infection. This is defined as excretions in the stool for longer than 1 year. Chronic infection serves as a nidus for subsequent infection in others and can be extremely difficult to eradicate. Some children respond to high-dose intravenous ampicillin or oral amoxicillin. In adult chronic carriers, ciprofloxacin is used, often with adjunctive cholecystectomy.
Invasive Salmonella: Salmonella typhi
· Causes enteric fever
· Person-to-person spread
· Chronic carrier state
· Fever, leukopenia, abdominal pain, hepatomegaly
· Treatment always indicated
Nontyphoidal salmonella refers to noninvasive disease. The most common illness caused by nontyphoidal salmonella is gastroenteritis. These organisms are found principally in food and animals. A percentage of food products, including eggs and chicken, are contaminated with salmonella strains. Pets, including turtles and iguanas, are also a well-described reservoir for nontyphoidal salmonella.
Once infected, prolonged excretion can occur, particularly in children. Almost half of children younger than 5 years of age continue to shed salmonella months after initial infection. It has been found that antibiotic therapy can actually prolong this excretion. It has been speculated that antibiotics suppress the protective effects of indigenous intestinal bacteria, which results in the continued survival and excretion of the salmonella bacteria. Unlike with S. typhi, chronic infection does not occur. Routine administration of antibiotics for salmonella gastroenteritis is not recommended because they are not thought to reduce clinical illness and can prolong excretion of the organism.
The most common manifestation of nontyphoidal salmonella infection is gastroenteritis. Although nontyphoid salmonella infections are usually confined to the gastrointestinal tract, there are variations in the clinical course. In young children, nontyphoid salmonella can behave very much like invasive salmonella strains. A percentage of young children with salmonella gastroenteritis have concurrent bacteremia. The reported incidence of bacteremia in children with nontyphoid salmonella gastroenteritis has been reported to be from 5% to 45%. This bacteremia can potentially result in severe morbidity and mortality from resultant osteomyelitis, sepsis, and meningitis. In general, a higher incidence of bacteremia is found in children younger than 1 year of age.
Given the incidence of concurrent bacteremia, an index of suspicion for salmonella gastroenteritis and bacteremia should be had in evaluating a young child, particularly in the first year of life. Numerous studies have addressed the clinical predictors that can be used for acute bacterial diarrhea in young children. The best predictive variable for a stool culture positive for bacterial pathogen is the presence of polymorphonuclear cells in the stools. Three symptoms useful in distinguishing bacterial from viral gastroenteritis are an abrupt onset of diarrhea, more than four stools per day, and no vomiting before the onset of diarrhea. A young child with high fever and gastroenteritis in whom bacterial disease is possible should have a stool and blood culture obtained.
Bacteremic illness should always be treated, usually for 10 to 14 days. In febrile children in the first year of life with proven salmonella gastroenteritis, even without bacteremia, treatment should be considered. This is particularly true in children with underlying conditions such as human immunodeficiency virus (HIV) infection or sickle cell anemia and in those receiving immunosuppressive therapy.
· Food-borne illness
· No chronic carrier state
· Usually causes self-limited gastroenteritis
· Antibiotics usually not indicated
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