Salmonella

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Salmonella typhi and S. paratyphi are Gramnegative, motile bacilli that colonize only humans. Therefore, disease is acquired through close personal contact or through the ingestion of water or food contaminated with human excrement. Typhoid fever continues to represent a global health problem, with more than 12.5 million annual cases, and subequatorial countries reporting mortality rates of up to 32% despite antibiotic treatment.13 In the USA, substantial progress has been made in their eradication, with better sanitation, food-handling and water treatment. These bacteria cause systemic illnesses characterized by fever, gastrointestinal symptoms and occasionally psychosis, confusion or rose spots on the trunk.14 The incubation period varies between 5 and 21 days (depending on the inoculum ingested), and chills, headache, cough, weakness and muscle pain are frequent prodromes. Most symptoms resolve by the 4th week without antimicrobial treatment. However, some patients relapse with high fever, abdominal pain from inflammation of Peyer's patches, and intestinal microperforation followed by secondary bacteremia with normal enteric flora. The definitive diagnosis of enteric fever requires the isolation of S. typhi or S. paratyphi from blood, stool, urine, rose spots, bone marrow, or gastric or enteric secretions. Chloramphenicol is the treatment of choice and has been shown to reduce the duration of fever and mortality.

In contrast to S. typhi, the cases of infection with non-typhoidal salmonellae infections have been increasing in the developed world. Patients as higher risk for infection include those with immunodeficiencies, age younger than 3 months, alterations in intestinal defenses (achlorhydria, antacids, rapid gastric emptying post-gast-rectomy), impaired reticuloendothelial function, (sickle cell and hemolytic anemias) and ingestion of antibiotics to which the organism was resistant. Reservoirs include a wide range of domestic and wild animals, including poultry, swine, cattle, rodents and reptiles. S. enteriditis is the leading reported cause of food-borne disease outbreaks in the USA, with eggs and contaminated raw fruits and vegetables identified as vehicles.15 Transmission from person to person and from pets has also been reported. The incubation period is 6-48 h, after which fever, headache, vomiting, abdominal pain and watery diarrhea (which may contain blood, mucus, and leukocytes) occur for about 1 week. Severe extraintestinal infections can range from life-threatening sepsis to focal infections in the meninges, bones and lungs. The micro-organism is easily isolated from fresh stools or blood culture. Antimicrobials are not indicated to treat asymptomatic carriage or uncomplicated infections in the normal host, as they may prolong excretion or induce relapse. Although efficacy is unproven, it is common clinical practice to administer oral or parenteral antibiotics to high-risk patients or to those who have an extraintestinal focus of infection. Increasing resistance to commonly used antibiotics is seen, so the choice of regimens should be guided by susceptibility data. Suggested therapies include TMP-SMX (Bactrim®, Septra®, Sulfatrim®), ampicillin (10-20% of isolates in the USA are resistant), cefotaxime, ceftriaxone or chloramphenicol.

Hygienic practices for preventing food-borne transmission is the most efficient prevention for non-typhoidal Salmonella infections, since the vast majority of outbreaks and sporadic cases result from culinary practices that allow the organisms to survive and multiply in food. Parents should be instructed to avoid serving food containing raw or undercooked eggs and meat (especially poultry). Food should be thawed in the refrigerator, microwave, or under cold water but not at room temperature, because surface bacteria begin to multiply when the outer layers warm. Eggs should be cooked until both the yolk and the white are firm, and meats must reach an internal temperature of at least 74 °C (165 °F). Frequent hand washing is important. High-risk pets (especially chicks, ducklings and reptiles) are not advisable for young children.

An extremely problematic situation is the management of an infected child who is attending day care. Excretion can go on for weeks and create a hardship to working parents if the child must be excluded from day care. While the decision to admit such a child must be made in concert with day care and public health officials, it is generally recommended that the infected children be excluded from day care if they are symptomatic or if adequate hygiene cannot be ensured. There is no vaccine to prevent non-typhoidal salmonellosis.

Enteric fever

The definitive diagnosis of enteric fever requires the isolation of S. typhi or S. paratyphi from the patient. Cultures of blood, stool, urine, rose spots, bone marrow and gastric and enteric secretions may all be useful in establishing the diagnosis. Chloramphenicol has been the treatment of choice since its introduction, given its low costs and its high efficiency after oral administration. Treatment with chloramphenicol reduced typhoid fever mortality from approximately 20 to 1% and reduced the duration of the fever from 14-28 days to 3-5 days.16

The most effective attenuated vaccine for typhoid fever currently available, Ty21a, has proved to be free of adverse reactions in large-scale efficacy field trials involving almost 600 000 pediatric subjects.17 When administered as a liquid suspension, Ty21a protected both young (82% vaccine efficacy) and older children (69% vaccine efficacy).17 Currently, there are three new-generation attenuated vaccines, genetically engineered by deleting different pathogenic factors, that are undergoing extensive phase II trials.

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