Salmonellosis PDF

lliiiii - Salmonellosis David A. Pegues, Samuel I. Miller Bacteria of the genus Sa/111011el/a are highly adapted for growth in both humans and animals and cause a wide spectrum of diseases. The growth of se...

lliiiii - Salmonellosis David A. Pegues, Samuel I. Miller Bacteria of the genus Sa/111011el/a are highly adapted for growth in both humans and animals and cause a wide spectrum of diseases. The growth of serotypes Sa/111011el/a Typhi and Sa/111011,lla Paratyphi is restricted to human hosts, in whom these organisms cause enteric (typhoid) fever. The remaining serotypes (nontyphoidal Salmonelln, or NTS) can colonize the gastrointestinal tracts of a broad range of ani- mals, including mammals, reptiles, birds, and insects. More than 200 serotypes of Salmonella are pathogenic to humans, in whom they often cause gastroenteritis and can be associated with localized infections and/or bacteremia. ETIOLOGY This large genus of gram-negative bacilli within the family Enterobac- teriaceae consists of two species: Salmonella euterica, which contains six subspecies, and Sa/111011elln lxmgori. 5. enterica subspecies I includes almost all the serotypes pathogenic for humans. Members of the seven Sn/111011,lln subspecies are classified into >2500 serotypes (serovars); for simplicity, Salmo11elln serotypes (most of which are named for the city where they were identified) are often used as the species designation. For example, the full taxonomic designation S. enterica subspecies enterica serotype Typhimurium can be shortened to Salmonella sero- type Typhimurium or simply S. Typhimurimn. Serotyping is based on the somatic O antigen 0ipopolysaccharide cell-wall components), the surface Vi antigen (restricted to S. Typhi and S. Paratyphi C), and the flagellar H antigen. Salmonellae are gram negative, non-spore-forming, facultatively anaerobic bacilli that measure 2-3 µm by 0.4--0.6 µm. The initial identi- fication of salmonellae in the clinical microbiology laboratory is based on growth characteristics. Salmonellae, like other Enterobacteriaceae, produce acid on glucose fermentation, reduce nitrates, and do not pro- duce cytochrome oxidase. In addition, all salmonellae except Snlmonelln Gallinarum-Pullorum are motile by means of peritrichous flagella, and all but S. Typhi produce gas (H,S) on sugar fermentation. Notably, only 1% of clinical isolates ferment lactose; a high level of suspicion must be maintained to detect these rare clinical lactose-fermenting isolates. Although serotyping of all surface antigens can be used for formal identification, most laboratories perform a few simple agglutination reactions that define specific 0-antigen serogroups, designated A, B, C,, Cl' D, and E. Strains in these six serogroups cause ~99% of Salmonella infections in humans and other warm-blooded animals. Molecular typ- ing methods, including pulsed-field gel electrophoresis, multiple-locus variable-number tandem repeat analysis, and whole-genome sequenc- ing, are used in epidemiologic investigations to differentiate Sa/111011elln strains of a common serotype. PATHOGENESIS All Snlmonelln infections begin with ingestion of organisms, most com- monly in contaminated food or water. The infectious dose ranges from 200 colony-forming units (CFlJ) to HJ' CFU, and the ingested dose is an important determinant of incubation period and disease severity. Conditions that decrease either stomach acidity (an age of 0,000 deaths annually. The annual incidence is high- est (>100 cases/100,000 population) in South-Central and Southeast Asia; medium (10-100 cases/100,000) in the rest of Asia, Africa, Latin America, and Oceania (excluding Australia and New Z.ealand); and low in other parts of the world (Fig. 160-1). A high incidence of enteric fever correlates with poor sanitation and lack of access to clean drink- ing water. In endemic regions, enteric fever is more common in urban than rural areas and among young children and adolescents than among other age groups. Risk factors include contaminated water or ice, flooding, food and drinks purchased from street vendors, raw fruits and vegetables grown in fields fertilized with sewage, ill house- hold contacts, lack of hand washing and toilet access, and evidence of prior Helicolxicler pylori infection (an association probably related to chronically reduced gastric acidity). It is estimated that there is one case of paratyphoid fever for every four cases of typhoid fever, but the incidence of infection associated with S. Paratyphi A appears lo be increasing, especially in India; this increase may be a result of vaccina- tion for S. Typhi. Multidrug-resistant (MOR) strains of S. Typhi emerged in the 1980s in China and Southeast Asia and have since disseminated widely. These strains contain plasmids encoding resistance to chlorampheni- col, ampicillin, and trimethoprim-antibiotics long used to treat enteric fever. With the increased use of fluoroquinolones to treat MOR enteric fever in the 1990s, MOR strains of S. Typhi and S. Paratyphi with decreased susceptibility to ciprofloxacin (DSC; minimal inhibitory concentration IMICI, 0.125--0.5 µg/mL) or ciprofloxacin resistance (MIC, 21 µg/mL) have emerged on the Indian subcontinent and have spread with human migration to southern Asia and now to eastern and southern Africa. These strains represent clone H.58, which increasingly has been associated with clinical failure of quinolone treatment. Test- ing of isolates for resistance to the first-generation quinolone nalidixic acid detects many but not all strains with reduced susceptibility to ciprofloxacin and is no longer recommended. Strains of S. Typhi and S. Paratyphi producing extended-spectrum ~-lactamases have emerged, primarily in India and Nepal. Approximately 300 cases of typhoid and 150 cases of paratyphoid fever are reported annually in the United States. Of 3499 cases of S. Typhi-associated enleric fever reported to the Centers for Disease Control and Prevention in 1999-2010, 82% were associated with recent international !rave.I, most commonly to India, Pakistan, and Bangladesh, and occurred predominantly in young to middle-aged adults. Only 6% of travelers diagnosed with enteric fever had received S. Typhi vaccine. Overall, 15% of recent S. Typhi isolates in the United States were resistant to ampicillin, chloramphenicol, and trimethoprim· sulfamethoxazole (TM P-SMX), whereas 60% of isolates exhibited DSC. Infection with DSC S. Typhi was associated with travel lo the Indian subcontinent. During this period, 18% of reported cases of enteric fever in the United States were domestically acquired, and these cases were less often due to MDR or DSC strains than were travel associated cases. Most cases of domestically acquired enteric fever are sporadic, but outbreaks linked to contaminated food products and previously unrecognized chronic carriers continue to occur. CLINICAL COURSE E11teric feve.r is a misnomer, in that the hallmark features of this disease-fever and abdominal pain-are variable. While fever is docu· mented at presentation in >75% of cases, abdominal pain is reported in only 30-40%. Thus, a high index of suspicion for this potentially fatal systemic illness is necessary when a person presents with fever and a history of recent travel to a developing country.  The incubation period for S. Typhi 1175 averages 10-14 days but ranges from 5 to 21 days, depending on the inoculum size and the host's health and immune status. The most prominent symptom is prolonged fever (38.8'-40.S'C; 101.8'- 104.9'F), which can continue for up to 4 weeks if untreated. S. Paratyphi A is thought to cause milde.r disease than S. Typhi, with predominantly gastrointestinal symp- toms. However, a prospective study of 669 consecutive cases of enteric fever in Kathmandu, Nepal, found that the infec- tions caused by these organisms were clinically indistinguishable. In this series, symptoms reported on initial medical eval- uation included headache (80%), chills (35-45%), cough (30%), sweating (20-25%), Low (