Clinical Syndromes of Enteric Fever

Clinical Syndromes of Salmonella

• Enteric fever (typhoid fever) is a systemic illness caused by S. Typhi and S. Paratyphi A, B, and C.
• Gastro-enteritis and food poisoning are characterized by vomiting, diarrhea, fever, and abdominal pain.
• Bacteraemia is a common feature of enteric fever, but rare with other strains.

Characteristics of Salmonella

• Gram-negative bacilli, non-capsulate, non-sporing.
• Oxidase negative, aerobic and facultative anaerobic.
• Grow on a wide range of simple media, motile, and non-lactose fermenting.
• Urease negative.

Antigenic Structure (Kauffmann-White Classification)

• Serotyping is done according to O (cell wall) and H (flagellar) antigens.
• Detected by agglutination test with specific antisera.
• Each Salmonella strain is described as a formula with three parts: O antigens, Hantigens phase 1, and Hantigens phase 2.

Examples of Kauffmann-White Classification

• S. Typhi: O antigens: 9,12, Hantigens phase 1: d, Hantigens phase 2: -
• S. Paratyhi A: O antigens: 1, 2, 12, Hantigens phase 1: a, Hantigens phase 2: 1,5
• S. Enteritidis: O antigens: 1, 9, 12, Hantigens phase 1: g, m, Hantigens phase 2: 1,7

Vi Antigen

• Almost all S. Typhi isolates have a covering polysaccharide layer outside the cell wall, known as the Vi antigen.
• May cause O antigen to be undetectable by agglutination test.
• Heat labile antigen that can be removed by heating the bacterial suspension for 1 hour at 100°C.

Laboratory Diagnosis

• Isolation of the organism from patient's blood, faeces, urine, or vomit.
• Culture media: blood agar, MacConkey agar, xylose lysine deoxycholate (XLD) agar, salmonella shigella (SS) medium, deoxycholate citrate agar (DCA).
• Enrichment broths: Selenite F broth, Tetrathionate broth.
• Identification: culture characteristics, motility, biochemical tests, serotyping by latex agglutination, and serotyping by slide agglutination.

H Antigen Phase Conversion

• In many Salmonellae, production of H antigen is diphasic, with strains varying spontaneously and reversibly between two phases with different sets of antigens.
• Definitive identification requires the detection of antigens of both phases.
• The alternative phase can be obtained by selective culture of the isolate in semi-solid agar containing monophasic antiserum to the original phase antigens.

Serological Diagnosis by Widal Test (SAT)

• Standard agglutination test to detect Salmonella antibodies in patient's serum.
• In diagnosis of enteric fever, but not useful in Salmonella food poisoning.
• Patient's serum is tested by tube agglutination for titers of antibodies against H, O, and Vi antigens.

Treatment

• Ciprofloxacin (always with nalidixic acid disk for extra-intestinal isolates).
• Chloramphenicol, ampicillin, cotrimoxazole, and ceftriaxone/cefotaxime.