Typhoid and Enteric Fever Diagnostics

Enteric fever is a systemic illness caused by Salmonella enterica serovar Typhi (typhoid) or Paratyphi A, B and C (paratyphoid). Diagnosis rests on isolating the organism. Blood culture is the main test, bone marrow culture the most sensitive, and stool and urine culture add yield later in the illness.

Serology (the Widal test) and rapid antibody tests (Typhidot, TUBEX) are widely used where culture is unavailable, but their performance is poor. Nucleic acid amplification is used in research and reference laboratories. Antimicrobial susceptibility testing is essential, because resistance now shapes treatment.

About 9 million cases and 110 000 deaths occur each year, mostly in South Asia and sub-Saharan Africa, with the highest incidence in children aged 2 to 15 years. Extensively drug resistant S. Typhi (resistant to chloramphenicol, ampicillin, co-trimoxazole, fluoroquinolones and third generation cephalosporins) has spread from Pakistan since 2016.

Untreated disease carries a case fatality of 10 to 20%, falling under 1% with appropriate treatment. Chronic carriage develops in 2 to 5% of those infected, mainly in the gallbladder.

Indications. Fever lasting 3 days or more in a patient from or returning from an endemic area, with headache, abdominal pain, constipation or diarrhoea, relative bradycardia, hepatosplenomegaly or rose spots. Fever of unknown origin. Suspected intestinal perforation or haemorrhage. Outbreak investigation. Screening of food handlers and suspected chronic carriers.

Specimen. Blood culture before antibiotics: 2 sets from separate venepunctures, 8 to 10 mL in each bottle in adults and 2 to 4 mL in young children, with a blood to broth ratio of about 1 to 10. Bone marrow aspirate 0.5 to 1 mL in culture broth. Stool and urine in sterile containers. Serum for serology.

Culture.

• Blood culture: sensitivity 40 to 80%, highest in the first week (up to 70 to 80%) and falling to 20 to 30% by week 3. Prior antibiotics reduce yield by 30 to 50%. Larger blood volume raises yield, and sensitivity in children is higher because bacteraemia is heavier.
• Bone marrow culture: sensitivity 80 to 95%, and remains positive for 5 days after starting antibiotics. It is used when blood cultures are negative with strong clinical suspicion, or after antibiotics.
• Stool culture: positive in 30 to 40% from week 2 to 3, and used for carrier detection. Urine culture positive in about 25%.
• Duodenal string or aspirate culture: useful for biliary carriage.
• Clot culture and bile culture: used in specialised settings.

Susceptibility testing. Reported for ampicillin, chloramphenicol, co-trimoxazole, ciprofloxacin or nalidixic acid screening (a nalidixic acid resistant isolate has reduced fluoroquinolone susceptibility and a worse response to ciprofloxacin even with a sensitive MIC), ceftriaxone, azithromycin and carbapenems.

Multidrug resistance means resistance to chloramphenicol, ampicillin and co-trimoxazole. Reduced ciprofloxacin susceptibility is now present in most South Asian isolates. Azithromycin and carbapenems remain options for extensively drug resistant strains.

Widal test. Measures agglutinating antibodies to O (somatic) and H (flagellar) antigens of S. Typhi and, in some versions, paratyphoid antigens. A fourfold rise between paired sera taken 7 to 10 days apart is diagnostic. A single titre is interpreted against a local baseline, with typical cut offs of 1 in 160 or higher for O and H in endemic areas.

Antibodies appear after the first week, peak in weeks 3 to 4, and the O titre falls in a few months while H persists longer. Sensitivity is 50 to 70% and specificity 60 to 90%, with wide variation by population.

Rapid serological tests.

• Typhidot detects IgM and IgG against a 50 kDa outer membrane protein. IgM positive suggests acute infection, IgG alone suggests past infection or convalescence. Sensitivity 70 to 95% and specificity 65 to 90%.
• TUBEX detects IgM against O9 lipopolysaccharide and is read by colour on inhibition of particle agglutination. Sensitivity 60 to 80%.
• Both tests are not accurate enough to exclude or confirm disease and should not replace blood culture.

Molecular tests. PCR on blood detects flagellin (fliC-d), Vi (viaB) and other targets, with sensitivity exceeding culture in some studies, but is not routine. Multiplex gastrointestinal panels on stool detect Salmonella without serovar separation.

Critical values and findings needing immediate action.

• Positive blood culture: notify the clinician at once, start or adjust treatment by susceptibility, and report to public health.
• Features of complications: intestinal haemorrhage or perforation (abdominal rigidity, free gas on radiography, falling haemoglobin), encephalopathy, myocarditis, osteomyelitis and relapse.
• Ciprofloxacin non-susceptible or extensively drug resistant isolate: change to azithromycin or a carbapenem and inform infection control.
• Persistent positive stool culture 1 year after illness: chronic carrier, with exclusion from food handling.
• Relapse (2 to 3 weeks after stopping treatment, in 5 to 10%): repeat culture and susceptibility.

Interferences and pitfalls.

• Culture taken after antibiotics, or with small blood volumes, causes false negatives. Antibiotics are not delayed for culture in a sick patient, but cultures are drawn first.
• Widal cross-reacts with other Salmonella, Enterobacteriaceae, malaria, dengue, brucellosis and rheumatological disease, and is raised by earlier typhoid infection or vaccination (H and O titres).
• Baseline titres vary by area, so the cut off should be set locally.
• Rapid IgM tests stay positive for months, so a positive result does not prove acute infection in endemic areas.
• Vi vaccination does not cause Widal rises. Oral Ty21a and parenteral typhoid vaccines can cause a response in some tests.
• Fever with a negative blood film does not exclude co-existing malaria, which occurs in the same populations.
• Intestinal perforation occurs in 1 to 3% in the third week, and surveillance for abdominal signs and falling counts continues during treatment.
• Leucopenia, relative bradycardia, elevated transaminases and thrombocytopenia are supporting findings only.

Special groups.

• Children: milder disease but greater culture yield, with higher risk of complications under 5 years.
• Pregnancy: bacteraemia can cause miscarriage, preterm delivery and neonatal infection, so cultures are taken early. Fluoroquinolones are avoided.
• Chronic carriers and food handlers: 3 stool cultures taken at least 1 month apart after treatment, at 48 hours intervals.
• HIV-infected patients: risk of relapse and recurrent non-typhoidal bacteraemia.
• Regions: South Asia has high fluoroquinolone and extensively drug resistant rates. Africa has increasing multidrug resistance and invasive non-typhoidal Salmonella as a competing cause of febrile illness.

Follow up tests. Repeat blood culture if fever persists beyond 5 days of appropriate treatment, full blood count, liver function tests, creatinine, chest and abdominal radiography if complications are suspected, abdominal ultrasound for gallbladder disease in carriers, stool cultures 1 month after treatment, HIV test, and Vi capsular polysaccharide or conjugate vaccination for contacts and travellers.