Introduction
- Enteric fever is a significant global health concern primarily caused by Salmonella enterica serotype Typhi (S. Typhi) and less commonly by Salmonella enterica serotype Paratyphi A, B, and C.
- It is endemic in many developing countries and is characterized by a spectrum of clinical symptoms, ranging from mild gastrointestinal discomfort to severe systemic illness.
- Accurate laboratory diagnosis is essential for timely treatment and management of the disease and public health interventions.
Clinical Presentation
The symptoms of enteric fever typically appear 1 to 3 weeks after exposure and can include:
- Fever: A sustained, high-grade fever that may reach 39-40°C (102.2-104°F).
- Abdominal Pain: Often in the right lower quadrant, it can be diffuse.
- Diarrhea: Occurs in about 30% of cases; some patients may experience constipation instead.
- Headache: Common, often accompanied by fatigue and malaise.
- Rose Spots: Painless, flat, salmon-colored spots that appear on the abdomen in about 20% of patients.
- Enlarged Spleen (Splenomegaly): Occurs in many cases and can be detected on physical examination.
Sample Collection
- Specimen Types
- Blood Samples: Primary specimens for diagnosing enteric fever.
- Bone Marrow Aspirates: Used especially when blood cultures are negative.
- Stool Samples: Helpful in identifying carriers or late-stage infections.
- Urine Samples: Sometimes tested to identify the pathogen in chronic carriers.
- Collection Technique
- Blood Cultures:
- Collect 2-3 blood cultures, ideally during febrile episodes, to enhance detection rates.
- Use sterile techniques and appropriate blood culture bottles.
- Bone Marrow Aspirate:
- Perform under sterile conditions. This involves using a specialized needle to obtain marrow from the posterior iliac crest and then inoculating it into culture media.
- Stool Samples:
- Collect in sterile containers, particularly from individuals suspected of being carriers or with prolonged illness.
- Urine Samples:
- Obtain clean-catch urine specimens, particularly in cases of suspected chronic infection.
- Transport and Handling
- Blood Samples: Should be transported to the laboratory immediately or kept at room temperature for short periods.
- Bone Marrow Samples: Processed immediately if possible; if not, they should be refrigerated.
- Stool and Urine Samples Should be processed within 24 hours or refrigerated to maintain viability.
Laboratory Techniques for Diagnosis
Blood Culture
A. Culture Media
- Nutrient Broth: Blood is inoculated into enriched broth (e.g., tryptic soy broth or brain heart infusion) to support bacterial growth.
- Selective Media: Following broth culture, subcultures on selective media such as:
- MacConkey Agar: Differentiates lactose fermenters from non-fermenters, indicating the presence of Salmonella.
- XLD (Xylose Lysine Deoxycholate) Agar: Useful for isolating Salmonella and Shigella.
B. Incubation Conditions
- Blood cultures are incubated at 35-37°C for 24-48 hours. After this period, subcultures are performed on selective media.
C. Identification of Colonies
- Colonies resembling Salmonella will be further tested for biochemical properties, such as:
- Indole Production: Positive for S. Typhi.
- Hydrogen Sulfide Production: Indicates Salmonella species.
Bone Marrow Culture
- Higher Yield: Bone marrow cultures are more sensitive than blood cultures, particularly in chronic cases.
- Cultures are performed similarly to blood cultures, often yielding positive results even after multiple negative blood cultures.
Stool Culture
- Stool samples may be inoculated onto selective media, although they are less sensitive to acute infections. The stool is tested for:
- Salmonella: Isolation confirms the diagnosis, particularly in chronic carriers or late presentations.
Urine Culture
- Urine cultures can reveal Salmonella in chronic carriers. The urine sample is cultured on selective media and incubated under appropriate conditions.
Serological Tests
- Widal Test:
- Measures agglutination of antibodies against S. Typhi O and H antigens.
- While a positive result may indicate typhoid fever, it has limitations, including cross-reactivity with other infections and variable sensitivity. It is more useful for seroepidemiological studies rather than acute diagnosis.
- Typhidot Test:
- Detects IgM and IgG antibodies to S. Typhi, providing quicker results than traditional serological methods.
Molecular Methods
A. Polymerase Chain Reaction (PCR)
- PCR techniques can identify S. Typhi DNA from blood, stool, or other specimens. This method offers:
- High Sensitivity and Specificity: Especially valuable when cultures are negative or delayed.
- Rapid Results: Provides results within hours, crucial for prompt treatment.
B. Multiplex PCR
- Allows simultaneous detection of multiple pathogens, making it useful in cases where co-infection is suspected.
Interpretation of Results
- Blood Culture Results
- Positive Culture: Isolation of S. Typhi confirms the diagnosis of enteric fever. Cultures may remain positive for weeks in untreated cases, and a single positive blood culture in the context of clinical symptoms is sufficient for diagnosis.
- Negative Culture: Negative results can occur if antibiotics were administered before culture or samples were collected early in the illness.
- Bone Marrow Culture Results
- Bone marrow cultures are particularly sensitive and can confirm enteric fever even when blood cultures are negative. A positive result provides strong evidence for the diagnosis.
- Stool and Urine Culture Results
- While useful for identifying carriers, stool cultures are less effective in acute cases. Isolation of S. Typhi from stool indicates infection but may not be definitive for acute diagnosis.
- Serological Test Results
- Widal Test: Interpretation must consider the clinical context; a significant titer rise indicates infection, but false positives are common.
- Typhidot Test: A positive IgM result suggests recent infection, while IgG may indicate past infection.
- PCR Results
- Positive PCR: Confirms the presence of S. Typhi, particularly useful in early stages or when cultures are negative.
- Negative PCR: May not exclude infection, especially if performed shortly after symptom onset.
Clinical Considerations
Diagnosis and Treatment
Timely diagnosis and initiation of appropriate treatment are critical in managing enteric fever. Antibiotic resistance is a growing concern, making susceptibility testing vital.
- Antibiotic Therapy: Commonly used antibiotics include:
- Ceftriaxone: A broad-spectrum cephalosporin effective against severe infections.
- Azithromycin: Increasingly used due to its efficacy and lower resistance rates.
- Fluoroquinolones: Such as ciprofloxacin, although resistance is rising in some regions.
Follow-Up and Monitoring
- Monitor for complications such as intestinal perforation, septic shock, and response to treatment.
- Follow-up blood cultures may be necessary to ensure the infection has resolved.
Public Health Implications
Enteric fever poses significant public health challenges, particularly in low-resource settings. Surveillance for cases, antibiotic resistance monitoring, and public hygiene and vaccination education can reduce incidence rates.