African trypanosomiasis, commonly known as sleeping sickness, is a parasitic disease transmitted by tsetse flies in sub-Saharan Africa. Accurate and timely diagnosis is crucial for effective treatment and management of the disease. The diagnostic approach varies slightly depending on the subspecies of trypanosome involved: Trypanosoma brucei rhodesiense and Trypanosoma brucei gambiense. This article outlines the key diagnostic methods employed for both forms of African trypanosomiasis and the staging process.
Diagnosis of Rhodesiense HAT
Infections caused by T. b. rhodesiense typically present with a more acute and rapidly progressing illness. Diagnosis often begins with clinical suspicion based on the patient’s symptoms and history of potential exposure to tsetse flies, particularly in known endemic areas. Due to higher parasitemia levels, especially in the early stages, direct parasitological detection in blood is frequently successful.
Microscopic Examination of Blood
The primary diagnostic method for Rhodesiense HAT involves the microscopic examination of blood samples. Trypanosomes are often detectable in peripheral blood smears. Concentration methods can enhance sensitivity. Centrifugation of blood allows for the examination of the buffy coat, the layer between plasma and red blood cells, where parasites tend to concentrate. Fresh wet preparations are particularly useful as motile trypanosomes are easier to identify. Giemsa-stained slides can also be used for parasite identification in blood films. It is critical to minimize delays between blood collection and microscopic examination, as trypanosome motility decreases within hours.
Chancre and Lymph Node Aspirate Examination
In addition to blood, samples from trypanosomal chancres (the initial lesion at the site of the tsetse fly bite) and lymph node aspirates can also be examined microscopically for trypanosomes. Fluid expressed from a chancre or material aspirated from enlarged lymph nodes may contain detectable parasites.
Serological Testing
Serological tests are generally not utilized for diagnosing T. b. rhodesiense infections due to their limited sensitivity and specificity in this context. Direct parasite detection methods are favored due to the typically higher parasitemia.
Diagnosis of Gambiense HAT
Diagnosing T. b. gambiense infection presents unique challenges due to lower parasitemia levels and the chronic, slowly progressing nature of the disease.
Card Agglutination Test for Trypanosomiasis (CATT)
The CATT test is a serological screening tool widely used in endemic regions of Africa for population screening of T. b. gambiense. While not available in the United States and not sufficiently specific for definitive diagnosis, CATT is valuable for identifying potential cases requiring further investigation. A positive CATT result necessitates parasitological confirmation.
Parasitological Confirmation
Definitive diagnosis of Gambiense HAT relies on parasitological confirmation, primarily through microscopic examination.
Microscopic Examination of Chancre Fluid and Lymph Node Aspirate
Similar to Rhodesiense HAT, microscopic examination of chancre fluid and lymph node aspirates is crucial. However, the yield from lymph node examination in Gambiense HAT can vary considerably, ranging from approximately 40% to 80%.
Blood Examination and Concentration Techniques
While trypanosomes can be found in the blood of individuals with T. b. gambiense infection, the parasite load is typically low, making direct detection challenging. Therefore, concentration techniques are essential to enhance diagnostic sensitivity. These techniques include:
- Centrifugation and Buffy Coat Examination: Similar to Rhodesiense HAT, centrifuging blood and examining the buffy coat layer increases the chances of detecting parasites.
- Mini-anion Exchange Centrifugation Technique (mAECT): This technique concentrates trypanosomes by selectively separating them from blood components.
- Microhematocrit Centrifugation Technique (mHCT): mHCT also concentrates parasites, improving the likelihood of microscopic detection.
Serial examinations of blood samples collected on consecutive days may be necessary to increase the sensitivity of parasite detection in Gambiense HAT.
Staging of African Trypanosomiasis
Staging is a critical step in managing both T. b. gambiense and T. b. rhodesiense infections. Staging determines the extent of disease progression, particularly whether the infection has reached the central nervous system (CNS). This distinction is crucial as it dictates treatment strategies. Staging is achieved through analysis of cerebrospinal fluid (CSF) obtained via lumbar puncture.
Cerebrospinal Fluid (CSF) Examination
CSF is examined microscopically in a wet preparation to assess for motile trypomastigotes and white blood cells (WBCs). The staging criteria are as follows:
- First Stage: Patients with 5 or fewer WBCs per microliter of CSF and no trypomastigotes are classified as being in the first stage, indicating the infection is primarily in the peripheral blood and lymphatic system, without CNS involvement.
- Second Stage: Patients with more than 5 WBCs per microliter of CSF or the presence of trypomastigotes in the CSF are classified as being in the second stage. This indicates CNS involvement, requiring different and often more intensive treatment regimens.
Elevated protein levels and increased non-specific IgM in the CSF can also support a diagnosis of second-stage disease.
Conclusion
Accurate diagnosis of African trypanosomiasis requires a combination of clinical suspicion, parasitological techniques, and serological tests, particularly for T. b. gambiense. Microscopic examination remains the cornerstone of diagnosis, with various concentration techniques enhancing sensitivity, especially in Gambiense HAT. Staging, determined by CSF analysis, is essential for guiding appropriate treatment and patient management. Resources like DPDx (https://www.cdc.gov/dpdx/) offer valuable diagnostic assistance for healthcare professionals.
