Classification and Morphological Features
The genus Trypanosoma includes species pathogenic to humans discovered in the early 20th century. These include the subspecies T. brucei (gambiense and rhodesiense), which cause African trypanosomiasis, and T. cruzi, responsible for American trypanosomiasis.
Morphologically, a trypanosome is a narrow, elongated cell. Movement is provided by a flagellum and an undulating membrane. The parasites reproduce asexually by longitudinal binary fission.
Depending on the life cycle stage, three main forms are distinguished:
- Epimastigotes (crithidial stage). Found in the vector's gut. The flagellum originates near the middle of the cell, and the undulating membrane is poorly developed.
- Trypomastigotes (trypanosomal stage). Located in the blood of vertebrates. The flagellum originates at the posterior end, and the undulating membrane is well-developed.
- Amastigotes (non-flagellated stage). Characteristic of T. cruzi. They are oval-shaped without a flagellum and develop strictly intracellularly (in muscle and other tissues).
African Trypanosomiasis (Sleeping Sickness)
The disease is transmitted by blood-sucking tsetse flies. Two forms of the infection are recognized:
- Gambian (T. brucei gambiense). Characterized by a chronic course. The main reservoir is humans, with pigs as a secondary reservoir.
- Rhodesian (T. brucei rhodesiense). Proceeds acutely and severely. The reservoirs are animals (antelopes, cattle, goats), and human infection is accidental.
Pathogenesis: By the end of the first week, an ulcerating papule (a "trypanosomal" chancre) forms at the bite site. The pathogen enters the bloodstream, multiplies, and disseminates into lymph nodes and cerebrospinal fluid (CSF). Clinically, this manifests as fever, meningoencephalitis, progressive fatigue, wasting, and characteristic hypersomnia. Without treatment, the infection is fatal (though asymptomatic carriage can occur).
American Trypanosomiasis (Chagas Disease)
The causative agent is T. cruzi. Triatomine ("kissing") bugs serve as vectors. Infection occurs not directly through the bite, but when bug feces are rubbed into the bite wound or mucous membranes.
The incubation period lasts 1–3 weeks. A firm, dark-red infiltrate forms at the site of entry. The life cycle in the human body is divided into two phases:
- Blood phase: trypomastigotes circulate in the bloodstream but do not replicate.
- Tissue phase: the parasite invades host cells, transforms into an amastigote, and actively divides by binary fission. The engorged cell ruptures, releasing new trypomastigotes to infect adjacent cells.
During the acute phase, lymphadenopathy, fever, myocarditis, hepatosplenomegaly, and central nervous system involvement are observed. The chronic stage may feature a latent period lasting several decades.
Immune Response and Microbiological Diagnostics
During the acute phase, high levels of protective and nonspecific IgM are produced, while IgG appears in the chronic phase. However, trypanosomes effectively evade immunity due to high antigenic variation (constantly switching surface glycoprotein variants). Autoimmune pathological reactions are also characteristic of trypanosomiasis.
Diagnostic Methods:
- Microscopy: examination of blood smears, CSF, or lymph node aspirates (Giemsa or Wright staining).
- Biological and culture methods: inoculation of laboratory rats/mice or culture on blood-supplemented nutrient media.
- Serology: ELISA, complement fixation test (CFT), and indirect immunofluorescence assay (IFA) to detect antibodies.
Principles of Treatment and Prevention
Treatment depends on the species and stage of the disease. For the African form, suramin or pentamidine are used. If the central nervous system is involved, melarsoprol (an arsenical drug capable of crossing the blood-brain barrier) is used. Treatment for Chagas disease (benznidazole, nifurtimox) is effective only in the acute phase.
Prevention includes vector control, elimination of animal reservoirs, and clearing vector breeding sites. For personal protection, insect repellents, protective clothing, window screens, and bed nets during sleep are recommended.