Trypanosomes

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 fission.

Depending on the stage of the life cycle, three main forms are distinguished:

• Epimastigotes (critidial stage). They inhabit the vector's gut. The flagellum originates from the middle of the cell, and the undulating membrane is poorly developed.
• Trypomastigotes (trypanosomal stage). They localize in the blood of vertebrates. The flagellum originates from the posterior end, and the membrane is prominently developed.
• Amastigotes (non-flagellated stage). Characteristic of T. cruzi. They have an oval shape 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 infection are distinguished:

1. Gambian (T. brucei gambiense). Characterized by a chronic course. The main reservoir is humans, with pigs as an additional reservoir.
2. Rhodesian (T. brucei rhodesiense). Proceeds acutely and severely. Animals (antelopes, cattle, goats) serve as the reservoir, and human infection is rare.

Pathogenesis: By the end of the first week, an ulcerating papule (the "trypanosomal" chancre) forms at the site of the bite. The causative agent enters the blood, multiplies, and disseminates into the lymph nodes and cerebrospinal fluid. Clinically, this manifests as fever, meningoencephalitis, progressive fatigue, exhaustion, and specific somnolence. Without treatment, the infection ends in death (although asymptomatic carriage is also possible).

American trypanosomiasis (Chagas disease)

The causative agent is T. cruzi. Triatomine ("kissing") bugs serve as vectors. Infection occurs not through the bite itself, but by rubbing the bug's feces into the wound or mucous membranes.

The incubation period lasts 1–3 weeks. A dense 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 multiply.
• Tissue phase: the parasite penetrates inside cells, turns into an amastigote, and actively divides by binary fission. The overcrowded cell is destroyed, and new trypomastigotes emerge to infect neighboring cells.

In the acute phase, lymphadenitis, fever, myocarditis, hepatosplenomegaly, and brain damage are observed. The chronic stage can proceed with a latent period lasting several decades.

Immune response and microbiological diagnostics

In the acute phase of the disease, a high level of protective and nonspecific IgM is produced; in the chronic phase, IgG appears. However, trypanosomes are able to effectively evade the immune system due to high antigenic variation (constantly changing antigenic variants). The development of autoimmune pathological reactions is also characteristic of trypanosomiases.

Diagnostic methods:

• Microscopy: examination of blood smears, cerebrospinal fluid, or lymph node aspirates (Romanowsky-Giemsa or Wright staining).
• Biological and culture methods: infection of laboratory rats/mice or inoculation onto nutrient media supplemented with blood.
• Serology: ELISA, complement fixation test (CFT), and indirect immunofluorescence assay (IFA) for antibody detection.

Principles of treatment and prevention

Treatment depends on the species of the pathogen and the stage of the disease. For the African form, suramin or pentamidine is used. If the central nervous system is affected, melarsoprol (an arsenic-based 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 the eradication of reservoir animals and vector breeding sites. For personal protection, the use of repellents, protective clothing, window screens, and bed nets during sleep is recommended.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Molecular mechanisms of trypanosome antigenic variation
• Pathogenesis of autoimmune processes in Chagas disease
• Mechanisms of T. brucei penetration across the blood-brain barrier
• Specifics of parasite cultivation on blood-based nutrient media
• Pharmacodynamics and toxicity of arsenic drugs (melarsoprol)

Other topics in "Flagellates (Leishmania, Trypanosoma, Giardia, Trichomonas)"

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