Causative agents and epidemiological forms
African trypanosomiasis is caused by two species of flagellated protozoa that inhabit the blood and tissues: Trypanosoma gambiense and Trypanosoma rhodesiense. Depending on the causative agent, two forms of the disease are distinguished:
- Gambian form (T. gambiense). This is an anthroponosis, meaning the primary source of infection is an infected human.
- Rhodesian form (T. rhodesiense). This is a zoonosis. The primary reservoir for the parasite is antelopes, while secondary reservoirs include other wild animals and livestock. This form is considered more dangerous. Human infection occurs less frequently, typically in natural foci where the pathogen circulates among ungulates.
Life cycle and transmission route
Trypanosomes undergo a complex life cycle involving alternation of hosts and morphological transformations. Human infection occurs via vector-borne transmission (inoculation) through the bite of the vector — the tsetse fly (Glossina sp.).
- In the tsetse fly: The insect becomes infected by feeding on the blood of an infected human or animal. The ingested blood forms of trypanosomes multiply in the midgut and then migrate to the salivary glands. There, they transform into epimastigotes, which actively divide and mature into metacyclic trypanosomes — this is the infective stage for mammals.
- In the human body: During a bite, the fly injects metacyclic forms with its saliva into the victim's bloodstream. Inside the new host, the parasites transform into blood-stage forms, spread throughout the body, and multiply by binary fission in the blood, lymph, and cerebrospinal fluid.
Clinical presentation
Sleeping sickness proceeds in stages.
Early stage begins with local manifestations: a characteristic reddish nodule, the so-called trypanosomal chancre, forms at the site of the infected fly bite. As the pathogen disseminates, general symptoms appear: irregular fever, lymph node enlargement (lymphadenopathy), and hepatosplenomegaly (enlargement of the liver and spleen). Patients complain of increasing weakness.
Late stage (neurological stage) develops when trypanosomes penetrate the cerebrospinal fluid and affect the central nervous system. It is characterized by severe neuropsychiatric disorders: mental depression, progressive somnolence (the patient may fall asleep even while eating), marked muscle weakness, and extreme emaciation (cachexia). Without treatment, the disease inevitably results in death.
Briefly: answers to topical questions
What morphological forms does the trypanosome pass through in the tsetse fly?
In the tsetse fly, the trypanosome goes through several sequential developmental stages. When feeding on blood, the vector ingests the initial forms of the parasite, which then transform.
- Trypomastigotes — the initial trypanosome form entering the vector's gut.
- Epimastigotes (critidial form) — the stage into which trypanosomes transform in the salivary glands and actively multiply.
- Metacyclic trypomastigotes — the infective form into which epimastigotes mature for subsequent infection of the vertebrate host.
What drugs are used for etiotropic treatment of the Gambian and Rhodesian forms?
For the etiotropic treatment of African trypanosomiasis, specific antiparallel agents are used, the choice of which depends on the stage of the disease and the type of pathogen.
- Suramin — one of the primary drugs of choice.
- Pentamidine — used for treating early stages (prior to CNS involvement) caused by Trypanosoma gambiense and certain strains of Trypanosoma rhodesiense.
- Melarsoprol — an arsenical drug that crosses the blood-brain barrier; used for CNS involvement.
- Eflornithine — a less toxic alternative to melarsoprol, which is highly effective for the Gambian form but ineffective for the Rhodesian form.
Class and exam questions
Which stage of trypanosome is infective for humans?
The infective stage is represented by metacyclic trypanosomal forms that accumulate in the salivary glands of the tsetse fly.
How does sleeping sickness transmission occur?
Infection occurs via vector-borne transmission (inoculation) through the bite of an infected tsetse fly.
What is the difference between the Gambian and Rhodesian forms?
The Gambian form is an anthroponosis (the source of infection is humans), while the Rhodesian form is a zoonosis (the primary reservoir is antelopes). The Rhodesian form is more severe and considered more dangerous.
What else to study on this topic
- Morphological differences of trypanosome stages
- Features of the immune response in trypanosomiasis
- Laboratory diagnostic methods
- Specific therapy for sleeping sickness
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