Notes on Trypanosoma brucei and Human African Trypanosomiasis (HAT)

Introduction

  • Focus of midterms: parasitic protozoans; all the parasitic protozoans discussed are endoparasites.

  • Protozoans are unicellular microorganisms capable of many life processes (e.g., aerobic respiration).

  • Classification overview:

    • Free-living protozoans: can survive without a host.

    • Parasitic protozoans: cannot survive without a host; further divided into facultative parasites or obligate, permanent, or accidental parasites.

    • All discussed parasitic protozoans in this lecture are endoparasites.

  • For this set of notes, we concentrate on the class that includes Trypanosomatida and, within that, the genus Trypanosoma.

Taxonomy and focus of the lesson

  • Class: likely a mis-spelled reference to the group containing parasitic flagellates; four orders discussed under the umbrella of this class:

    • Retinotoplastica

    • Retorta monadida

    • Diplomona dida

    • Trichomonadida

  • Under order: Trypanosoma (the focus of this video).

  • Family: Trypanosomatidae (as mentioned in the transcript).

  • Suborder: Trypanosomatina (as mentioned in the transcript).

  • This video specifically discusses Trypanosoma, within the order Trypanosomatida.

Morphology of Trypanosoma (key identifying features)

  • General shape: slender, elongated, often forming a curved (C-shaped) body.

  • Movement structure: undulating membrane with a flagellum.

    • Flagellum originates near the posterior end, runs toward the anterior, and extends outward as a whip-like locomotive structure.

  • Kinetoplast: a dark, small structure situated near the posterior end; contains mitochondrial DNA. This is a distinctive feature used for classification within Trypanosoma.

  • Nucleus: typically centrally located within the cell.

  • Cytoplasm: granulated with vacuoles.

  • Overall appearance: darkly stained, larger cell with a defined kinetoplast and nucleus.

Reproduction

  • Primary mode: binary fission (asexual).

    • Process (longitudinal division):
      1) The kinetoplast duplicates.
      2) The new flagellum develops.
      3) Nuclear division occurs (mitosis), yielding two nuclei.
      4) Cytokinesis happens, producing two identical daughter cells containing complementary organelles (nucleus, kinetoplast, flagella, undulating membrane).

  • Result: two daughter Trypanosoma (identical to parent in structure).

  • Important note: during division, you observe two kinetoplasts, two flagella, and two nuclei in the parent cell before cytokinesis.

Nutrition and energy metabolism

  • Typical nourishment sources when in host: blood plasma, cerebrospinal fluid (CSF), products of cellular disintegration.

  • Blood plasma components accessed by the parasite: glucose, amino acids, lipids, proteins.

  • Energy metabolism context:

    • Trypanosoma spp. utilize aerobic respiration and especially aerobic glycolysis in glucose-rich environments (e.g., bloodstream).

    • In the bloodstream, nutrients (notably glucose) are absorbed directly through the parasite’s cell membrane.

    • Implication: infection can be associated with hypoglycemia due to glucose consumption by the parasite.

  • Lymphatic system: parasites spread from initial bite site into the lymphatic system, which provides a sustained nutrient supply (proteins, lipids, white blood cells) to support replication and dissemination.

  • Involvement of tissues and organs: invasion leads to cellular damage and release of intracellular contents that parasite can scavenge for energy and building blocks.

Life cycle overview (host-vector dynamics)

  • Primary life cycle involves two hosts: insect vector (tsetse fly, Glossina spp.) and mammalian host (humans or other mammals).

  • Infective stage for mammals: metacyclic trypomastigotes injected by the bite of an infected tsetse fly.

  • In the mammalian host:

    • Metacyclic trypomastigotes enter the bloodstream and transform into bloodstream forms (trypomastigotes).

    • They multiply by binary fission in blood, lymph, and CSF (diagnostic stage when circulating in blood).

    • Circulation of slender and stumpy trypomastigotes occurs; slender form is highly proliferative; stumpy form is shorter and pre-adapted for uptake by the fly.

  • Insect vector (tsetse fly) cycle:

    • Ingested blood containing stumpy trypomastigotes from an infected mammal.

    • In the midgut, they transform into procyclic trypomastigotes and multiply by binary fission.

    • Procyclic forms leave the midgut and transform into epimastigotes in the fly’s gut or salivary glands.

    • Epimastigotes multiply in the salivary glands and transform into metacyclic trypomastigotes, which are the infective stage for new mammalian hosts when the fly bites again.

  • Summary of infective cycle: Metacyclic trypomastigotes injected -> bloodstream slender trypomastigotes replicate -> stumpy forms taken up by fly -> midgut procyclics -> salivary gland epimastigotes -> metacyclic trypomastigotes in saliva -> new mammalian host infected.

  • Diagnostic and clinical correlation:

    • Acute phase: parasites circulate in blood; diagnostic stage is when parasitemia is detectable in blood.

    • Latent or dilated phases refer to periods when parasites may be less detectable in blood but not completely cleared.

Forms observed in humans and in vectors (life-cycle forms)

  • In humans (mammalian host):

    • Slender trypomastigotes: highly proliferative, responsive for initial expansion of parasite population.

    • Stumpy trypomastigotes: shorter, pre-adapted for uptake by the tsetse fly; circulate at high levels but are less proliferative.

  • In the insect (tsetse fly):

    • Procyclic trypomastigotes: form in the midgut; feed on glucose for energy; replicative stage in midgut.

    • Epimastigotes: in the salivary glands; replicative stage.

    • Metacyclic trypomastigotes: final infective form that is transmitted to mammals via saliva during a bite.

  • Other forms cataloged in life-cycle schematics (less common in some texts):

    • Amastigotes, koanomastigotes, promastigotes, epimastigotes, metacyclic forms; these terms reflect stages seen in various Trypanosoma spp. life cycles, with the transcript noting them as part of the illustrative development forms.

Insect vector and hosts (Glossina spp.)

  • Primary vector genus: Glossina (tsetse fly).

  • Vector species mentioned (and their contexts):

    • Glossina palpalis

    • Glossina tachinoides

    • Glossina morsitans

  • Transmission ecology by form:

    • Gambiense (West/Central Africa): human reservoir predominates; often associated with chronic sleeping sickness; tends to be maintained in human populations.

    • Rhodesiense (East/Southern Africa): animal reservoirs predominate (domesticated and wild animals, notably cattle and wildlife); can lead to more acute disease in humans.

  • Reservoirs:

    • Gambiense: humans are the main reservoir.

    • Rhodesiense: cattle and other animals serve as reservoirs; zoonotic potential is notable.

  • Geographic distinctions (epidemiology):

    • Gambiense: endemic in West and Central Africa; constant presence in humans.

    • Rhodesiense: endemic in East and Southern Africa; more animal reservoir-driven transmission.

Diseases caused and clinical forms

  • Disease: Human African Trypanosomiasis (HAT), also known as African sleeping sickness.

  • Forms by geographic/clinical presentation:

    • Gambiense (West Africa): chronic sleeping sickness; slower progression.

    • Rhodesiense (Rhodesian, East Africa): acute sleeping sickness; rapid progression.

  • Disease progression and signs:

    • Initial or early symptoms: fever, headache, joint and muscle pains, rashes at bite site (reddish sores or cutaneous signs).

    • Hematologic effects: microcytic anemia (small red blood cells); leukocytosis (increased white blood cells) as part of immune response.

    • Lymphatic involvement: superficial lymph node swelling, notably Winterbottom's sign (posterior cervical lymphadenopathy).

    • Progressive invasion of the central nervous system (CNS): blood-brain barrier compromise leads to perivascular infiltration by lymphocytes and plasma cells; encephalitis and meningomyelitis with inflammation of meninges; neurological symptoms such as confusion, personality changes, slurred speech, lethargy, and progression to coma if untreated.

    • Severe stages: coma, death, often due to primary disease or intercurrent infections (malaria, dysentery, pneumonia, malnutrition).

  • Prognosis:

    • Possible recovery if treated before nervous system involvement; prognosis worsens significantly after CNS invasion.

Diagnosis (methods and clues)

  • Gross examination clues:

    • Painless chancre at bite site (painless sore).

    • Winterbottom's sign (swollen posterior cervical lymph nodes).

  • Laboratory diagnosis:

    • Blood smear to detect circulating trypomastigotes during acute phase.

    • CSF examination (in late or CNS involvement) to assess CNS infection.

    • Lymph node aspirates or other tissue aspirates.

  • Molecular diagnostics (confirmatory/supportive):

    • PCR (polymerase chain reaction) to detect parasite DNA.

    • LAMP (loop-mediated isothermal amplification) as a rapid and field-friendly method.

  • Clinical decision framework:

    • Acute phase: parasitemia detectable in blood; diagnosis via blood smear.

    • CNS involvement: CSF findings crucial; diagnosis supported by molecular tests.

Treatment (drugs and stage-specific therapy)

  • First-stage (before CNS involvement):

    • Pentamidine (used for gambiense form of HAT in early stage).

    • Ceramine (referenced in transcript; historically used in some contexts; note potential spelling variation in sources).

  • Second-stage (CNS involvement):

    • Nifurtimox-eflornithine combination therapy (NECT): eflornithine IV plus nifurtimox orally; highly effective for CNS involvement.

    • Eflornithine alone (alternative in some guidelines).

    • Melarsoprol: effective in both stages but highly toxic; IV administration; used when other options are unavailable or ineffective.

  • Special considerations:

    • Ceramine/melarsoprol require careful monitoring for side effects due to toxicity.

    • NECT preferred for its balance of efficacy and safety in CNS disease.

  • Stage-specific rationale:

    • Bloodstream parasites without CNS invasion can be targeted by drugs that do not need BBB penetration.

    • Once parasites are in CSF, drugs must cross the blood-brain barrier to be effective.

Prevention and control

  • Personal protective measures:

    • Use insect repellents.

    • Wear long sleeves and clothing to reduce skin exposure.

    • Avoid known endemic areas when possible.

  • Vector control measures:

    • Traps to capture or kill Glossina flies.

    • Insecticide spraying in habitats; baiting animals with insecticides.

    • Ethical considerations involving animal welfare in vector control strategies.

  • Case detection and treatment as a control strategy:

    • Identify infected humans and animals and treat promptly to reduce reservoir and interrupt transmission cycles.

  • Public health implications:

    • Control efforts aim to reduce transmission by breaking the cycle: human/animal reservoir ↔ fly vector ↔ human/animal host.

Epidemiology and geographic distribution

  • Endemic regions and country counts:

    • Gambiense: West and Central Africa; endemic in about 24 countries including Angola, Cameroon, Central African Republic, Chad, Congo, Democratic Republic of Congo, etc.

    • Rhodesiense: East and Southern Africa; endemic in about 13 countries including Burundi, Ethiopia, Zimbabwe, etc.

  • Transmission cycle summary:

    • Cyclical transmission: human or mammal → tsetse fly → new human or mammal.

    • Primary reservoir varies by form (humans for gambiense; animals for rhodesiense).

  • Notes on transmission routes:

    • Vector-borne transmission via tsetse fly bite is primary.

    • Less common routes include sexual transmission (coitus) and congenital transmission (mother to fetus/newborn).

Key life-cycle sequence (concise recall aid)

  • Start with an infected tsetse fly taking a blood meal and injecting metacyclic trypomastigotes into the mammalian host.

  • In the mammalian host: metacyclic trypomastigotes transform into bloodstream slender trypomastigotes → multiply by binary fission in blood/lymph/CSF → circulate as slender and stumpy forms.

  • Tsetse fly ingests bloodstream trypomastigotes (predominantly stumpy forms).

  • In the fly midgut: transform into procyclic trypomastigotes → multiply.

  • Move to the salivary glands: transform into epimastigotes → multiply.

  • In salivary glands: transform into metacyclic trypomastigotes (infective form).

  • New mammalian host infection occurs when the fly bites again, injecting metacyclic trypomastigotes.

Remarks on terminology and next topics

  • The speaker indicates that the next video would cover Trypanosoma cruzi (the agent of American trypanosomiasis, Chagas disease).

  • Overall emphasis: comparative morphology, life-cycle forms, pathogenesis, diagnosis, treatment, prevention, and epidemiology of Trypanosoma brucei spp. (causing African sleeping sickness).