Parasitology - Blood and Tissue Protozoa (Trypanosoma) Flashcards
Outline Trypanosomes.
- Trypanosomes have a complex life cycle involving an insect vector.
- The diseases that are caused in man, African trypanosomiasis (sleeping sickness) and South American trypanosomiasis (Chagas’ disease), are restricted in distribution according to the habitat of the insect host.
- African sleeping sickness is caused by trypanosomes that are subspecies of Trypanosoma brucei, spread by the tsetse fly. The human parasites are T. brucei gambiense, which occurs in riverine areas of west and central Africa, and T. brucei rhodesiense, a parasite of the savannah plains of east Africa.
- Chagas’ disease is caused by Trypanosoma cruzi, spread by the kissing bug.
Pictured: trypomastigotes.
What is the vector for African trypanosomiasis (sleeping sickness)?
The tsetse fly
What is the vector for American trypanosomiasis (Chagas’ disease)?
The kissing bug
What is the causative agent for African trypanosomiasis (sleeping sickness)?
Trypanosoma brucei subspecies:
The human parasites are T. brucei gambiense, which occurs in riverine areas of west and central Africa, and T. brucei rhodesiense, a parasite of the savannah plains of east Africa
What is the causative agent for American trypanosomiasis (Chagas’ disease)?
Trypanosoma cruzi
Outline African trypanosomiasis (sleeping sickness).
- The human parasites are T. brucei gambiense, which occurs in riverine areas of west and central Africa, and T. brucei rhodesiense, a parasite of the savannah plains of east Africa, where cattle and wild antelope act as reservoirs of infection.
- 6,000 to 10,000 human cases are documented annually
- 35 million people affected
- Regional epidemics of the disease are cause of major health and economic disasters
- Following the bite of an infected tsetse fly, a localized trypanosomal chancre may appear transiently, but invasion of the bloodstream rapidly occurs.
Describe the lifecycle of Tryposoma brucei.
- Tsetse fly takes blood meal, injecting metacyclic trypomastigotes
- Injected metacyclic trypomastigotes transform into bloodstream trypomastigotes, which are carried to other sites
- Trypomastigotes multiply by binary fission in various body fluids, e.g. blood, lymph, CNS
- Trypomastigotes in blood
- Tsetse fly takes blood meal, bloodstream trypomastigotes are ingested
- Bloodstream trypomastigotes transform into procyclic trypomastigotes in tsetse fly’s midgut. Procyclic trypomastigotes multiply by binary fission.
- Procyclic trypomastigotes leave the midgut and transform into epimastigotes
- Epimastigotes multiply in the salivary gland. They transform into metacyclic trypomastigotes.
Outline the morphology of Trypanosoma.
What are trypomastigotes?
This stage is found in all species of Trypanosoma, and in most species it is the only stage that reproduces in the vertebrate (human) host.
Reproduction in blood, lymph, spinal fluid by fission.
Flagellum and undulating membrane.
How does Trypanosoma brucei evade the immune response?
Antigenic variation.
The Surface Coat of every individual trypanosome is formed by 10 million copies of a single molecular species of antigen: Variant Surface Glycoprotein (VSG). It is the frequent switching of the VSG forming the Surface Coat that accounts for antigenic variation.
Trypanosoma brucei contains hundreds of VSG Genes, accounting for about 10% of all its genes.
If this system could be disrupted, our immune systems might be able to eliminate trypanosome infections
What are the three stages of African trypanosomiasis (sleeping sickness)?
- Bite reaction (chancre: pictured.
A nonpustular, painful, itchy chancre, ulcer that forms 1-3 weeks after the bite and lasts 1-2 weeks. It leaves no scar)
- Parasitemia (blood and lymphoid tissue infection)
- CNS stage
What are changes that can be observed in a person in stage three of African trypanosomiasis?
Stage three: CNS involvement.
- Lack of interest and disinclination to work
- avoidance of acquantances, melancholic attitude alternating with exaltation, mental retardation and lethargy (lack of energy)
- Low and tremulous (shaking) speech, tremors of tongue and limbs, slow shuffling gait, altered reflexes
- Males become impotent
- Slow progressive involvement of cardiac tissue
- Drowsiness and uncontrollable urge to sleep
- Terminal stage marked by wasting and emaciation (abnormally thin)
- Death results from coma, infection, or cardiac failure
How is African trypanosomiasis (sleeping sickness) diagnosed?
- Detection of parasite in the bloodstream, lymph secretions and enlarged lymph node aspirate provides definitive diagnosis in early (acute) stages.
- Bloodstream parasite can be concentrated by centrifugation or by the use of anionic support media
- CSF examination
- ELISA for definite diagnosis
Serological tests available today are only useful for screening T.b.gambiense infections and establishing suspicion of infection. Confirmation of infection requires the performance of parasitological tests to demonstrate the presence of trypanosomes in the patient.
However, the number of parasites can be so low (mainly in the gambiense form of the disease) that available parasitological methods may not be sensitive enough to find them.
A negative parasitological result in the presence of a positive serological test does not necessarily indicate absence of infection, and tests may have to be repeated over time to achieve diagnosis.
How is African trypanosomiasis treated and prevented?
The blood stage of African trypanosomiasis can be treated with reasonable success with Pentamidine or Suramin
CNS involvement should be treated with Melarsoprol, an organic arsenic compound, and eflornithine (T. gambiense)
Avoid contact with tsetse fly, immunisation not effective due to antigenic variation
In April 2009, WHO added the combination of eflornithine and nifurtimox (currently used to treat American trypanosomiasis) on its ‘Essential Medicines List’ to improve case management of human African trypanosomiasis.
Clinical trials have confirmed that a combination of both drugs reduces the duration of eflornithine monotherapy treatment and is easier to administer, while maintaining the same level of efficacy and safety.
Outline America trypanosomiasis (Chagas’ disease).
- Caused by Trypanosoma cruzi.
- The insect vectors are various species of kissing bugs. The trypanosomes are present in the bug’s faeces, which the unwitting sleeper rubs into the bite wound.
- They do not multiply in the bloodstream, but invade cells of the reticulo-endothelial system (phagocyte system) and muscle, where they lose their flagellum and associated undulating membrane and adopt a more rounded shape.
- This morphological form is called an amastigote.
- The amastigotes multiply in muscle and are liberated from ruptured cells as trypanosomal forms (trypomastigotes), which disseminate the infection and provide the parasitaemia needed to infect fresh reduviid bugs when they next feed.
- Found in Central and South America, stretching from parts of Mexico to Argentina.
- 7-8 million people are estimated to be infected and about 50,000 people die each year from the disease
How can the trypomastigotes of T. cruzi and T. brucei be told appart?
Trypomastigotes of T. cruzi are shorter than those of the T. brucei group and have a characteristically large kinetoplast.
What is shown in this image?
Amastigotes of T. cruzi in heart muscle
Describe the lifecycle of Tryposoma cruzi.
- Kissing bug takes blood meal, passing the metacyclic trypomastigotes in faeces, trypomastigotes enter bite wound or muscosal membranes, such as the conjunctiva.
- Metacyclic trypomastigotes penetrate various cells at bite wound site. Inside cells they transform into amastigotes.
- Amastigotes multiply by binary fission in cells of infected tissue.
- Intracellular amastigotes transform into trypomastigotes, then burst out of the cell and enter the bloodstream. Trypomastigotes can infect other cells and transform into intracellular amastigotes in new infection sites.
- Kissing bug takes a blood meal, trypomastigotes are ingested.
- Epimastigote stage in midgut.
- Multiplies in midgut.
- Metacyclic trypomastigotes in hindgut.
What are the three forms that Tryposoma cruzi can be found in?
- Trypomastigote
- Amastigote
- Epimastigote
How does T. cruzi (American tryposomiasis/ Chagas’ disease) escape the immune response?
- Binding: Trypomastigotes bind to the cell surface (e.g. a monocyte) via a variety of receptor proteins including fibronectin. Sialic acid is important as cells deficient in sialic acid are not penetrated. Interestingly, T. cruzi has an enzyme called trans-sialidase which actually puts more sialic acid onto cells, thereby enhancing uptake.
- Entry into the cell: Uptake is by a process of induced phagocytosis. Lysosomes migrate to the cell surface when cells come in contact with T. cruzi and the parasite enters a cytoplasmic vacuole, the parasitophorous vacuole, that is formed from a lysosome. T. cruzi releases a protein toxin that inserts into the membrane of the lysosome. As a result T. cruzi escapes into the cytoplasm as the lysosomal membrane is destroyed.
What are the three stages of American tryposomiasis (Chagas’ disease)? Give a breif description of each stage.
- Primary lesion (Chagoma: pictured;
Ramana’s sign: unilateral conjunctivitis and orbital oedema)
* Primary lesion (chagoma) appears soon after bite as raised, erythematous plaque surrounded by hard oedema. * Usually found on the face, eyelids, cheek, lips or the conjunctiva, but may occur on the abdomen or limbs. * When the primary chagoma is on the face, there is an enlargement of the pre- and post- auricular and the submaxillary glands on the side of the bite. * Infection in the eyelid, resulting in a unilateral conjunctivitis and orbital oedema (Ramana's sign) is the commonest finding.
-
Acute phase
- The acute stage appears 7-14 days after infection.
- It is characterized by restlessness, sleeplessness, malaise, increasing exhaustion, chills, fever and bone and muscle pains.
- There is a general oedematous reaction associated with lymphadenopathy. Diffuse myocarditis, sometimes accompanied by serious pericarditis and endocarditis, is very frequent during the initial stage of the disease.
- In children, Chagas’ disease may cause meningo-encephalitis and coma. Death occurs in 5-10 % of infants. Hematologic examination reveals lymphocytosis and parasitemia.
-
Chronic phase
- 10-20% of victims develop a chronic disease.
- They alternate between asymptomatic remission periods and relapses characterized by symptoms seen in the acute phase.
- Cardiac arrhythmia is common.
- The chronic disease results in an abnormal function of the hollow organs (heart, oesophagus and colon).
- The cardiac changes include myocardial insufficiency, cardiomegaly, disturbances of atrio-ventricular conduction and the Adams-Stoke syndrome (episode of syncope).
- Disturbances of peristalsis lead to megaesophagus and megacolon.
What are the differences in morphology of the different forms of trypomastigotes?
