Protozoa-Flagellates/Chagas disease Flashcards
What are the flagellates?
Protozoa with one or many flagellae.
Flagella are similar to cilia, except that they are longer and only a few are found per cell.
The flagellum originates at the basal body and consists of central pair of microtubules surrounded by 9 microtubule doublets.
The motor protein dynein is attached along one side of the microtubule and moves along following hydrolysis of ATP. This causes the flagella to beat and the cell moves.
What are the kinetoplastids? What are the three main species?
Group of free-living protozoa with flagella.
Include three species that cause disease in humans, all of which parasitize blood or tissues of human and have insect vectors –>
- African trypanosomes: T. brucei, T. congolense, T. vivax - causes sleeping sickness.
- Trypanosoma cruzi: causes Chagas disease in humans
- Leishmania spp
Describe Chagas disease - its parasite, vector, epidemiology
- American trypanosomiasis caused by the T. cruzi which infects a wide range of mammals.
- Transmitted by several species of the triatomine bug via blood feeding (‘kissing bugs’ due to its tendency to bite around the face).
- Most important triatomine species are Triatoma infestans and Rhodnius prolixus.
- Zoonotic disease with many reservoir host species, including rodents, armadillos and monkeys.
Epidemiology –> Chagas disease is distributed across the Americas, southern USA to southern Argentina.
- Usually found in poor rural areas and it’s the leading cause of heart disease in Latin America.
- Estimated up to 18 million people infected.
Describe life cycle of Trypanosoma cruzi
- T. cruzi is transmitted to humans via blood feeding by the triatomine bug. They are relatively large bugs that tend to feed at night when human is sleeping.
- As it’s taking the blood meal, the bug defecates and releases metacyclic trypomastigotes (which develop in its hindgut) which can then enter the bloodstream.
- They can also enter via the mucosal membranes in the eye – often enter when rubbed into eye.
In the human:-
- The trypomastigote then invades a host cell and becomes intracellular. Inside the cell it will convert into the amastigote form.
- The amastigote will replicate via binary fission in the cytoplasm of the host cell and its progeny will fill up the cell.
- After several rounds of division, these amastigotes can transform back into trypomastigotes and be released from the infected cells. They then travel through blood and can repeat cycles of division.
- The trypomastigotes can also be taken up by the triatomine bug during feeding.
In the triatomine bug:-
- In the bug’s midgut they will convert into epimastigotes and undergo multiple rounds of binary fission.
- Some of the epimastigotes will then travel to the hindgut and differentiate into metacyclic trypomastigotes where they will be excreted in the faeces.
Describe the vectors for Chagas disease
two species - Triatoma infestans and Rhodnius prolixus
- They reside in houses of rural areas where they take shelter in roof and walls.
- Each bug takes up to 1ml of blood during a feed – can cause chronic anemia.
- Surface of parasite is covered in thick glycoprotein which allows it to bind to the triatomine’s gut membrane, essential for its development.
- Because transmission via defecation is relatively inefficient compared to the African trypanosomes, a lot of contact is needed.
Describe the stages of Chagas disease
Acute infection - often asymptomatic or mild symptoms:
- A hallmark is Romana’s eye: a swelling of the eye characterized by conjunctivitis, when the parasite enters the eye.
- Skin lesion known as chagomas, if it enters through skin.
- Symptoms of a systemic infection can also appear: fever, fatigue, abdominal pain, diarrhea, enlarged liver/spleen, swollen lymph nodes, heart failure.
Indeterminate stage – latent period
– Absence of any overt clinical symptoms and can last for decades.
Chronic stage - 30-40% will progress to this stage 10-20 years after infection
– 60% are asymptomatic
- Cardiac problems (30%) – all parts of the heart can be affected. Include arrhythmias, cardiomegaly (enlarged heart – due to hypertrophy and dilation) and heart failure. The cardiac nerves are frequently affecting leading to abnormal ECG.
- Gastrointestinal problems (10%) – characterized by enlargement of the GI tract, most commonly in esophagus and colon. This leads to problems swallowing, prolonged constipation and heartburn. It’s thought to be due to neuronal dysfunction.
- –> The basis of Chagas pathogenesis is inflammatory-mediated damage of parasympathetic nerves in heart and digestive system.
What are the methods of prevention and control against Chagas disease?
- Avoid sleeping in mud or thatch houses: Because a lot of vector-human contact is required, high levels are associated with houses becoming infected with triatomine bugs.
- -Thatched roofs and mud houses provide ideal habitats.
- Housing improvements: crack free walls, cement floor, metal roof, seal windows and doors. Poor housing is primary risk factor for Chagas disease – houses with cracked walls etc allow triatomine to infest house.
- -Having domestic animals living in or near house will allow a source of a reservoir host for the parasite.
- Destruction of natural habitat of triatomine bug in the forest led to the bugs to occupy human habitats.
- Using insecticide to kill triatomine bugs
- Blood transfusion screening - this is second most common mode of transmission. Because of the long latent period, the blood donor may be unaware they are infected. Therefore blood donors from endemic areas should be screened for T. cruzi infections.
- –In South America, contamination of blood banks with T.cruzi was more common than HIV and hepatitis.
- In Ecuador, serological screening was introduced in 2003.
What was the southern core initiative?
A program launched in 1991 by six governments in South America.
- The goal was to restrict transmission of T. cruzi by eliminating the T. infestans from domestic settings.
- Control efforts consisted of spraying (uninfested) houses with insecticide.
- Blood donors were also screened to prevent transmission via transfusion.
- This was very successful and led to reduction in disease prevalence and eradication in Uruguay, Chile and large parts of Brazil.
- The success of this initiative was mainly due to the vector being exclusively domestic and that its slow reproduction and variability meant that it had low selection for insecticide resistance.
- Two other initiatives have been set up as result of SCI’s success: Andean pact and central American initiative which mainly target R. prolixus as it’s the main vector there.
List the two drugs approved for Chagas and outline their mechanisms of action.
The two approved drugs for Chagas are Nifurtimox and Benznidazole.
– Nifurtimox - introduced in 1965, it acts by generating ROS and inhibiting certain essential enzymes. Adverse side effects due to damage to host tissues: nausea, vomiting, weight loss, abdominal pain, muscle ache, neuropathy, insomnia.
– Benznidazole - introduced in 1971, it acts by inhibiting protein and RNA synthesis in trypanosomes. Side effects - nausea, vomiting, insomnia, rashes etc.
–> both are not that effective for chronic stage of the disease and the side effects (in 2/3 of patients) mean that adherence is poor.
What are the potential drug targets for Chagas disease?
Reductive metabolism –> the reducing thiol-based redox reactions based on glutathione are essential for detoxifying free radicals.
- However in kinetoplastids, an unusual form of glutathione called trypanothione (which is 2 glutathiones joined together by a spermidine group) replaces the glutathione system for producing reducing equivalents.
- -The uniqueness of this system poses as a good drug target.
Ergosterol bioysynthesis –> Cytochrome P-450-dependent C14 sterol demethylase is the enzyme that catalyses the major sterol compounds.
- Sterols are required for a lot of cell physiology. (Inhibition of sterol synthesis - cannot be replaced by host sterols).
- -Drugs acting in this way have shown promise against acute and chronic stages
– The parasite’s Cysteine protease –> Cruzipain [the major cysteine protease in T. cruzi – also potential antigen for vaccine development]
Purine salvage –> Hypoxanthine-guanine phosphoribosyl transferase (HGPRT) is an important enzyme in purine salvage pathway.
this is due to a lack of de novo synthesis in parasite - relies on host for preformed purines; also in Leishmania.
Purine salvage is a pathway in which purines are synthesized from intermediates in degradative pathway of RNA/DNA.
What are the novel methods for vector control against Chagas disease?
- Transgenic methods of genetic manipulation
- - Bacteria (Rhodococcus rhodnii) in gut of Rhodnius help the insect to get necessary nutrients from blood
- - the parasite obtain this bacteria from eating faeces of nearby adults
- - “Cruzigard”, a kind of fake faeces, is inoculated with genetically modified bacteria. When placed on walls of houses (where adults rest after feeding), it is taken up by the parasitic larvae.
- - Bacteria can be transformed with cecropin A (a gene of the immune response) to kill parasites in gut [cercropin A is a peptide that lyses cells] - Biopesticides for control of vector of Chagas –>
Isolates of a pathogenic fungal species, Beauveria bassiana, have some potential.
