South American trypanosomiasis and triatomines Flashcards
Describe the lifecycle of T cruzi.
- Epimastigotes divide in triatomine stomach (clonal reproduction)
- Differentiate into infective metacyclics in digestive tract
- Deposited on skin via faeces
- Penetrate skin through breaks or mucosal membranes
- Infects any cell type (but prefers skeletal and cardiac muscle)
- Pseudocysts form
- Differentiate into amastigotes
- Division so that pseudocysts contain up to 400 parasites
- Lyse and release trypomastigotes into the blood
What is the kinetoplast?
Mass of mitochondrial DNA, lots of circular DNA. Maxicircles and minicircles.
How long between pseudocyst rupturing to release trypomastigotes until they can be taken up for further transmission?
2 weeks.
Why is microscopy diagnosis not so good?
Because the parasites are mainly intracellular and blood parasitemia is low.
What are the acute and chronic presentations of chagas?
Acute: non-specific symptoms such as flu like symptoms, malaise
Chronic: mega organs e.g. mega colon, heart distension leads to failure.
What is a romana’s sign?
An inflammation of the eye
What is a chagoma?
A local inflammation at the site of infection. (reaction to insect bite as well as parasite).
What may explain why not everyone presents with a chagoma?
Different parasite strains and differences in immune response.
What is thought to be the cause of pathology and why is this contentious?
Thought to be due to the direct presence of the parasite. However, potentially not becuase some people show pathology despite the apparent absence of amastigote nests.
What do parasites secrete?
Antibodies such as cruzipain. This elicits a powerful immune response and is thought to cause pathology.
How is chagas diagnosed?
Gold standard is ELISA. Supplemented with pcr of k dna
Xenodignosis (feed on patient and check gut contents).
other:
- dipstick
- rapid diagnostic tests-immunographic
- indirect immunofluorescence test
WHy do we have to be careful with PCR of kDNA?
as there are thousands of copies, it can be overly sensitive and cross contamination if not careful can ruin results.
Why is xenodiagnosis good?
CAn reclaim the parasite and culture the parasites taken from insects so we can see their full life cycle.
WHat is the issue with peptide based test diagnosis?
Variable results, such as with malaria. QUality control issues. Sensitivity and specificity change with location (may be because parasite strains are very genetically diverse))
How is Chagas treated? HOW are these treatments ineffective?
Benznidazole, nifurtamox. Both prolonged treatments and dose is dependent on age and physical health because they are so toxic (only 40-60% of patients can finish the course).
Doesn’t work as well as parasites enter tissues from the blood as the dose must be increased and this increases the associated side effect/toxicity.
What are the three main genera and species of triatomine bug?
Rhodnius prolixus, Triatoma infestans, and Panstrongylus megistus.
Which of the species of triatomine bugs are most medically important
T infestans and R prolixus.
What are the feeding preferences of the triatomines?
Depending on the species, some prefer narrow host ranges whereas some are more promiscuous.
What makes a triatomine a competent vector?
It defecates as it feeds.
How many nymphal stages are there and what is unusual about this?
- Require a full blood meal before each metamorphosis/ transition into the next larval stage.
Why can people become anaemic in endemic areas?
The sheer number of bugs means that a lot of blood is sometimes taken.
Why is chagas more infective in tropical areas?
The climate means the faeces doesn’t dry up as quickly (which would kill the fragile parasites) so they are infective for longer.
Why are older bugs more likely to be infective than younger ones?
Despite newly born bugs having potential to be infective (as they are immediately haematophagic), as K strategists they live up to 2 years (species dependent) so the older they are, the more likely they will have picked up an infection.
When do triatomine bugs fly?
Don’t really like flying so only will when they are starving and looking for food.
How are triatomine bugs caught?
Noireau traps- put with gauze over and mouse in the middle as bait.
What are the preferred climates for T infestans and R prolixus?
T infestans prefers dryer arid areas (southern S America?)
R prolixus likes more tropical regions (north S America, Brazil?)
What are the two types of transmission cycle?
Overlapping or separate transmission cycles.
Describe the separate transmission cycle?Give an example.
Domestic and wild cycles do not overlap. E.g. T infestans is almost exclusively associated with domestic environments.
Describe the overlapping transmission cycle. Give an example.
R prolixus.
Feeds in sylvatic environments (e.g. palm trees) as well as domestic environments (e.g. bringing palm leaves into the house for thatching the roof).
How does the transmission cycle type inform control strategies?
Separate transmission cycles are more likely to be able to be interrupted by spraying as getting rid of the domestic population means re infection is unlikely whereas spraying indoors is unlikely to prevent reinfection in overlapping cycles.
Describe the most well-known control initiative. What were its objectives? What were the phases of the program?
Southern cone initiative. Argentina, Bolivia, Brazil, Chile, Paraguay and Uruguay.
Objectives: to interrupt vectorial and transfusional transmission.
Phases: planning, mapping and sampling, attack (spraying all houses and outhouses every 3-6 months if still infected), vigilance (rapid respraying if found again).
How many lineages of T cruzi? What is the significance of this?
- TcI-VI.
Each lineage has different transmission cycles/ methods.
Which lineage has been associated with Didelphis (Latin American marsupial)?
TcI
Which lineage is associated with armadillos and ground-living triatomines (panstrongylus)?
TcIII
Which lineages are the most important and why?
TcV and VI because they are the most overwhelmingly represented in human cycles by infecting T infestans.
How can parasites hide?
Initial population explosion, followed by a decline in number so you think you are cured but they are hiding (e.g. in the digestive tract) and can reemerge e.g. if immunocompromised.
What is isoenzyme characterisation?
Spin down parasite and extract enzymes. Run lysate on gel with the substrate on and enzymes will react with their defined substrates and produce bands. Gives an indication of genetic diversity.
Are parasites clonal?
Thought not! Can genetically recombine.
What is gene drive? Why is this relevant?
The idea that altered genes are almost always inherited due to gene drive. Can be used to genetically engineer insects to be resistant to the parasite. Even a small no of transformed bugs may be useful as it will spread through the population quickly due to the gene drive. Means we don’t have to develop potentially toxic drugs.
Where do triatomines usually infest? (Which types of habitat and how many countries are endemic)?
Poor quality housing, endemic in 21 countries
What is the lifecycle of t cruzi in the human and in the insect?
Human: Metacyclic trypomastigotes are in the insect faeces which get scratched into an open wound or penetrate mucous membranes (invades any tissue in mammals)–> forms pseudocysts and replicates inside (replicative phase) –> pseudocysts rupture to release differentiated trypomastigotes into the blood –> taken up in a blood meal.
Insect: trypomastigotes taken up in blood meal –> transform into the insect replicative phase which is epimastigotes, they multiply in the midgut –> differentiate into metacyclic trypomastigotes as they pass down the gut tract and replicate in the midgut. Differentiate into infective metacyclic trypomastigotes in the hind gut –> defaecated and infect human host.
What are the preferred human tissues of the t cruzi parasites?
Preferentially infects skeletal muscle and intestinal tract and cardiac tissues.
Which is the most dangerous triatomine and why?
R prolixus becuase it defaecates while feeding. Also T infestans because is closely associated with human/domestic environments and also defaecates while feeding.
where is R prolixus found?
Tropical regions such as northern South America
What are the possible modes of transmission of T cruzi?
- Blood transfusions
- Oral (rare) e.g. from juice presses with trapped bugs
- Accidental -e.g. inoculation in lab settings
- Sexual (unlikely)
- Vectorial
Describe the phenotype of triatomine eggs
- How many are laid
- How big are they
- How long does it take them to hatch?
200-300 laid individually. 1-2mm. Hatch after 10-30 days.
How many nymph instars are there? How are they differentiated from adults?
- Only the adults have wings.
What is the lifespan of a triatomine bug?
4 months to a year.
How can nymphs be vectors? Why are older individuals more likely to be vectors?
Need a blood meal between nymphal instars.
Which species, found globally, carries Trypanosoma cholorini? How is it spread and what does it feed on?
Triatoma rubrofasciata. Spread by shipping, feeds on rodents.
Why do the Andes and the Amazon lack transmission?
Low human population.
WHich ecotypes do triatoma like?
ROcky.
What are panstrongylus and Rhodinus prolixus associated with?
R prol: marsupials
Pan: burrows and armadillos
What is the result of the bugs taking a large volume of blood and being present in large numbers?
- Anaemia
- Malnutrition by proxy and stunted growth
Describe the transmission cycle of triatoma sordita.
Separate.Sylvatic cycle feeding on marsupials and avians. Separate human cycle.
Where are the majority of overlapping cycles found?
Tropical countries in the North of S America.
Where can enzootic transmission come from?
- Undercooked/ uncooked meat
- Juice presses
- Accidental infection (e.g. lab)
- Humans working in close proximity to cycles
Where are R prolixus, T infestans and T sordita found?
R prol: Central to Northern South America
T infest: Southern cone
Sordita: Brazil
How do triatomines find their prey?
Visual acuity in the longer range. Lock onto their prey using sense of CO2 and heat at short range.
What does Rhynocorus ventralis feed on?
Originally plant feeders and predatory insects.
Describe the differential antennae of the three tribes (i.e. how we can tell them apart).
Rhodnius: antenna detaches at the end of the head
Triatoma: antenna is half way between the eye and the head.
Panstrongylus: antenna much closer to the eye
Describe the terminal segment of triatoma males and females
Pointed in females. More rounded in males.
Describe the terminal segment of rhodnius males and females
Pointed in females and more rounded in males. Additionally, the females have a crescent shape.
What are the techniques we could use to identify variations in species?
- Microsatellites
- Isoenzyme assays
- RAPD profiles (random amplified polymorphic DNA)
- Multilocus sequence typing
What features make triatomine bugs amenable to control?
- All life stages found in the same place (e.g. a house)
- Low genetic diversity within populations
- Don’t fly often so have decreased capability for dispersal
- K strategists so spread slowly
How do we go about controlling triatomine bugs?
- Health education
- House improvements (e.g. smoothing over house walls, corrugated iron instead of palm leaf roofs) and IRS
- Screening blood for transfusional control.
How can gene drive be used to control triatome bugs? Why is this an advantage against current strategies?
CRISPR CAS9 to edit genes and cause the bug to produce antiparasite peptides so they are resistant to infection and thus reduce transmission. Less need for IRS or dangerous human treatment! Biological change is inherited and so spreads QUICKLY through the population.
Can also change olfaction genes so they cannot sense humans as well.
What is cruzipain?
An antigen secreted by T cruzi. It is a cysteine protease that helps aid the parasite entering the host cell and evading the immune response.
