Introductions to parasites, helminths and vectors Flashcards
What is a direct and indirect parasitic lifecycle?
Direct- requires one host.
Indirect- 2 or more hosts.
What is a mechanical vector?
A vector which spreads the parasite non-biologically i.e. on the feet of a fly.
What is the difference between an intermediate host and a vector?
Both involved for indirect parasite lifecycles. Intermediate host is usually big and slow-moving. A vector is usually smaller and quick moving.
What are Koch’s 3 postulates that he applied to bacteria?
- The bacteria must be present in every case of the disease.
- The bacteria must be isolated from the host with the disease and grown in pure culture.
- The specific disease must be reproduced when a pure culture of the bacteria is inoculated into a healthy susceptible host.
Give an example of the outcome of infection depending on what is already in the body.
Haemophilus can cause pustules or may not depending on the preexisting flora.
What are the two main phyla of helminths?
Platyhelminths
Nematodes
What is the phyla platyhelminth further subdivided into?
Trematoda and Cestoda both come under platyhelminths
What is the global burden of helminth infection?
1/3 of the world is infected.
What are the general features of parasitic helminths?
Multicellular, extracellular, often zoonotic, often from oral ingestion of eggs (sometimes skin penetration), they go through developmental stages (such as moulting, eggs/ larvae etc).
How are helminths different from protozoan parasites?
They often do not multiply inside the host and so the disease burden correlates with exposure, meaning that disease accumulates.
Furthermore, the disease is often persistent/chronic as it is long lived.
What is the definition of a definitive host?
The host that supports the adult or the sexually mature stage of the parasite.
What type of helminth is a fluke?
Trematode
What type of helminth is a tape work?
Cestode
What does digenetic mean? Describe the generic digenetic lifecycle of trematodes?
Means alternation between sexual reproduction inside a vertebrate and asexual reproduction inside a mollusc.
Eggs hatch in water to release miracidia –> infect snails –> multiply in snail intermediate host and form cercaridae–> cercariae released to infect new definitive host or encyst into metacercariae on plants which are eaten by the definitive host.
How are schistosomes exceptions to general trematode features?
Schistosomes are blood flukes in the blood vessels. trematodes are usually hermaphrodites but schisto are male and female (female rests inside a slit in the male’s body where they copulate 24 hours a day).
Describe which cestode stages are in the intermediate and definitive hosts.
Larvae stage in intermediate. Adult in definitive host.
Describe the structure of roundworms.
Long, unsegmented, cylindrical. Are often intestinal nematodes.
What is DALY?
Disability-adjusted life year. Quantifies the burden of disease from mortality and morbidity.
What is the function of a peritrophic matrix in blood feeding?
Is secreted by the midgut when blood feeding to start the digestion of the blood meal. They aid digestion, provide a mechanical barrier to pathogens and protect against mechanical and chemical damage.
Describe the two types of peritrophic matrix.
Type I and type II.
Type I is secreted by the entire midgut and is formed simply from the delamination of the surface of the midgut epithelium. Usually in response to feeding.
Type II is produced by a specialised group of cells in the anterior midgut (on the proventriculus). Occurs in the presence or absence of food. Continually secreted as an unbroken sleeve like structure.
What is the primary vector for cerebral malaria in sub Saharan Africa?
Anopheles gambii.
Why is anopheles gambii such a good vector?
- Night feeds so is hard to defend against
- Can fly up to 2km
- RAPID lifecycle from egg to adult in 6 days
- Unlike most mosquitoes, A. gambii goes UPwards when it hits a wall which makes it more likely to enter a house via the eaves.
- Extremely competent in hosting the full lifecycle of the malaria parasite.
- Acquires and transmits infection v quickly
- Rapid reproduction rate
- High percentage of mosquito survival from day to day.
- Can breed in very transient breeding grounds.
What is the extrinsic incubation period?
The extrinsic incubation period is the time taken by an organism to complete its development in the intermediate host
What is the fat store in insect vectors used for?
- Energy store
- Produces specialist immune cells.
What are the routes of transmission from vectors?
Faeces, saliva, coxal fluid, regurgitation, active penetration through proboscis.
How do soft ticks transmit borrelia?
Thorugh coxal fluid and saliva. The function of the coxal fluid is to anchor the tick during the blood meal.
Where in the sandfly does leishmania development occur?
Limited to the mid and foregut. However some species reside in the hindgut but these are not considered medically important.
What is the function of the stomodial valve?
Squeezes the meal into the midgut.
What are the constituents of saliva that aid feeding?
Anticoagulants, anaesthetics, immunomodulators, vasodilators.
How does sandfly saliva promote leishmania infection?
Anti-inflammatory cytokines increase in the presence of saliva (IL6, IL10 and IL5).
Pro-inflammatory cytokines decrease in the presence of saliva (IL12 and IFN gamma).
Saliva also inhibits complement activation and also causes a long term dampening of peripheral T cells.
Where in the insect does T cruzii develop?
Midgut and rectum. (Metacyclics are excreted in faeces and rubbed into bite).
Where do filarial worms develop in the insect vector?
Penetrate midgut and migrate to the flight muscles where they moult. When a blood meal is sensed they complete their 4th and final moult and penetrate flight muscles into proboscis for transmission.
Why are blackfly larvae harder to target? How was this eventually successfully done?
They are more specialist and require fast-flowing water for their larvae. Was overcome by bombing watercourses with larvaecides.
Give an example of how vectors can be manipulated by the parasite they are carrying.
Mosquitoes became more persistent when they became infectious for malaria (had sporozoites) when exposed to human hands for a blood meal and were interrupted.
Which spp carry dengue and chickungunya arboviruses?
Aedes aegypti and Aedes albopictus.
What is the main defining morphological/ colourational feature of Aedes albopictus and aegypti respectively?
Albopictus- white stripe on back
Aeypti- black line on the back- b/w stripes on back like a tiger
Which virus causes rabies?
Lyssavirus
What is the latin name of the pork tapeworm?
Taenia solium.
What is the latin name of Guinea worm?
Dracunculus medinensis
Which organism gives alveolar disease. What happens during the course of this disease?
Echinococcus multilocularis.
Asymptomatic inculation period, followed by a chronic cause where disease presents as jaundice and epigastric pain. Disease may be triggered by falling over and releasing the liquid causing anaphylactic shock.
Why could some people have a higher worm burden than others?
- Hygiene differences
- Dietary and nutrition factors
- Genetic component
What is the parasite that causes river blindness and which vector transmits this?
Onchocerca volvulus. Blackflies.
What is a problem when trying to control mycobacterium ulcerans (a mycobacterium that is increasing in prevalence in Australia)?
It has an unknown vector- not sure how it is transmitted so is difficult to put control measures into place.
What are the symptoms of Chagas (T. cruzi)?
Mild and non specific symptoms if acute. Later in life get dilated cardiomyopathy and die.
What does a DALY measure?
Years lost to disability, not just mortality.
Does a disease that kills children give a high or low daly score?
High because lots of years lost.
Which disease has the highest DALY score, after malaria?
Schistosomiasis.
Why are most of the diseases we study prevalent in tropical and subtropical regions?
They often require the warmer temperatures to complete stages of their lifecycles.
What are the two main pathogen species for filariasis?
Wucheria and Brugia
Where are culex mosquitoes often found?
Urban areas.
Where areanopheles mosquitoes often found?
More rural areas.
What are the vectors for yellow fever in:
- The jungle
- Urban areas
Jungle: egypti
Urban: haemagogus
Which mosquito species is the most important arboviral vector?
Aedes aegypti
Describe the hind wings of mosquitoes. What is the function of these structures?
They are vestigial, meaning they have been reduced to halteres.
They are used for body positioning, orientation and flight balance.
Which type of mosquito can withstand higher levels of environmental pollution?
Culex (although pollution tolerance is species specific).
What is a spermatheca?
Package of sperm storage that allows for only one mating being necessary for multiple fertilisations.
How many days from picking up the pathogen to the mosquito transmitting it?
Usually around 10 days.
WHich age of mosquito is involved in transmission?
Older females. AS they will have to have fed and laid eggs around twice during these ten days for the disease to become transmissible.
What are the two major mosquito families?
Culicinae and anophilinae
Compare the posture of anophelinae and culicinae.
Anoph- at a slant/ angle
Cul- horizontal
What are the two main genera in culicinae? What is the third?
Aedes and Culex.
Mansonia.
Describe differences in eggs from anopheles, culex and aedes.
Anopheles: single eggs with floats.
Culex: rafts of eggs bound by surface tension.
Aedes: no raft, dessication resistant eggs, only hatch when exposed to water.
Descrbie the differences in larval morphology of anophelinae and culecinae eggs
Anoph:
- Horizontal to water
- Spiracle to take water from surface through trachea
- Siphon not present
Cul:
- Found on top of water to take atmospheric oxygen.
- Respiratory siphon
- At an angle, hang down from surface as they use siphon.
Describe the larval posterior of aedes and culex.
Both have a siphon (a short barrel) with subventral tufts. There are 3/4 pairs of tufts in culex but only one pair in aedes.
Which developmental stage of mosquito has respiratory trumpets?
Pupal stage.
Describe the palps in culecine and anopheline mosquitos.
Culex: Shorter than proboscises (females). (Males have long club shaped palps).
Anopheles: same length as proboscis
Where are the breeding grounds for anopheles gambiae?
Small temporary bodies of water (this means they do not have time to develop predators). Often puddles in rural areas.
What is the vector for malaria in the UK?
Anophleles maculipennis
Describe the structures within the probiscis and their functions.
Labarum- takes up blood.
Hypopharynx- tube transports saliva.
Mandiblex and maxillae- skin penetration.
Labium- encases all of this, bends back/ folds during feeding.
What is mansonia spp. a vector for?
Lymphatic filariasis.
