Parasitology (1) Flashcards
(21 cards)
What are protozoa?
Eukaryotic, single-celled animals
How are parasitic protozoa transmitted and give examples?
Faecal-oral route
1) Entamoeba histolytica
2) Giardia duodenalis
What are some features of the giardia classification of parasites?
1) 6 species with specific host type
2) G.muris - rodents, G.psittaci - birds
3) Has number of different genetic groups species
4) Giardia duodenalis types have been found in humans, cattle, sheep, dogs, cats, beavers, guinea pig, chinchillas and other mammals
How are the genetic groups of Giardia revealed?
1) Molecular typing - ‘assemblages’, appear to have restricted host range
2) G. duodenalis isolates group into 8 assemblages (A-H)
3) Further group into sub-types
4) Only A and B found in humans
5) Human-specific sub-types
Are animal subtypes of Giardia different to those found in humans?
1) Yes
2) Specifically human types of G duodenalis - generally considered strictly human pathogen
3) Analysis of sub-types indicates allows possibility of cross-species infection (zoonosis/anthroponosis)
What is the life cycle of Giardia duodenalis?
1) Cysts ingested in contaminated food/water
2) Cysts travel through alimentary canal to SI, where they excyst into flagellate form
3) In SI, flagellates attaches to gut wall using suction
4) Absorb nutrients from host’s gut contents
5) Start to multiply and detach gut wall to move around intestine
6) Patient has loose bowel movements producing greyish, fatty faeces and has characteristic smell (H2S)
7) Passing to the LI, flagellates close up into resistant cyst form - passed in faeces and can persist in water and on food for several weeks
8) Resistant to storage at 4-6C and Cl
9) Faeces may contain flagellates but cannot survive long outside body/infect another host
What is the lab identification for Giardia using a faecal sample?
1) Fixed faecal smear - small pea-sized amount of emulsified saline, fixed in alcohol and stained with trichrome stain/fluorescent labelled Ab
2) Wet preparation - small pea-sized amount emulsified in saline, cover slip placed on top, stained with iodine
3) Concentration method (e.g., formol-ether)
What is the appearance of G. duodenalis cysts?
1) 2 nuclei
2) Remains of flagellae
3) 9-7 microns
What is the treatment once getting a diagnosis of Giardia duodenalis?
Metronidazole - common antimicrobial drug, used to treat anaerobic bacterial infections
What is an example of parasitic protozoa transmitted in blood?
Plasmodium spp
What is plasmodium spp.?
1) Malaria
2) About 120 species
3) Found in mammals (includes humans, primates and bats), birds, reptiles
4) Only 5 species cause disease in humans
How is malaria transmitted?
1) Plasmodium spp. can be transmitted by a variety of insect vectors (mosquitoes, sandflies, biting midges)
2) Plasmodium spp. which infect humans are all transmitted by mosquitoes of Anopheles species
What is the life cycle of Plasmodium spp.?
1) Infection transmitted to host via bite of infected Anopheles mosquito
2) Mosquito carries sporozoites in saliva, so injected when insect takes blood meal
3) Blood required for egg production so only female mosquitoes bite hosts
4) Sporozoites enter blood stream and migrate to liver, where surface proteins attach to hepatocytes
5) Circumsporozoite protein (CSP) basis of RTS, S vaccine
6) Multiply exponentially to produce schizonts (collection of merozites, a nucleus surrounded by cytoplasm)
7) Hepatocyte bursts, releasing merozoites into blood stream
8) In P. vivax and P. malarie some sporozoites take a dormant form in liver and reactivate months/years later
9) Has specific surface proteins (merozoite surface proteins, MSPs) which allow attachment to glycoproteins on RBC membrane
10) Each merozoite develops into the ‘ring form’ trophozoite
What happens to Plasmodium once inside the RBC?
1) Produces its own proteins (RBCs do not have nuclei, ‘anucleate’)
2) Parasite proteins expressed on surface of RBCs, called knobs
3) Trophozoites feed on Hb, grow and divide to form schizonts (containing more merozoites
4) Break down product of this metabolism is called haemozoin (malaria pigment)
5) Burst out of RBCs and infect new RBCs
6) Cycle of infection of RBCs, growth of trophozoites and bursting of RBCs
7) Host immune response causes symptoms
8) Pyrexia (38C), rigours (hot and cold), aching, headache, lethargy, sometimes diarrhoea and vomiting
9) Loss of RBCs —> Anaemia
How does Plasmodium move around the body?
1) Plasmodium parasites ‘hide’ in liver cells and RBCs so immune system cannot detect them
2) Infected RBCs can move around systemic circulation
3) Can get stuck in small blood vessels, due to knobs, e.g., in brain
What does Plasmodium do so the immune system cannot respond adequately?
1) At different life cycle stages, parasite expresses different cell surface Ags
2) By the time the host has made suitable Ab, the parasite has moved on to the next stage
3) One sporozoite can produce vast numbers of progeny.
What happens to trophozoites in the later stages of Plasmodium infection?
1) Trophozoites start to develop into gametocytes instead of more merozoites
2) Gametocytes taken up by mosquito when host is bitten - as long as individual mosquito taken up a male and female gamete, Plasmodium can continue life cycle inside mosquito
3) Male and female gametes fuse inside mosquito’s gut to form zygote - develops into ookinete, which can invade mosquito gut wall
4) Inside gut cell, forms oocyst, grows and divides to form sporozoites
5) Oocyst bursts and sporozoites are released into mosquito’s body and enter salivary gland
What is immunity to malaria like?
1) Human immune response to Plasmodium spp. is NOT well understood
2) Young children living in areas where malaria transmission is high, ‘holoendemic’ area, are at risk from severe disease
3) This is why majority of deaths are in children under age of 5
4) Women can experience serious effects from malaria during pregnancy - due to changes in immune response
What is the process of lab identification of Plasmodium spp. in a thick blood sample?
1) 25microL blood, centre of microscope
2) Spread out across slide
3) Air dry
4) Stain with Giemsa stain
5) RBCs are lysed, releasing any parasites present
6) Examine relatively large volume of blood
What is the process of lab identification of Plasmodium spp. for a thin blood film?
1) 10microL blood, 1 end of microscope slide
2) Spread with spreader (with another slide)
3) Air dry, then fix in alcohol
4) Stain with Giemsa stain
5) RBCs remain intact, so morphology of parasites clearer, allowing identification of species
6) Examine smaller volume of blood
What is the Rapid Diagnostic Test for Plasmodium?
1) OptiMAL test
2) Ag capture test method
3) Exploits fact that Plasmodium spp. produce a Lactate Dehydrogenase enzyme (pLDH) which has no antigenic cross reaction with human LDH
4) Plasmodium falciparum produces antigenically distinct pLDH
