Parasitology Flashcards
What is a parasite
An organism that lives and feeds on or in an organism of a different species and causes harm to its host
What are the only types of parasite we learn about
eukaryotic endo-parasite groups classified as protozoa and helminths.
What are indirect parasite life cycles
direct, involving a single definitive host
(sexual cycle)
indirect, involving both definitive and intermediate hosts
some lifecycles rely on an invertebrate vector host
How does pathology of parasites differ from most pathogens
Like most pathogens, parasites can cause acute disease, but unlike most pathogens, almost all parasites will cause chronic disease as well.
disease will be caused by direct damage from the parasite, and indirect damage caused by immunopathological response to infection.
How was the evolution of parasites driven
by natural selection for optimizing replication within a host and transmission between hosts.
Selection will determine the spectrum, intensity, geographic range and persistence of disease.
How can helminths be divided
into flatworms and roundworms
What is the most common route of parasite infection
why
oral-faecal route
most endo-parasites will have been originally derived
from consumption of free-living organisms by one of the hosts
What is the host, transmission and life cycle of whip worm
how common is it
where is it mostly distributed
Human parasite, direct life cycle, oral-faecal transmission
3rd most common nematode infection worldwide, an estimated 1 billion people carry an infection.
Mostly tropical and subtropical geographic distribution
Describe the pathology of whip worm
Pathology ranges from asymptomatic to severe (possibly leading to rectal prolapse, or anaemia in children)
Infection may persist for years
Hygiene and sanitation key to control
What is Trichuris trichuria
What is its lifespan
whipworm
1 year
How is trichuris treated
with helminth microtubule inhibitor albendazole
Describe in detail the life cycle of whipworm (5)
unembryonated eggs are passed with the stool
In the soil, the eggs develop into a 2-cell stage T
an advanced cleavage stage and then they embryonate
eggs become infective in 15 to 30 days.
After ingestion (soil-contaminated hands or food), the eggs hatch in the small intestine, and release larvae that mature and establish themselves as adults in the colon
adult worms (approximately 4 cm in length) live in the cecum and ascending colon
What is the proper name for pinworm
Enterobius vermicularis
What is the most common worm infection in the UK
What is interesting about its behaviour
pinworm
crawls out of the rectum to the peri-anal region to lay eggs at night. The eggs are then spread by scratching and touching bed clothes, furniture, doors etc.
Give an exmaple of a large roundworm
How do they tend to cause damage
the giant round worm Ascaris lumbricoides
can be very large and cause disease by simple mechanical blockage of the intestine.
Describe the pathology of toxocara canis
what is it an example of
a helminth parasite with the dog as definitive host, causes human toxocariasis when the eggs are ingested. The larval worms do not develop, but migrate to the eye and brain causing inflammation which may result in blindness and epilepsy respectively
Oral-fecal routes of infection between definitive and intermediate hosts can have severe pathologies as sometimes the worm does not develop properly in the intermediate host
What is tanenia solium and what is its definitive host
What are the larvae called
the pork tapeworm
human is the definitive host with pig as a common intermediate host.
cystercerci
How can a human develop taeniasis
When humans ingest the larval stage of tanenia solium (cystercerci) from infected meat, a cystercircus may develop into a
hermaphroditic adult worm intestinal infection
What are some of the effects of having a pork tape worm
Why is this surprising
generally asyptomatic but in severe cases anaemia and emaciation may occur
surprising considering they can be several meters long
How does pork tape worm pass from humans to pigs
what do they do once they enter the intermediate host
The adult worm sheds gravid proglottids (containing up to 50,000 embryolated eggs) that are excreted by the human host. The proglottid/eggs are ingested by the intermediate host (pig) where the the eggs hatch and larval worms migrate to a variety of tissues including muscle and brain to settle as cystercerci
When does severe disease occur in the tanaenia solium life cycle
How does pathology differ between humans and livestock
when the eggs are ingested and cystercerci grow causing cysticercosis.
Further, since the large cystercerci are fluid filled bladders containing worm proteins, they may cause a fatal massive inflammatory response upon accidental rupture.
Most livestock do not live long enough to suffer serious disease, but in humans some cystercerci may grow into an intermediate form that can be very large (20 cm) and obstruct organ function
What happens if a pork tape worm reaches the brain
Where is this a real problem
epilepsy, seizures and blindness.
In regions with poor sanitation, cysticercosis is thought to be a major cause of epilepsy.
What are the 3 primary classes of anti-helminth drug
Briefly describe the modes of action
ivermectin - NT inhibitor
albendazole/ mebendazole - microtubule assembly inhibitor
praziquantel - permeability of membranes to Ca2+
What are the drug targets for antihelminth drugs
ivermectin - glutamate gated chloride channels
albendazole - colchicine
praziquantel - unknown
Describe schistosomiasis
an ancient tropical/subtropical disease first described by
the Egyptians noting symtoms including skin rash, chills, fever, abdominal
pain, enlarged liver and spleen, blood in stool or urine.
caused by Schistosoma spp (flatworm, trematode)
Why is Bilharzia
another name for schistosomiasis
Theodor Bilharz in
the mid-1850s made the connection between the worm and disease
Where is most of the disease burden of schistosomiasis focussed?
sub-Saharan Africa
harbors 80-90% of global disease burden.
Worldwide, 600 million are at risk from schistosomiasis, 260 million infected and an estimated 200,000 deaths annually
What is the socio-economic impact of schistosomiasis?
Morbidity due to infection may be a greater socio-economic burden than mortality as severe disease leading to mortality is about 5-10% of cases.
How can schistosomiasis be divided
What does severe disease in these categories lead to
into intestinal and bladder disease.
Severe intestinal disease includes hepato-splenomegaly, varicies, fibrosis and calcification of the liver.
Bladder disease includes bladder fibrosis, calcification
and cancer, with urogenital “sandy patches on the cervix
What is the lifecycle of schistosoma spp (7)
S. mansoni cercaria in the water will come in contact with swimmers or waders.
After penetration of the skin and loss of tail, the larval schistosomula undergo development into the adult worm.
The adult male and female pair will migrate and attach in the mesenteric venules living 5- 10 years.
They will deposit 300-3000 eggs/day depending upon species.
Egg size and shape are diagnostic for each species.
The eggs hatch in the water releasing
miracidia that invade the snail
host.
Sporocysts in the snail will release the motile cercaria into the water
What is diagnostic of the different types of schistosoma spp.
Egg size and shape are diagnostic for each species
What are the organ preferences for two of the different species of schistosoma
S. japonicum is more frequently found in the superior mesenteric veins draining the small intestine image
S. mansoni occurs more often in the inferior mesenteric veins draining the large intestine image
However, both species can occupy either location and are capable of moving between sites.
What causes morbidity in schistosomiasis
egg deposition
Most of the eggs will transit the gut wall into the intestinal lumen and shed with faeces.
However, a fraction will reflux with blood flow back into the liver causing granuloma formation. Severe disease will include fibrosis and calcification
How will schistosoma eggs try to transit the gut wall
via the induction of some of the tissue repair pathways, particularly fibrin coating and then plasmin fibrinolysis.
Along with wound healing mechanisms, the eggs may form granulomas.
Why does a heavy worm load lead to more severe disease
After reflux
to the liver, granuloma formation dominates and in severe disease, the wound healing
cytokine milieu persists leading to fibrosis and calcification of the liver. A heavy worm
burden, hence egg burden and granulomas will contribute to severe disease.
What is required from the body to avoid severe schistosomiasis
a balanced immune response involving a range of cell types
How does the immune response to schistosoma eggs begin (5)
Egg antigens will be picked up by dendritic cells and processed to present antigen and activate Th2 cells that in turn will produce a Th2 cytokine profile.
Basophils will also be activated by egg antigen and reinforce the Th2 cytokine profile.
Alternatively activated macrophages and eosinophils will support wound-healing mechanisms including collagen
deposition.
B-cells responding to egg antigens produce igE and IgG antibodies that may aid in destruction of eggs in granulomas.
However, egg antigens will participate in Treg formation that, with the alternatively activated macrophages, produce IL-10 to limit granuloma size and liver tissue wound healing response.
How does the immune response to schistoma change over time
What can go wrong
After an initial Th1 inflammatory response to cercaria penetration and larval worm migration, egg deposition initiates a switch to Th2 response that peaks about 9 weeks p.i. Treg cells are formed and IL-10 production starts a modified Th2 response.
A continued strong Th2 response may lead to severe disease, fibrosis and calcification.
Define parastitic protozoa
Unicellular eukaryotic organisms that are endoparasites and cause harm to their hosts.
What does natural selection act to improve in parasite fitness
1) Replication within a host
2) Transmission between hosts.
Why does leishmania not really have a definitive host
conventional meiosis is not part of its life cycle
What acts as a definitive host for toxoplasma
the cat family
What are the intermediate hosts for leishmania
range of intermediate hosts for leishmania (that infect humans) appears to be fairly wide, but is probably limited by what the vector will feed on.
The sand fly vector will specialize in both host preference and location
What is the invertebrate vector used by toxoplasma
does not use one because transmission is via contaminated vegetables or carnivory.
What are the potential intermediate hosts for toxoplasma
Virtually all warm-blooded animals
Give a surprising animal we have found to be infected with toxoplasma
Even marine mammals like sea otters that never come in contact with cats or land-based prey can be infected.
Where does natural selection toxoplasma and leishmania act for parasites
within the host immune system and regional ecosystems.
How old is leishmaniasis
How has it been found throughout history
an ancient disease and has been depicted in clay figures found in Mexico dating back to 400 AD
In Africa during the 1700’s the disease was named Kala Azar, but the parasite was only described in the early 1900s by William Leishman, a Glaswegian army doctor based in India
Where does leishmaniasis occur
The disease is
primarily tropical and subtropical but occurs in temperate environments from
Spain to the Middle East through northern China/southern Mongolia.
Where is the distribution of leishmania restricted to
The distribution is probably limited by where the sand fly vector occurs.
Which organisms does the leishmania species life cycle involve
Which stages of the lifecycle are associated with each host
alternates between the mammalian host and the sand fly vector
flagellated promastigote forms are associated with growth in the sand fly midgut
the ovoid non-flagellate amasitgote form occupies macrophage host cells within the mammalian host.
Describe the structure of promastigotes in the leishmania life cycle
have a single flagellum connected with the axoneme and basal body.
Directly adjacent to the basal body is the kinetoplast
What is a kinetoplast
a DNA-containing structure that is held within the single
mitochondrion
Which parts of the promastigotes can be easily seen in a LM
The kinetoplast and nucleus are easily seen under light microscopy when appropriately stained
Where are leishmania amastigotes found
the ovoid form seen within macrophages in the mammalian host in areas of tissue damage.
How do amastigotes enter the sandfly
What happens next
ingested by the sandfly and held within the peritrophic matrix while the host cell is broken down
parasite transforms into a promastigote, replicates and the flagellum attaches to the sand fly midgut
Why does the new promastigote attach to the wall of the sand fly’s midgut
to prevent loss via defaecation
What happens to the promastigote after it has attached to the sand fly’s midgut wall
replicates and then further transforms into an infective promastigote, detaching from the midgut and migrating to just behind the mouthparts and can be injected into the mammalian host on the next blood meal
How do promastigotes enter a mammal
injected into the
mammalian skin tissue and taken up by
macrophages (Mϕ) and dendritic cells (DC)
Why is the promastigote not destroyed as soon as it is injected into the mammal
the Mϕ does not respond to is as it isn’t activated - this allows replication of the amastigote
What happens to DC cells after they are infected with a promastigote
What is the next sequence of events
Why is this sequence of events important in the parasite’s lifecycle
migrates to a local
lymph node, presents antigen to a Th1 cell activating the Th1 cell to secrete IFN-γ, activating Mϕ.
Nitric oxide bursts kill the parasites but some will escape to re-infect new Mϕ.
Inflammation and Mϕ death produces localized tissue damage, attracting more Mϕ host cells
this is the major amplification of the parasite
How does the Leishmania parasite protect its self from loss and attach in mammals and sandflies
The outer surface of the parasite (including flagellum) are coated with the carbohydrate lipophosphoglycan (LPG).
LPG maintains the parasite in the sand fly midgut and protects the parasite from immune attack in the mammalian host.
How does Leishmania LPG act in the sand fly
In the sand fly midgut, LPG initially binds a midgut galactin to anchor it from loss via defecation
As the parasite divides and develops, the LPG
changes length and side chains, releasing it from attachment, allowing it to be injected into the mammalian host.
LPG helps the parasite resist complement attack and during transformation into the amastigote, LPG gets longer and more decorated helping it resist oxidative bursts within the macrophage
What are the 2 key Leishmania species
Compare the pathologies of each
Old world (Europe and Asia) and New world (Americas) species
in both cause human pathologies ranging from a simple selfhealing cutaneous lesion, to severe facial tissue destruction, to a serious and
fatal (if untreated) visceral disease
What has the recent evolution of New and Old world Leishmania species been due to
recent evolution of these parasites and concomitant disease pathologies has been largely due to human activity.
What is the origin of leishmania
Visceral leishmaniasis was not recorded in the Americas before the European explorers, so the origin of L. chagasi has been a question. Recent data suggests that L. chagasi is likely to have been brought to Brazil by the early Portuguese explorers, specifically their dogs as reservoir host
Which other Leishmania species is most closely related to L. chagasi
L. infantum from Portugal
What can L. braziliensis cause
What is the natural host
both a simple cutaneous lesion and severe mucocutaenous disease.
two-toed sloth, an animal that spends the vast majority of its life in the upper canopy of the rainforest and has little contact with humans
Why is it initially surprising L. braziliensis affects humans
transmission cycle for L. braziliensis is in an ecological niche that doesn’t include humans.
However with rainforest destruction for roads and agriculture, the sloth, L. braziliensis and the sand fly vector came in close contact with humans and their companion animals.
What is the peri-urban life cycle of L. braziliensis
human occupation of towns that are surrounded by rainforest have developed a peri-urban transmission cycle that includes other intermediate hosts like rodents and horses.
L. braziliensis has adapted to use different sand fly vectors so the parasite is now a common human parasite.
What is the pathology of L. braziliensis an example of
severe pathologies that occur when parasites infect a non-natural host like
humans (although severe disease does only develop in a minority of cases)
L. major
what pathology does it cause?
where is it found?
what is interesting about its disease pattern?
causes simple cutaneous disease and is a common species from southern Europe to India and China.
The disease pattern mirrors the distribution of the primary L. major sandfly vector, Phlebotomus papatasi.
What are the different vectors for L. major
primary vector = P. papatasi (restrictive vector)
also, P. arabicus (permissive host and transmits a variety of Leishmania species)
Compare the vector hosts of L. major
By using the restrictive P. papatasi as the primary vector but also using P. arabicus as a permissive vector, L. major can avoid competition for the most part, but can also expand into other ecological
niches
Compare the distributions of L. major vectors
P. papatasi has greater global geographic range than P. arabicus.
Where the sand fly species overlap locally like in the Sinai, P. arabicus tends to live at higher elevations than P. papatasi .
L. major can use both vectors.
What makes P papatasi a restrictive vector
because no other leishmania species is known to grow in it
How does L major settle in the P. papatasi vector
P. papatasi has the lectin galactin on the midgut villi that binds L. major – specific LPG so the parasite can reproduce.
Why is P. arabicus considered permissive
is thought to have
a common mechanism that allows a variety of Leishmania species to bind in its midgut.
What is the most common protozoan on earth
Toxoplasma gondii
Describe toxoplasma gondii as a protozoan
obligate intracellular protozoan parasite that infects any nucleated cell in virtually all warm-blooded animals
apicomplexan
What is the definite host for toxoplasma gondii where the sexual cycle takes place
cat family
What is toxoplasma classified as
What is it related to
As a gut-infective parasite, toxoplasma is classified as a coccidian
related to
other important livestock pathogens like Eimeria species (e.g. avian coccidiosis) and Neospora caninum (cattle neosporosis)
How does a cat get an acute infection of toxoplasma
The cat can eat a tissue cyst (bradyzoites) or oocyst (sporozoites) and the parasite will transform into an actively dividing tachyzoite resulting in an acute infection
What happens after the acute infection of toxoplasma in cats
cat immune system will clear the tachzoites but many will have transformed into micro and macro gametocytes that fuse to form an oocyst that is shed in feces. The oocyst will go through meiosis in the environment and produce an octet of
sporozoites
tachyzoite will also transform into the bradyzoite form and reside in a tissue cyst, thousands of parasites in a single cell. Bradyzoites are generally quiescent but may re-activate upon immune suppression and cause acute disease
Can you become immune to toxoplasma
Healthy individuals are immune from further
infection after the first exposure to the parasite.
What does acute toxoplasmosis look like
what about in pregnancy
mild flu-like symptoms in healthy individuals, but with immunosuppression, can result in a fatal encephalitis and chorioretinitis (blindness).
If the parasite crosses the placenta during pregnancy, birth defects and spontaneous
abortion may occur.
What is surprising about the population structure of toxoplasma
Although toxoplasma is a global parasite that has a sexual cycle and therefore recombination, the population structure is remarkably clonal, particularly in North America and Europe, where the population is dominated by 3 clonal lineages, called types I, II and III.
How similar are the 3 clonal lineages of toxoplasma
What do we think caused this
90-95% similarity.
An analysis of the type strains suggest that in NA and Europe, the parasite went through a population bottleneck about 6-10,000 years ago.
How does diversity of toxoplasma strains change across the world
why is this
Asian isolates show somewhat greater diversity but mostly variation within a basic type strain and recombination between the three type strains.
South American isolates show considerably more diversity. Although the NA and European type strains are present, there are more clonal lineages.
thought to be due to the comparatively high number of cat species in SA and
therefore ecological niches
Why was there a bottle neck for toxoplasma 10,000 years ago
Agriculture was developing at this time in the Fertile Crescent, with previously unseen stores of grain with the associated high numbers of rodent pests and domesticated cats
new transmission route - parasite escaped the definitive host and could pass directly between intermediate hosts via carnivory
What is probably responsible for the massive expansion of toxoplasma and its global distribution
its unique transmission between intermediate hosts:
its is able to transmit within definitive hosts, transmit from definitive to intermediate host and between intermediate hosts
What is the defining feature of apicomplexan parasites
the apical complex - the structural feature at which the parasites can control motility and host cell invasion
How do apicomplexans control motility and invasion
what are the key organelles
hrough an evolved set of secretory organelles that allow the parasites to store and then secrete proteins and membrane precisely when required.
the micronemes, rhoptries and dense granule
Describe the structure of toxoplasma
has an inner membrane complex consisting of a corset of microtubules and microfilaments that radiate from the apical complex, where the organelles secrete proteins and membrane
What do micronemes secrete in apicomplexans
proteins that make transient attachments to the host cell surface and bridge attachment to the actin-myosin motor that drives gliding motility.
The parasite propels itself by a kind of ‘rowing’ action.
How do apicomplexans access the host cell
glides through microneme secretion on the host cell surface, finds an appropriate attachment site and blows a transient hole in the host cell PM,
rhoptries then secrete protein and membrane
These migrate back to the host cell PM and fuse creating a ring through which the parasite pushes host cell membrane to form a PV that includes parasite proteins and membrane
How do apicomplexans differ from viruses, bacteria and other protozoans
can actively invade host cells without using any host cell functions , e.g. cytoskeletal rearrangements
What can the apicomplexans do once the parasitophorous vacuole is formed (4)
parasite membrane and protein secreted by the rhoptries form a ring though which the parasite moves through.
The ring excludes many host cell transmembrane proteins, including those that normally promote vacuole acidification and fusion with lysosomes.
The PV also retains rhoptry proteins that will be useful in protecting the PV.
Dense granules
secrete proteins that decorate the membrane network within the PV and process host cell molecules for uptake by the parasite.
What is ROP18
whhat is its purpose
s a kinase that will phosphorylate IRGs and other host proteins
inactivate IRGs, protecting the PV from membrane attack
What is the mouse LD100 so different for type 1 apicomplexans compared to II and III
Type I parasites have ROP18 whereas Type II and III parasites do not
the LD100 is massively lower for type 1 parasites
What are IRGs
Immune Related GTPases (IRGs), some of which attack vacuolar membranes allowing other intracellular mechanisms to kill pathogens
Why do most toxoplasma strains have a very high LD100
they can be killed by IRG mechanisms
How does immune resistance to toxoplasma differ between species
Humans do not have the same number or function of IRGs, and it is likely that the variation in immune systems across warm-blooded animals will require different levels of specific IRG protection. Hence, polymorphism for
different immune-modulating mechanisms including ROP18 will be selected for overall.
How can toxoplasma adjust its resistance to the immune system of the current host
Combinations of different immune modulators can “tune” the response for specific hosts.
Toxoplasma can take advantage of both clonal growth via
transmission between intermediate hosts and sexual recombination via the cat.
What is the causative agent of malaria and when was this established
plasmodium
was not
until the late 19th century that plasmodium species were shown to be the
causative agents of malaria and the link with mosquitos as vectors established
Which species of mosquito transmits malaria
Anopheles
How prevalent is malaria
is the world’s most prevalent vector-borne disease, endemic in about 91 tropical and subtropical countries
About 41% of the world’s population is at risk, with 3-500 million clinical cases of malaria annually,
where over 90% are in sub-Saharan Africa. Of the approximately 2 million deaths each year, half are in children 5 years old and under.
What is the brief history of how malaria has evolved with humanity
plasmodium species that cause human malaria have co-evolved with humans for about 100,000 years, with a rapid expansion about 10,000 years ago around the time that agriculture developed. Since malaria has its greatest impact in pre-reproductive mortality, natural selection for resistance to the disease has left a lasting impact on the human genome.
What are the 4 main plasmodium species
Which is the most important
P. falciparum, P. vivax, P. malariae and P. ovale
P. falciparum is by far the most important malarial species on the basis of proportion of cases and ultimately disease burden
What is important to remember when diagnosing which type of malaria a patient has
Notably, a high proportion of infections are not
diagnosed at the species level and some are mixed infections.
where is there a low incidence of P. viviax
Why?
How does P. falciparum compare?
conspicuously absent or in low frequency across sub-Saharan Africa.
Human resistance has been selected for in this region
P. falciparum dominates sub-Saharan Africa and is therefore responsible for most of the malarial disease.
Describe the heat map for P. vivax
peak at just over 7%
infection rate across all age groups in peak transmission
season.
The primary foci are in Brazil and Indonesia
Describe the heat map of P. falciparum
has a peak at just over 70% infection rate in just the 2-10 year old age group in peak transmission season. This
is an order of magnitude higher than for P. vivax.
The foci are in West Africa and Mozambique but most of sub-Saharan Africa is high and given the comparative rate of
infection, is high in the Amazon basin and Southeast Asia.
Describe the first stage of the P. falciparum life cycle in the human
the liver stage
infection by the mosquito borne sporozoites is a relatively cryptic injection of 10-100 parasites that travel to the liver and invade hepatocytes. The amplification of the parasite is massive during liver stage merogony
Merozites are released into the circulation to infect RBCs (moves into 2nd stage of lifecycle in humans)
What happens in merogony in the P falciparum human life cycle
the growing parasite is no longer a single entity but a mass of dividing nuclei and other organelles that re-form into single merozoite parasites.
A single liver hepatocyte cell can contain about 10,000 merozoite
parasites in a schizont
What does schizont refer to
the group of parasites that represent a single original parasite PV
What is a problem with functional sporozoites vaccines
even if the vaccine kills all but one of the parasites, the infection will still have amplified by 4 orders of magnitude
What happens in the 2nd stage of the P falciparum lifecycle in humans
parasites replicate at a rate 3 orders of magnitude lower than in the hepatocyte
free merozoites invade a new RBC and transform into a “ring form” trophozoite. Trophozoite growth is the beginning of merogony in the RBC resulting in only tens of parasites being generated in a single schizont.
During trophozoite growth, the RBC surface is decorated with parasite-derived proteins that will assist the infected RBC (hence parasites) from destruction in the spleen. Other merozoites are pre-committed to become sexual cells and the ring form will develop into either a microgametocyte (male) or macrogametocyte (female). These if taken up together in a mosquito blood meal, they will fuse to carry out the sexual cycle within the mosquito
How does P falciparum enter the human
what happens once the parasite has entered the human
sporozoites may be injected into the human host by the mosquito during a blood meal.
The sporozoites invade liver hepatocytes
What happens to the P falciparum sporozoites once they have entered the hepatocyte
what happens next
transform and initiate a single round of merogony where the parasite amplifies
The released parasites infect RBCs, transform into ring form trophozoites and then growing
trophozoites to initiate a new round of merogony. Merozoites are released to re-infect new RBCs. Some of the merozoites transform into a single crescent-shaped microgametocyte or a single macrogametocyte.
What happens to the micro- and macrogametocytes after the mosquito has been infected with P falciparum
After ingestion the micro- and macrogametocytes exflagellate and fuse to form an ookinete.
The diploid ookinete traverses the gut wall and becomes an oocyst that attaches to the outside of the stomach
What happens to diploid ookinete after it reaches the outside of the mosquito during the P falciparum life cycle
parasite within goes through meiosis and replication, culminating in the release of sporozoites that travel to the salivary glands for injection into the human host upon next blood
meal.
What are the stages of the mosquito phase of the P falciparum lifecycle begin
with the ingestion of about 100 each
of micro- and macrogametocytes that exflagellate and pairs of micro and
macrogametocytes fuse to form an ookinete that traverses the stomach wall and develops into an oocyst that attaches to the outside of the stomach where
meiosis, transformation into sporozoites and parasite replication occur
How many sporozoites are released from each oocyst
Why is this an estimate
1000-5000
Since the maturation of the oocysts is not synchronized, estimating the number of sporozoites per oocyst is difficult. Evidence suggests that about 20% will get to the salivary glands for injection and only 10-100 will actually make it into the human host during a single feeding event.
Why do the symptoms of P falciparum vary so much (5)
because of factors including the current infection level, immunological experience, age, nutritional status and co-infections.
the most important correlate with severe disease is age.
What are the symptoms assocaited with the different phases of the P falciparum life cycle
Commonly the liver phase of infection is close to asymptomatic, but with the initiation of the RBC phase, common symptoms
include muscle ache/pain, nausea & vomiting and high fever.
Severe disease includes anemia, respiratory distress, renal failure and cerebral inflammation
What does immunity look like in areas where P falciparum is endemic
adults may be clinically immune and infants are protected by maternal care, which limits exposure and also often contributes maternal antibodies for immune protection
When do the symptoms of P falciparum begin
10-15 days post-exposure and sporozoites infection
How do symptoms of P falciparum vary with age in endemic areas
the reduction in symptoms generally correlates with age. Maternal protection of infants is lost, but over time into adulthood, the symptoms may cease altogether.
However, this appearance of clinical immunity does not mean that the individual is free of parasites.
When is the age of greatest danger of severe disease and mortality from P falciparum
Why
ages 1- 5
Maternal protection is lost and exposure to the parasite is limited, so both parasitemia and immunopathology are not very well modulated, resulting in a high frequency of respiratory distress, renal failure, anemia and cerebral inflammation.
What is the most important part of the P falciparum lifecycle
Initiation of the RBC cycle is critical for disease. The ability of the parasite to
firmly attach to the RBC is critical for invasion.
Describe the process of RBC invasion of Plasmodium species
How does it compare to that of toxoplasma
The plasmodium merozoite will move along the RBC using microneme secretion and then re-orient to have the apical end irreversibly attach to the RBC plasma membrane. RBC surface receptors and the cognate receptors at the apical end of the merozoite interact to achieve this.
Secretion by the
rhoptries is similar to toxoplasma and
the initiation of the PV begins.
The process of invasion is similar to toxoplasma, except that Plasmodium species’ selection of host cell receptors for binding is much more specialized.
What is the sole or major RBC receptor used by P vivax
The Duffy Antigen Receptor for Chemokines (DARC)
Why do we think P vivax was important for human evolution in West Africa
The virtual lack of P. vivax in sub-Saharan Africa suggests that P. vivax was historically a very important selective factor in West Africa as there is
a high frequency of DARC negative alleles in the population
Can you become resistant to P falciparum by evolving DARC negative RBCs
no: P. falciparum uses a variety of receptor interactions to achieve RBC binding. The ubiquitous presence of P. falciparum globally with intense foci in sub-Saharan Africa suggests that selection for human resistance to this species does not operate via a mechanism like DARC negative RBCs.
What is the reason P falciparum has a much higher rate of infection and parasitemia than P. vivax
P falciparum uses multiple receptors
What do we need to know about the immune response to P falciparum
very complex and beyond the scope of this lecture series. However, it is useful to illustrate how, like other pathogens with highly variable pathologies, infection can lead to variation in cytokine production and hence immunopathology in severe disease
What is the problem with the immune response to P falciparum for young children
What can this lead to
An imbalance of cytokines may occur and this will be particularly problematic in 1-5 year olds with little or no previous immunological experience with P. falciparum because many immunomodulatory mechanisms haven’t been developed.
over-production of either (or both) pro- and antiinflammatory cytokines such that the cytokine profile may inappropriately activate a variety of cells and tissues that conspire to generate the range of severe disease pathologies.
What has P falciparum had to evolve in the RBC phase of its lifecycle
Selection for the parasite to replicate includes destruction of its host cell. RBCs are exquisitely metabolically balanced to maintain the ability to carry oxygen and therefore ultrastructure/shape. Infection will disturb this balance to the extent that infected RBCs can be cleared in the spleen
evolved to avoid clearance by cytoadherence to uninfected RBCs and vascular endothelium
How does P falciparum affect the structure of the RBC
growing trophozoites will produce
and secrete proteins (eg PfEMPs) that will decorate the RBC plasma membrane and also cause the RBC to acquire “knobs” on the surface
these mediate cytoadherence for rosetting
What is rosetting in the P falciparum life cycle
adherence of infected RBCs to uninfected RBCs
What is a problem with local inflammation during a P falciparum infection
can this become systemic
consequence of cytokine imbalance such as excess IFN-γ and TNF-α is the activation and upregulation of adhesive molecules on the vascular endothelium as normally seen in local inflammation and neutrophil recruitment
yes - this can become systemic including brain vasculature, leading to cytoadherence of infected RBCs to the vascular endothelium
What does PfEMP interact with
CR-1 on RBCs
and
iCAM-1 on activated vascular endothelium
What happens when the P falciparum infected RBCs get sequestered in systemic infection
they are sequestered and kept from clearance in the spleen, so the parasites can mature into merozoites and be released for new infection. The sequestration probably exacerbates the brain swelling due to inflammation
What is a key feature of P falciparum infection from a diagnosis standpoint
the rarity of late trophozoites and schizonts in peripheral blood smears as a consequence of sequestration.
What are PfEMPs composed of
Which genes encode them
Where are the genes located
mixed set of adhesive domains
var gene family
at the subtelomeric regions of the majority of the 14 chromosomes
Why may a given P falciparum have a unique set of var genes
Why is this a problem for the immune system
The var genes undergo substantial intra-gene somatic recombination leading to a high variation
makes it difficult for the immune system to keep up with the antigenic variation in any given infection.
it has been hypothesized that for PfEMP variation is selected for in immunologically experienced individuals with clinical immunity to P falciparum. Why is this important?
for maintaining continuity in the parasite’s life cycle between high transmission seasons, where it may be months in between wet seasons and expansion of the mosquito vector - mosquito population goes down in dry season so have to survive until the next wet season so it varies its antigens to persist in the host
How does natural resistance to malaria appear
as selection for haemoglobinopathies in malaria endemic regions such as the haemoglobin sickle cell anaemia allelem(HbS), α and β thalassemias
What are the potential mechanisms for protection against malaria in a heterozygotic Sickle cell patient (4)
Impairment of P. falciparum RBC invasion and growth under low oxygen tension
• Enhanced removal of parasite-infected HbAS RBCs
• Reduced pathogenicity of P. falciparum infected RBCs via reduced PfEMP expression
• Improved acquisition of malaria-specific immunity
What are the mechanisms of protection against malaria provided by α thalassemia (3)
Protection against malaria-induced anemia
• Reduced pathogenicity of P. falciparum infected RBCs
via reduced rosetting & sequestration
• Immunological cross protection between species
What are the mechanisms of protection against malaria provided by β thalassemia (3)
• Enhanced removal of parasite infected RBCs
• Reduced invasion and growth of P. falciparum
• Reduced pathogenicity of P. falciparum infected RBCs
via reduced rosetting & sequestration
What is a potential protection mechanism provided by G6DP deficiency against malaria
• Loss of reducing power, free radicals hostile to parasite
What protection against malaria do most haemoglobinopathies provide
impairment of the parasite to replicate within the affected RBC:
Haemoglobin disorders affect critical balances in things like water potential and solute concentration, particularly in low oxygen tension. In turn parasite growth is impaired so the parasitemia is lower and the expression of critical adhesive proteins like PfEMPs are lower so severe disease is less likely
Give a notable example of where each of the following were once endemic:
US
Japan
Malaria was once endemic to the eastern US and
schistosomiasis in Japan (now both locally extinct)
Why were polio and smallpox easy targets for complete eradication
How does control differ for schistosomiasis and malaria
Protection of the individual and herd immunity via mass vaccination worked because the viruses were sufficiently conserved that vaccination programmes could break the transmission cycle
vector control via improved water, drainage and hygiene were critical infrastructure measures,
but mosquito control tipped the balance for malaria in the US
Which drug is known as the ‘wonder drug’
why
Ivermectin
its spectrum of use ranges from worm parasites to arthropod ectoparasites
How was Ivermectin discovered
1970s
Streptomyces avermitilis
contained a Macrocyclic lactone that binds to glutamate gated
chloride channels and kills parasitic worms and some arthropods
What is the difference between avermectin and ivermectin
the original active compound named avermectin was further developed for better activity and safety into ivermectin, classified by itself as an “endectocide”.
What is Onchocerciasis
African River Blindness caused by the round worm Onchocerca volvulous
Was ivermectin given to treat human onchoceriasis immediately
no originally seen to be effective against the microfilariae of onchocerca cervicalis in horses
How is onchocerca volvulus transmitted
The black fly will pick up microfilaria upon a blood meal from an infected human.
In the black fly, the microfilaria will undergo a transformation into an infective larval worm.
In a subsequent blood meal, the larval worm will enter the human tissue and develop into an adult worm.
What is the vector for the transmission of onchocerca volvulus
the black fly insect vector (Simulium spp.)
Where do adult onchocerca volvulus worms reside
what do they do
in subcutaneous nodules
produce microfilaria that will travel in blood and lymphatics to distal tissue
How long do adult O. volvulus adults live for
microfilaria ?
What causes disease?
Adults may live as long as 15 years
microfilaria live about 2 years
inflammatory response to the microfilaria - this can lead to blindness if in the eye
Where is onchocerciasis focussed
in sub-Saharan Africa but occurs sporadically in South America
How is global control of River lindness going
eliminated in Colombia, Ecuador, Mexico and Guatemala
• 100 million people treated in 2013
delivery/coverage
and some drug resistance has hampered some of the efforts in sub-Saharan Africa.
Nevertheless, ivermectin use has at least locally eliminated river blindness
Why is control/ treatment of malaria so important
40% of the world’s population is at risk
How long have treatments for malaria been around
why is this surprising
Although it has only been 130 years since Plasmodium spp. were identified as the causative agent of malaria and the life cycle confirmed, treatments for malaria have been around for millennia
How do Native South Americans treat malarial fever
with a preparation from the bark of the quina quina or Cinchona tree
Jesuit priests brought this back to Europe in the 1600s
What is the active ingredient in the quina quina preparation to treat malarial fever?
For how long was it the treatment of choice for malaria
Quinine (an alkaloid)
for the next 100 years until synthetic drugs like chloroquine
What is the first line drug against malaria
Despite drug resistance in P. falciparum, chloroquine is still the first line drug for P. vivax and P. ovale infections.
Which remedy for malarial symptoms has been used for 2000 years in Asia
How was it used in the 20th century
qinghoa
fractionated to find an extract that was 100% effective against P. berghi in mice and P cynomolgi in monkeys
was even effective against P falciparum (which is resistant to chloroquine)
What is the active ingredient in a qinghoa preparation
what has been further developed from it
a sesquiterpene lactone, named qinghaosu, now known as artemisinin
dihydroartemisinin (better activity)
What forms the basis for synthesising artesunate and artemether
dihydroartemisinin
What are the 2 proposed hypotheses for the MOA of artemisinin and its derivatives
The endoperoxide bridge breaks and creates a free radical which puts a lot of oxidative and metabolic stress on the parasite
• PfATP6 inhibition leading to intracellular accumulation of calcium and subsequent cell death.
What is ACT
What is it important for
Artemisinin-based Combination Therapies
reducing the development of drug resistance
What is the first line of therapy for uncomplicated P falciparum infections
What are the possibilities to take in one tablet (2)
ACT
Artemether/lumefantrine
• Artesunate/amodiaquine
Name 6 companion drugs for ACT
lumefantrine, mefloquine, amodiaquine, sulfadoxine/pyrimethamine, piperaquine chloroproguanil/dapsone.
Why is cultivating Artemisia annua difficult
what does this mean
it is a weed so is not reliable in terms of biomass and yield of artemisinin.
Importantly, the market is unstable so prices and shortages will fluctuate accordingly.
How have we tried to stabilise the market for A. annua
genetically modify microorganisms to produce artemisinin
yeast designed to produce artemisinic acid as a precursor for SSA
What made malaria elimination possible in the US
water and drainage control,
but mosquito control probably tipped the balance towards elimination
Describe the steps involved in eradication of malaria in the US
The project commenced in 1947
- By end of 1949, 0ver 4.6 million houses sprayed with DDT
- In 1950 the US was declared free of malaria
• Drainage and aircraft spraying in
areas of recent cases
- In 1951 the CDC ceased to actively participate
- In 1952 the CDC went to surveillance only
What happened to the Global Malaria Eradication Programme in the late 1950s
The success in the US provided inertia
The campaign succeeded in eliminating malaria from Europe, most of North America, parts of Central and South America, the Caribbean and parts of Asia but failed is sub-Saharan Africa where the bulk of malaria cases are today.
What are the major emphases of the Roll Back Malaria initiative (beginning 1998)
Long Lasting Insecticidal Nets (LLINs) and Indoor Residual Spraying (IRS)
Give 5 facts about Long Lasting Insecticidal Nets
LLINs contain pyrethroid insecticides only
• LLIN minimum lifespan of 20 washes or 3 years use in field conditions
• 2000-2015 over a billion LLINs were delivered to endemic countries
• 29% households could protect all family members
• Coverage of children under 5 years in sub-Saharan Africa
– <2% in 2000
– ~68% in 2015
Give 2 facts about Indoor Residue Spraying coverage
DDT is used for IRS as it lasts ~6 months
• 16 million people protected in 2014 (4% at-risk)
Where are the remaining hotspots for P falciparum infection
areas of recent civil war/ unrest
5 things WHO and UNICEF have managed to do since 2000
- 60% reduction in malaria deaths, focused in sub-Saharan Africa
- Under 5 malaria death rate decreased by 65%
- Caucuses and Central Asia fastest decreases reported zero cases in 2014
- Six countries reported fewer that 10 cases
- 13 countries reported zero cases
What vector control effective for control of malaria
local elimination
What have Oxitec done in the fight against malaria
engineered a 2nd generation Anopheles mosquito that carries a female lethal gene that is active only when inherited in
non-laboratory settings.
The process is self-limiting so unlike gene drives is not designed to drive populations to extinction but rather simply knock down local populations for several generations.
How do gene drives differ from the Oxitec method
Gene drives based on CRISPR/Cas 9 genome editing tools, new twist on the sterile male mosquito control has the potential to knock down mosquito populations locally and even to extinction. Clearly there are ethical and environmental concerns to be addressed
