Protozoan Parasites (7-8)

Question 1

What are ampicomplexan parasites?

Answer 1

→ eukaryotic obligate parasites (one or multiple hosts depending on species)
→ disease agents: malaria parasites (Plasmodia), coccidia (inc. cryptosporidium), piroplasms (babesia)

Question 2

What is apicomplexan’s evolutionary journey?

Answer 2

2 endosymbiosis events → ancestral cell consumed red algae, then underwent secondary endosymbiosis
→ endosymbiotic gene transfer, loss of nucleus and mitochondria of symbiont
→ loss of photosynthetic ability and loss of organelles lead to apicomplexans e.g. Cryptosporidium

→ relevant for development of treatment e.g. herbicides could target apicomplexa and not their host

Question 3

What is the apical complex of apicomplexans?

Answer 3

Distinctive structure at apical end to amicomplexans
→ key cellular machinery for host cell invasion (amicoplexans must invade host cells)
→ essential for movement
→ includes secretory organelles: micronemes, rhoptries, polar rings (composed of micro tubules), conoid (very tip - pokes out in response to Ca)
→ subpellicular membranes enable parasite feeding from host

Question 4

What is involved in host cell invasion of amicoplexans?

Answer 4

Conoids → protrude into host cells in response to calcium
Rhoptries → secretory organelles releasing rhoptry proteins into host during invasion
Micronemes → secrete proteins into host cell enabling entry
A parasitophorus vacuole is formed → parasite moved into this vacuole
Polar ring → microtubule organising centre

Question 5

What is the amicoplexan life cycle?

Answer 5

Can involve one or more hosts
Sexual → gametes fuse to form zygote
Asexual → sporozoites differentiate to form merozoites
Merozoites undergo multiple rounds of replication → don’t differentiate again just multiply by merogony
→ some differentiate into gamonts to invade
Gamonts differentiate into gametes (sporozoites are typically invasive part)

Question 6

What is malaria?

Answer 6

A life-threatening disease → caused by plasmodian amicoplexan parasites
Mosquito vectors are necessary for disease transmission
Half of the world’s population is at risk → mainly in tropical areas, 247 million cases in 2021 (huge no.), 619000 deaths
Disease can cause considerable morbidity and mortality
WHO Africa region carries disproportionately high share of global burden (95%) → 80% of deaths were children under 5

Question 7

Where is malaria distributed globally?

Answer 7

Band across the tropics where malaria is dominant

Question 8

What does malaria impact on?

Answer 8

Impact on → individuals, families, communities, countries, regions
Multiple interconnected impacts → health, productivity, economic, education, health services

Question 9

What are the causative agents of malaria?

Answer 9

Plasmodium falciparum - most virulent species
Plasmodium vivax - concurrent infection is not uncommon
Plasmodium malariae
Plasmodium ovale
Plasmodium knowlesi - emergent zoonotic (crossing from monkeys to humans)
→ top two cause majority of mortality and morbidity

Vector: female (only ones that take blood meals) Anopheles mosquito

Question 10

What is involved in plasmodium life cycles?

Answer 10

Always involves mammal and a female mosquito as hosts
In insects: gametes fuse to form zygote which forms oocyst
In humans: sporozoites differentiate to form merozoites, - undergo multiple rounds of replication, some differentiate into gamonts - differentiate into gametes
Has human liver and blood stages

Multiple rounds of merogony occurs in RBC → burst into blood, causes disease symptoms, synchronous RBC bursting causes spike in symptoms

Question 11

What is the disease presentation of malaria?

Answer 11

Asymptomatic malaria → circulating parasites but not symptoms (may have level of immunity)
Uncomplicated malaria → nonspecific symptoms: fever, shaking chills, profuse sweating, headache, nausea, vomiting, diarrhoea, anaemia (RBC bursting)
Complicated malaria → as above with additional organ damage and anaemia, associated with higher levels of parasetemia and increased mortality
→ impaired consciousness, multiple convulsions, difficulty breathing, dark or bloody urine, jaundice, abnormal bleeding

Question 12

What causes malaria disease symptoms?

Answer 12

Blood stage parasites → undergo synchronous schizogony, organ damage and anaemia due to ruptured erythrocytes
→ the simultaneous rupture of the infected erythrocytes and release of antigens and waste products accounts for the intermittent fever associated with malaria occurring either 48 or 72h intervals
Pathology and clinical manifestations associated with malaria are almost exclusively due to the asexual erthrocytic stage parasites (tissue schizonts cause little pathology)
Disease has a tendency to relapse or precedence over months or years - dormant phase in liver
Most susceptible → children, pregnant women, unborn children, those living with HIV/AIDS
Long term impact on health life chances and economics

Question 13

What are the specific complications with Plasmodium falciparum?

Answer 13

Cerebral malaria → often fatal, extremely high temp associated with convulsions and coma
→ infected erythrocytes adhere to capillary endothelial cells become blocked - become knobbly as a result of parasite proteins exported to the membrane, bind to ligands on host cells
Backwater fever → massive lysis or erythrocytes causes high levels of free haemoglobin in the blood and renal failure, the presence of haemoglobin in the urine gives the condition its name

Question 14

What adaptations of plasmodia and humans has the co-evolution lead to?

Answer 14

Plasmodia → array of hypervariable membrane proteins to switch between and evade immune system
→ P.falciparum genome contains large family of 60 var genes that encode hypervariable erythrocyte membrane protein 1 (PfEMP1) - during erythrocytic stage each parasite expresses one of its var genes, switching enables it to evade destruction by host immune system

Humans → higher proportions of people lacking Duffy blood group receptor in endemic areas of P.vivax which cannot bind to receptors and invade cells
→ several inherited erythrocyte disorders in endemic areas offering protection against infection e.g. sickle cell anaemia - having heterozygous alleles offers protection - a glutamic acid residue is replaced by a valine which reduces oxygen carrying capacity (homozygous often die before 30)

Question 15

How is malaria diagnosed?

Answer 15

WHO criteria: fever and presence of parasites
Light microscopic examination of blood smear and rapid diagnostic tests

Question 16

What are the treatment recommendations for malaria?

Answer 16

P.falciparum → artemisinin combination therapy (ACT) - artemisinin derivative plus quinine derivation
P.vivax and P.ovale → chloroquine (cheaper) unless drug resistance issue in area then ACT
→ drug resistance is a major concern so combination therapy sometimes used

Question 17

What are the vector control methods for malaria?

Answer 17

Widespread insecticide/larvicide (spray walls)
Destruction of breeding grounds
Indoor residual spraying (IRS)
Long-acting insecticide treated bednets (ITN)
Experimental methods in pipeline
→ take organisational, economic and political drive

Question 18

How do insecticide treated bed nets help control malaria?

Answer 18

Have been shown to reduce severe disease and mortality due to malaria in endemic regions
→ reduce all-cause mortality by about 20%
→ currently only pyrethroid insecticides approved - very low mammalian toxicity, highly toxic to insects and have a rapid knock-down effect even at low doses - have a high residual effect: don’t rapidly break down unless washed or exposed to sunlight
→ need for frequent retreatments (6-12 months) - barrier for full implementation in endemic counties
Long lasting insecticidal nets are the preferred form for public health programmes

Question 19

How is malaria controlled on the human level?

Answer 19

Chemoprotection → prophylactic dose to travellers to endemic areas
Chemoprevention → full treatment dose to children and pregnant women, seasonal (rainy season), mass drug administration
Vaccine → possibility
Treatment of disease → rapid diagnosis + treatment prevents transfer

Question 20

How can the drugs used to treat malaria work?

Answer 20

Kill parasites in the liver → causal prophylaxis
Kill asexual parasites in RBC → suppressive prophylaxis
Kill sexual parasites (gametocytes) in RBC → gametocytocidal prophylaxis

Question 21

What are the recent advances for a malaria vaccine?

Answer 21

RTS,S → virus-life particle (VLP) vaccine based on the P. falciparum circumsporoziote protein (CSP) - which attacks the liver
→ target’s sporozoites injected by mosquito, three primary doses plus booster, 55% efficacy

Question 22

What was the impact of COVID-19 on malaria?

Answer 22

Fragile health system capacity overwhelmed
Misdiagnoses as COVID-19 or flu
Decreased funding
Malaria control delivery systems impacted by: absenteeism, supply chain disruption, collecting ITNs

Question 23

What are the experimental controls for malaria?

Answer 23

Endectocides (systemic insecticides e.g. ivermectin)
Sterile insect techniques
CRISPR/CAS9 gene drives
Biocontrol using bacteria or fungi that infect the mosquito and represent development of the plasmodium

Question 24

What causes cryptosporidiosis?

Answer 24

Cryptosporidium parvum
Cryptosporidium hominis
→ multiple species of cryptosporidium that infect humans, two main ones
→ waterborne pathogens
→ symptoms: diarrhoea and flu-like

Question 25

What is cryptosporidiosis?

Answer 25

Serious and potentially life-threatening in immunodeficiency patients (especially AIDS) → characterised by profuse watery diarrhoea → second leading cause of diarrhoea in young children and a major contributor for diarrhoeal deaths in LMICs (malnutrition) → community outbreaks can occur → first human cases reported in 1976

Question 26

What is the cryptosporidium life cycle?

Answer 26

One host is required → for *C. hominis* host is a human → for *C. parvum* host is a human or animal (e.g. cattle) 1. Thick-walled oocyst (sporulated) exits host 2. Contamination of water and food with oocysts 3. Thick-walled oocyst ingested by host Merozoites undergo multiple rounds of replication in the gut

Question 27

What are the transmission routes for cryptosporidium?

Answer 27

→ swallowing water or beverages contaminated by stool from injected humans or animals → swallowing recreational water contaminated → eating contaminated uncooked food → touching your mouth with contaminated hands (Important that public water supplies are monitored and treated)

Question 28

What are the risk factors of cryptosporidium?

Answer 28

Small size of oocysts → resistant to sedimentation Wide range of host specificity Monoxenous development (only 1 host needed) → life cycle easy to maintain Large number of oocysts excreted (up to 100 billion per calf) Low infective dose Robust oocysts which are resistant to chlorine A complex protective barrier consisting of a double layer of a protein-lipid-carbohydrate matrix → makes it harder to manage

Question 29

How is cryptosporidiosis acquired occupationally?

Answer 29

Workers in sewage and waste water Workers in outdoor leisure industries in contact with water Farm workers Abattoir workers, meat processing plant and butchers Veterinary surgeons Healthcare and care workers Construction/demolition/building renovation workers e.g. where there is stagnant water

Question 30

How is cryptosporidiosis distributed?

Answer 30

The number of reported cases in England and Wales around 4000 annually → many cases are thought to be unreported, the actually number of cases is probably several times higher

Question 31

What is the disease presentation for cryptosporidiosis?

Answer 31

Symptoms start 2-10 days after infection → some are asymptomatic In immuno-competent people the main symptom is water diarrhoea for 1-2 weeks → stomach cramps or pain, nausea and vomiting (absorption of nutrients affected), weight loss, dehydration, fever In immunodeficient people it can become more serious, chronic and life-threatening Children under 5 at more risk especially where malnourished → even a single episode can result in growth deficits during the first 2y of life and impact growth long term

Question 32

How does cryptosporidium cause disease symptoms?

Answer 32

Exact mechanisms still unclear Invasion of parasites in gut epithelial trigger disease and malabsorption of nutrients

Question 33

How is cryptosporidiosis diagnosed?

Answer 33

Microscopic examination of stool samples for presence of oocysts Immunoassay Molecular methods → PCR, real-time PCR

Question 34

What is the treatment recommendations for cryptosporidiosis?

Answer 34

Self-limiting illness in immunocompetent individuals → general supportive care is only treatment → oral or intravenous rehydration and replacement of electrolytes may be necessary → nitoxazonide is partially effective in immunocompetent patients, not immunosuppressed Patients with HIV/AIDS are advised to take anti retrovirals to improve immune response

Question 35

How is cryptosporidiosis controlled?

Answer 35

Primary focus is to prevent contamination of the source → remove contamination at source - water supply, swimming pool - monitoring Hygiene measures as for prevention of all faecal-oral diseases → especially for high risk professions - nursery staff, agricultural workers No vaccines on the horizon currently

Question 36

Where was there an outbreak of cryptosporidiosis in 1993?

Answer 36

Community outbreak in Milwaukee in spring of 1993 → estimated 400,000 people developed symptomatic cryptosporidiosis with 50 deaths (underrepresented)

Question 37

What are the future directions for cryptosporidiosis?

Answer 37

Development of an effective drug → particularly for children and immunosuppressed people

Question 38

What is toxoplasmosis?

Answer 38

T. gondii causes chronic infections in up to one third of the human population and in animals → in healthy individuals a primary infection with toxoplasma causes relatively mild symptoms → in immunocompromised patients or developing foetus it can cause life-threatening infection with severe neurological and ocular manifestations → felines are the only definitive host → Toxoplasma has a complex life cycle consisting of intestinal and tissue phases - intestinal phase: occurs in felines and exhibits typical intestinal life cycle, merogony and gametogony, sexual cycle - produces oocysts which are excreted in the faeces

Question 39

What is the lifecycle of toxoplasma?

Answer 39

Intermediate hosts → ingest sporulated oocysts Sporozoites are released, penetrate the intestinal epithelium and invade macrophages and other types of cells The parasite undergoes binary fission (i.e. merogony) to form tachyzoites (Tachy means rapid) The host cell will rupture and release the tachyziotes which will invade new hosts cells and repeat the replicative cycle Infected macrophages will disseminate the tachyzoites throughout the host during this acute infection

Question 40

How is toxoplasmosis distributed?

Answer 40

In the UK about 30% of people will be infected by the age of 30 In the US about 30-35% have antibodies In France more than 65% of women tested for antibodies has toxoplasmosis

Question 41

What are the transmission routes of toxoplasma?

Answer 41

Ingestion of material contaminated with sporulated oocysts (cat faeces) Ingestion of undercooked meat containing tissue cysts or tachyzoites → particularly lamb and pork Infections rates of 50% or higher in domestic chickens, geese, cattle, goats, pigs and sheep → 38% of meat samples in UK positive Congenital transmission → can only occur during an acute infection (I.e. tachyzoites) acquired during pregnancy, mothers with chronic infection acquired before pregnancy are not at risk of transmitting → 2 in 1000 women catch toxoplasmosis each year during pregnancy, about 1400 each year in the UK → 30-40% pass it into the unborn baby, risk is greatest in the third trimester at 70%, early it’s 15%

Question 42

How does acute toxoplasmosis disease present?

Answer 42

In immunocompetent individuals → doesn’t normally cause symptoms, can cause flu-like symptoms, typically self resolving within 6 weeks Those more at risk are: those with weakened immune systems, pregnant women and their unborn children → infection may lead to miscarriage, stillbirth or survival with growth problems, blindness, water on the brain (hydrocephalus), brain damage, epilepsy or deafness - often develops after birth so infants kept under observation

Question 43

How does chronic toxoplasmosis disease present?

Answer 43

Effects of congenital toxoplasmosis can be life long: Ocular disease → can reactivate causing more damage each time Reactivation of dormant tissue cysts can occur In immunocompromised people → fever, confusion, headache, seizures, nausea and poor coordination Can lead to toxoolasmic encephalitis Chronic psychological and neurological effects

Question 44

What is the dormant / resting stage of toxoplasma?

Answer 44

As the host develops immunity the replication stage will slow and infected host cells will become encapsulated (I.e. tissue cysts) → these slowly replicating forms are called bradyzoites (Brady means slow) and represent a dormant stage - secrete chitin and other components to form a cyst wall (very resistant to immune system)

Question 45

What is reactivation of toxoplasmosis?

Answer 45

Reactivation is associated with waning immunity → malignancies, transplantation, AIDS Results from the release of the encysted organisms and the initiation of the tachyzoite stage of infection → followed by local tissue damage and inflammation During the 1980s toxoplasmic encephalitis emerged as a common complication associated with AIDS - progressed to neurological defects and convulsions, dementia in late stage AIDS associated with toxoplasma

Question 46

What is ocular toxoplasmosis?

Answer 46

Occurs from activation of cysts deposited in or near the retina

Question 47

Where was a toxoplasmosis outbreak in 1994/1995?

Answer 47

Canadian community → serological evidence consistent with acute toxoplasmosis was demonstrated in 100 people → epidemiologic evidence implicated contamination of the city water supply with oocysts

Question 48

How is toxoplasmosis diagnosed?

Answer 48

Serologic testing antibody IgG or IgM Molecular testing e.g. PCR Can detect parasite in biopsies Ocular disease by case history and serologic testing

Question 49

How is toxoplasmosis treated

Answer 49

Synergistic combination of pyrimethamine and sulfadiazine Spiramycin for prophylactic use during pregnancy

Question 50

How is toxoplasmosis controlled?

Answer 50

Cooking meat to safe temperatures Wash fruit and vegetables Avoid drinking untreated water Wear gloves for gardening and wash hands afterwards Clean cat little trays immediately (avoid if pregnant) - oocysts don’t have time to develop into sporozoites

Question 51

Is there a vaccine for toxoplasmosis?

Answer 51

Toxovax → prevention of toxoplasmosis abortion in ewes, may revert to pathogenic strain so not suitable for humans Vaccine of great importance for cats → prevent or reduce the shedding of oocysts → effective oral vaccine was developed and validated, but production and purification was costly and unprofitable for the manufacturer No promising vaccine

Question 52

What are the chronic effects of latent toxoplasmosis?

Answer 52

Rodent psychology → host behavioural changes could benefit parasite, infected rodents more likely to investigate novel stimuli and appear less cautious when presented with cats - increase the odds of completing the *T. gondii* life cycle Human psychology → no adaptive benefit to parasite, case-control studies show interesting correlations with human behaviours, mental health disorders and toxoplasma seropositivity - schizophrenia, autism, risk taking e.g. risk of traffic accident