Parasitology

Question 1

What two forms can parasites occur in?

Answer 1

single celled – protozoa

- multicellular – metazoan - helminths or worms

Question 2

What is the definitive host?

Answer 2

One in which the sexual cycle occurs or the adult is present

Question 3

What is the immediate host?

Answer 3

One in which the asexual system occurs or the larva is present

Question 4

How many species of plasmodium parasite are there?

Answer 4

5

Question 5

What do the life cycles of all plasmodia consist of?

Answer 5

A sexual phase and an asexual phase

Question 6

Where does the sexual phase mainly occur?

Answer 6

Mosquito vectors

Question 7

Where does the asexual phase mainly occur?

Answer 7

Humans

Question 8

Where can the life cycle be said to begin?

Answer 8

When the mosquito acts as a vector and transmits the plasmodium parasite in the sporozite stage into the host during a blood meal

Question 9

What happens after the sporozoites enter the body?

Answer 9

They go to the liver and hepatocytes → they invade the hepatocytes by forming a parasitophorous vacuole where they differentiate into many thousands of merozoites

Question 10

What happens after the sporozoites have become merozoites?

Answer 10

The infected heptocyte ruptures and merozoites are released into the blood → invade RBCs and begin the ASEXUAL reproductive phase

Question 11

What does asexual reproduction involve?

Answer 11

Merozoites multiply → differentiae into ring-shaped trophozoites → these then mature into schizonts → these rupture, release merozoites that can infect other RBCs

Question 12

What can a small number of merozoites differentiate into?

Answer 12

into male (microgamete) and female (macrogamete) gametocytes

Question 13

What can happen to the gametocytes?

Answer 13

Taken up by the mosquito in her blood meal

Question 14

What happens to the gametocytes once they are ingested?

Answer 14

Migrate to the midgut of the mosquito → pass through the wall of the midgut to form oocysts

Question 15

What happens to the oocysts in the midgut?

Answer 15

They divide meiotically to form the sporozoites, which further move to the salivary glands and they are ready to restart the cycle of transmission - this is the sexual cycle

Question 16

Summarise the asexual cycle

Answer 16

sporozoite → hepatocyte → merozoite → RBC → trophozoite -→ schizonts → merozoites → gametocytes

Question 17

Summarise the sexual cycle

Answer 17

In mosquito stomach, microgametes penetrate macrogamates → zygotes → become motile ookinetes → invade midgut wall + form oocysts → sporozoites in salivary glands

Question 18

Which stage accounts for malaria symtpoms?

Answer 18

Erythrocytic phase

Question 19

Describe pathogenesis

Answer 19

haemolysis of the infected RBCs, releasing the parasites as well as malaria endotoxin

These trigger TLR9 and this leads to high levels of TNF production and clinical symtpoms, such as fever, heache, myalgia, arthralgia and chills

These symptoms recur every third day, due to the fact that the P. faliciparum asexual erythrocytic phase takes 48h to complete, hence there is periodic erythrocyte bursting and release of merozoites

Furthermore, the membrane of the infected RBC stiffens, and this can lead to capillary obstruction, which can affect vital organs in severe malaria

Anaemia is another common feature, due to erythrocyte loss and hence loss of oxygen-carrying capaciy

P. falciparum has an incubation period of about 2 weeks - this is the period it spends replicating in the liver before it invades erythrocytes

Question 20

What is chloroquine resistance mediated by?

Answer 20

Mutation in the gene encoding chloroquine transport in the cell membrane of the organism

Question 21

Mechanism of chloroquine

Answer 21

Caps the hemozoin to prevent biocrystallisation of heme - leads to heme build up - chloroquine binds to heme which is very toxic to the cell

Question 22

What are trypanosomes characterised by?

Answer 22

Unicellular parasitic flagellate protozoa

Question 23

What is T.brucei responsible for?

Answer 23

African trypanosomiasis (also known as sleeping sickness) in humans and the wasting disease nagana in livestock ONLY in sub-Saharan Africa

Question 24

What is T.cruzi responsible for?

Answer 24

Chagas’ disease, most prominent in Central and South America

Question 25

What three components is the disease caused by T.brucei composed of?

Answer 25

• the trypanosome, the tsetse fly and the host

Question 26

Describe T.brucei life cycle

Answer 26

injected metacyclic trypomastigotes → bloodstream trypomastigotes → proliferate in bloodstream as slender forms which are replaced by non-proliferative stumpy forms when numbers increase (increases chances of host survival) → blood meal by tsetse fly → transformation into procyclic trypomastigotes in midgut (kinetoplast repositioning, surface coat changes, metabolism changes) → they arrest division and migrate to tsetse salivary gland → attach as epimastigotes (elaborationof flagellum) → generation of non-proliferative metacyclic forms (new coat)

Question 27

How is the T.brucei able to survive in the bloodstream?

Answer 27

Evading immune system w antigenic variation → sequential expression of antigenically distinct variable surface glycoproteisn

Question 28

How do trypomastigotes replicate?

Answer 28

Binary fission

Question 29

Pathogenes of T.brucei

Answer 29

Stage 1 → haemolymphatic (fever, headaches, joint pains)

Stage 2 → meningoencephalitic → pass BBB neurological complication (disruption of sleep cycle)

Question 30

What are the hosts of T.cruzi?

Answer 30

Humans (or some animals) and the triatomine bug as a vector

Question 31

Life cycle of T.cruzi?

Answer 31

Triatomine blood meal → ingests trypomastigotes form blood → epimastigotes in midgut → trpomastigotes in hindgut → during meal, trypanomastigotes enter wound or mucosal membrane → penetrate varios cells at the bite wound → transformation into non-flagellated amastigotes which multiply by binary fission in cells of infected tissues → differentiate into trypomastigotes which burst out of cells and enter blood → taken up by bug

Question 32

What are the most frequent cells infected by T cruzi?

Answer 32

Glial, myocaridal and reticuloendothelial

Question 33

Pathogenesis of T. cruzi?

Answer 33

Acute phase of infection (both trpomastigotes in blood and amastigotes in cells) → local inflammation → mengingoencephalitis, hepatosplenomegaly etc
Cardiac muscle targetted → arrhythmias

Question 34

What happens after acute T. cruzi infection?

Answer 34

Most people aymptomatic → some porgress to chronic

Nerve damage, cardiomyopathy, megacolon Death

Question 35

What is leishmaniasis?

Answer 35

vector-borne disease that is transmitted by sandflies and caused by
obligate intracellular protozoa of the genus Leishmania.

Question 36

Life cycle of Leishmania

Answer 36

Sandfly take blood meal, injects promastigote into skin → promastigotes are phagocytosed by macrophages → promastigotes transform into amastigotes inside macrophages → amastigotes multiply in cells of various tissues → sandfly takes blood meal and ingests macrphages with amastigotes → amastigotes transform into promastgote stage in midgut → divide in midgut and migrate to proboscis → blood meal

Question 37

Clinical features of Leishmaniasis

Answer 37

1. In cutaneous forms, skin ulcers usually form on exposed areas, such as the face, arms
   and legs. These usually heal within a few months, leaving scars
2. In mucocutaneous forms, the lesions can partially or totally destroy the mucous
   membranes of the nose, mouth and throat cavities and surrounding tissues.
3. Visceral leishmaniasis, also known as kala azar, is characterized by high fever,
   substantial weight loss, swelling of the spleen and liver, and anaemia. If left
   untreated, the disease can have a fatality rate as high as 100% within two years.