Protozoan Infections

Question 1

Below is a list of protozoal parasites. Match each parasite with a mechanism to avoid destruction by the host immune system.

Toxoplasma gondii
Plasmodium falciparum
Human African Trypanosomes
Leishmania ssp

A. Formation of dormant forms in immune-privileged sites
B. Sequestration in blood vessels deep within major organs
C. Residing intracellularly within a modified host cell organelle
D. Antigenic variation by the switching of variant surface glycoproteins

Answer 1

Toxoplasma gondii A
Plasmodium falciparum B
Human African Trypanosomes D
Leishmania ssp

1. T. gondii forms a long-term chronic infection by conversion from the replicative and lytic tachyzoite stage to the quiescent bradyzoite stage. This ensures transmission when the host is predated.
2. P. falciparum avoids destruction of infected erythrocytes during circulation through the spleen by exporting PfEMP1 to the erythrocyte surface. These mediate cytoadhesion to ligands on blood vessel epithelia.
3. HATs encode 1500-2000 VSGs which are expressed sequentially to avoid detection by host antibodies.
4. During uptake by phagocytic cells, Leishmania ssp release proteins that neutralise the phagolysosome and inactivate proteases.

Question 2

Which one of the following statements is TRUE?

a. Both male and female tsetse and male and female mosquitoes are involved in the transmission of sleeping sickness and malaria respectively
b. Only female tsetse and female mosquitoes are involved in the transmission of sleeping sickness and malaria respectively
c. Only male tsetse and male mosquitoes are involved in the transmission of sleeping sickness and malaria respectively
d. Both female and male tsetse, and only female mosquitoes are involved in the transmission of the causative agents of sleeping sickness and malaria respectively

Answer 2

D

Both male and female tsetse feed on vertebrate blood and as such can be involved in the transmission cycle, only female mosquitoes feed on blood, male mosquitoes feed on sugar from plants.

Question 3

African trypanosomes are covered in a coat of VSGs - what does VSG stand for?

Very stable glycoprotein

Variant surface glycoprotein

Virtual surface glycoprotein

Vaccinal stable glycoprotein

Answer 3

VSG stands for variant surface glycoprotein.

Question 4

Which of the following statements are TRUE?

A. Leishmania modifies a host organelle to form its parasitophorous vacuole
B. Plasmodium falciparum actively invades host erythrocytes
C. Leishmania actively invades host macrophages
D. Toxoplasma gondii modifies its host cell membrane with variant antigens to escape immune surveillance
E. Plasmodium falciparum modifies its host cell membrane with variant antigens to escape immune surveillance

Answer 4

A– True. Leishmania ssp modify the host phagolysosome to form an intracellular niche

B– True. During invasion, inward motion is driven by an actin-myosin motor underlying the parasite membrane. The parasite enters at the moving junction and invaginates the host cell membrane to form the parasitophorous vacuole membrane.

C– False. Leishmania are taken up by phagocytosis, and then modify the phagolysosome to escape destruction.

D– False. T. gondii switches from a proliferative to dormant form to avoid clearance by the immune system.

E– True. P. falciparum traffics members of the variant PfEMP1 family to the surface of infected erythrocytes to mediate sequestration and avoid destruction in the spleen.