Malaria Transmission and Life Cycle

Malaria Transmission

• Anopheles mosquito transmits malaria to humans
• Sporozoites are the infectious stage of malaria transmitted to humans

Sporozoite Life Cycle

• Sporozoites travel to mosquito's salivary glands
• When mosquito feeds on host's blood, it injects saliva to prevent blood clotting
• Some sporozoites in saliva enter host during feeding process

Exoerythrocytic Cycle

• Sporozoites enter blood/peripheral blood and migrate to liver
• Specifically, sporozoites target parenchymal cells of the liver
• Sporozoites multiply inside liver cells, marking the start of the exoerythrocytic cycle

Exoerythrocytic Cycle

• The first cycle of Plasmodium sporozoites in the human body occurs before replication in Red Blood Cells (RBC)
• This cycle involves reproduction outside the RBC
• The duration of the exoerythrocytic cycle varies between 8-25 days, depending on the species of Plasmodium
• A single liver cell can produce thousands of sporozoites during this cycle

Transition to Erythrocytic Cycle

• The liver cell eventually ruptures, releasing Merozoites into the bloodstream
• The Merozoites then travel through the bloodstream, searching for RBC to replicate in, marking the start of the Erythrocytic cycle

Malaria Parasite Life Cycle

• Malaria parasites exist as merozoites when they are in the bloodstream.
• In this form, they infect and multiply Red Blood Cells (RBC).

Erythrocytic Cycle

• The erythrocytic cycle is the second phase of reproduction in the malaria parasite's life cycle.
• During this phase, replication of the parasite occurs inside the Red Blood Cells (RBC).

Plasmodium Life Cycle

• In the bloodstream, Plasmodium exists in the form of merozoites, which infect and multiply within Red Blood Cells (RBCs).
• The erythrocytic cycle is the second phase of reproduction, where Plasmodium replicates inside the RBC.
• Inside the RBC, Plasmodium feeds on hemoglobin and undergoes multiple growth stages.

RBC Fates

• Some RBCs rupture, releasing merozoites, which continue the erythrocytic cycle.
• Some RBCs develop into Microgametocytes and Macrogametocytes.
• The immune system destroys some RBCs.

Lifecycle of Malaria Parasite

• Malaria parasite lifecycle has three possibilities after infection:
• Merozoites rupture from infected Red Blood Cells (RBCs) and search for new RBCs
• Merozoites develop into microgametocytes and macrogametocytes
• The immune system eliminates the parasite within the infected RBC

Transmission of Malaria

• Mosquitoes ingest mature microgametocytes or macrogametocytes when they feed on the blood of an infected person

Gametocytes in Mosquito

• Microgametocytes: male/sperm cells
• Macrogametocytes: female/egg cells

Formation of Zygote and Oocyst

• 2 types of gametocytes replicate in the stomach of the mosquito
• Replication forms a zygote, also known as an ookinete
• Ookinete encysts and matures into an oocyst

Plasmodium Life Cycle in Mosquito

• Gametocytes develop into ookinetes in the mosquito's midgut.
• Ookinetes are the 2nd form of Plasmodium in the mosquito.
• Ookinetes develop into oocysts, which is the 3rd form of Plasmodium in the mosquito.
• Oocysts rupture and release sporozoites, marking the 1st formation of Plasmodium in the mosquito.

Malaria Transmission

• Oocysts rupture, releasing sporozoites that travel to the mosquito's salivary gland, awaiting their next host.

Alternative Forms of Transmission

• Transfusion malaria: a rare form of transmission through contaminated needles used for blood transfusions.
• Mainline malaria: a form of transmission among intravenous drug users who share infected needles.
• Congenital malaria: a rare form of transmission from mother to child during pregnancy or childbirth.

Hypnozoites and Relapse Infection

• Hypnozoites can form with Plasmodium vivax or Plasmodium ovale, leading to a relapse infection or recrudescence.
• Hypnozoites contain dormant Plasmodium-infected liver cells that can remain dormant for months or years.
• When hypnozoites rupture, they release merozoites, which then initiate the erythrocytic cycle or a relapse infection.
• Relapse infection, also known as recrudescence, has never occurred in humans.
• Hypnozoites are responsible for the reactivation of the parasite, leading to multiple episodes of malaria.