Follow the Plasmodium parasite's intricate and, occasionally bizarre, 13 steps to transmitting malaria.
With her recent blood meal, the female Anopheles mosquito consumed dozens of stowaways: gametocytes male and female forms of the parasite Plasmodium falciparum.
In the mosquito’s midgut, male gametocytes produce sperm-like microgametes. The female macrogametes soon are fertilized by the males, transform into zygotes and lengthen into sausage-shaped ookinetes.
A few ookinetes pass through the midgut wall and form oocysts.
For 8 to 15 days, the oocysts produce thousands of thread-like sporozoites. Perhaps 20 percent of them reach the mosquito’s salivary glands.
As the mosquito bites another person, about 100 sporozoites swim with the saliva into the victim.
The sporozoites ride the bloodstream. Only one or two reach their target: the liver. The human victim isn’t yet aware of the enemy within.
After infiltrating a liver cell, each sporozoite transforms into a schizont that produces thousands of merozoites, which will invade red blood cells.
After 5 to 7 days, the merozoites burst from the infected liver cell, enter the bloodstream and invade red blood cells. The infected person still doesn’t feel any symptoms.
The parasite first takes on a signet-ring shape inside the red blood cell and later makes knobs on the red blood cell's surface, causing it to adhere to blood vessel lining and impede blood flow.
The rings and the later form—trophozoites—feast on the red blood cell’s cytoplasm and hemoglobin. This stage ends with the formation of a schizont that produces up to 32 new merozoites. These exit and in a burst, invade still more red blood cells.
The parasite’s numbers increase tenfold every 48 hours. From the one or two sporozoites that entered the liver, trillions of parasites may teem in the body. Two weeks after the mosquito’s bite, the patient experiences fever, headache, malaise and nausea.
The knobby red blood cells stick like Velcro to the endothelial cells lining the blood vessels of the brain, heart and lung—and, in pregnant women, the placenta—which often leads to death.
During the blood stage, some merozoites develop into yet another form of the parasite: the infective male and female gametocytes—seeds of destruction for malaria’s next victims.