THE Plasmodium falciparum sporozoite is injected into the human host during a mosquito's blood meal. It makes its way into the blood stream, where it is freely swimming (extra-cellular). Within minutes of entering the blood stream, it penetrates the relative immuno-protected environment inside liver cells (hepatocytes). It takes the parasite two weeks to mature during the liver stage. The liver and sporozoite stages together are called the pre-erythrocytic stage (that is, the stage before it enters the blood cells). During this stage, an infected person shows no symptoms of the disease and the malaria cannot be detected. Unchecked, each sporozoite develops into up to 40,000 merozoites that burst out of the liver cells and enter the bloodstream.
The merozoites begin cycles of invading red blood cells, multiplying and rupturing out of the host cells roughly every two days. It takes only a few seconds for merozoites to invade new blood cells, so merozoites spend much more time inside them, protected from the immune system. It is during this stage, the blood stage, that infected persons develop the periodic fevers, chills and other symptoms of malaria.
After roughly 10 days, some of the merozoites mature into sexual forms. When a mosquito during a blood meal, takes up these sexual forms, the relatively colder mosquito gut allows them to mate and after 10-14 days mature into sporozoites. The parasite is poised to begin the cycle again with the mosquito's next blood meal.
