Sporozoites

Sporozoites are the motile, elongated life-cycle stage of certain apicomplexan parasites, including Plasmodium species and Toxoplasma gondii, that are produced in the sexual stage within a mosquito or other definitive host and represent the infective form for vertebrate hosts.

In Plasmodium, sporozoites form in oocysts on the inner surface of the mosquito midgut after fertilization; mature sporozoites migrate to the salivary glands and are transmitted to humans during a bite. Once in the bloodstream, sporozoites rapidly travel to the liver, where they invade hepatocytes and develop into exoerythrocytic schizonts that release merozoites, initiating the blood-stage infection. A subset of sporozoites in some species may become dormant hypnozoites in liver cells, explaining relapse in P. vivax and P. ovale.

Sporozoites possess an apical complex of secretory organelles, including micronemes, rhoptries, and dense granules, enabling host-cell recognition and invasion. They move by gliding motility and use surface proteins such as circumsporozoite protein (CSP) to attach to hepatocytes; successful invasion triggers transformation into liver-stage forms and loss of sporozoite identity.

In Toxoplasma gondii and related genera, sporozoites develop in oocysts within the intestinal epithelium of felids and are released into the environment, where they infect intermediate hosts and continue development as intracellular tachyzoites and bradyzoites.

Sporogony and sporozoite transmission are central to disease epidemiology. Vaccines and interventions that block sporozoite invasion, notably CSP-based approaches such as RTS,S, aim to prevent liver infection. Vector-control measures reduce exposure to sporozoites and interrupt transmission.