Which stage of the malaria parasite is responsible for transmission by mosquitoes?

The sporozoite stage of the malaria parasite is responsible for transmission by mosquitoes. When a female Anopheles mosquito bites an infected human, it ingests the gametocytes present in the human's bloodstream. These gametocytes develop and mature in the mosquito's gut, ultimately forming sporozoites. Upon maturation, the sporozoites migrate to the mosquito's salivary glands. When the mosquito bites another human, the sporozoites are injected through the saliva, leading to the infection of the new host.

Each of the other stages plays a role in the lifecycle of the malaria parasite but does not directly participate in transmission from mosquitoes to humans. For instance, the meront (or schizont) and trophozoite stages occur within human hosts, specifically inside liver cells and red blood cells, respectively, and are not involved in the mosquito vector's transmission process. Only the sporozoite is conveyed from the mosquito to the host, making it the critical stage for initiating the malarial infection in a new human host.