Species-specific escape of Plasmodium sporozoites from oocysts of avian, rodent, and human malarial parasites

Abstract

<p><h3>Background</h3> <p id="x-x-Par1">Malaria is transmitted when an infected mosquito delivers <em>Plasmodium</em> sporozoites into a vertebrate host. There are many species of <em>Plasmodium</em> and, in general, the infection is host-specific. For example<em>, Plasmodium gallinaceum</em> is an avian parasite, while <em>Plasmodium berghei</em> infects mice. These two parasites have been extensively used as experimental models of malaria transmission<em>. Plasmodium falciparum</em> and <em>Plasmodium vivax</em> are the most important agents of human malaria, a life-threatening disease of global importance. To complete their life cycle, <em>Plasmodium</em> parasites must traverse the mosquito midgut and form an oocyst that will divide continuously. Mature oocysts release thousands of sporozoites into the mosquito haemolymph that must reach the salivary gland to infect a new vertebrate host. The current understanding of the biology of oocyst formation and sporozoite release is mostly based on experimental infections with <em>P.</em> <em>berghei</em>, and the conclusions are generalized to other <em>Plasmodium</em> species that infect humans without further morphological analyses. <h3>Results</h3> <p id="x-x-Par2">Here, it is described the microanatomy of sporozoite escape from oocysts of four <em>Plasmodium</em> species: the two laboratory models, <em>P. gallinaceum</em> and <em>P. berghei</em>, and the two main species that cause malaria in humans, <em>P.</em> <em>viv</em>ax and <em>P. falciparum</em>. It was found that sporozoites have species-specific mechanisms of escape from the oocyst. The two model species of <em>Plasmodium</em> had a common mechanism, in which the oocyst wall breaks down before sporozoites emerge. In contrast, <em>P. vivax</em> and <em>P. falciparum</em> sporozoites show a dynamic escape mechanism from the oocyst via polarized propulsion. <h3>Conclusions</h3> <p id="x-x-Par3">This study demonstrated that <em>Plasmodium</em> species do not share a common mechanism of sporozoite escape, as previously thought, but show complex and species-specific mechanisms. In addition, the knowledge of this phenomenon in human <em>Plasmodium</em> can facilitate transmission-blocking studies and not those ones only based on the murine and avian models.</p>