Background Co-infections with multiple parasites are widespread in wild populations and can influence both host-parasite and vector-parasite interactions. Among haemosporidian parasites, co-infections involving different genera or occasionally, multiple species within the same genus – such as Plasmodium – have been documented in birds. However, the impact of such co-infections on sporogonic development within vectors remains poorly understood.Methods This study investigated the sporogony, transmission and virulence of single and co-infections of Plasmodium relictum (genetic lineage pSGS1) and Plasmodium elongatum (genetic lineage pERIRUB1) in Culex pipiens pipiens form molestus mosquitoes, using microscopic examination and PCR testing. Mosquitoes were experimentally infected by allowing them to take a blood meal from canaries carrying either a parasite of a single species or exposed to birds infected with two species, resulting in double infection. After blood meals, the insects were maintained until the parasite reached the sporozoite stage.Results Our findings reveal that both parasite species can complete sporogonic development within the same insect vector and can be simultaneously transmitted to a healthy avian host through a single mosquito bite. Additionally, the study shows distinct effects of single and co-infections on mosquito survival, demonstrating specific interactions between parasites during sporogonic development in the vector. Co-infected mosquitoes experienced lower mortality than those infected with a single P. elongatum infection. This could be due to a potential suppressive effect of P. relictum on the development of P. elongatum within the mosquito, thereby reducing harm to the vector.Conclusions This study deepens our understanding of avian malaria co-infections by focusing on co-infections within vectors, and highlights their potential impact on transmission efficiency, vector fitness and the ecological dynamics of disease spread.