Mixed-species Plasmodium infections of Anopheles (Diptera:Culicidae).

Mixed-pathogen infections of vectors rarely are considered in the epidemiological literature, although they may occur in nature. A review of published reports shows that many Anopheles species are capable of carrying sporozoites of > 1 Plasmodium species, of doing so simultaneously in field conditions, and of acquiring and transmitting these in experimental situations. Mixed-species infections in mosquito populations occur at frequencies greater than or equal to the product of the constituent species prevalences, whereas human populations have apparent mixed-species infections at frequencies less than or equal to their corresponding expected values. We present a model for the accumulation of parasite infections over the lifespan of a mosquito that explains this surplus of mixed-species infections. However, the expected frequencies of mixed infections on the basis of our model are greater than those found in nature, indicating that the sampling by mosquitoes of Plasmodium species from human malaria infections may not be random.