Normal numbers of oocysts are produced. Sporozoite formation within the oocysts is blocked. The diameters of oocysts were significantly larger than that of wild type on days 14 and 21 of infection. Light microscopy revealed the presence of two distinct populations of oocysts: those that displayed a phenotype reminiscent of immature wild type oocysts (i.e., non-sporulated), and those that appeared vacuolated/degenerate compared to wild type. Transmission electron microscopy analysis of oocysts further confirmed these findings and revealed that oocysts of these parasites possessed an endoplasmic reticulum that was highly vacuolated compared to that of wild type parasites. On day 13 p.i., the nuclear organization of oocysts appeared immature as indicated by the presence of few but large nuclei. By comparison, wild type oocysts of the same age had formed sporozoites, each with their own (haploid) nucleus.

No midgut sporozoites were observed on day 10/11 p.i. By day 18 p.i., reduced numbers (typically 0%12%) of sporozoites were observed in dissected midguts. The number of sporozoites in salivary gland preparations infections was consistently reduced to <1% of wild type. The expression and targeting of the circumsporozoite protein in midgut sporozoites was indistinguishable from that in wild type.To test if the observed sporozoites were infectious to mice, infected mosquitoes were allowed to feed on mice on days 21 and 28 p.i. Blood stage parasites were observed in all mice bitten by wild type-infected mosquitoes when screened on day 4/5 post-bite. In contrast, mice bitten by mutant-infected mosquitoes remained uninfected.