Host haematological factors influencing the transmission of Plasmodium falciparum gametocytes to Anopheles gambiae ss mosquitoes

We investigated the relationship between selected host haematological and p arasitological parameters and the density and infectivity of Plasmodium fal ciparum gametocytes. 143 individuals (age range 1-62 years) attending an ou tpatient clinic in Farafenni, The Gambia, who had peripheral blood gametocy taemia were recruited (mean gametocyte density 123.7/mu l, range 5-17,000/m u l). Of the parameters measured, packed cell volume (PCV), reticulocyte co unt (RetC) and lymphocyte count (LyC) were significantly associated with ga metocyte density (r = -0.17, P < 0.05; r = 0.21, P < 0.01; r = 0.18, P < 0. 05, respectively). Data from membrane feeding experiments in which 15 or mo re mosquitoes were dissected showed that 60.7% (53/87) of gametocyte carrie rs infected one or more mosquitoes. Gametocyte density was strongly correla ted with transmission success (TS) (r = 0.3, P < 0.005) and, in successful infections, with both mosquito prevalence (MP) (r = 0.36, P < 0.005) and me an oocyst burden (MOB) (r = 0.65, P < 0.0001). None of the other factors me asured were significantly associated with any of these indices in bivariate analysis. Regression modelling showed that both gametocyte density and PCV were positively associated with gametocyte carrier infectivity to mosquito es (LRchi 2 = 100.2 and 47.2, respectively) and, in successful infections, with MOB (beta = 0.16, t = 4.9, beta < 0.001; beta = 0.02, t = 2.3, beta < 0.05, respectively). The positive association with PCV suggests that blood meal quality influences infection probably as a nutritional requirement, ho wever, as this effect was most apparent at high gametocyte densities, its e pidemiological significance is questionable. Though the haematological para meters associated with gametocyte density are a direct consequence of asexu al infection, they may also represent an adaptive mechanism for optimizatio n of sexual stage development.