Résumé
Malaria, a vector-borne disease caused by Plasmodium spp., remains a major
global cause of mortality. Optimization of disease control strategies
requires a thorough understanding of the processes underlying parasite
transmission. While the number of transmissible stages (gametocytes) of
Plasmodium in blood is frequently used as an indicator of host-to-mosquito
transmission potential, this relationship is not always clear. Significant
effort has been made in developing molecular tools that improve gametocyte
density estimation and therefore prediction of mosquito infection rates.
However a significant level of uncertainty around estimates remains. The
weakness in the relationship between gametocyte burden, measured from a
blood sample, and the mosquito infection rate could be explained by a
non-homogeneous distribution of gametocytes in the bloodstream. The
estimated gametocyte density would then only be a single snapshot that
does not reflect the host infectivity. This aspect of Plasmodium
infection, however, remains largely neglected. In both humans and birds,
we found here that the gametocyte densities differed depending on which
side of the body the sample was taken, suggesting that gametocytes are not
homogeneously distributed within the vertebrate host. We observed a
fluctuating asymmetry, in other words, the extremity of the body with the
highest density of parasites is not always the same from one individual to
another. An estimation of gametocyte density from only one blood sample,
as is commonly measured, could, therefore, over- or underestimated the
infectivity of gametocyte carriers. This might have important consequences
on the epidemiology of the disease since we show that this variation
influences host-to-mosquito transmission. Vectors fed on the least
infected body part had a lower parasite burden than those fed on the most
infected part. The heterogeneous distribution of gametocytes in
bloodstream should be considered to improve diagnosis and test new malaria
control strategies.