Susceptibility of Anopheles stephensi to Plasmodium gallinaceum: A Trait of the Mosquito, the Parasite, and the Environment

Vector susceptibility to Plasmodium infection is treated primarily as a vector trait, although it is a composite trait expressing the joint occurrence of the parasite and the vector with genetic contributions of both. A comprehensive approach to assess the specific contribution of genetic and environmental variation on "vector susceptibility" is lacking. Here we developed and implemented a simple scheme to assess the specific contributions of the vector, the parasite, and the environment to "vector susceptibility." To the best of our knowledge this is the first study that employs such an approach.We conducted selection experiments on the vector (while holding the parasite "constant") and on the parasite (while holding the vector "constant") to estimate the genetic contributions of the mosquito and the parasite to the susceptibility of Anopheles stephensi to Plasmodium gallinaceum. We separately estimated the realized heritability of (i) susceptibility to parasite infection by the mosquito vector and (ii) parasite compatibility (transmissibility) with the vector while controlling the other. The heritabilities of vector and the parasite were higher for the prevalence, i.e., fraction of infected mosquitoes, than the corresponding heritabilities of parasite load, i.e., the number of oocysts per mosquito.The vector's genetics (heritability) comprised 67% of "vector susceptibility" measured by the prevalence of mosquitoes infected with P. gallinaceum oocysts, whereas the specific contribution of parasite genetics (heritability) to this trait was only 5%. Our parasite source might possess minimal genetic diversity, which could explain its low heritability (and the high value of the vector). Notably, the environment contributed 28%. These estimates are relevant only to the particular system under study, but this experimental design could be useful for other parasite-host systems. The prospects and limitations of the genetic manipulation of vector populations to render the vector resistant to the parasite are better considered on the basis of this framework

Immunogenic Salivary Proteins of Triatoma infestans: Development of a Recombinant Antigen for the Detection of Low-Level Infestation of Triatomines

Chagas disease, caused by Trypanosoma cruzi, is a neglected disease with 20 million people at risk in Latin America. The main control strategies are based on insecticide spraying to eliminate the domestic vectors, the most effective of which is Triatoma infestans. This approach has been very successful in some areas. However, there is a constant risk of recrudescence in once-endemic regions resulting from the re-establishment of T. infestans and the invasion of other triatomine species. To detect low-level infestations of triatomines after insecticide spraying, we have developed a new epidemiological tool based on host responses against salivary antigens of T. infestans. We identified and synthesized a highly immunogenic salivary protein. This protein was used successfully to detect differences in the infestation level of T. infestans of households in Bolivia and the exposure to other triatomine species. The development of such an exposure marker to detect low-level infestation may also be a useful tool for other disease vectors

Infection prevalence and intensity in unselected lines of <i>An. Stephensi</i>.

<p><i>Anopheles stephensi</i> prevalence and mean oocyst load in unselected lines over a total of 28 infection experiments. Overall mean is shown by the horizontal line. Number of experiments is shown above each box-whisker plot (sample size range per experiment 50–175, except n = 20 in one experiment with the NIH line). The differences among lines in overall prevalence was not significant (χ2 = 3.96, df = 2, P<0.137) as was the case for the oocyst load (ANOVA with Experiment treated as blocking factor: F_2,2135 = 0.58, P>0.55).</p

Infection prevalence and intensity in selected lines.

<p>The changes in the prevalence and mean oocyst load in the selected lines (A and E) of <i>An. stephensi</i> and their corresponding controls (C and F) over generations. Vertical bars represent standard error of the means (SEM, calculated separately for each generation). Only four generations were required by the A line to reach its target phenotype (prevalence>85%), whereas the E line did not reach this target after 9 generations, when the experiment was terminated.</p

Response of <i>An. stephensi</i> selected for susceptibility to <i>P. gallinaceum</i> based on mean oocyst load.

<p>The cumulative response to selection is regressed on the cumulative selection differential for the selected <i>An. stephensi</i> lines. Regression is forced through the origin.</p

Effect of selection on mean oocyst load and prevalence of <i>An. stephensi</i> and <i>P. gallinaceum</i> and their realized heritability (h²_r).

a<p>Denotes the number of generations selection was carried out.</p>b<p>Phenotypic values before selection (<b>Z₀</b>) and after the last generation of selection (<b>Z_t</b>).</p

The changes in prevalence and intensity of infection over parasite selection.

<p>The changes in the prevalence and mean oocyst load following selection in a line of <i>P. gallinaceum</i> vectored exclusively by <i>An. stephensi</i> compared with that in <i>Ae. aegypti</i> that was simultaneously fed on the same chicken. Note the different Y-axes for the different mosquito species. Prevalence values of <i>An. stephensi</i> were adjusted by the mean prevalence of <i>Ae. aegypti</i> (0.95). Values of oocyst load of <i>An. stephensi</i> were adjusted by the mean oocyst load of <i>Ae. aegypti</i> (19.5). Vertical bars represent SEM (calculated separately for each generation). To avoid clutter, bars representing mean-SEM of <i>Ae. aegypti</i> were excluded.</p

Response of <i>P. gallinaceum</i> selected for compatibility with <i>An. stephensi</i> based on mean oocyst load.

<p>The cumulative response to selection is regressed on the cumulative selection differential for the selected <i>An. stephensi</i> lines. Regression is forced through the origin.</p

Selection protocol for increasing vector susceptibility.

<p>Schematic illustrating selection protocol for increased vector susceptibility (nVC) in <i>An. stephensi</i> infected with <i>P. gallinaceum</i>. (1) <i>An. stephensi</i> colony mosquitoes randomly chosen for the selection experiment; (2) feed on <i>P. gallinaceum</i> infected chicken (side by side with <i>Ae. aegypti</i>, used as positive control); (3) Mosquitoes separated out individually on day 5 p.i for oviposition. On day 6 p.i., a subset of the females that laid eggs (50</p