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Meta-Analysis
. 2009 Apr 7;276(1660):1369-78.
doi: 10.1098/rspb.2008.1709. Epub 2009 Jan 13.

Mode of transmission and the evolution of arbovirus virulence in mosquito vectors

Louis Lambrechts  1 , Thomas W Scott
Affiliations
Meta-Analysis

Mode of transmission and the evolution of arbovirus virulence in mosquito vectors

Louis Lambrechts et al. Proc Biol Sci. .

Abstract

The traditional assumption that vector-borne pathogens should evolve towards a benign relationship with their arthropod vectors has been challenged on theoretical grounds and empirical evidence. However, in the case of arboviruses (arthropod-borne viruses), although a number of investigators have reported experimental evidence for virus-induced vector mortality, others have failed to detect any significant impact. Whether this variation in the observed level of arbovirus virulence depends on biological traits or experimental design is unclear. Here, we perform a meta-analysis of studies across a range of mosquito-virus systems to show that, overall, arboviruses do reduce the survival of their mosquito vectors, but that the magnitude of the effect depends on the vector/virus taxonomic groups and the mode of virus transmission. Alphaviruses were associated with highest virulence levels in mosquitoes. Horizontal transmission (intrathoracic inoculation or oral infection) was correlated with significant virus-induced mortality, whereas a lack of adverse effect was found for Aedes mosquitoes infected transovarially by bunyaviruses-a group of viruses characterized by high natural rates of vertical transmission in their enzootic vectors. Our findings are consistent with the general prediction that vertically transmitted pathogens should be less virulent than those transmitted horizontally. We conclude that varying degrees of virulence observed among vector-virus systems probably reflect different selective pressures imposed on arboviruses that are primarily transmitted horizontally versus vertically.

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Figures

Figure 1
Figure 1
Relative contribution of transmission mode to arbovirus-induced mortality in mosquitoes. The weighted distribution of the standardized magnitude of virus-induced mortality (effect size) is shown as a function of the mode of viral transmission. The height of the bar in each class is made up of the combined weight of experiments falling in this class (weight=1/variance). Black bars represent experiments in which viruses were horizontally transmitted (n=21) and grey bars correspond to experiments based on vertical transmission (n=12). On the x-axis, positive values represent higher mortality of infected compared with control mosquitoes, whereas negative values correspond to a positive effect of viral infection on mosquito survival.
Figure 2
Figure 2
Effect of infection mode and vector/virus taxonomic groups on arbovirus-induced mortality in mosquitoes. The graphs show the weighted mean of the standardized magnitude of virus-induced mortality as a function of (a) virus genus, (b) mosquito genus and (c) infection mode. Vertical bars represent the bootstrapped, bias-corrected 95% confidence intervals of the cumulative effect sizes. The number of experiments in each category is indicated above each bar. Positive values represent higher mortality of infected compared with control mosquitoes, whereas negative values correspond to a positive effect of viral infection on mosquito survival.
Figure 3
Figure 3
Correspondence analysis of infection mode and vector/virus taxonomic groups. The association between (a) virus genus and infection mode, (b) mosquito genus and infection mode and (c) mosquito and virus genera are displayed using correspondence analysis plots (Benzécri 1973; Greenacre 1983). Different symbols represent different variables (crosses, virus genus; diamonds, mosquito genus; circles, infection mode), and the categories of each variable are indicated. The plots show the entries in the tables of relative frequencies in terms of the distances between individual rows and columns in a two-dimensional space. The C1 axis tends to score response levels linearly, whereas the C2 scores indicate non-neutrality. Overall, spatial clustering indicates similar patterns of counts in the contingency table.
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