The costs of infection and resistance as determinants of West Nile virus susceptibility in Culex mosquitoes

In contrast to our previous findings with a relatively resistant Cx. tarsalis colony, WNV infection did not alter fecundity or blood-feeding behaviour of Cx. pipiens, yet results do indicate that resistance to infection is associated with a fitness cost in terms of mosquito survival.The identification of species-specific differences provides an evolutionary explanation for variability in vector susceptibility to arboviruses and suggests that understanding the costs of infection and resistance are important factors in determining the potential competence of vector populations for arboviruses.A comprehensive understanding of the relationships between arthropod fitness and infection with arthropod-borne viral pathogens (arboviruses) is vital to describing the ways in which selective pressures might drive the co-evolution of vector and virus. With the ultimate evolutionary goal of pathogens being dispersal and transmission, and the probabilities of these generally increasing with increased vector fitness, evolution should tend to favor an innocuous relationship between arboviruses and their mosquito hosts [1]. Indeed, many studies have demonstrated that such a benign association often exists [2-4]. This is particularly the case with viruses which rely more extensively on vertical transmission for maintenance [5]. Despite this, evolution away from virulence is not always the rule if, as initially proposed in the trade-off hypothesis, increased virulence is itself coupled with increased transmission [6,7]. Specifically, increases in transmissibility are generally associated with increased replication, yet increased replication may be inherently coupled with increased virulence. Additional strategies such as infection causing increases in feeding rate as documented for malaria [8] and West Nile virus (WNV;[9]) may partially overcome the costs of virulence on pathogen transmission, yet ultimately pathogen evolution should move towards the balance between virulence and patho