Impact of the interactions between hosts vectors and pathogens on the transmission of avian malaria and flavivirus by mosquitoes

Abstract

Vector-borne pathogens cause important diseases such as malaria and are nowadays a major public health concern, because they cause human –and animal- fatalities worldwide and have a significant impact on local economies. Factors associated to global change, such as habitat alteration and introduction of invasive species, have largely contributed to the spread of potential insect vectors and the pathogens they are able to transmit, thus creating novel epidemiological scenarios. Therefore, it becomes essential to study the factors that modulate the transmission risk of these disease agents involving the interactions between vertebrate hosts (humans and other animals), pathogens and insect vectors in natural ecosystems. In this thesis, I used a multidisciplinary approach combining molecular tools, experimental bioassays and statistical analyses to assess the ecological and evolutionary factors that affect the transmission success of two mosquito-borne pathogens. In particular, I considered the interactions between insect vectors, vertebrate hosts, the avian malaria parasites and the flavivirus Zika virus. I focused on two major steps directly influencing the pathogen transmission success: i) the contact rate between mosquitoes and infected/susceptible vertebrate hosts and ii) the development of the pathogen in the mosquito and its consequences on the pathogen transmission risk. To do that, first I tested the potential causes underlying differences in the biting patterns of mosquito species. I exposed two bird species to two mosquito species to determine the role of mosquito species identity and effect of three host-related factors on host-vector contact rates (i.e. body mass, gender, and infection status by avian malaria). I found clear interspecific differences in the biting rates of mosquitoes, which were also influenced by variation in hosts’ traits, although these effects differed depending on the particular mosquito-host assemblage. Therefore, the biting patterns of mosquitoes are far from being generalizable. Secondly, I assessed the vector competence of different mosquito species for the transmission of Zika virus and avian malaria using mosquito saliva. I found that the ability of mosquito-borne pathogens to develop in mosquitoes differed between insect species, which may be the result of complex co-evolutionary processes. In addition, I assessed the consequences of parasite development in the mosquito vectors and their implications for the pathogen transmission risk. I found that host parasite load and parasite identity play affect the impact of parasites on mosquito longevity finally determining the transmission risk of the parasites. With this information mostly derived from studies under controlled conditions, I assessed the importance of environmental conditions affecting the host-parasite-vector assemblages in the wild. I found that habitat characteristics, which determine the existence and abundance of insect vectors, and host related factors (i.e. immune-competence) determines the prevalence of avian malaria parasites in insular ecosystems. Altogether, in this thesis I identified key factors affecting the transmission success of vector-borne pathogen affecting humans or wildlife allowing a better understanding the complex transmission dynamics of vector-borne pathogens.