title: Blackfly (Diptera: Simuliidae) vector ecology in Scotland: environmental drivers of Leucocytozoon infection and an investigation of the role of Wolbachia
Creators: name: Kirby, Georgia M.
Publisher: name: University of Glasgow
Year: 2022

description: Vector-borne disease transmission depends on complex biological variables that dictate relationships between vectors, hosts and pathogens. These variables, which include vector abundance and life history traits, are highly sensitive to environmental influences. While research has elucidated many of the processes governing transmission of tropical diseases of humans, much less is known about how environmental conditions impact vector-borne diseases of wildlife in temperate ecosystems. Understanding these relationships is important for wildlife conservation and for the prediction and control of vector-borne diseases in a changing climate. As well as influencing vector survival and activity, environmental conditions can affect the presence, density and behaviour of endosymbiotic bacteria within vectors. Endosymbionts such as Wolbachia have the capacity to interact with pathogens and affect reproduction in insects, and some strains are therefore able to alter vector competence and life history traits in vector species. Artificial transfection of Wolbachia into mosquito vectors can inhibit pathogen replication, and Wolbachia has been deployed as an intervention to control arbovirus-transmitting mosquito populations. However, the pathogen-interfering capacity of many Wolbachia strains in their natural hosts remains unclear. Questions also remain regarding interactions between Wolbachia and protozoan parasites, which respond inconsistently to Wolbachia. There is evidence to suggest that Wolbachia-mediated phenotypes are dependent on specific ecological conditions. Establishing these conditions is crucial in order to evaluate the potential of Wolbachia as a vector control intervention. To address these knowledge gaps, this thesis made use of an avian malaria study system in Scotland. Several blackfly (Diptera: Simuliidae) species harbour natural Wolbachia infections which may affect their ability to transmit the avian malaria parasite Leucocytozoon. By investigating variation in Leucocytozoon and Wolbachia infections in blackflies in association with environmental conditions, this thesis sought to identify the environmental drivers of parasite prevalence and natural Wolbachia phenotypes in blackfly vectors in a temperate ecosystem. Knowledge of the ecology of vector populations is necessary to understand disease transmission risk. Here, variation in blackfly abundance, species composition and phenology was investigated. Blackfly adults and larvae were sampled over five consecutive years at a single site in south-west Scotland. Blackfly abundance was temporally variable and seasonal, and was associated with rainfall at the adult stage and with ambient temperature at both the adult and larval stage. Temperature was also associated with species composition. In order to study natural Wolbachia infection heterogeneity, Wolbachia was detected and quantified here for the first time in blackfly adults and larvae using a novel qPCR assay that amplified the coxA gene. Wolbachia prevalence in adults varied inter-annually and was negatively associated with previous high temperatures. In larvae, prevalence was associated with blackfly species and larval age. Wolbachia density also varied by blackfly species at the larval stage and was highest in Simulium vernum. Previous high temperatures were positively associated with Wolbachia density in larvae. To investigate Wolbachia-parasite interactions and infer the impacts of environmental conditions on Leucocytozoon infection, Leucocytozoon prevalence and density were measured in blackfly adults. Wolbachia presence was not associated with Leucocytozoon presence or density. Mean Leucocytozoon infection prevalence during the study period was 48.1% ± 4.9%. Prevalence was positively associated with temperature and rainfall. Leucocytozoon density varied between blackfly species and was highest in S. vernum. The study of key vector life history traits such as longevity can be facilitated by rearing vectors in the laboratory. Here, a magnetic stirrer system was developed and trialled for the rearing of S. vernum, S. cryophilum and S. variegatum larvae for the first time. Larval survival was positively associated with temperature and negatively associated with larval crowding. Successful pupation and emergence were observed in S. vernum and S. cryophilum, indicating that this system could be optimised for the characterisation of life history traits in these vector species. The findings from this work increase understanding of blackfly vector ecology in Britain and the environmental determinants of avian malaria transmission by blackflies. The findings also provide insights into variation in natural Wolbachia infections and the implications this has for vector control. This work highlights the importance of studying vector-parasite relationships and natural Wolbachia phenotypes in wild populations.