Introduction

This book concerns the population biology of vector-borne diseases. Vector-borne diseases of people are a perennial challenge for public health. Although recent decades have enjoyed major declines in the incidence of diseases like malaria and onchocerciasis (river blindness), vector-borne diseases continue to claim the lives of more than 700,000 people per year and exact costs of tens of billions of dollars in expenses for control and through lost productivity (...

Vector Control, Optimal Control, and Vector-Borne Disease Dynamics

Understanding methods of vector control is essential to vector-borne disease (VBD) management. Vaccines or standard medical interventions for many VDBs do not exist or are poorly developed so disease control is focused on managing vector numbers and dynamics. This involves understanding not only the population dynamics but also the population genetics of vectors. Using mosquitoes as a case study, in this chapter, the modern genetics-based methods of vector control (self-limiting, self-sustaining) on mosquito population and disease suppression will be reviewed. These genetics-based methods highlight the importance of understanding the interplay between genetics and ecology to develop optimal, cost-effective solutions for control. The chapter focuses on how these genetics-based methods can be integrated with other interventions, and concludes with a summary of regulatory and policy perspectives about the use of these approaches in the management of VBDs.

Direct and Indirect Social Drivers and Impacts of Vector-Borne Diseases

Vector borne diseases (VBDs) are often seen by the highly developed nations of the world as an issue of poor tropical countries. While framing the problem this way—through the paradigm of a poverty-trap—may leverage aid and motivate political will toward disease control, it misses a wide range of socio-political contexts both driving, and driven by, vector borne diseases. In this chapter, we present a series of global vignettes, to illustrate different facets of the broad remit of social drivers and interactions with VBDs. We approach the urban social-ecological context in Latin America and the Caribbean, impacts and aftermath of natural disasters such as earthquakes and tropical storms, struggles with trust in intervention implementation in Haiti, and drivers and impacts of ruminant arbovirus emergence events in Europe. We conclude that incorporating an understanding of social context, including political history and cultural perceptions, is a key part of VBD research and intervention practice.

Ecological Interactions Influencing the Emergence, Abundance, and Human Exposure to Tick-Borne Pathogens

Tick-borne pathogens pose the greatest vector-borne disease burden in temperate areas of Europe and North America. We synthesize key aspects of tick life history that enable ticks to persist, spread and impact human health, including a two-year life cycle, multiple transmission pathways and dependence on hosts for tick feeding, movement and pathogen transmission. We discuss modeling advances that incorporate these traits in the context of climate-driven variation in tick feeding phenology. For established pathogens, such as the Lyme disease agent in the United States, we disentangle the linkages between land use change, habitat fragmentation and host diversity influencing human risk of infection along an urbanization gradient. We propose a coupled natural-human system framework for tick-borne pathogens that accounts for nonlinear effects and feedbacks between the enzootic cycle and human spillover. A deeper understanding of the eco-bio-social determinants of these diseases is required to develop more effective public health interventions.

Environmental Drivers of Vector-Borne Diseases

The transmission of vector-borne diseases is sensitive to environmental conditions, including temperature, humidity, rainfall, and land use/habitat quality. Understanding these causal relationships is especially important as increasing anthropogenic changes drive shifts in vector-borne disease dynamics. In this chapter, we first briefly describe the biology of vectors and pathogens that underlies environmental influences on transmission of vector-borne diseases. Next, we review the impacts of each of the major environmental drivers (as previously mentioned), synthesizing and comparing mechanisms across different vector-borne disease systems. Then, we discuss key challenges and standard approaches to research in the discipline. Finally, we highlight areas where research is advancing in promising new directions and suggest areas where new approaches are needed.

Mosquito—Virus Interactions

Vector-borne viruses (arboviruses) are emerging threats to both human and animal health. The global expansion of dengue virus, West Nile virus, chikungunya, and most recently Zika virus are prominent examples of how quickly mosquito-transmitted viruses can emerge and spread. We currently lack high quality data from a diversity of mosquito-arbovirus systems on the specific mosquito and viral traits that drive disease transmission. Further, the factors that contribute to variation in these traits and disease transmission remain largely unidentified. In this chapter, we outline and explore the following: 1. the specific mechanisms governing the outcome of vector-virus interactions 2. how genetic variation across mosquito populations and viral strains, as well as environmental variation in abiotic and biotic factors shape the mosquito-virus interaction and 3. the implications of these interactions for understanding and predicting arbovirus transmission, as well as for control of mosquito species that transmit human pathogens.

Force of Infection and Variation in Outbreak Size in a Multi-Species Host-Pathogen System

The size of annual outbreaks in seasonally forced host-pathogen systems is poorly understood. We studied contributing factors to the six-fold observed variation in the number of human cases of West Nile virus in New York City in the years 2000–2008. Sampling error and intrinsic noise (demographic stochasticity) explain roughly half of the observed variation. To investigate the remaining sources of variation, we estimated the monthly force of infection from data on the distribution and abundance of mosquitoes, virus prevalence, vector competence, and mammal biting rate at two spatial scales. At both scales, the West Nile virus force of infection was remarkably consistent from year to year. We propose that fine scale spatial heterogeneity is the key to understanding the epidemiology of West Nile virus in New York City.

Population Biology of Culicoides-Borne Viruses of Livestock in Europe

Culicoides biting midges are the principal vectors for a number of internationally important arboviruses that infect animals. Over recent decades two viruses they transmit, bluetongue virus (BTV) and Schmallenberg virus (SBV), have emerged as major threats to European livestock. Here, the host, vector, viral, and environmental factors which influence the transmission of these viruses are reviewed. The influence of these factors on the patterns of spread that followed their emergence are explored, both for different viruses (BTV and SBV) and for different strains of the same virus (BTV). Finally, consideration is given to the longer-term dynamics of Culicoides-borne viruses and, in particular, their ability to persist from one vector season to the next.

Kindling, Logs, and Coals: The Dynamics of Trypanosoma cruzi, the Etiological Agent of Chagas Disease in Arequipa, Peru

The forces that lead to the emergence of Trypanosoma cruzi, the etiologic agent of Chagas disease, are often distinct from those that maintain its transmission, and these are distinct again from those that allow the parasite to persist over decades. Just as kindling, logs, and coals all play discrete roles in the growth of a fire, a myriad of mammalian hosts contribute differently to epidemics of Trypanosoma. cruzi. Chagas disease affects millions of people in the Americas, and, through migration, thousands more on other continents. The agent of the disease, Trypanosoma cruzi, is a slender, highly-motile, unicellular parasite. T. cruzi does not migrate to the salivary glands of its insect vector–the blood-sucking triatomine insects–as many other vector-borne parasites do.

Incorporating Vector Ecology and Life History into Disease Transmission Models: Insights from Tsetse (Glossina spp.)

Accurate models are crucial for predicting the spread of vector-borne diseases, and for developing appropriate control policies. Simple models often ignore finer details of vector biology, commonly due to lack of pertinent field data. However, for tsetse (Glossina spp), vectors of the parasites causing debilitating human and livestock trypanosomiasis in Africa, extensive field and laboratory data facilitate improved models and predictions of vector control outcomes. We review studies on the effects of environmental temperature, and fly age and sex, on survival and reproduction in tsetse, savannah species particularly–emphasizing the extreme maternal investment and sensitivity of early life stages to high temperatures. We consider implications of these results for predictive models of tsetse populations, and of the transmission and control of African trypanosomiasis. We discuss how further research on vectors, and improved models of vector populations and disease dynamics, can lead to improved predictions of vector abundance and disease spread.