Climate and Vectorborne Diseases

Abstract

Climate change could significantly affect vectorborne disease in humans. Temperature, precipitation, humidity, and other climatic factors are known to affect the reproduction, development, behavior, and population dynamics of the arthropod vectors of these diseases. Climate also can affect the development of pathogens in vectors, as well as the population dynamics and ranges of the nonhuman vertebrate reservoirs of many vectorborne diseases. Whether climate changes increase or decrease the incidence of vectorborne diseases in humans will depend not only on the actual climatic conditions but also on local nonclimatic epidemiologic and ecologic factors. Predicting the relative impact of sustained climate change on vectorborne diseases is difficult and will require long-term studies that look not only at the effects of climate change but also at the contributions of other agents of global change such as increased trade and travel, demographic shifts, civil unrest, changes in land use, water availability, and other issues. Adapting to the effects of climate change will require the development of adequate response plans, enhancement of surveillance systems, and development of effective and locally appropriate strategies to control and prevent vectorborne diseases.

Introduction

Global climate change poses the threat of serious social upheaval, population displacement, economic hardships, and environmental degradation. Human health also could be influenced by increased variability and sustained changes in temperature, rainfall patterns, storm severity, frequency of flooding or droughts, and rising sea levels.1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13 Climate change and increased climatic variability are particularly likely to affect vectorborne diseases (Table 1). Much of the impact of climate on vectorborne diseases can be explained by the fact that the arthropod vectors of these diseases are ectothermic (cold-blooded) and, therefore, subject to the effects of fluctuating temperatures on their development, reproduction, behavior and population dynamics.3, 7, 99, 100, 101, 102, 103, 104, 105, 106 Temperature also can affect pathogen development within vectors and interact with humidity to influence vector survival and, hence, vectorial capacity. The seasonality and amounts of precipitation in an area also can strongly influence the availability of breeding sites for mosquitoes and other species that have aquatic immature stages. For those diseases that are both vectorborne and zoonotic (i.e., have vertebrate reservoirs other than humans), climatic variables can affect the distribution and abundance of vertebrate host species, which can, in turn, affect vector population dynamics and disease transmission.¹⁰⁶

The purpose of this article is to review the influence of climate on the transmission and spread of vectorborne diseases and identify the most important gaps in our knowledge, including the degree to which studies on the impact of climatic variability on vectorborne diseases can help us understand the likely alterations of vector–host–pathogen relationships under conditions of sustained climate change. We also discuss how human behaviors, land use, and demographic factors can interact with climate to determine the actual burden of vectorborne disease among humans. Finally, we suggest approaches for adapting to the potential effects of climate change on the occurrence of vectorborne diseases in humans.

The article is organized into three main sections that provide examples of the effects of climatic variability on diseases transmitted by mosquitoes, other flying arthropods, and ticks and fleas, and discuss the potential effects of climate change on the distribution and incidence of the diseases. The article is divided in this manner because the vectors in each group exhibit distinct differences in their mobilities and host-seeking behaviors, their abilities to disperse over long distances, and the degree to which their distributions are tied to specific hosts or habitats.

Some mosquito species, for example, are highly mobile and can fly many kilometers or be transported over long distances aboard planes, ships, or other vehicles.¹⁰⁷ Their distributions also are often associated more closely with the availability of suitable breeding habitats and climatic conditions than the presence of a narrow range of hosts or host habitats. Although capable of flight, certain other arthropod vectors, such as sand flies or triatomine bugs, typically disperse over more limited distances and can be largely restricted in distribution to certain host habitats that provide both suitable breeding areas and homes or resting sites for their hosts.108, 109 Similarly, fleas and ticks are wingless and, because of their limited mobility, often rely on the movements of their hosts for dispersal. Many of these species also spend considerable time in host nests or burrows, awaiting the return of specific hosts to these sites. Those tick and flea species that quest for hosts in more exposed environments, such as grassy fields or forest floors, also are likely to be exposed to the life-threatening effects of high temperatures or low humidity, or be forced to limit their questing to brief periods when conditions are less extreme.54, 98

Although the mobility of ticks and fleas, as well as certain winged insects such as sand flies and triatomine bugs, is generally much more limited than that of mosquitoes, it should be noted that they can be transported over long distances by human-related activities such as the shipment of infested animals or the transport of luggage or other goods. This last factor poses the risk that even low mobility vectors, as well as the disease agents they transmit, could become established in new areas, including those that might become suitable habitats as a result of climate change. However, as noted below, successful introduction and establishment of local vectorborne disease cycles relies on not just the dispersal of the vectors but also many other ecologic and human-related factors.