In the years to come, the rapidly changing climate is positioned to intensify existing threats and introduce novel threats to human health. In light of the COVID-19 pandemic, numerous concerns have surfaced regarding the capacity of governmental institutions to protect public health. The pandemic should serve as an example of the nature of infectious diseases that have the potential to reach a cataclysmic state in a rapidly changing climate. In addition, it should serve as a novel focal point for global environmental politics and mitigation efforts. As a result, understanding the link between disease and climate change has become increasingly urgent.
Epidemiologists who study the distribution of various infectious diseases generally agree that climate change will likely lead to an increase in future epidemics caused by pathogens. Climate change stands as one of the largest factors that influences the distribution of vector-borne and zoonotic diseases (VBZD). Zoonotic diseases refer to infectious diseases that involve transmission between humans and vertebrae mammals, such as bats. Vector-borne diseases refer to infectious diseases that involve transmission between humans and an arthropod vector, such as mosquitos. Both vector-borne and zoonotic diseases are caused by various pathogens such as viruses, bacteria, and parasites. Zoonotic diseases can either be vector-borne or non-vector borne depending on the route of transmission, and similar, vector-borne disease can be zoonotic or non-zoonotic. The broad term of vector-borne and zoonotic diseases (VBZD) is utilized to encompass any of the above conditions.
Epidemiologists and researchers have proposed four primary mechanisms through which climate change is likely to impact host pathogen and vector populations including: geographic distribution, population density, the prevalence of infection by zoonotic pathogens, and the pathogen load in individual vectors and hosts.
In response to climate change, range shifts have been observed for a variety of species, contributing to the spread of VBZD’s. Broadly speaking, these range shifts have been directed towards higher elevations, and poleward. This has altered the total area that is occupied through expanding or reducing the species range. For example, this shift has occurred for Ixodes Ricinus (known as the castor bean tick) across Europe, which is a vector for Lyme disease and tick-borne encephalitis (TBE), and has been associated with an increase in the incidence of Lyme disease and TBE.
In response to climate extremes, shifting temperatures can also result in dramatic alterations to population densities of various pathogens. For example, increases in mosquitos have been associated with El Niño Southern Oscillation (ENSO) events in southern Africa and South America, which have been associated with an elevation in the prevalence of mosquito-borne malaria. Warming temperatures have also been associated with a decreased maturation period for mosquitos, effectively increasing the population density.
As host and vector populations are altered, this directly affects the frequency of contact between hosts and vectors. Broadly speaking, the spread of infectious diseases is exacerbated by an increased prevalence of hosts. This has been observed in North America, as an increased prevalence of hantavirus was observed following an increase in the deer mouse population as a result of an ENSO event. Climate change can also have significant implications for the development of pathogens in hosts and vectors. For example, mosquito parasites are known to develop in mosquitos only within a certain range of temperatures. In addition, viral replication, which is the generation of viruses in the target host cell, is known to be affected by heat stress.
These climate variables are intensified by anthropogenic factors such as human migration, transportation, and disturbances of ecosystems. This holds great implications for the influence of environmental politics in preserving public health. Local conservation efforts upheld by municipal governments must transcend state boundaries to prevent the exacerbation of VBZD’s. Addressing climate change has consequences that are inherently visible, but the often-understated impacts of climate change on human health must also be a focal point of international mitigation efforts.