66 Ecological and Environmental Factors

Controlling these methods of transmission is important to reducing the incidence of new disease in humans. One of the most important ways that this can be achieved is by reducing human-animal interactions. Habitat destruction and urbanization have been driving an increase in these potentially disease-spreading interactions. As human populations increase, we seek to increase our land footprint to build housing, farms, and other services necessary to support that growth. This inevitably expands into wildlife habitat, displacing organisms. Sometimes, these animals adapt, finding ways to live in cities. This further increases the chance of human-animal contact, risking transmission of dangerous pathogens. Decreasing forest cover has been linked to an increasing incidence of disease, particularly in biodiverse tropical areas (Mathur, 2022).  (NC)

Specifically, human encroachment into wild places is increasing the prevalence of lyme disease. Lyme disease is spread to humans by deer ticks, who live in forested areas and feed on the blood of small rodents, like chipmunks and squirrels. These animals become reservoirs for the disease, not becoming infected but allowing each tick that feeds on them to become a carrier. Through this mechanism, more ticks become infectious and can pass the disease to humans who enter their habitats. Historically, these rodent populations have been kept under control by large predators, like foxes, decreasing the prevalence of lyme disease in small animal populations. As humans have expanded and developed suburban areas, large forests have become fragmented. These forests, now significantly reduced in size, still house lots of small rodents but can no longer support predatory species. Thus, rodent populations rapidly grow, increasing the reservoir for lyme. When humans enter these environments or when ticks spread close to their homes, they have a significantly increased risk of contracting the disease (Allan et. al., 2003). (NC)

Unfortunately, climate change has also been a driving factor in the increase in tick populations and the prevalence of lyme disease. As global temperatures warm, small mammal populations – as well as the ticks that use them for food and transportation – can move to new locations. Some, the species that can only survive in very specific temperature ranges and habitats, are forced to relocate. More adaptable species, like the white-footed deer mouse, are able to expand their range an

d move towards higher altitudes and more extreme latitudes. These habitats were previously too cold or too harsh for survival, but warming has allowed new ecosystems to change and support species that could previously not survive in these conditions. If these species are hosts to ticks, the insects can expand their range along with their food source, bringing a reservoir of lyme disease into an area that did not previously have it (Mills et. al., 2010). (NC)

Climate change can affect the frequency of vector-borne diseases through a variety of mechanisms. Changing temperatures and rainfall patterns can shift the ranges of hosts and vectors, bringing them into contact with new human populations. Weather patterns can alter the population density of disease-carrying animal populations, again altering human exposure to hosts and vectors. Climate change can also alter the amount of virus that hosts and vectors carry. Infection prevalence in host or vector populations can change alongside climate, modifying the frequency of interaction with an infected host or vector. Finally, shifting climate can change rates of pathogen reproduction, replication, or development in hosts and vectors, altering pathogen load in infected individuals and affecting the likelihood that host or vector contact would result in pathogen transmission (Mills et. al., 2010). (NC)

One of the easiest examples of these phenomena to understand is the expansion of the range of Aedes aegypti as a result of climate change. This mosquito species is incredibly prevalent in tropical and subtropical regions of the world. It thrives in warm, wet environments, and uses standing water, such as puddles, lakes, and water collection tanks to lay its eggs. Importantly, these aegypti mosquitoes are vectors for the viruses that cause severe diseases, including yellow fever, dengue, chikungunya, and zika (Laporta et. al., 2023). Climate change has caused a global rise in temperature and shifting precipitation patterns, converting more of Earth’s land area into habitat that can support these disease-carrying mosquitoes. Increases in mosquito populations can be caused by increased temperatures, which increase breeding and growth processes, and periods of high rainfall, which increase areas of standing water and give mosquitoes areas to breed and lay their eggs (Mills et. al., 2010). (NC)

The environmental suitability for Aedes aegypti, the amount of land area that can support species populations, has expanded by 1.5% every ten years since 1950. Extreme events of above-average temperatures and rainfall have already allowed mosquitoes to move into the Caribbean and Latin America, putting inhabitants of these regions at risk of infection from viruses carried by Aedes aegypti (Laporta et. al., 2023). These mosquitoes have already achieved northward expansion into the southern United States, taking advantage of rising temperatures to increase their spread (Mills et. al., 2010). Regions of Europe are also at risk of mosquito-borne disease outbreak. In 2005, the island of Madeira, Portugal was free from Aedes aegypti. By 2012, this island had become hospitable to these mosquitoes, resulting in an outbreak of dengue infection. As warming accelerates, the rate of expansion of aegypti’s range is only expected to increase. By the end of the century, its range is predicted to increase by 10-30%. Temperate areas of the northern hemisphere where global populations are concentrated, like the United States and Europe, are likely to see the effects of climate change through increased mosquito populations. Without proper interventions to control mosquito populations, their ability to spread disease, or even to slow the effects of climate change, rising temperatures and changing precipitation patterns could be driving factors in an increase of vector-borne viral disease instance in the near future (Laporta et. al., 2023). (NC)