Impact of Climate Change on Vector-Borne Diseases and Other Infections

Environmental and other stressors that influence disease pathogenesis and propagation

Over the past couple of decades the global landscape of infectious disease risk has been altered by a number of factors, both aggravating and diminishing in terms of disease pathogenesis and propagation.

Ø Anthropogenic climatic: change in climate caused by the activities of mankind. Primarily global warming induced by increased levels of greenhouse gases.

Ø Demographic: factors such as increased urbanization, population growth, land-use change, migration, ageing, and changing birth rates.

Ø Technological: changes caused by cheaper, faster global travel, increased international trade, and improved health care.

These three groups of stressors do not act independently, but are interlinked in often complex and poorly understood ways. We can consider the impacts of these stressors in terms of their effects on disease emergence (pathogenesis) into human populations, local- scale disease dynamics, and global spread.

Recent global changes affecting pathogenesis:

Ø Increased patterns of contact between human and wildlife reservoirs.

Ø Behaviours that increase the potential for spillover, such as consumption of wild meat.

Ø Intensifying contact between wild and domestic animal hosts.

Ø Expanding agriculture and its intensification.

Ø Changing nature of human populations that are exposed to potential spillover.

Ø Globally ageing populations with poor immune systems at risk of spillover.

Ø Evolutionary selection for drug resistance.

Ø Climate change may play a role in the risk from pathogen spillover. Changing environmental conditions can alter species range and density, leading to novel interactions between species, and increase the risk of zoonotic emergence.

Ø Rapid rates of urbanization in low-income and middle-income countries.

Recent global changes have affected each of these drivers of local-scale dynamics:

Ø School attendance not only modulates transmission of childhood infections but also shapes human mobility.

Ø Age specificity of the burden of disease.

Ø Demographic changes to population size and density via urbanization may also affect dynamics.

Ø Changes in the occurrence of immunomodulatory infections, which, in turn, may affect other infections.

Ø Global change in climatic conditions will play a key role in driving the local-scale seasonal disease dynamics.

Ø Exposure to local air pollution in rapidly urbanizing locations, e.g. PM2.5 airborne microparticulates.

Ø Living in an urban location may bring benefits in terms of increased access to health care.

Ø Rapid rates of urbanization in low-income and middle-income countries.

Ø For VBDs, biological traits of both the vector and the pathogen may be sensitive to climate. Many transmission-related life cycle traits of the mosquito (biting rate, adult lifespan, population size and distribution) and the pathogen (extrinsic incubation rate) are temperature sensitive, and oviposition patterns depend on water availability.

Ø Vaccination. At the local scale, one of the strongest footprints detectable on the dynamics of many endemic infections in recent years is declines in incidence associated with access to vaccinations.

When considering the impact of global change on infectious disease risk, the effects of three forms of global connectivity can be considered:

Ø International travel. The total number of airline passengers doubled from just below two billion in 2000 to more than four billion in 2019. This step increase in global connectivity brings with it new risks from emerging and endemic pathogens that can circulate via transit routes. For example seasonal influenza circulation in the USA can already be predicted by flight patterns.

Ø Human migration and local-scale mobility. It is estimated that globally the number of permanent international migrants is almost 272 million, representing 3.5% of the world’s population. The rate of migration continues to increase owing to social, economic, political and environmental drivers. Climate change sea level rise and extreme weather will likely provide an escalating push factor in this respect.

Ø The international trade of animals, animal products and plants. This has expanded rapidly in the modern era and has been matched by a global proliferation of infectious diseases affecting not only humans but also animals and plants. Trade drives this pattern by facilitating the translocation of hosts and pathogens across the geographical and ecological boundaries that constrain their spread.