The Impact of Climate Change and Vector-Borne Diseases: The Case for Dengue Fever and Lyme Disease
Vector-borne diseases are among the most well-studied diseases, with an extremely high global disease burden of approximately 96 million. Vector-borne diseases, such as malaria, refer to a class of diseases caused by interaction with a vector, most commonly mosquitoes, ticks, mites and flies, which are infected with a pathogen. There are many factors which contribute to the emergence and persistence of vector-borne diseases and climate change has emerged as a major barrier to the effort to reduce the burden of these diseases. The three primary elements of the disease transmission pathway which are vulnerable to climate change are i) vector survival and reproduction, ii) biting rate, and iii) the extrinsic incubation period (EIP), which represents the time taken by the pathogen to become infectious and transmissible. Natural disasters further contribute to vector-borne diseases, specifically, those spread by mosquitoes through increasing breeding sites and destroying infrastructure.
The spread of Dengue Fever has been impacted by recent climate change. Dengue Fever, a mosquito-borne tropical disease is propagated by the species Aedes aegypti. While, primarily found in Southern Asia the species is now endemic to more than 100 countries across the globe. These mosquitoes occupy environmental niches that are highly influenced by climate change and fluctuations in both precipitation and average daily temperatures can affect the reproductive cycle and niches that Ae. aegypti can inhabit. Specifically, ovarian development and survival of the mosquito is optimized between 20-30 degrees Celsius and the EIP temperatures are even higher at 32-35 degrees Celsius. Further, frequency of feeding for these mosquitoes are observed to increase with higher temperatures, making them more likely to bite humans. As such, the continual rise in global temperatures creates an opportunity for Ae. aegypti to expand their environmental niche and to increase disease transmission to humans. Therefore, it comes as no surprise that dengue has been increasing dramatically, with a 30-fold increase within the past 50 years.
Precipitation poses as a climatic factor which influences reproductive environments and aquatic stages of Ae. aegypti. Water-holding containers are a favourite habitat for Ae. aegypti to lay eggs. During drought and related natural disasters, the number of water-holding containers increase to ensure a basic supply of water. As the Ae. aegypti eggs are able to withstand drought conditions for several months and hatch immediately after the first rainfall, the vector is extremely difficult to eliminate. As such, they are captured and persist in the water-holding containers when dry and later hatch upon rain exposure.
Natural disasters can increase human susceptibility to mosquito-borne diseases owing to the destruction and lack of health infrastructure. Indeed, following Hurricane Katrina, Caillou et al. found an increase of West Nile Virus throughout Mississippi and Louisiana that subsequently led to both increased flooding and damaged infrastructure. Climate change may not be the sole contributor to the catastrophes of natural disasters but it is definitely an underlying factor.
Climate change poses a problem in the Americas and has notably had an impact on the spread of Lyme disease. Cases of lyme disease, spread through the bite of Ixodes scapularis ticks, have been increasing with particular spikes in prevalence observed in the United States and Canada. Research models have shown that climate change can alter the redistribution of the tick by changing suitable environmental conditions. For example, a rise in the minimum temperature at higher latitudes can cause the ticks to inhabit more northern regions. Coinciding with this temperature change, environments which ticks currently reside in can become uninhabitable when the maximum threshold temperature is reached. Figure 1 shows predicted changes in habitable regions for Ixodes scapularis modeled by Brownstein et al. Notably, here he shows that by 2080, ticks would have spread well throughout northern Ontario and the American mid-West, and is no longer contained in the constant that the 2020 projection shows.
Figure 1: Projected distribution of Ixodes scapularis in 2020, 2050 and 2080 based on climate change predictions by the Canadian Global Coupled Model
While climate change is not the sole factor in altering the fates of vector-borne diseases, changes in agricultural practices, urbanization and global travel and their intersection with climate change needs to be better studied. The fact remains, to robustly address the issue of vector-borne diseases, it is critical that strategies to combat climate change be included on the agenda.