Many effects of climate change are well known, such as increasing global temperatures, shrinking glaciers, and rising sea levels. However, one impact may be lesser-known but is no less important – the expanding geographical range of many diseases.
Zoonotic diseases, also called zoonoses, are infectious diseases that spread from non-human animals to humans. According to this United Nations report, about 60% of infections in humans originate from animals. In addition, the report estimates that about 75% of new and emerging diseases jump from animals to humans.
Transmission of zoonotic diseases may be through direct contact with vertebrate animals, such as in HIV, influenza (including bird flu and Spanish flu), Ebola, and COVID-19. Other zoonoses may be transmitted indirectly, through insect vectors or via the food system. Examples of indirectly transmitted zoonotic diseases include Lyme disease, West Nile fever, and Zika fever.
The emergence of COVID-19 has brought the issue of emerging infectious diseases into focus. In addition to COVID-19, other recent novel diseases include bird flu, H1N1, and severe acute respiratory syndrome (SARS). Research has shown that zoonotic diseases have been emerging from wildlife at an increasing rate as shifts in land use, deforestation, and climate change bring humans into closer contact with wildlife. In addition to the establishment of novel diseases, changing climate conditions have also led to the geographical expansion of existing diseases.
Increasing temperatures have multiple effects that can impact the geographical range of zoonotic diseases. For vector-borne zoonoses, the ranges of insect vectors are expanding as temperatures rise. Insects, such as mosquitoes and ticks, are now able to survive and breed in regions that were previously too cold. In addition, mosquitoes, which can transmit many diseases such as West Nile fever, yellow fever, and Chikungunya, are more active, breed more frequently, and mature faster with warmer temperatures.
Changing Rainfall Patterns
Heavier rainfall due to climate change has been associated with local conditions that lead to favourable conditions for the transmission of zoonotic diseases. For example, outbreaks of Rift Valley Fever in Africa follow periods of heavy rainfall and subsequent floods because the mosquitoes that transmit Rift Valley Fever lay eggs in floodwaters. More rain can also lead to increased crop yields, which then results in more rodents and rodent-borne diseases.
On the other hand, droughts can also lead to disease outbreaks. Droughts can decrease the number of mosquito predators, as well as facilitate easier transmission of zoonotic diseases between animals as animals will congregate around watering holes.
Human Actions in Response to Climate Change Effects
As climate change leads to changes in local weather and climate, human responses to the changing conditions can lead to unintended consequences. For example, in areas with more drought periods, residents may choose to collect and conserve water by installing water storage tanks. These water storage tanks can become breeding grounds for mosquitoes, leading to increased transmission of mosquito-borne diseases.
Canadian Example: Lyme Disease
In Quebec, the ticks that transmit Lyme disease have expanded their range by an estimated 35 to 55 km per year, which has been shown to be directly linked to warming temperatures and increasing rainfall. The expansion of tick habitats has led to an increased risk of human-tick contact, resulting in more cases of Lyme disease in humans. This trend is expected to continue in the future as average temperatures are expected to rise by an additional 2 to 4°C by 2050.
Even though zoonotic diseases will continue to emerge in the future, the One Health approach can be used to reduce and control the risk posed by emerging zoonoses. The One Health approach requires looking at human, animal, and environmental health in a holistic way and designing risk control measures that address all three areas. For example, we know that diseases can be passed from wild animals to domestic animals and humans. Therefore, preemptive treatment of diseases in wild animals, while seemingly lower priority compared to investments in human health, can help to prevent human infections downstream.
Investments in surveillance and coordinated human, animal, and environmental health services are needed to control the spread of disease and prevent future large-scale epidemics and pandemics. The economic and social costs of the COVID-19 pandemic has highlighted the value of investing in measures for surveillance, prevention, and early responses to emerging diseases.
Environmental conditions must also be considered when addressing the risks posed by emerging zoonoses. For example, habitat loss can lead to increased chances of contact between wild animals and humans and in turn lead to increased risk of disease transmission. So, habitat restoration, while seemingly unrelated to human health, can help to reduce the risk of new diseases emerging from wild animals.