How vector borne diseases are spreading with climate change

Friday, September 21, 2018, 1:37 PM - Scientific models show that climate change will bring vector-borne diseases to places far from their typical habitats, which challenges health care systems to quickly act on treating previously uncommon infections and developing long-term care options.

Temperatures are not only getting warming - more and more regions are now considered tropical. While some might welcome the toasty temperatures with open arms, the changing conditions have significant implications for public health risk and health care systems. A new study reports that tropical regions are increasing both northwards and southwards, and the diseases that thrive in these regions are spreading with them.

The tropics have been widening at an average rate of approximately 0.2 degrees latitude, or about 27 kilometres per decade in both hemispheres, which is causing an expansion of hot, dry deserts and lowering marine productivity in effected oceans. The expansion of these regions will have significant environmental changes, as well as the types of insects and animals that live in these areas. Nearly half of all humans live within or near subtropical, semi-arid regions, so billions of people could be affected by more vector-borne disease. 

WARM, WET CONDITIONS ARE GREAT FOR VECTOR BORNE DISEASES

Weather conditions have direct and indirect influences on the ability a vector has to acquire, maintain, and transmit the virus. Recent changes in climate conditions, such an increased rainfall, humidity, and temperature have contributed to the maintenance of West Nile Virus in locations such as the Canadian Prairies, Southern Europe, parts of the United States where it was recognized in New York City in 1999.

Malaria, West Nile Virus, and Zika are all examples of vector-borne diseases that are commonly transmitted through vectors, such as insects like mosquitoes. Insects carrying the parasites of these infections typically transmit them to humans through a bite. These infections are generally confined to tropical regions because that is the optimal habitat for insects like mosquitoes - warm temperatures and lots of moisture. According to the IPCC Fourth Assessment Report, there is evidence that the geographic ranges of vectors like ticks have changed in response to climate change.

Flooding in Laos, where malaria and other vector-borne diseases are a significant socio-economic challenge. Credit: Wikimedia Commons

While malaria is a preventable and curable disease, it is considered to be the world's most important and deadly tropical mosqutio-borne parasitic disease - and it's geographic range is spreading. This disease significantly hinders the efforts that nations in Africa, Asia, and Latin America make in reaching the Millennium Development Goals, and outbreaks in nations that do not have adequate resources for vector management and treatment have shown trends of severely negative impacts on economic development and societal well-being.

There are two critical factors for the spread of malaria - favourable conditions for vector breeding and the immunity of the exposed population. The widening of the tropics expands large populations of people who have no immunity to parasites like malaria, which could rapidly drive the rate of disease increase. Projections of temperature increase show that if global temperatures increase by an average of 2-3 degrees Celsius the number of people who would be at risk of malaria would increase between 3-5 per cent of the entire global population or tens of millions of people.

Temperature rise will happen faster in some regions, and the range of predicted temperature rise varies between 1.5 to 5.8 degrees Celsius, according to IPCC projections. A warmer temperature and an increased frequency of extreme weather events will likely result in storms releasing larger amounts of precipitation, which creates ideal conditions for moist environments and stagnant water that serve as breeding grounds for some vectors like mosquitoes.

EL NIÑO PLAYED A ROLE IN SPREADING ZIKA

The Zika virus was first identified in Uganda in 1947 and local infections across Africa and Asia occurred and typically involved mild illness. In 2015 Brazil experienced locally transmitted outbreaks of the infection and a spike in the amount of babies born with microcephaly, characterized by a smaller than average head size, to mothers that may have contracted the disease through Aedes mosquito transmission, which prompted widespread travel concerns and recommendations that women delay becoming pregnant if they were at risk of the infection.

By February 2016 over 20 countries and territories in the Americas had reported local transmission and the World Health Organization declared a public health emergency of international concern, and studies suggest that El Nino played a role in the sudden explosion of infected mosquitoes. Research from the University of Liverpool showed that the El Niño of 2015, which was one of the strongest events on record, caused dramatic changes in weather patterns and climate change created conditions that were exceptionally conducive for mosquito-borne transmission of Zika.

The warm temperatures support growing mosquito populations, increases their feeding frequency, lengthens their lifespan, and reduces the time required for the virus to develop inside the vector. Zika was is believed to have entered Brazil in 2013, yet the disease become widespread after the anomalously hot temperatures caused from both El Nino and climate change. Malaria outbreaks in Ecuador, Peru, and Bolivia in 1983 were also found to be linked to the strong El Niño event in 1983 that brought heavy rains and flooding.

How climate change is projected to spread malaria by 2050. Source: De Souza et. al. (2012), Impact of Climate Change on the Geographic Scope of Diseases.

In addition to extreme weather events and climate change, the variation in seasons plays an important role in the patterns of vector-borne diseases. One study states patterns of human behaviour related to changing seasons determines can largely determine their outdoor exposure, occupation, lifestyles, physical exercises, and overall risk of contracting a vector-borne disease. In some of the poorest countries lifestyles rely on travelling long distances to obtain food and water, limited access to health care resources, and face a disproportionate risk to climate change.

Countries in Europe and North America are some of biggest contributors to climate change and are beginning to see an increased risk for vector-borne diseases, whereas other nations that have been battling these diseases at large for decades have hardly contributed to the unprecedented amount of human-released greenhouse gases. The impact that climate change has on vector-borne diseases demonstrates the catastrophic effects that altering the climate has on human health, and how health care systems will need to increasingly consider climate change in outreach, treatment, and resource management.

Sources: Nature | IPCC | PNAS | Environmental International

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