Climate Change and the Spread of Vector-Borne Diseases
By: John O. Davies-Cole, PhD, MPH, Professor at The George Washington University
A vector-borne disease is one in which an arthropod or other agent is responsible for transmitting a pathogen or disease-causing organism like, bacteria, virus or protozoa from an infected individual to another individual. According to the World Health Organization (WHO), the most deadly vector-borne disease, malaria, kills over 1.2 million people annually, mostly African children under the age of five, and dengue fever, together with associated dengue hemorrhagic fever (DHF), is the world's fastest growing vector borne disease. Nearly half of the world's population suffers from vector-borne diseases, resulting in high morbidity and mortality. The most seriously affected parts of the world are the developing countries located in tropical and subtropical areas. Some examples of neglected vector-borne diseases include dengue/dengue hemorrhagic fever, human African trypanosomiasis (sleeping sickness) and onchocerciasis (river blindness).
It is projected that climate change will cause the spread of vector-borne diseases, and place additional stresses on ecosystems, thus bringing untold suffering to the rural poor, those in urban slums or in conflict zones. The most vulnerable in the population are the elderly, the young, the socially isolated, lower socioeconomic status families, those with preexisting conditions and the Immunocompromised (Having an immune system that has been impaired by disease or treatment).
Climate change will effectively weaken our present efforts to accelerate and sustain progress toward the Millennium Development Goal (MDG) 6 that focuses on control of malaria and other major diseases. Increased temperature and humidity will provide favorable conditions for disease vectors to thrive and expand their range. Heavy rains, abundance of breeding grounds and availability of natural reservoirs will exacerbate the situation. For example, it is projected that temperature changes, especially warming at night in temperate regions, will favor the growth and development of the mosquito vector, Aedes sp, and will cause dengue fever to spread.
Dengue is a mosquito-borne infection transmitted by Aedes mosquitoes that in recent decades has become a major international public health concern. It is found in tropical and sub-tropical regions around the world, and causes fever, rash, muscle and joint pains. Scientists now say that climate change is accelerating the spread of dengue throughout the Americas and in tropical regions worldwide. They are attributing this spread to more rainfall and warmer temperatures that are providing optimal conditions for the mosquito to breed and expand into new territories. It is now estimated that by 2085 climate change will put about 3.5 billion people at risk of dengue fever.
Human African trypanosomiasis or sleeping sickness is spread by the bite of an infected tsetse fly (Glossina sp). It is characterized by fever, swollen lymph glands, aching muscles and joints. In advanced stages, the disease attacks the central nervous system. Scientists have predicted increased temperatures in the Sahara and semi-arid parts of southern Africa including equatorial countries such as Cameroon, Kenya and Uganda. As ecosystems change, the distribution of the tsetse will change. Factors that alter the resting sites for adult tsetse flies, such as long-term changes in rainfall, will affect the epidemiology and transmission of trypanosomiasis. This will put millions of people at risk of the disease in affected areas.
Onchocerciasis is the world’s second leading infectious cause of blindness and afflicts approximately 42 million people worldwide, with well over 99 percent of its victims in sub-Saharan Africa. The disease causes chronic suffering and severe disability. The blackflies (Simulium sp) that transmit the disease breed in rivers and streams, especially in fast-flowing waters.
Since 1975, the WHO, international foundations, nongovernmental organizations, and governments have worked cooperatively to reduce the burden of onchocerciasis, and tremendous progress has been made in the last 35 years. The effects of climate change will severely reverse this situation as projections now show that black flies could increase as much as 25 percent if temperature and precipitation patterns change.
Given the current projections, state governments including national and international organizations must work together to develop suitable intervention strategies to address this issue. This would require improvements in the health infrastructure and provision of additional resources.