Dengue 'serious threat' to large parts of Europe, South America

A new report from the United Nations University warns that people living in large areas of Europe and mountainous parts of South America face a "serious threat" of dengue virus exposure as climate change causes global temperatures to rise.

UN report warns that dengue virus threatens large areas of Europe and mountain regions in South America as global temperatures rise.

Currently, these parts of the world are too cold to allow mosquitoes to survive all year round.

However, as climate change warms the planet, more regions will escape falling to the sub-zero temperatures at which eggs of the dengue-carrying Aedes mosquitoes die.

The report - from the UN University's Institute for Water, Environment and Health in Canada - contains the first ever maps of global vulnerability to dengue.

The colored maps show how dengue vulnerability contracts and expands worldwide during the year, revealing the hotspots that occur in January, April, July and October.

While not all the regions identified as vulnerable in the report have endemic dengue, the new maps show where such potential exists.

Dengue is the world's fastest-growing vector-borne disease

The report notes that dengue is endemic in over 100 countries and is the world's fastest-growing "vector-borne disease" - one that is transmitted by blood-sucking arthropods, such as mosquitoes, fleas, biting flies, mites and ticks.

The authors acknowledge that a rise in global temperatures could make some places too hot for the dengue-carrying mosquitoes and so shrink exposure in some regions. However, their view is that generally, the dengue virus will spread north and south of its current equatorial range under climate change.

Dengue is a fast-spreading tropical viral infection that is transmitted from human to human via the bites of Aedes mosquitoes.

The symptoms include fever, headache, muscle and joint pains and a measles-like rash. A small proportion of those infected can develop life-threatening dengue hemorrhagic fever. There are currently no licensed vaccines or treatments.

As there is no vaccine, the primary preventive measure to reduce dengue infections is the control of Aedes mosquito population - for example, by "insecticide fogging" of sites where they are known to breed.

Rate of dengue infections has grown dramatically in recent decades

The incidence of dengue has grown dramatically in recent decades. Currently, around 400 million people are infected worldwide every year, a figure that is increasing by around 50-100 million a year. This includes between 250,000 and 500,000 severe cases, resulting in hundreds of thousands of hospitalizations and 20,000 deaths a year, placing huge burdens on national economies and health care systems.

In South East Asia, the direct annual health care costs attributed to dengue are estimated to be in the region of $950 million.

The UN estimate that today, around 2.5 billion people are at risk of dengue - about 40% of the world's population - and that this will rise to about 6 billion by 2085, taking into account climate change and population growth.

The top 10 countries where dengue is endemic are: Brazil, Indonesia, Vietnam, Mexico, Venezuela, Thailand, Philippines, Colombia, Malaysia and Honduras.

Between 2004 and 2010, Brazil reported the largest number of cases of dengue - about 450,000.

South and South East Asia most vulnerable to dengue

However, the report notes that the people of South Asia and South East Asia are probably the populations most vulnerable to dengue.

Countries near the equator - like Colombia, Sierra Leone, Nigeria, Southern India and Indonesia - tend to remain vulnerable to dengue throughout the year because they have moderate to high susceptibility and exposure conditions are consistently high.

The report defines vulnerability as a function of exposure and susceptibility. Exposure is determined by conditions that favor the mosquito and depends on environmental factors, such as temperature, rainfall and land cover. 

Susceptibility is determined by factors such as access to health care, clean water, good housing and other social factors. Dengue control and government policies can also affect susceptibility.

High population density can also increase exposure because there are more potential human hosts to act as a reservoir for the virus, and this increases the rate of spread.

References:

1.  Mapping Global Vulnerability to Dengue using the Water Associated Disease Index (PDF), Laura M. Fullerton, et al., United Nations University, December 2014.
2.  United Nations University Institute for Water, Environment and Health news release accessed 23 December 2014.