The recent dengue outbreaks, notably in the Philippines, USA (Key West), India (Delhi), and China (Guangdong province) got us thinking about why infection rates are on the increase and how the disease is spreading to areas previously believed safe from the virus.
The dengue virus is primarily spread by infected female Aëdes aegypti (urban domestic) mosquitoes that bite during the day (dawn to dusk) both indoors and outdoors. The disease has become a major economic burden and serious public health concern in tropical and sub-tropical areas. According to the World Health Organization, there are approximately 50 million infections worldwide each year and 2.5 billion people are at risk of contracting dengue. However, under-reporting (because the patient did not receive medical treatment) and misdiagnosis (the illness mimics the flu) make it difficult to monitor the spread of the disease.
Dengue can be asymptomatic, meaning that some people do not exhibit any symptoms and may unknowingly be carriers of the disease. If a person does get sick, the illness is characterized by flu-like symptoms generally occurring 4 to 7 days after the bite. High fever (usually lasting 2 to 7 days), pain behind the eyes, muscle and joint pain, severe headache, and a rash (bright red spots) are typical signs of the illness. While there are no permanent physiological effects from these symptoms, a person can be ill for up to six weeks. The infection may progress to a more severe illness called Dengue Hemorrhagic Fever (DHF). Symptoms include severe abdominal pain, vomiting, diarrhea, convulsions, bruising, and uncontrolled bleeding. It can lead to circulatory system failure and shock, and possibly death.
Transmission occurs when mosquitoes bite an infected person. The virus incubates in the mosquito for 8 to 10 days and can transmit the disease for the rest of her life. There are four distinct, but closely related dengue viruses (DENV) belonging to the Flavivirdae family that are responsible for the illness. Contracting one type of virus provides immunity for life for that specific dengue virus, but does not provide immunity to the other three types. If a person becomes ill with dengue more than once, they are at higher risk of developing Dengue Hemorrhagic Fever.
Dengue is now endemic – transmission occurs continuously – in 100 countries in the Caribbean, Central and South America, Southeast Asia, Oceania, Eastern Mediterranean, and Africa. In 2009, Argentina saw a major outbreak and so far this year, Brazil, Venezuela, Thailand, Malaysia, and the Philippines have reported the most cases. Since the risk of infection is based on cyclical weather patterns – higher transmission rates during rainy seasons – sudden infection of thousands of people overburdens the healthcare system and causes major work and school absenteeism.
Over the past couple of years, we’ve seen a drastic rise in dengue cases. Why the sudden increase?
Urbanization: As more people move to cities, high population density means that human-mosquito contact increases contributing to risk of infection. Also, deteriorating urban infrastructure (old plumbing and sewer systems), poor sanitary conditions (inadequate waste disposal, unattended water in artificial containers, old tires), and lack of adequate housing (broken or no door / window screens, little or no air circulation, no plumbing) are all responsible for the proliferation of mosquito breeding sites.
Lack of funding, interrupted fumigation campaigns, and poor management of mosquito eradication programs contribute to the mosquito resistance. Moreover, public health education programs do not always reach the people most at risk of contracting dengue and directives are not always carried out effectively and consistently.
International Travel: Globalization – mainly worldwide trade and international travel – is also a contributing factor to the rapid spread of dengue. People and goods (and mosquitoes) are now being transported within hours rather than days and weeks. We are now finding disease vectors in regions that did not previously have them. In fact, the jungle-dwelling A. albopictus mosquito which is also a dengue carrier, was originally found in Asia, but is now well established in the United States, Central and South America and Caribbean, Europe and Africa.
Global Warming: As global weather patterns are changing and some local temperatures get warmer, the Aëdes aegypti and A. albopictus mosquitoes adapt to higher altitudes and move to more northern latitudes where infection was previously unknown or rare (the Key West, Florida outbreak being an example).
If you are travelling to an area affected by dengue, take measures to prevent mosquito bites during the daytime. Insect-bite prevention includes wearing light coloured clothing, using a DEET-containing repellent on exposed skin, and applying a permethrin spray or solution to clothing and gear. Remove any artificial containers that can hold stagnant water from your living space. Proper solid waste disposal is also key to preventing mosquitoes from laying eggs. You may also want to apply insecticides inside the room or place where you are staying. There is currently no vaccine or medication to prevent dengue.
See the list of countries where dengue is present: IAMAT Dengue information.