02/26/2014 - 13:53

Stopping a new mosquito-borne viral threat

Laura H. Kahn

Laura H. Kahn

A general internist who began her career in health care as a registered nurse, Kahn works on the research staff of Princeton University's ...

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In the Makonde language of Tanzania and Mozambique, chikungunya means “that which bends up,” a phrase that describes how people look when suffering from the disease. Before December 2013, the chikungunya virus was found in Africa, southern Europe, Southeast Asia, and on islands in the Indian and Pacific Oceans—but not in the Americas. That changed when two laboratories identified cases on the French side of the Caribbean island of St. Martin in residents who had not recently travelled. Since then the virus, which causes extreme debilitating pain, has spread to nearby islands and sickened thousands.  

Mosquito-borne viruses like dengue, yellow fever, and chikungunya are so good at traveling and terrorizing people that the US government’s Centers for Disease Control and the National Institute of Allergy and Infectious Diseases (NIAID) consider them potential agents of bioterrorism. While chikungunya is not usually fatal, the rate of suffering it causes is high, and there is no approved vaccine. A widespread chikungunya outbreak could keep people home from work and spread fear. Military operations could also be seriously affected if enough service members became infected during a deployment. To attempt to deliberately spread the disease, an attacker would have to release either mosquitoes or people infected with the virus into the environment, though this approach certainly wouldn’t guarantee success.

However the virus takes hold, containing it isn’t easy. Human behavior has helped these diseases spread. As encroachment into the natural world continues, human behavior will have to change in order to reverse course.

World travelers. Like its more deadly cousins dengue and yellow fever, the chikungunya virus is spread by two species of Aedes mosquitoes. While dengue has already been present in the Caribbean for hundreds of years, the appearance of chikungunya is a major development, because once a disease has appeared in one place on a new continent, it is very likely to spread further.  

How it arrived in the Western Hemisphere is anybody’s guess, but the most likely explanation is that an infected traveler or mosquitoes arrived in the region by airplane. Mosquito-borne viruses have a long history of traveling with humans to new continents. Four different dengue viruses originated in monkeys and spread to humans—most likely in Asia but possibly in Africa—hundreds of years ago. Dengue-like outbreaks were first reported in the Americas in 1635 in Martinique and Guadeloupe. But it wasn’t until World War II that the Aedes mosquito, and probably infected people, inadvertently played a crucial role in spreading the disease as they were transported around the world in ships. One of the two types of the mosquito, Aedes albopictus, originated in Asia and spread to the Americas and Europe through the trade in used tires, which make excellent mosquito incubators because it’s difficult to remove water—which the mosquitoes need to breed—from them. Also known as “break bone fever,” dengue is now present on every continent except Antarctica. The World Health Organization estimates 50- to 100- million infections annually result in 22,000 deaths. Several potential dengue vaccines are now in clinical trials, but attempts to develop one have been frustratingly slow.

Yellow fever, meanwhile, which got its name because it can turn the skin yellow, originated in Africa and probably spread to the Americas via the slave trade. It has taken root, and is now widespread on these two continents. The disease has never been reported in Asia, but the region is at risk because it does have the Aedes mosquito. Though a vaccine exists, epidemics still occur annually.

The human factor. The Aedes albopictus, known as the “tiger mosquito,” is aggressive but prefers outdoor, forested environments. It is very resilient and difficult to control. The other Aedes mosquito, Aedes aegypti, has adapted to urban environments and is difficult to control because poor sanitation in slums provides good breeding sites. Aedes aegypti mosquitoes live for about two weeks, biting at dawn and dusk, with a preference for humans over other animals. This mosquito is the primary dengue vector worldwide.

Global travel and trade is one way humans help spread mosquito-borne diseases. Environmental destruction and poor sanitation are two more. All three viruses—chikungunya, dengue, and yellow fever—originated in wild primates and were carried by forest-dwelling mosquitoes before humans altered the environment. By eroding ecosystems where disease-carrying insects and animals live, deforestation allows emerging infectious diseases to spread. Climate change, too, will likely widen the mosquitoes’ range as parts of the world grow warmer.

There is no simple solution. Mosquito control programs won’t stop the introduction of mosquito-borne viruses into new areas, but they can help reduce the spread. Eliminating breeding areas and killing larvae are far more effective at controlling mosquito populations than spraying neighborhoods with adulticides, which kill only mature insects. By the time trucks and helicopters come around dispersing chemicals, it’s too late.

Changes to urban environments, such as adding screened windows and doors and eliminating larval breeding sites, might contribute to a solution. Widespread use of personal protection would help, whether in the form of mosquito nets, protective clothing, insect repellents, or novel technologies like the Kite Mosquito Patch, which is being developed to prevent bites.

One of the greatest challenges for public health programs, particularly those that require sustained efforts like vector control, is long-term support. Success at prevention results in the absence of disease, but when there is no disease, government officials often figure that there is no need to fund prevention. Convincing them that preventing disease not only reduces human suffering but is also cost effective is not easy, especially when money is tight. One bright spot on the horizon is that the United States recently launched a $40 million global initiative to prevent infectious disease threats in developing countries. For it to succeed against chikungunya, dengue, and yellow fever, it will have to zero in on mosquito-borne diseases, and maintain support over the long term.