08/19/2007 - 22:00

The spread of mosquito-borne diseases

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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There are approximately 2,500 mosquito species in the world, but a mere fraction of them feed on human blood. Of this fraction, only the females are vampires, as they require blood to nourish their eggs. When she's ready to lay these eggs, which usually number in the hundreds, the female typically does so on a small, still body of water. In some mosquito species, she creates little rafts for the eggs. They float until they hatch as tiny larvae a few days later. Like butterflies, they eventually turn into pupae, which ultimately metamorphose into the insects we know.

In their book Mosquito, Andrew Spielman, the late mosquito expert at Harvard University's School of Public Health, and Michael D'Antonio, a Pulitzer-Prize winning journalist, wrote that these insects serve no useful purpose in the ecosystem other than to perpetuate their species. Unfortunately, in the course of reproducing, they kill millions of people each year by transmitting some of the most deadly diseases on Earth. Think of them as flying hypodermic syringes drawing blood and injecting disease from one individual to the next.

Mosquitoes transmit malaria, yellow fever, dengue, and Rift Valley fever. They also spread encephalitic viral diseases (such as West Nile virus, St. Louis encephalitis, and Eastern Equine encephalitis) that cause inflammation of the brain's lining. And different mosquito species transmit different diseases. For example, only the genus Anopheles transmit the malaria parasite. (Note: A genus is a subcategory of a species.) Save for Antarctica, these mosquitoes exist everywhere on the planet. Malaria is geographically distributed in tropical and subtropical regions; Sub-Saharan Africa bears the greatest malaria burden.

Temperature is critical for the parasite's transmission. Of the four types of malaria parasites,Plasmodium falciparum is the most deadly and widespread. It requires temperatures above 20 degrees Celsius (68 degrees Fahrenheit) in order to grow inside the mosquito. Therefore, these mosquitoes require warmer climates. It's important to note that malaria was once endemic in the United States. Economic development and stringent mosquito-control measures contributed to its eradication.

Mosquitoes of the genus Aedes (particularly Aedes aegypti) can transmit yellow fever and dengue. Yellow fever is a zoonotic disease specific to Africa and South America that can infect monkeys, making eradication impossible. Fortunately, a vaccine exists. However, the vaccine can cause serious illness in a small fraction of individuals. There is no vaccine for dengue, which can cause hemorrhaging, shock, and death. Dengue's average death rate is about 5 percent, which is 3 percent higher than the 1918 flu pandemic. Like the Anopheles mosquitoes, Aedes aegypti mosquitoes transmit yellow fever and dengue only in areas where temperatures rarely fall below 10 degrees Celsius.

Some mosquito-borne diseases are spreading geographically, and given the required temperature for the pathogens, it would seem that many of these diseases should spread with warmer global temperatures. However, it isn't that simple.

Take inadequate mosquito control for example. West Nile virus originally appeared in the Western Hemisphere in 1999 in New York City. The virus was first isolated in Uganda in 1937 and was typically found in Africa, the Middle East, West Asia, and Europe. No one knows how the virus crossed the Atlantic. But once in New York City, it exploded across the United States. It didn't help that New York City drastically cut its mosquito-control program for almost a decade prior to the outbreak, with the pest control unit of the New York City Department of Health cut so severely that any surveillance or control of the disease-carrying mosquitoes was minimal.

The mosquito that transmits West Nile virus is Culex pipiens, the most common mosquito in urban and suburban settings. It prefers to feed on birds, which is why birds died before the first humans became infected. The virus is now endemic in the United States and appears to be heading to the Caribbean and South America.

Surprisingly, the shipping and dumping of used tires also contributes to the spread of mosquito-borne diseases. A 1992 Institute of Medicine report ("Emerging Infections: Microbial Threats to Health in the United States") found that the United States generates 250 million used tires each year and imports several million more. Fewer than 5 percent of these tires are recycled. As it turns out, used tires are perfect mosquito incubators; once water gets inside of them, it's virtually impossible to remove it. In 1985, the Asian tiger mosquito, which can transmit yellow fever, dengue, and viral encephalitides, was found to have hidden in a shipment of scrap tires from Japan to the United States. The mosquito is now established in 26 U.S. states. A year later, it was found in Brazil for the first time. Since then, it's also been found in Mexico, Guatemala, and the Caribbean. To date, there are no federal laws dealing with scrap tires.

In addition, mosquitoes can hitchhike on long flights from tropical regions to temperate regions. (See "Estimating the Malaria Risk of African Mosquito Movement by Air Travel"). For example, it's possible that a mosquito carrying West Nile virus hitchhiked on a transatlantic flight in 1999, although there's no proof of this. There have been a number of "airport malaria" cases reported in North America, Europe, and elsewhere. Deforestation, urbanization, and intensive agriculture practices can also lead to the spread of mosquito-borne diseases.

Rigorous mosquito surveillance and control is crucial to preventing these mosquitoes from spreading throughout the world. Elected officials must not limit their mosquito-control programs, especially if the programs have been successful in preventing widespread outbreaks. To develop effective control measures, it's important to understand the mosquito life cycle and behavior. Eliminating breeding sites (still, brackish water) and stocking ponds with fish that eat mosquito larvae help keep mosquito burdens down; effective larvicides and pesticides are another option.

In the dog days of summer, the last thing anyone wants is to be bitten by a potentially deadly mosquito.