It was hardly surprising, really, since she had been spending her days catching the very mosquitoes that carry what western scientists call dengue fever. Pandemic around the world, dengue fever is spreading rapidly to places it has never been seen before. Harrington, at the time a graduate student from the University of Massachusetts at Amherst, was training to be a medical entomologist and wanted to know more about these ubiquitous insects.

Worldwide there are thought to be more than 2500 species of mosquitoes, with 150 in the United States. Mosquitoes are one of the most efficient vectors for human pathogens, likely responsible for transmitting more than 100 of them, causing diseases from viral encephalitis to elephantiasis. Malaria, of course, is the illness most associated with mosquitoes, but New Yorkers, in particular, came to associate mosquitoes with West Nile virus when two years ago seven elderly people died of the disease.

Experts in the United States were fairly confident that West Nile virus was primarily transmitted by mosquitoes here because it is transmitted by mosquitoes in the other areas of the world where the virus has occurred.

Harrington, spurred on by her own brush with death from dengue fever (she’s had malaria too), finds insect vectors fascinating for the complex role they play in human disease systems. She is a medical entomologist in the college’s Department of Entomology.

Medical entomologists use the tools of fields including epidemiology, ecology, evolutionary biology, genetics, parasitology, and arthropod behavior to conquer human disease. (The phylum Arthropoda includes more than 17,000 species of bloodsucking insects such as mosquitoes, sand flies, deer flies, lice, and fleas and over 800 species of ticks.)

Medical entomologists are up against a mighty adversary in the mosquito, an insect that needs to consume blood as a source of proteins essential to producing its own offspring. Malaria, for example, a disease once thought nearly conquered, is coming back with a vengeance.

"A vaccine for malaria is the Holy Grail of tropical medicine," Harrington says. "Many people believe it will never be accomplished because there’s so much we don’t understand about the immunology and the complexity of the parasite, which has developed resistance to drugs both to prevent and to treat the disease."

Take one of the long-held truths that Harrington has overthrown through her research in Thailand on the dengue fever vector, the yellow fever mosquito.

"For a long time we thought these mosquitoes fed once every time they laid a batch of eggs–that is once every three days," Harrington says. "But we found that they will feed almost daily on people, so that exponentially increases the potential for disease transmission."

Harrington has a National Institutes of Health grant to continue her work (in collaboration with Thomas Scott and John Edman from the University of California at Davis) on how dengue fever is spread. Since you can’t band mosquitoes as you can birds, and tracking them with paint is difficult, Harrington and her colleagues have turned to the same technique of DNA fingerprinting used by the FBI to match up identities.

In her case, she takes cheek swab samples from all human residents in a given area and runs DNA profiles on them. Then she collects mosquitoes and does the same thing with the blood in their abdomens. In this way, she can tell exactly whom they fed on, how old the people were, where they live, and whether they are male or female.

"This provides us with detailed information about the feeding behavior of the mosquito and tells us if certain people are at greater risk of infection than others. It also gives us an idea of how far the mosquitoes move around the community, and it turns out they don’t move very far," Harrington says. "The finding supports the hypothesis that people move the virus around as they visit friends and family."

There are less than 20 species of mosquitoes that cancarry the virus of the 80 or so in the Northeast, but the exact vector remains unknown.

"We can select certain species and see how efficient they are in picking up the virus and transmitting it under controlled laboratory conditions," Harrington explains. "So we can identify the most likely species, but we haven’t been able to pinpoint which ones are responsible for human infection in nature."

Nor is it clear how the virus got to North America. It may have arrived with infected mosquitoes. Some scientists think it traveled in the bloodstreams of exotic birds imported–often illegally–for the pet trade. Others think it may have been brought by tourists. One of the many unanswered questions is whether people produce enough of the virus in their circulating blood to infect mosquitoes that bite them.

There is precedent for this idea. Every year in the United States, there are a few cases of airport or locally acquired malaria, most recently on Long Island. In this mode of transmission, people traveling out of the country bring the parasite back and are bitten by mosquitoes once they are home. These mosquitoes, in turn, bite other people who then come down with malaria. No major outbreaks occur because the environmental conditions aren’t conducive for the pathogen to replicate. But were the conditions right, it certainly could.

Harrington is trying to discover the so-called "bridge" vector, the mosquito species that transmits the West Nile virus from birds to people, which may well be different from Culex p. pipiens, the species that researchers are fairly certain transmits it from birds to birds. This is the species that pesticide control efforts are currently targeting.

Research done in the past shows Culex p. pipiens rarely feeds on people. It much prefers birds. So Harrington’s work will involve behavioral studies examining the feeding preferences of Culex p. pipiens and several other likely species. Using antibody and DNA testing techniques, she will analyze blood from the mosquitoes’ abdomens to determine whether it came from a person or other animal.

"This is a critical question because when we know which mosquitoes transmit the virus to people, then we can look at where they breed and target control strategies accordingly," Harrington says. "What’s more, we’ll know whether they feed during the day or at night and advise the public about the times of highest risk."

In some respects, Harrington sees the West Nile virus as a blessing in disguise because it has raised the awareness of both the public and public health agencies to the possibility of introductions of non-native vectors and exotic diseases. While the chance of contracting West Nile virus is very slight, the next disease may be much more serious.

Furthermore, there are many vector-borne viruses percolating in the environment just below the public’s radar screen. Eastern equine encephalitis has what Harrington calls a "silent" transmission cycle from mosquitoes to birds, with humans and horses rarely involved. But when humans do contract the disease, the mortality rate can be as high as 90 percent.

Too, tourists and business travelers bring in their fair share of diseases. And as the human population expands, residents and eco-tourists encroach on environments where humans hadn’t been before. The spread of Lyme disease is a case in point. In the past, only hikers were at risk. Today, suburbanites working in their own backyards contract Lyme disease.

"We know there are all sorts of diseases out there," Harrington says. "Recently, researchers collected mosquitoes in the Amazon and found numerous unnamed viruses that could, at some point, be important for human health."

Yet Harrington sees no need to panic, rather to learn the risks and precautions. Then protect yourself. And in the meantime, Harrington and her collaborators in the field of medical entomology are slowly unraveling the complex interconnectedness between the human and animal worlds.

Consider the statistics: In 1999, the first year that West Nile virus was discovered, only 62 people in all of the United States were hospitalized for it and they all lived in the New York City metropolitan area. Seven of them, all quite elderly, died. Last year the virus had spread to 12 states and the District of Columbia yet only 20 people were hospitalized and just two–again elderly people–died.

One in 150 infected people are estimated to have become seriously ill, and about 10 percent of those cases have been fatal in the United States.

"This represents a less than 0.1 percent chance of an infected person contracting a fatal case of West Nile virus," says Levitan, who has gathered the largest compilation of information on West Nile virus available on the Internet. "Most mosquito bites will not lead to a West Nile infection," she points out.

If this is so, why has there been such a dramatic response by the media (The New Yorker has featured the disease on two covers) and public health authorities, some of whom began large-scale spraying of pesticides?

Exotic diseases of unknown origin are frightening. As is aerial spraying of chemicals in populated areas. But within several months it became clear that West Nile wasn’t a plague.

As with all insect-borne diseases, you can protect yourself if you know what to do. First, look around your house. Because mosquitoes breed in wet places, eliminate all standing water such as in rain gutters or car tires. Fill puddles, but do not fill natural wetlands. Aerate decorative ponds or stock them with fish, mosquitoes’ natural predators. Change water weekly in bird baths. Consider using pesticides only when the disease is suspected or detected in the human population in your area.

New York State Department of Health web site at http://www.health.state.ny.us/nysdoh/westnile/index.htm.

You can also call the New York State Health Department’s environmental health INFOLINE at 1-800-458-1158.

West Nile virus is a zoonotic disease–that is, part of the pathogen’s life cycle goes through animals other than humans. In the case of West Nile virus, mosquitoes infected with the disease feed on birds and, in doing so, infect them with the virus which continues to live in their bloodstreams. Sometime later, an uninfected mosquito bites the same bird and the mosquito becomes infected. Should it, in turn, bite a person, then the person can become infected.

Although crows are the most conspicuous victims of West Nile virus, the virus is most likely present in all types of birds: residents and migrants and birds living in all types of habitats. Crows are large and live near people, hence they tend to be found most often, according to Kevin McGowan, curator of Cornell’s bird and mammal collections in the Department of Ecology and Evolutionary Biology and one of the world’s foremost authorities on crows.

"In the case of most species, some birds will die, but the resistant ones will pass on those genes, and a state of evolutionary equilibrium will eventually be reached where the overall impact will be small," McGowan says. "But if endangered species such as the bald eagle were hit as hard as crows, they would be in real trouble as a species."

If you find a dead bird and want to report it, contact the National Wildlife Laboratory at USDA Wildlife Services Hotline: 1-866-537-2473 (1-866-537- BIRD).