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Behind the Bite 

City entomologist Greg Thompson explains that, to stop the West Nile virus, it helps to know about the mosquitoes that carry it.

The main mosquito carrying the West Nile virus behaves like a social climber. At night, the female mosquito (only the females bite you) flies high into the sky where she can easily rub up against some of the city's finest -- those with French Quarter balconies or open upper-story windows.

That's unusual. The typical biting mosquito most often dines on the masses of people at street-level.

"Most mosquitoes fly around at ground level," explains Greg Thompson, entomologist for the city of New Orleans. "But the primary mosquito we suspect for West Nile is the Culex, which is a bird feeder -- it really prefers feeding on birds." If there are no birds in sight, then the Culex mosquito will feed on something else with blood, like a human.

So the evening flight patterns of the Culex are focused not on blue blood, but rather on bird blood. "At night," says Thompson, "Culex mosquitoes spend most of their time up at the tops of trees, looking for sleeping birds. So if you're on a third-floor balcony, to a Culex mosquito, you look just like a bird, roosting."

The Culex, also called the Southern house mosquito, has always been around here, says Thompson. But we know a limited amount about this bug because it, like most mosquitoes in America, was a pest, not a killer. "When mosquitoes are only pests, there's not a whole lot of money spent on research," he notes. Louisiana has sporadic and occasionally fatal outbreaks of other mosquito-borne diseases such as St. Louis encephalitis and eastern equine encephalitis, but the outbreaks are brief enough that no one calls for additional funding or research, says Thompson.

Local mosquitoes are now considered more dangerous because of this year's outbreak of the West Nile virus, which can cause encephalitis -- inflammation of the brain -- and, in some cases, death. According to data from the Centers for Disease Control and Prevention (CDC) in Atlanta, the outbreak in Louisiana far outpaces any other state. As of last week, there were 156 cases of West Nile within the United States -- more than half were in Louisiana. During 2002, officials in Louisiana have reported seven deaths attributed to West Nile. All other states combined have had two.

In response, the CDC recently announced that Louisiana will receive $3.4 million to prevent and control West Nile. Most of this initial funding will go to spraying adult mosquitoes. Nationwide, in CDC funding alone, the feds are spending $27 million this year to combat the spread of the virus.

West Nile virus was first identified in 1937, in a blood sample taken from a feverish woman in the West Nile district of Uganda. Since that time, there have been outbreaks of the disease in the Middle East, western Asia, and Europe. Then, in 1999, New York City had an outbreak -- 62 cases and seven deaths.

To Thompson, the crucial moment came the following spring, when West Nile re-appeared. "If the virus had managed a winter in New York and come back again," says Thompson, "we knew that it was going to be a problem eventually down here."

Neither Thompson nor anyone else doing mosquito research had any idea how quickly West Nile would travel to Louisiana. And it's still not clear why this outbreak is happening here. "It just as well could have been any place else, and probably next year it will be someplace else. We just happen to be the unlucky one this year."

In fact, very little about West Nile in the United States is well understood. For instance, we don't know just how West Nile traveled to Louisiana. One common theory -- yet unproven -- is that West Nile arrived via the many migrating birds that pass through this state. Thompson describes the scenario. "Let's say that you're a bird in New York, and you get bit by a mosquito that gives you West Nile just before you migrate and fly south." You, the bird, may have to drop out on the way if you get sick. Or you may make it all the way south before getting sick.

"Once you arrive," says Thompson, "if you're just sitting there being ill, you're a very easy bird for mosquitoes to feed off of."

That's all it would take to trigger the virus-transmission cycle, as Louisiana-bred mosquitoes take blood-meals from sick migrating birds and then fly off to feed on and infect the local bird population.

There are other theories, says Thompson. For instance, it's possible that infected mosquitoes have been hitchhiking on trucks, trains and ships or flying the friendly skies as stowaways, he says.

Over the years, birds have proven to be useful in the fight against diseases. Most mosquito control districts across the United States and Canada, including Orleans Parish, routinely test wild birds and raise their own "sentinel chicken flocks" -- caged hens tested weekly for mosquito-borne-disease antibodies. Last year, sentinel chickens and wild birds had tested positive for West Nile in Kenner, but they weren't yet testing positive in Orleans. It was just a matter of time. "We knew that all the birds with West Nile were not just hanging out in Kenner," Thompson says. "Birds fly around."

Neither the wild birds nor the sentinel chickens were testing positive for West Nile. Finally, earlier this year, the chickens tested positive. Soon afterward, Orleans Parish reported its first human case.

Birds have long been used as sentinels because West Nile is not generally thought to be fatal in birds. Yet in 1999, hundreds of American crows were found dead in New York. There are now some theories that address that. "Most tests are being done on crows, blue jays and raptors -- predatory birds, like hawks and owls," explains Thompson. "It seems that, not only can this disease be passed from bird to bird by a mosquito, but apparently, it may be passed from bird to bird from a larger bird eating a smaller bird. Raptors eat lots of other birds. And crows and blue jays tend to raid the nests of other birds."

A shrill noise prompts Thompson to excuse himself for a moment. It's one of several interruptions he'll make this morning. No theory is needed to explain this phenomenon.

"The ringing phone is definitely a function of West Nile," he says. People are calling, asking questions, he says, and the six other people in his office are out in the field doing work.

Within the New Orleans Mosquito Control headquarters are cages of mosquitoes, some of which occasionally spring a leak. Thompson's reaction to mosquitoes is instinctive, he says. "The truth is, when one lands on me I tend to swat it and then wish I'd looked at it better."

When observed under a microscope, some mosquitoes are "very interesting little creatures," says Thompson. Some feature iridescent coloring or racing stripes, but Culex is an almost dreary-looking specimen. It's a dark brown, almost black, and doesn't have any striking color differences or stripes.

More exciting in appearance are two black-and-white striped mosquito cousins called Aedes (pronounced ay-dees), both considered invaders to the Americas. They're now quite common here and spend their time breeding in "the small containers that people who apparently love mosquitoes continue to put in their backyard," says Thompson.

The first cousin, Aedes aegypti, commonly called the yellow fever mosquito, is thought to have arrived on slave ships a few centuries ago, bringing the disease with it. On its back, it has a distinctive lyre shape.

As a result of Aedes aegypti's arrival, New Orleans saw yellow-fever outbreaks that killed one-quarter of the population in the matter of a month, says Thompson. Malaria, spread by a different mosquito, was endemic -- there were a couple thousand cases every year throughout the South.

Once people discovered that mosquitoes caused these diseases, the installation of sanitation systems and screened doors and windows led to less severe outbreaks. But both malaria and yellow fever still existed here until they were pushed out by the pesticide DDT in the 1950s.

The yellow fever mosquito's cousin, Aedes albopictus, is known as the Asian tiger mosquito because it has black and white stripes on its back that are prominent enough to be visible to the naked eye. It only arrived here in the past few decades. But it also can be dangerous -- it's been fingered as a carrier of several serious diseases, including West Nile. Researchers don't know for sure that the Asian Tiger is carrying West Nile in Louisiana, but it has in other places. Here, Culex is still considered the primary carrier.

Asian tigers probably hitched a ride here during the 1980s on used tires -- specifically, some of the thousands of tires the United States imports from Japan after their treads are too worn by strict Japanese standards. Like its name, it has a reputation for fierceness. When it first arrived, Asian tiger mosquitoes came into the backyards occupied by its cousin, the yellow fever mosquito, and somehow made the yellow fever mosquito disappear for awhile.

"One of the theories," Thompson explains, "was that the males of the Asian tiger species were more 'macho' so that they were able to out-compete by not only mating with the females of their species, but also mating with the females of the other species."

In any case, the yellow fever mosquito did eventually adapt and is now able to co-exist with its cousin amid the saucers and containers left in local backyards. It's this re-united Aedes family that can make Greg Thompson's job tough. "When we send a truck down the street, we can kill all the mosquitoes flying around on Tuesday, but if you have a container in your backyard, you're going to have new mosquitoes by Wednesday."

Every day, another mosquito-borne illness could be entering the United States. Thompson explains one other mosquito incident that slipped under the public's radar. "During the first two years of the West Nile outbreak, we actually had cases of malaria occurring on Long Island," he says. "Luckily, it apparently has disappeared; it's been controlled. But it shows that we have the potential all the time of a traveler bringing back malaria or yellow fever."

Occasionally, there have been small malaria outbreaks. These are called airport-related malaria because the disease shows up in people who live around airports. Thompson says that there haven't been any such cases here in New Orleans, but that those outbreaks are possible because "mosquitoes carrying disease can basically hop onto a plane and fly."

Mosquitoes may be flying here on jet airplanes, but they were here even before the wheel was invented, says Thompson. "Just like Jurassic Park shows, we know that mosquitoes were around at that time, biting dinosaurs."

In fact, nearly every living thing has some mosquito feeding on it -- even a fish that only occasionally ventures out of the water. "There's actually a species of mosquitoes," says Thompson, "that specializes on biting the mud skipper, the fish that comes out of the water and hops around on mud flats."

Basically, all humans can do is put on long pants and long sleeves, slather on the DEET, and hope for the best, says Thompson. "If you're on dry land and have a backbone, mosquitoes are going to try and feed on you."

click to enlarge CUTLINE PHOTO #2 IF NEEDED: 'Sentinel chickens' are tested weekly in the effort to track and combat mosquito-borne diseases. -
  • CUTLINE PHOTO #2 IF NEEDED: 'Sentinel chickens' are tested weekly in the effort to track and combat mosquito-borne diseases.

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