Mosquitoes are attracted to carbon dioxide. They also seem to be more attracted to some people than others, but no one knows whether this is because of personal chemistry or diet or cologne or blood health. Without a doubt, mosquitoes possess a higher level of natural awareness than humans can understand.
“A mosquito will land in a dry spot,” says Dice, “and she’ll know that water will return. And she’ll put her eggs there in a dry area, knowing that it will fill with water soon.”
Some mosquitoes are day-biters, but most feed from dusk until dawn. The first thing on a female mosquito’s mind is blood. She needs a blood meal to procreate.
Male mosquitoes lack the saberlike proboscis of the female and are harmless to humans and animals. The males eat only nectar.
“Female mosquitoes fill their stomachs with blood,” Department of Health supervising vector ecologist Chris Conlan explains. “When a mosquito finishes her meal, she’s loaded. It’s a big, red abdomen full of blood. And at that point, she’s going to go off and find a nice, quiet corner to rest. And she’ll spend two or three days converting the protein in that blood meal into eggs. At some point during that time, of course, she’s mated with a male, and that happens pretty quick, and she can store sperm from the male. So she really only has to mate once. And once she’s done all that, then she goes on a hunt to look for a nice place to lay her eggs. The whole process might take about four days, give or take, and then it starts all over again. Theoretically, under the best of all possible circumstances, a mosquito can lay a hundred or so new eggs every four days.”
And each mosquito will continue to eat and ripen eggs and lay them for five or six blood meals or more, depending on temperature, food availability, and, of course, whether or not it can avoid being slapped in mid-meal.
Most mosquitoes only live for a couple of weeks, but Conlan says that some will buck the odds and hibernate through a winter. “The mosquitoes that live longer are a greater risk for disease transmission,” says Conlan, “because they may have picked up an infection somewhere along the line that’s finally incubated long enough that they can transmit it to whatever else they bite.”
GETTING TO KNOW YOUR “BUGS”
Ever since he was in medical school at UCSD 20 years ago, Dr. Dale Lieu has been fascinated by “bugs and the drugs that kill them.” Although, when Dr. Lieu says “bugs,” he doesn’t mean mosquitoes, per se. He means viruses, bacteria, and parasites.
Dr. Lieu is a specialist in infectious diseases for Kaiser Permanente Medical Group.
“Like a lot of diseases that mosquitoes carry, West Nile doesn’t hurt the mosquito,” Dr. Lieu says. “And traditionally, it hurts animals a lot more than it hurts people. In a way, you could say that people aren’t the real target; we just kind of get in the way.”
The first time West Nile virus was discovered in the United States was in 1999 near the Bronx Zoo.
“They started having birds die off,” Mary Dice says. “Not their collection, but other birds, crows. And they were kind of confused, and they have an expensive collection of exotic birds to protect, so they got very interested. Because they’d never seen West Nile virus in this country before, the New York health department assumed that some exotic animal from the zoo had imported some disease. That turned out not to be the case. And the zoo collection was never affected. But they helped track it down. They discovered that it was, in fact, West Nile virus.”
Because birds are the carriers of the disease, and they migrate so widely, once a bird becomes a reservoir, then West Nile can go from New York City to Mexico in a week or less.
“West Nile has been sweeping through the country,” Dr. Lieu says, “but the interesting thing is this: Wherever it has peaked, and then the number of cases has started to go down, the virus has never circled back. If you look at New England and New York, where they had a whole bunch of cases seven or eight years ago, in the past couple of years, they hardly had any. And we think both people and the animal community eventually develop immunity.”
But Dr. Lieu is quick to state that we don’t have enough experience with re-exposure to the disease to guarantee that we develop complete immunity. “There’s probably a good chance that people who get it once become less likely to get real sick,” he says. “But we don’t know how protective it is.”
Has San Diego reached its peak?
“There’s no way of telling that,” Dr. Lieu says. “We’re certainly on an upswing, as far as the number of cases. We’ve had 23 cases so far this year, and we had 15 in 2007. But who knows where that’ll stop, or when it will start going down?”
Lieu wears a white lab coat with his stethoscope in his pocket. He expressly does not wear a tie, because it’s “the one article of men’s clothing that we never wash,” and Lieu is nothing if not fastidious. “I think most infectious disease doctors are like that,” he laughs.
Dr. Lieu himself has treated “a few cases” of West Nile virus in the past year. “The most susceptible people are very young, very old, or have compromised immune systems. Only about one in five people who are exposed to the disease actually get sick. And for the majority, they only get a flulike illness: fever, chills, muscle aches, headaches, fatigue. And that starts about three to seven days after the bite. Most people just think they have the flu, and they get better after a week or ten days, and they don’t think much of it. The vast majority are never even checked for West Nile. But about 1 in 100 people who get infected develops encephalitis. And that’s the most dangerous manifestation of West Nile. That’s where the brain tissue itself becomes infected with the virus, and it gets inflamed. And people come in real, real sick.”