The Southeast Asian tiger mosquito carries encephalitis and a few other things. It's constantly being introduced to the state in these shipments of 'Heavenly Bamboo' -- they sell it at the Del Mar fair.
As the hysteria over the West Nile virus moves westward, California appears to be the inevitable destination of the mosquitoes that carry the deadly virus. Even though San Diego has a relatively dry climate and is not known as a mosquito enclave, one local bug expert says that the question is not if the virus will get here, but when.
David Faulkner: "At this point I wouldn't be too concerned about the West Nile Virus, just because there are so many things we don't know about it yet."
David Faulkner, 51, is a forensic entomologist. A research associate of the Museum of Natural History in Balboa Park, Faulkner's most recent notoriety came as an expert witness in the David Westerfield trial. A bug fanatic since the age of five, Faulkner has been doing forensic research since 1979. "West Nile virus is definitely coming this way, but it's probably not going to be as intense as in other parts of the U.S. — particularly eastern parts of the U.S., where they have this thing called water! We don't have a lot of that in San Diego County, particularly with the drought, the Santa Ana conditions, and everything else we've had for the last couple of years."
Mosquito, Aedes aegypti. "I can go out in the back yard at dusk, and two or three mosquitoes will come after me, mostly Aedes, which is a good biter, and it's because we have water around here."
Water is the link between the insect and the spread of the disease. "We have an artificial environment, because we supply our water from outside, and that will restrict the activity of the mosquitoes, which would obviously restrict the spread of the virus. The places where we traditionally have a lot of problems with biting insects would be more of the coastal areas, like the estuaries, because a lot of these mosquitoes are able to survive in brackish water. Any of the lagoon systems where you have polluted or unpolluted fresh water feeding it, you have a potential for mosquitoes surviving. That's where you're going to have a lot of problems."
Faulkner says that normally the County Department of Environmental Health (formerly known as Vector Control) would be responsible for addressing this kind of problem, although it might not be the agency that tries to control it. "Often the counties do interception, and the state does control. The federal government may also regulate how it is done. In mosquito control, they used to use oil and all these other things that they would put across water surfaces. Right now, the most common techniques involve using biological control agents. They have 'mosquito fish,' which are very effective in certain situations, and the other thing is a bacillus, or bacteria, that gets into mosquitoes and kills them as well as the larval stages. We don't do a lot of fogging or spraying. Under certain circumstances, they might try to do that, but you'd have to have a reason for doing it, because fogging and spraying is pretty heavy-duty. We have had malaria here in the past few years, which is also transmitted by mosquitoes. What usually happens is that people will come in who are infected, and if you already have mosquitoes and water, you have the potential for that occurring. Even though we've had about 40 malaria cases in San Diego, most of those were brought in. We only know of 2 that originated here, from people who had not been out of the country.
"First, you have the actual vector of the disease, which is the mosquito. Then you have the introduction of a causal organism, which in this case is the virus or malaria or encephalitis, or whatever it is. Then you have the habitat that they can survive in, which would be aquatic situations -- then you have the potential for having a problem. The thing about West Nile virus is that it is spread about by birds. They are what we would call the 'reservoir hosts.' I know that in Orange County and throughout the state, they biopsy a lot of dead birds to find out what they died of. If you have a large die-off in one area, then you have a suspicion that it could be an accidental or potential poisoning, but it could be something else. There's always the potential that it's a disease that's being carried. Chickens are being used, because you can do blood tests on them while they are alive. And we have a lot of chicken ranches around here, too, which I imagine are tested from time to time as well. So a mosquito that can bite a bird that is infected can then bite a person, and that's how it gets spread around. The birds are the ones that are moving the disease, but it's only when it affects people that we have a real concern."
Another problem unique to San Diego is the movement of its water supply in a drought season. "The mosquitoes, birds, and everything else are moving more to the urbanized areas, because that's where the water is right now. You can go out in the chaparral where it's dry, and you can't find a lot of these insects, but I can go out in the back yard at dusk, and two or three mosquitoes will come after me, mostly Aedes, which is a good biter, and it's because we have water around here. You may not even be aware of the water around. We water our plants and use the sprinklers. There can be small pools of water around that are persistent, and that's enough."
What the small pool of water is "enough" for is the mosquito's life cycle -- much of which depends on mosquito "bites," which are not really bites but blood feasts. "The adult mosquitoes will live two, maybe three weeks at the most. It's a tough life: they've got to find a host, a blood meal, a mate, water to deposit eggs in, and then start the cycle over. The host is what they feed on. Many types of insects are pretty specific as far as what kind of animals they can take a blood meal from. The reason for the blood meal in the female mosquito is for egg maturity. The better the egg mass, the better a blood meal the mosquito has had. They pierce the skin, they add an anticoagulant to keep from clogging up their mouth parts and tube, through which they are feeding, and that's what people react to -- the anticoagulant," Faulkner explains. "They have to find a mate then, and the population is a lot less dense than it would be in a more optimal environment -- say, where there were lots of animals and a water source. But if you take an area that's diverse, that has a lot of dry area -- like San Diego -- they have to find a mate, and after mating, they have to find a water source where they can lay their eggs.
Faulkner continued, "There's another threat from mosquitoes in California that's potentially much worse, and that's the Southeast Asian tiger mosquito. It carries encephalitis and a few other things. It's constantly being introduced to the state in these shipments of 'Heavenly Bamboo' -- they sell it at the Del Mar fair -- where you have these little bamboo stalks. They're shipping it in water in vast amounts, and the mosquitoes are coming with it. They've been intercepted a number of times in the past few years, and they've become established in a couple of places. It's mainly nurseries, but they're trying to get them under control."
A phone call interrupts our conversation. When Faulkner hangs up the other line, he explains its significance to the topic at hand. "That was Jim Lang. He's with County Environmental Health. They've had a big turnover of employees recently, and he's one of their last entomologists. He says that it looks like the state does everything -- collects dead birds, sends them to a lab in San Bernardino, and does the necropsies on them there."
Even though Faulkner says the West Nile virus is inevitable for San Diego, he cautions against panic. "At this point I wouldn't be too concerned about it, just because there are so many things we don't know about it yet. We haven't narrowed down which mosquito is actually carrying it. Florida, which faces a potentially big problem with it, has 77 species of mosquitoes down there, and they're not sure which ones will carry it. We do know that a couple of them in this area are definite potential carriers, particularly the genera Aedes and Culex. But which ones are doing it, we don't know.
"We also don't know which hosts the mosquitoes will prefer. Humans may be a second or third choice. Maybe a bird or smaller mammal will be preferred. They have to bite more than once to transfer the disease. But if you only get one mosquito bite, you'd better make sure that you're the first person that mosquito has bitten. Mosquitoes are very opportunistic. They'll contract it in their body, then spread it in the next blood meal that they take. And if they don't get a full blood meal, which would expand their abdomen enough to let them know that the eggs will mature, they'll go for another host. The disease can build up in the body of the mosquito, and that's how they transfer it. There's a term called 'transovarian transmission' of pathogens, which means it goes from the ovaries of the female mosquito into the eggs, which would mean that the larvae would already be infected when they become adults, but I don't know if that's true for this particular disease. What I know about mosquitoes is probably more extensive than what I know about the West Nile virus."
One person who does know a lot about the West Nile virus is Dr. Leland Rickman, associate clinical professor of medicine for the infectious diseases division at UCSD Medical School. "It's a virus that can cause severe clinical symptoms in patients who acquire it. The virus has been around for a long time; however, in the United States, it was just recently recognized back in 1999. It's been traveling from the northeastern United States across the country, and so far, in California we haven't seen any cases. As of now, it's spread over 30 different states."
West Nile virus gets its name from the West Nile region of Uganda, where it was first isolated in 1937. "It was probably around before that, although we didn't have the technology to detect it. It has a worldwide distribution, but it's most commonly found in Africa, parts of Asia, the Middle East, and Europe. But 1999 was the first time it was recognized in the Western Hemisphere."
The "severe clinical symptoms" Dr. Rickman spoke of include meningitis and encephalitis. "These are infections of the brain or the lining surrounding the brain and spinal cord. It can cause a milder form of illness -- there's a large spectrum of illness, from asymptomatic infection, i.e., not knowing that you've caught it, to severe encephalitis and, potentially, death."
Fortunately, the odds of catching encephalitis when bitten by an infected mosquito are long. "Among patients and people who are infected, only about 1 in 150 actually develop severe disease. You have to be bitten by a certain type of mosquito, and that mosquito has to be carrying the virus. So, depending upon how much virus is circulating in that area, the chances of one mosquito actually transmitting the virus to a person can be remarkably low. For example, in California, where no virus has been detected, if you've got a thousand mosquito bites, you wouldn't get the virus, because there's no virus yet. It depends on how many of the vectors -- which are birds in this case -- are carrying the virus in their bloodstream."
Dr. Rickman is hesitant about saying "It's only a matter of when" about the West Nile virus. "That's a common term used for a lot of emerging infectious diseases. We basically don't know if or when it will arrive. We do have the mosquito vector here, but there is no virus identified so far in California. The birds -- who are actually the reservoir for the virus -- migrate, and they usually migrate in a north-to-south direction, and some of the migration paths for some of the more important bird reservoirs don't come over California.
"I live in San Diego, and I'm not alarmed at all at this point."