A Staff Report from the Straight Dope Science Advisory Board

What are my chances of getting West Nile virus?

December 2, 2003

Dear Straight Dope:

I live in Washington D.C., and out here on the East Coast I have been seeing many news stories relating to the West Nile virus. What are my chances of getting this disease sitting outside, as opposed to simply walking around? My ability to relax in the sun lies at stake.

Relaxing in the sun isn't a good idea, but the problem is more skin cancer than West Nile virus. It's entirely possible that you've already been exposed to the virus and either thought you had a mild summer cold or else showed no symptoms at all. About 20% of the people exposed to West Nile virus develop West Nile fever, symptoms of which include headache and body aches, a fever, and sometimes a body rash. Lymph glands may become swollen. Not exactly fun but it probably won't send you to the doctor as it lasts only a few days, and you end up producing antibodies that presumably protect you for the rest of your life. Most people exposed show no symptoms whatsoever.

While West Nile is not the bad boy on the arbovirus block (for that, we can look to something like eastern equine encephalitis, which is also transmitted by mosquitoes), it's no walk in the park for about 1 in 150. These poor folks contract West Nile encephalitis or meningitis, characterized by the mother of all headaches, big time fever, convulsions, muscle tremors, paralysis, disorientation, stupor, coma, and sometimes death. There's about a 50% chance that you'll end up with permanent disabilities if you survive. The mortality rate for West Nile in the New World ranges from 4 to 10%, with the chance of death increasing with age, particularly above 50 years old. It's still up in the air what your chances are if you're immunocompromised. Bad as all this sounds, don't panic--the Centers for Disease Control say that a bite from an infected mosquito has less than a one percent chance of making you seriously ill.

Why is the mortality rate so high if many of those exposed don't even get sick enough to know it? Because mortality rates are calculated by dividing the number who die from West Nile into the number discovered by a doctor to have been exposed. If you don't get sick enough to see a doctor, you don't get counted into that bottom value.

You ask what your chances are of getting exposed if you are sitting vs. walking around. First, simply being outside increases your chances of exposure. In an area where West Nile occurs, the longer you're exposed to biting mosquitoes, the higher your chances are of acquiring the virus. Beyond that, several factors determine whether you end up as a host (blood donor) or not. These include how attractive you are to the mosquito and how good the mosquito is at transmitting West Nile.

Mosquitoes often prefer certain types of animals (say, birds over mammals) and even certain types of individuals. We all know unhappy campers who are mosquito magnets while others can frolic in the altogether with nary a bite. Apart from the CO2 that initially attracts mosquitoes, acids in your perspiration confer varying degrees of attractiveness or repulsiveness. If you're working up a sweat, you're potentially giving off more host-seeking cues than if you were lying quietly on your chaise longue. On the other hand, don't fall asleep. You can't squash an offending mosquito while enjoying 40 winks.

West Nile exposure also depends on the species and quantity of mosquitoes in your area. As you probably know by now, West Nile virus is primarily passed among birds and mosquitoes. An infected bird is bit by a female mosquito looking for a bloodmeal so that she can develop her eggs and lay them. So, if she bites you after biting an infected bird, you get sick, right? Wrong. Not all mosquito species are equally competent at spreading the virus. The virus, an incidental particle in the blood, might stay within one species' stomach and get excreted. In other species, it passes through the stomach lining but gets no farther than the abdomen. These are "incompetent species" and won't spread the virus even if they bite you. In competent species, however, the virus enters the hemolymph (blood equivalent) of the mosquito and makes its way to her salivary glands. During a bite the virus gets ejected out with saliva and into you. In such cases you have less than 1 in 100 chance of getting really sick.

While any one (competent) mosquito has a low probability of carrying virus, millions of them are out there looking for an easy bloodmeal. But some simple measures on your part will reduce your risk of exposure. If mosquitoes are present, wear a repellent. A compound with 25% DEET is recommended. Repellents with higher DEET percentages will have more impact on your wallet than on mosquitoes. If you really like to swelter, you can wear long-sleeved shorts and pants to protect against bites--just make sure that the cloth is impenetrable to the proboscis of the probing mosquito.

In addition to repelling mosquitoes, eliminate potential breeding grounds. Keep your property clear of standing water such as in gutters, tires, and buckets. Change water in birdbaths once a week. If you still have mosquito problems, call your local mosquito control agency. They can determine which mosquitoes you've got (are they a threat?), map the nearby mosquito breeding habitats (where are they?) and control them using a larvicide (first choice), pupacide, or an adulticide (last choice).

Dear Straight Dope:

Why do health agencies tell us that it is virtually impossible for a mosquito to transfer the AIDS virus from one individual to another, when at the same time we are told that the West Nile virus is only passed to humans from a mosquito which has previously bitten an infected animal? Sign me . . . --"Dip me in DEET"

Guest contributor brachyrhynchos replies:

Why mosquitoes can transmit other viruses but not HIV pops up whenever arboviruses get media play. It's a reasonable question. Dr Wayne Crans of the Mosquito Research and Control Unit at Rutgers University has written a pamphlet (www-rci.rutgers.edu/~insects/aids.htm) explaining why HIV isn't suitable as an arbovirus (a disease transmitted by insects). Basically, it boils down to: (1) the mosquito picks up fewer HIV particles from infected humans than are required for transmission; (2) in contrast to arboviruses, which typically reproduce and migrate inside the mosquito, HIV remains inert in the mosquito's stomach, where it's digested within a fairly short time; and (3) contrary to what many imagine, a mosquito's proboscis doesn't work like a syringe, in which blood flows in both directions. Rather, blood from the host, plus any virus particles it may contain, flows in only one direction--into the mosquito. Saliva isn't injected into the host via the esophagus (the food pipe) but rather a different organ.

For a typical arbovirus such as West Nile, the virus particles are taken in as part of a bloodmeal and make their way through the stomach lining of competent mosquitoes into the hemolymph and eventually can be ejected through the saliva at the next bloodmeal. With HIV, the few particles taken up as part of a bloodmeal are destroyed during digestion. If a mosquito picks up some HIV particles during an interrupted bloodmeal, the second attempt at feeding won't introduce any particles from the proboscis since blood flows only into the mosquito, not back into the host. Mosquitoes don't regurgitate for feeding, unlike some of their more disgusting dipteran brethren.

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