Mosquito Sensitivity

(This 799th Buffalo Sunday News column was first published on July 23, 2006.)

Several years ago I wrote about my odd history with mosquitoes. When I was young, I was one of those who always attracted more of these insects than my companions, but then in my 20s I had major stomach surgery. (In those days that was the way they treated ulcers.) The surgery left me for over a year with some strange effects: sweet food tasted bitter and I no longer sweat. Best of all: mosquitoes no longer bothered me the way they had. I would occasionally be bitten, but not nearly as often as others.

Over time my taste for sweets returned (indirectly contributing to my girth) and I began to sweat again. But the mosquitoes didn't bother me in such numbers, thank goodness.

Because of that experience, I have been sensitive to the reports I receive from people who seem especially susceptible to mosquito and other insect bites, my wife among them.

My reason for retelling that story is a report about mosquitoes brought to my attention by Edward Willett. Researchers have confirmed what I have so long observed: people differ in their insect attraction.

A scientific team led by Professor John Pickett of the Rothamsted agricultural research station in England was studying flies that were biting cattle. It had become clear that the number of flies on a herd depended on the presence of certain insect attracting cows. The researchers found that those individuals gave out different chemical signals from other cows and when those cows were moved to another herd, the new herd was similarly pestered.

One of Professor Pickett's students, James Logan, decided that this effect might be related to human-mosquito interaction as well and he began, in collaboration with University of Aberdeen Professor Jenny Mordue, to investigate that problem.

He placed mosquitoes in an apparatus in which a y-shaped tube led in two directions. Those directions led to the hands of different volunteers who were protected behind screens.

He found that indeed some individuals attracted far fewer mosquitoes than others; that, in fact, "there is a strong match between people's perception of their attractiveness to mosquitoes and their attractiveness in the y-tube test."

Logan next set out to collect whole body odors by encasing volunteers in foil sleeping bags for two hours. He subjected these odors to analysis by sophisticated instrumentation to determine the specific components attractive and repulsive to various insects including not only mosquitoes but sand flies, mites, and biting midges.

According to his report, the results showed "that differential attractiveness is due to compounds in unattractive individuals that switch off attraction either by acting as repellents or by masking the attractive components of human odor." This was an important finding because he prior theory was that unattractive individuals simply lacked the attractive sweat components.

The United Kingdom investigators hope to develop a better and safer insect repellent based on their studies. One of the key repellents Logan has already identified is "a natural food additive, so it has proven safety" and he adds that, "because it can be made by plants, we may one day be able to mass produce it cheaply."

A new problem-free insect repellent will be an important contribution because DEET (the chemical N,N-diethyl-meta-toluamide), currently the active ingredient in most repellents, has associated problems. Over 10% of DEET applied to the skin is absorbed in the blood stream and some remains there for up to two months. For most people this creates no identifiable problem but some of us, especially children, have unpleasant reactions.

An alternative to skin application is to apply it to clothing, but it damages plastics including sunglasses.

We here in this country consider mosquitoes an irritation, but elsewhere they are accurately described in the sub-title of Andrew Spielman and Michael D'Antonio's book, Mosquito: A Natural History of Our Most Persistent and Deadly Foe (Hyperion). These tiny insects inflict upon humans many frightful diseases: malaria, yellow fever, dengue, West Nile virus, several types of encephalitis, Rift Valley fever, elephantiasis and Ross River virus, among others.

What we think of as pests cause in the rest of the world over 100 deaths each hour.

Let's hope then that Logan and his colleagues are successful.-- Gerry Rising