The Mathematics of Mosquitoes and West Nile Virus

Lying on my sleeping pad, I warily eye the mosquito perched above my head. I could reach up and squash it, but that would require extracting my arm from the warmth of my sleeping bag. So for now, it clings to the yellow nylon of my tent, unaware of its reprieve. Although I may triumph over this particular mosquito, I am all too aware of being vastly outnumbered outside my tent. Until recently, my interest in mosquitoes was largely pragmatic: avoid, repel, or swat. Lately, though, I have developed a grudging curiosity about how they make a living. Fact: a female mosquito overwinters with fertilized eggs so the first thing she does in spring, before even feeding, is lay eggs. Fact: Even if she doesn’t find a blood meal, she can survive by sucking plant juices. Fact: the combined meals of a mosquito horde can (and do) bleed a newborn calf to death. Fact: at best a mosquito bite simply itches; at worst, it means disease transmission—malaria, yellow fever, dengue fever, and West Nile virus are all mosquito-borne. It is West Nile virus that has piqued my interest of late. First identified in Uganda in 1937, the virus is well established in its native Africa where it lives primarily in birds and is transmitted among them by mosquitoes. Only occasionally does a mosquito transmit the infection to a mammal. From time to time a West Nile virus outbreak occurs in Europe and Africa—in Israel in the 1950s and South Africa in 1974, and more recently in Romania, Morocco, Tunisia, Italy, France, and Russia. Just recently, West Nile virus made its first known, and headline grabbing, North American appearance. In the summer of 1999, the birds of New York City began mysteriously to die, their bodies appearing conspicuously in the city zoo, parks, and backyards. At first the cause was unknown, but by December of the same year it had been identified, in two reports published in the same issue of Science magazine, as West Nile virus, a disease never before seen on this continent. In subsequent summers, West Nile virus spread west across the continent reaching Ontario in 2001, California and Washington in 2002, and Alberta in 2003. Corvids—crows and jays—were the hardest hit among the birds; other passerines such as sparrows also carried the virus but were dying in smaller numbers. Among mammals, horses appeared especially vulnerable, with a mortality rate of approximately 40%. Human cases were less common and less likely to be fatal, but were a growing health concern nonetheless. By the end of 2003, the virus had been identified in 7 Canadian provinces and 46 U.S. states, in at least 10