Rubbing on insect repellent and slapping at mosquitoes is a summertime tradition. Just the smell of a spritz of Off! gives some people Proustian recollections of fun-filled days at summer camp.
But until the 1920s, mosquitoes were a cause of deadly epidemics in the United States, and everyone now knows that they’re back again carrying yet another threat, West Nile virus. Malaria, according to the latest genetic research, is a relatively new disease, too.
It is now believed that malaria infections originated only about 8,000 years ago, when agricultural practices began to provide sufficient sunlit pools as breeding grounds for mosquitoes and sufficient population densities for humans to serve as hosts. Worldwide, there are an estimated 300 to 500 million cases of malaria, and about 2 million people a year die from it.
Some people are immune to malaria, in fact, the reason geneticists believe malaria only arose about 8,000 years ago is that that is when a gene variant arose that conferred immunity by causing changes in the genes that control red blood cells, which the malaria protozoan invades and destroys. Inheriting one copy of this gene from a parent confers immunity to malaria; inheriting two copies, one from each parent, causes sickle cell anemia, an often deadly blood disease.
The connection between mosquitoes and malaria was not always clear and was firmly established in 1897 by a researcher named Ronald Ross. He was awarded the 1902 Nobel Prize in Physiology or Medicine for his accomplishment. This led to using mosquito control as one weapon in the fight against malaria later in the century, but other factors helped contain the disease as well: population shifts to urban areas, improved drainage and housing, greater access to medical services, and the availability of quinine for treatment.
Large-scale mosquito control with DDT did not begin until the 1940s. But malaria is still endemic in many parts of the world, including South America, and some scientists believe that it will begin to make a comeback in the United States over the next decade or two. The parasite is becoming more and more resistant to drugs, global warming may send infected mosquitoes northward, and there is no border control that can stop them.
Malaria is on the increase in the United States even without South American mosquitoes, likely because of increased immigration from malaria-endemic parts of the world. Malaria was probably introduced into the United States by European colonists and Africa slaves in the sixteenth and seventeenth centuries. It moved with people and eventually affected many areas of the country, with the exception of northern New England (for reasons we’ll discuss in a moment).
In 1934, when systematic reporting of malaria cases began, there were 125,556 cases. The number of cases dropped steadily through the late 1970s. Since then, there has been a steady increase in rates, and presently about 1,800 cases a year are reported to the CDC, with between four and eight deaths caused annually by the illness. Most, but not all, of these cases are imported from other countries. Malaria is only carried by mosquitoes of the genus Anopheles, and there are several species within this genus. All 48 states in the contiguous United States have anopheles species capable of carrying malaria. So some of our plasmodium protozoa, like the one mentioned before that made the Long Island boy sick, are probably home-grown. Between 1957 and 1994, there were 56 episodes of transmission, mostly in rural areas, but at least three outbreaks occurred in densely populated areas of New Jersey, New York City, and Houston, Texas.
Malaria can only be transmitted when a female mosquito bites a person infected with malaria, and the person has to have mature male and female stages of the parasite in his or her bloodstream. The male and female stages meet in the stomach of the mosquito where, in a rather complicated process of development and multiplication, they produce sporozoites, the infective form of the parasite. These sporozoites wind up in the salivary glands of the mosquito, whence they can be injected into the next person the mosquito bites. Thus epidemics begin.
There are four different species of Plasmodium that transmit slightly different kinds of malarial infection. P. vivax and P. ovale can persist in human liver cells, periodically sending parasites into the bloodstream and causing relapsing illness. P. falciparum, and P. malariae don’t hide out in the liver, but if they aren’t sufficiently treated, they can persist for months in the case of P. falciparum, and for years in that of P. malariae, causing relapsing clinical disease long after the initial infection.
In all four kinds of infection, the symptoms are recurring chills and fever, sweating, malaise, headache, and muscle pain, all symptoms that look a lot like, and are often misdiagnosed as, the flu. Rapid diagnosis followed by treatment with chloroquine, sulfanomide-pyrimethamine, or both together usually has excellent results, although some resistant strains are now developing in Southeast Asia, sub-Saharan Africa, and the Amazon basin.
In cases where the concentration of parasites is high or where there is neurological dysfunction, intravenous quinidine gluconate is used, and this requires hospitalization in an intensive care unit. Otherwise, oral chloroquine is usually enough to wipe out the disease after a week of treatment. Therapy for vivax and ovale malaria, because the parasites continue to emerge from the liver, requires treatment for about two weeks to make sure all the parasites have been killed. The most dangerous kind of malaria is falciparum, which can have a death rate as high as 20 percent if it isn’t properly treated.
One of the things that limits contagion in the United States is that most mosquitoes don’t live long enough to allow the plasmodium parasite to go through its life cycle. Below about 62°F, plasmodium can’t survive to reproduce in mosquitoes. But warmer weather makes mosquitoes live longer and shortens the life cycle of plasmodium, and weather that is more humid and hotter than usual has been a factor in all recent outbreaks.
This is probably why northern New England has been spared endemic malaria, while almost all other parts of the country have been infected. Suffolk County, New York, where two kids (one described previously) got malaria in the summer of 1999, is one of the most heavily mosquito-infested areas in the northeast, and 1999 was one of the warmest and driest summers in history. But just before the two boys got sick, there was heavy rainfall, which may have resulted in a large population of adult female mosquitoes. If you’re a mosquito or a plasmodium protozoan, global warming may actually be good for you.
What are the odds of your getting malaria if you don’t leave this country? Very low. Considering how many people spend summers outdoors and how many of those are bitten by mosquitoes, it’s clear that 1,800 infections a year is a tiny percentage of those presumably at risk.
If you use insect repellent, and if you wear long pants and long sleeves and don’t go out at dawn and dusk when mosquitoes are feeding, your chances of being infected are reduced even further. Fear of malaria should not in any way interrupt your plans for outdoor activity in the summer.