A growing chorus has been chanting that global climate change will spread insect-born diseases, malaria, dengue fever, and yellow fever, to temperate latitudes. In the last year, the health effects of global warming have been the subject of lengthy journal articles in JAMA, the Journal of the American Medical Association, and Lancet, an international journal of medical science and practice. Last September, the Australian Medical Association sponsored a major conference on the subject. Professor Paul Epstein of the Harvard School of Public Health has claimed that mosquitoes carrying malaria and dengue fever have been found at higher altitudes in Africa, Asia, and Latin America. For North America, David Danzig, in a Sierra Club publication, has contends that only the tip of Florida is currently warm enough to support malaria carrying mosquitoes; global warming, however, could make most of us vulnerable. He should check his history.
Before the Second World War, malaria was widespread in the United States. The government recorded over 120,000 cases in 1934; as late as 1940 the number of new sufferers totaled 78,000. After the war, reported malaria cases in the U.S. plunged from 63,000 in 1945 to a little over 2,000 in 1950 to only 522 in 1955. By 1960, with the use of DDT, the disease was almost totally eliminated; only 72 cases were recorded in the whole country. The Centers for Disease Control reported a resurgence to around 3,000 cases annually in 1969 and 1970, brought in by returning service personnel from Vietnam. Subsequently, immigrants from tropical areas have spawned small upticks in new cases.
In the 1980s and 1990s, the number of reported cases has averaged around 1200 to 1300. The CDC reports that since 1985 approximately 1,000 of those cases are imported every year, with visitors and recent immigrants accounting for about half. The rest come from travelers returning from tropical countries, returning service personnel from infested areas, and a handful of individuals, typically those living near international airports, that are bitten by a mosquito that hitched a ride from a poor country. More stringent efforts to keep out these unwanted "immigrants" may be called for if the problem becomes worse.
Yellow and dengue fever were both common in the United States from the 17th century onward. Epidemics of yellow fever ravaged New Yorkers and killed tens of thousands of people. In one year 1878, of 100,000 cases reported along the East Coast, 20,000 people died. Between 1827 and 1946, eight major pandemics of dengue fever overran the United States. In 1922, the disease spread from Texas, with half a million cases, through Louisiana, Georgia, and Florida. Savannah suffered with 30,000 cases, of which nearly 10,000 had hemorrhagic symptoms, a very serious form of the disease. In contrast, last year the CDC listed 86 imported cases of dengue and dengue hemorrhagic fever and eight local transmissions, all in Texas. There were NO reported cases of yellow fever.
As a public health issue, those diseases, which did plague the United States in the reputedly colder nineteenth and early twentieth century, have been largely exterminated. There is no evidence that a resurgence is imminent. Certainly the climate is not keeping the spread of these diseases in check. If it was warm enough in the cold nineteenth century for the mosquitoes to thrive, it is warm enough now!
Is there any basis for these scaremongering prophecies? Is malaria rising worldwide? Not according to the World Health Organization. As the chart shows, from 1983 to the latest year for which data exist, 1992, the number of cases of malaria reported in Africa, the most heavily infested section of the world, has fallen sharply, especially in the most recent years. For the rest of the world, the news is a little less good. Malaria continues to be a problem, but there has been NO increase in cases reported even though the world's population has climbed. The good news is that the rate of malaria per 100,000 people has fallen for the whole world.
What brought an end to these scourges? The introduction of DDT clearly played a major role. From the end of World War II until it was banned in 1972, this pesticide worked wonders to eliminate harmful insects, espcially mosquitoes. But it wasn't just insecticides that did the trick. Simple steps, such as screens on windows, the elimination of standing water, and the movement to the suburbs, which reduced population density and thus the risk of transmission, have played a critical role in eliminating mosquitoe-borne diseases.
In 1995, however, a dengue pandemic afflicted the Caribbean, Central America, and Mexico, generating around 74,000 cases. Over 4,000 Mexicans living in the Tamaulipas state, which borders Texas, came down with the disease. Yet Americans living a short distance away remained unaffected. The contrast between the twin cities of Reynosa, Mexico, which suffered 2361 cases, and Hidalgo, Texas, just across the border, is striking. Texas reported only 8 non-imported cases for the whole state, including the border towns.
The only reasonable explanation for the difference between the spread of dengue in Tamaulipas and its absence in Texas is living standards. Where people enjoy good sanitation and public education, have the knowledge and willingness to manage fresh water around households, implement programs to control mosquitoes, and employ screens and air-conditioning, these mosquito-borne diseases cannot spread. If the climate does warm, these factors will remain. In short American's need not fear an epidemic of tropical diseases.
References:
January 17, 1996 issue of the JAMA, vol 275, No. 3.
Lancet, June 8, 1996 and August 31, 1996
David Danzig, "Global Warming = Health Hazard" Sierra Club, 1995
1980 2,062
1981 1,388
1982 1,056
1983 813
1984 1,007
1985 1,049
1986 1,123
1987 944
1988 1,099
1989 1,277
1990 1,292
1991 1,278
1992 1,087
1993 1,411
1994 1,229
1995 1,419
Source:
CDC and various editions of the Statistical Abstract of the United States.
Africa Total
excluding
Africa
1983 3168 5781
1984 4422 5742
1985 13207 5037
1986 17927 5602
1987 20588 5633
1988 23235 5364
1989 14124 5694
1990 2012 5648
1991 594 5800
1992 420 5340
Source:
WHO, Malaria Control Program, Geneva Switzerland