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Emerging Infections: Microbial Threats to Health in the United States
Epidemic Lyme disease is an increasing problem in Europe, fueled by a post-World War II trend of reforestation and proliferation of deer. Epidemic Lyme disease was first diagnosed in Europe in the mid-1980s, a few years after the epidemic was recognized in the United States (Matuschka and Spielman, 1989). Reports of Lyme disease in Europe rose rapidly, until by 1988, twice as many cases were reported there as in the United States (Matuschka and Spielman, 1989). At least 1,000 new cases occur annually in Sweden (Jaenson, 1991), and similar incidence levels have been reported in Switzerland and Austria. The comparable annual estimate of new cases for Germany is much higher—between 30,000 and 60,000 cases (Matuschka and Spielman, 1989). Indeed, Borrelia -specific antibody is said to be detectable in some 7 percent of German residents. This extraordinarily high frequency of Lyme disease may be related to the large number of Central Europeans who live in forested areas and visit the forests for recreational purposes.
Lyme disease has been reported in many temperate parts of the world, including northeastern China, Japan, South Africa, and Australia (Jaenson, 1991). Unconfirmed, anecdotal reports of Lyme disease have been received from tropical Africa and South America.
A variety of etiologically distinct infectious agents are transmitted by the same ticks and maintained in the same rodents that perpetuate the agent of Lyme disease (Spielman, 1988). In the United States, babesiosis, a malaria-like disease caused by Babesia microti, has been diagnosed in people living in or near the same areas in which Lyme disease is prevalent. In human hosts, both of these infections can be fatal. Simultaneous infection is common.
Although it is still a controversial issue, the potential effects of global warming on disease transmission must be considered. This is particularly true for diseases caused by mosquito-borne viruses, since temperature increases in cooler climates may enlarge areas suitable for mosquito breeding. Unfortunately, it is currently impossible to predict accurately the effect of warming on disease emergence. This does not mean, however, that the issue should not be addressed. It is thus disturbing to note the apparent lack of interest in global warming as a possible contributor to public health crises on the part of funding agencies and environmental groups.
According to the best estimates obtainable from mathematical modeling, the earth's temperature could increase by as much as 5°C by the year 2050 (National Research Council, 1992). Should this occur, the distribution of disease vectors and the organisms they transmit might very well change. Research has shown that replication of virus in the mosquito is temperature dependent (Hardy et al., 1983). A warmer planet would likely enhance the transmission of some viruses, while slowing or halting the transmission of