construction in the Senegal River Basin. Although Rift Valley fever had never been recognized in the region, surprisingly, a third of the local adult inhabitants were found to have antibodies to the virus (Tirrell et al., 1985). The pattern of infection indicated that the disease was endemic. Similar studies at the Institut Pasteur, Dakar, revealed widespread exposure to the virus among people and domestic animals in Mali and Mauritania, two of the countries bordering on the Senegal River. Early in 1987, the institute warned of ''a potentially important risk of amplification of the virus in relation to the migration of domestic animal herds and human populations" in southern Mauritania (Digoutte and Peters, 1989).
Later in 1987, after the Diama Dam was activated, an epidemic of Rift Valley fever occurred near the village of Keur Macene, upstream from the new dam. The epidemic was associated with abortions in sheep and cattle, and severe human disease in 1,264 inhabitants. There were 244 deaths (Jouan et al., 1988). The association of this epidemic with prior construction of a dam paralleled the experience in Egypt 10 years earlier. Whereas in Egypt, Rift Valley fever virus was introduced into an immunologically virgin population, in Mauritania it was almost certainly endemic in people and livestock living near the site of the dam. Prior to 1987, the virus caused either inapparent infection or more serious disease in too few people to be recognized. Once the dam was activated, however, the ecology changed. Nonimmune people and their domestic animals who were already in, or entering, the areas became infected. Under these favorable circumstances, the disease rapidly reached epidemic proportions.
Mosquitoes indigenous to both Europe and North America are potentially capable of transmitting the Rift Valley fever virus. In Egypt, the virus demonstrated its capacity to be transported, and in both Egypt and Mauritania, the virus was able to cause epidemics when ecological changes favored mosquito breeding. In the United States, the virus could enter in the blood of an infected person or by way of an animal imported into a wildlife park or zoo. This is an admittedly unlikely possibility; nevertheless, should Rift Valley fever virus become established in vectors in this country, its control would require expensive, integrated efforts including vaccination of domestic livestock and extensive measures to kill vector mosquitoes.
The emergence of Lyme disease, caused by the spirochetal bacterium Borrelia burgdorferi, is intimately tied to changing land use patterns that date back over the past two centuries. Early in the 1800s, the eastern United States was rendered virtually treeless when vast tracts of land were cleared to make way for agriculture (Jorgenson, 1971; Cronon, 1983). Deforestation was enhanced by the prodigious quantities of wood needed for domestic