In temperate zones, epidemic onset of a newly emergent vector-borne disease occurs most often in the spring or summer, since both vector and pathogen depend on higher temperatures to maintain a rapid rate of reproduction. The spread of infection during the summer months may be rapid, particularly if humans are an effective source of vector infection or if the agent has become widespread in a nonhuman reservoir population. Thus, to be effective, vector control efforts must be launched shortly after the disease is first recognized or, ideally, before the disease is apparent.

For most vector-borne infectious diseases, the onset of winter dampens transmission or can even eliminate the vector or infectious agent. The exception is pathogens that can survive in humans for long periods and produce chronic infection (e.g., malaria and typhus). Vectors native to temperate areas, if introduced into new regions, may be able to survive at low temperatures, while those native to the tropics may not. In much of North America, cold weather is a second line of defense against most newly emerged or introduced pathogens that depend on vectors to be transmitted to humans. A sudden decrease in incidence of an unidentified disease at the start of winter may be the first epidemiological evidence that the disease is vectorborne.

VECTOR-CONTROL RESOURCES

North America has extensive vector-control resources. In fact, vector control is an essential part of environmental health programs in many communities. California's mosquito control, for example, covers most of the state and involves some 72 agencies with a 1991 budget of more than $48.9 million for an area with a population of more than 20 million (California Mosquito and Vector Control Association, Inc., 1991). Statewide surveillance for mosquito-borne encephalitis, plague, malaria, and Lyme disease is coordinated by the California Department of Health Services.

There are approximately 1,000 additional regional and community vector-control and vector-surveillance programs in the United States and Canada (American Mosquito Control Association, 1991). Most of these programs are geared to protecting local populations from indigenous vector-borne diseases and arthropod pests. They may also provide an early line of defense against newly introduced or resurgent vector-borne diseases. In the United States, responsibility for organizing surveillance data and investigating epidemics of emerging vector-borne diseases, such as encephalitis, plague, and Lyme disease, rests with the CDC's Division of Vector-Borne Infectious Diseases in Fort Collins, Colorado.

The control methods used in a particular region depend on the vectors that are present and on what is known about their biology and behavior. Chemical and biological agents and environmental modification can be



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