vectors (ticks and mosquitoes), and infected amplification hosts (domestic birds, wild resident birds, and wild migratory birds). It is not difficult to envision ways in which this virus may quickly move around the country. Many cities have large populations of vector mosquitoes capable of transmitting this virus. These include Cx. pipiens in the northern half of the country, its close relative Cx. quinquefasciatus in the south, Cx. tarsalis in the west, and Cx. nigripalpus in the deep south. These species of mosquitoes are certainly among the most likely North American candidates to transmit WN to birds and humans. There are likely many other mosquito and tick species that could play a role in keeping WN circulating within a region. If WN is introduced into another region of the USA, it will, in time be observed in domestic avian populations. Nation-wide vigilance for WN is essential.WHAT CAN BE DONE? An important question concerning WN movement and transmission throughout North America is: What can be done to monitor the movement and introduction of WN in new cities and localities throughout North America? The answer is simple: surveillance, surveillance, surveillance, and more surveillance. Vector surveillance, amplification host surveillance, meteorological surveillance, and virus surveillance. Mosquito and vector control programs throughout the USA already have considerable experience monitoring WN's close relative, SLE (see the page on SLE). St. Louis encephalitis has been a continuing problem particularly in the upper mid west, Florida, Louisiana, Texas, and California where there have been many severe outbreaks. A comprehensive integrated arboviral surveillance program for SLE in Florida has been proposed and implemented (see Day and Lewis 1992). Programs similar to this should be considered for areas that are at risk for arboviral transmission, especially WN, SLE, and dengue viruses. The dengue viruses present another potential...
