PERSPECTIVE n engl j med 356; 8 www. nejm. org february 22, 2007 770 but it is unclear which vector was involved in most islands of the Comorian archipelago, where studies have not been conducted or are ongoing. There is recent evidence that the outbreak in India, where A. aegypti is the primary species of mosquito, was caused by the new variant of the virus. 3 A. albopictus is generally considered to have a lower vector capacity for arboviruses than A. aegypti. Specific mosquito populations, however, may have a high vector capacity, 4 as suggested by a massive outbreak of dengue that was propagated by A. albopictus in Reunion in 1977. It is also possible that the strain of chikungunya virus in the Indian Ocean became better adapted to the A. albopictus vector. Introductions of nonnative species of plants, invertebrates, and vertebrates are increasingly being recognized in countries with temperate climates. 5 Among migrating invertebrate species, mosquitoes that are capable of transmitting infectious diseases are of particular interest. The expansion of global air travel and seaborne trade removes geographic barriers to insect disease vectors, enabling the insects to move great distances in short periods. If they can adapt to the local environment, they establish themselves in new areas. It is thus that mosquitoes of the aedes (stegomyia) genus have gained an increasingly global distribution. In the past 50 years, the anthropophilic A. albopictus has spread to all continents (see map) and adapted to most climates. Although long considered a secondary disease vector, it has been shown to be capable of transmitting arboviruses under both laboratory and field conditions. Like epidemics of dengue and West Nile virus, the chikungunya outbreak is an example of the abrupt expression of vectorborne diseases in the global village. It involved an African virus and an Asian mosquito and started in the Indian Ocean. The establishment of new vectors makes possible the introduction of new pathogens. Malaria, yellow fever, African tickborne rickettsiosis, and more recently, West Nile virus have all been imported from Africa to the Americas. After all, the spread of such vectorborne diseases requires only a host reservoir and a specific vector. If humans are the host reservoir and the vector is widely distributed, globalization of the disease is just a matter of time.

The emergence in 1999 of West Nile virus in the United States and its subsequent rapid spread throughout the country demonstrated that arboviruses can present a threat in developed countries with temperate climates in the absence of herd immunity. One can therefore justifiably spec-