Transmission of arboviruses between invertebrates and vertebrates imposes constraints on evolution and dispersal, which are reflected in their phylogenetic relationships (7). However, the influence of modern life on arbovirus emergence cannot be overemphasized. The following human activities may influence arbovirus emergence: 1) increased transportation of animals, humans, plants, arthropods, and other materials; 2) increased outdoor leisure activities; 3) reduced or nonexistent arthropod control programs; 4) deforestation; 5) reforestation; 6) land reclamation; 7) altered farming practices; 8) urbanization programs; 9) irrigation projects, including building dams or creating reservoirs and lakes; 10) military activities; 11) movement of military personnel and local populations in war zones; 12) natural disasters, such as flooding; and 13) the early effects of climate change (8).

TBEV complex viruses rarely cause disease in indigenous forest animals but may emerge as pathogens when they infect introduced species. Members of the complex have evolved and dispersed westward across Asian and European forests during the past 3–5 millenia. This dispersion is driven by Ixodes spp. vectors that inhabit the moist forest undergrowth. Nymphal ticks may infect newly hatched larvae when they co-feed on forest animals. Since these vertebrates do not become sick or have detectable viremia, this direct method of vector infection is known as nonviremic transmission. In contrast, LIV and related viruses in Spain, Turkey, Greece, and nearby regions produce viremia and fatal encephalomyelitis in domesticated animals when infectious ticks feed on them (9). These animals have no genetic resistance or immunity to the TBEV-related viruses that have emerged from the forests (8).

Kyasanur Forest disease virus emerged in Karnataka, India, in 1957 after forests were cleared for urbanization and farmland reclamation. Arboreal monkeys scavenging on the exposed forest floor became infested with Kyasanur Forest disease virus–infected ticks from the undergrowth. Local inhabitants and veterinary scientists who examined dead and dying monkeys also became infected. A closely related virus, Alkhurma virus, emerged in Saudi Arabia in 1992, causing hemorrhagic disease in butchers who handled sheep imported into Saudi Arabia for the Hajj (10). Alkhurma virus was detected in ticks removed from imported sheep, but the country from which these sheep originated has not been identified.

CCHFV emerges sporadically in Africa, Asia, and Europe. This arbovirus may be transmitted to animals by ticks, especially members of the Hyalomma genus. Emergence of CCHFV largely depends on the transportation of livestock such as cattle and goats, on which infected ticks feed. Most cases of hemorrhagic disease occur in humans involved with the livestock industry. Since >21 tick species are present in the United Kingdom, and since viruses such as CCHFV can infect a wide variety of ticks, the introduction and emergence of CCHFV are not inconceivable.

NSDV (family Bunyaviridae, genus Nairovirus) is transmitted to sheep by Rhipicephalus appendiculatus and causes fever and gastroenteritis; the infection is often fatal. NSDV is distributed widely throughout Africa (1) but not elsewhere. However, the closely related Ganjam virus is found in India, which may imply that NSDV has been transported in ticks on animals imported to India from Africa. Both NSDV and Ganjam virus produce febrile illness with polyarthritis in humans.

YFV circulates among simians and Aedes and Hemagogus spp. mosquitoes in the forest canopy and adjacent savannah regions of Africa and South America. In Africa, nonhuman primates do not become ill from YFV infection, presumably because the virus has adapted to the simian hosts. Nevertheless, human yellow fever epidemics do occur in Africa because the virus emerges sporadically from the jungles and savannah into rural and urban areas, where most of the human population is susceptible. In South American jungles, moribund monkeys are usually found immediately before local epidemics of yellow fever, implying that the virus recently encroached into the region. YFV was introduced into South America on ships that sailed from Africa during the past few centuries (8). Many European and North American ports observed cases of yellow fever. In some cases, the virus was carried on sailing ships from Cuba to Europe, causing localized outbreaks where the ships docked (11). Infections in Swansea (Wales) and St. Nazaire (France) resulted from the bite of Aedes aegypti mosquitoes released from the newly arrived ships. However, the disease immediately disappeared when the local temperature dropped and the tropical mosquitoes died. Thus, such viruses are unlikely to emerge in the United Kingdom.

The 4 DENV serotypes emerged more recently than YFV (8) and can generate epidemics in humans without depending on a sylvatic cycle in the jungles. Therefore, in contrast to YFV, which is maintained in the jungles of Africa and South America, DENV continues to be dispersed by humans and mosquitoes throughout the tropics and subtropics worldwide (8,12). Presumably, as Ae. aegypti and Ae. albopictus continue to spread, the geographic distribution of these viruses will also expand.

Since birds are highly mobile and can travel great distances in a relatively short time, they play a role in arbovirus dispersal. SINV, for example, is carried annually between South Africa and Scandinavia by migrating birds (12). SINV is rarely associated with disease in Africa. However, when cases of arthritis, polyarthritis, and fever associated with rash were recorded in Scandinavia, the antigenically closely related Ockelbo virus was isolated. Presumably, Ockelbo virus evolved from introduced SINV and subsequently became established in a local transmission cycle. In the New World, St. Louis encephalitis virus is also carried long distances by migrating birds. It was probably introduced into South America from Africa during the past few centuries and gradually dispersed by migrating birds to North America (13). More recently, WNV was introduced into the New York area, probably in 1999. The virus became established in birds and was rapidly dispersed north to Canada; south to Florida, Mexico, and the Caribbean; and west, reaching California in 2004 (http://www.cdc.gov/ncidod/dvbid/westnile/). The WNV that was introduced into New York was related to an Egypt/Israel strain that is also present in the Volga River Delta. Despite its high virulence in birds (particularly corvids), horses, and alligators in North America, WNV circulates relatively harmlessly in the Old World, presumably reflecting co-evolution of virus and host over a long period.

Other emerging flaviviruses include Usutu virus, which appeared unexpectedly in Austria in 2000 (14), killing birds in Vienna and surrounding areas. Previously, Usutu virus had only been found in Africa. Israel turkey meningoencephalitis virus, isolated in Israel, is phylogenetically closely related to the African Bagaza virus (7). Thus, Israel turkey meningoencephalitis virus may represent the birdborne version of Bagaza virus.

RVFV (family Bunyaviridae, genus Phlebovirus) is usually transmitted by Aedes spp. mosquitoes and occasionally infects livestock, producing epizootics with high death rates in sheep and other ruminants. Humans in areas where RVFV is epizootic may also become infected. Until 2000, RVFV was most commonly found only in eastern, western, and southern Africa. However, in September 2000, an outbreak was reported in Saudi Arabia and subsequently in Yemen, representing the first cases identified outside Africa and showing the potential for global dispersal.

Bluetongue virus (family Reoviridae, genus Orbivirus) is transmitted to domestic and wild ruminants by culicoide midges. Bluetongue virus epidemics can threaten entire livestock populations and were first described in South Africa after Merino sheep were introduced from Europe in the late 18th century. The virus subsequently dispersed globally, recently reemerging in Italy, Spain, and Portugal (15), most likely through the transportation of ruminants. Whether bluetongue virus will emerge in the United Kingdom likely depends on the effects of climate change on vector dispersal.

The English Channel and North Sea are not barriers to migrant birds, bats, or commercial transportation systems. Therefore, most mainland European arboviruses could be introduced and established in the United Kingdom. The Table presents a summary of all recognized zoonotic mosquitoborne and tickborne arboviruses circulating in western Europe. While Uukuniemi virus has not been shown to be a human pathogen, it circulates widely in Europe, and seropositivity in humans has been demonstrated many times. YFV, DENV, RVFV, Ganjam virus, and NSDV are excluded from the Table because they only arise as imported viruses. Only LIV and Uukuniemi virus have been isolated from field specimens in the United Kingdom, probably because of limited investigation, although the other viruses in the Table have been isolated in Europe. Serologic evidence obtained from bird sera (2) implies that at least 3 additional arboviruses, WNV, Usutu virus, and SINV, circulate in the United Kingdom. Currently, TBEV has not dispersed west beyond the Alsace forests in France (29). LIV, Spanish sheep encephalomyelitis virus, and Greek goat encephalomyelitis virus, on the other hand, represent more recent lineages of western European and Far Eastern TBEV that have emerged on the moorlands after the introduction of sheep and goats for grazing (8).

Three alphaviruses are listed in the Table, SINV, Semliki Forest virus, and Chikungunya virus. SINV has been isolated from field material in Scandinavia, Estonia, Czechoslovakia, and Italy. This virus is widespread throughout the Old World. In Albania, seroconversion to Chikungunya virus was detected in 5 patients with mild flulike symptoms (12); other findings of antibodies to Chikungunya virus have been reported, but evidence of cross-reactivity with other alphaviruses was not always tested. Among the remaining zoonotic arboviruses included in the Table, Tahyna virus is the most common. This virus is widespread throughout western Europe in humans and many nonhuman mammalian species and birds (12). Inkoo virus has only been found in Scandinavia, but few investigations have taken place elsewhere. While investigations for Batai virus have not been extensive, this virus is also widespread throughout western Europe (12).

Arboviruses rarely cause disease in their maintenance hosts or vectors; consequently, viremic blood is not available for vectorborne transmission. Arboviruses have therefore developed other transmission strategies to ensure their survival. For years, the favorite mechanisms to explain virus survival in the absence of disease were vertical transmission, overwintering, or persistent infection in the arthropod or vertebrate host. We now recognize that the process of nonviremic transmission between co-feeding ticks or co-feeding mosquitoes, and perhaps ticks co-feeding with mosquitoes, is a powerful force for virus transmission even in the presence of immunity. Therefore, this repertoire of transmission strategies can ensure arbovirus perpetuation in the absence of disease.

Armed with this versatility, arboviruses can adapt to virtually any situation. Most arboviruses have evolved a sylvatic existence that is infrequently associated with disease in vertebrate hosts. However, in the presence of susceptible hosts, an environment that favors rapid arthropod amplification, an efficient mechanism for dispersal, and a suitable climate for transmission, DENV, RVFV, Japanese encephalitis virus, and YFV may cause epidemic outbreaks of immense proportions.

This variety of critical factors must come together at an appropriate time for an arbovirus to emerge as an epidemic pathogen, a situation that has rarely happened in the United Kingdom. Of course, if climate change has the effects that experts predict it will, the situation could change during the 21st century. In conclusion, despite compelling evidence that potentially highly pathogenic arboviruses are circulating or, alternatively, are being introduced annually into the United Kingdom, no imminent threat exists that they will cause epidemic outbreaks of serious concern to public or environmental health.