Clement Ramsdale and Keith Snow look at the susceptibility of the British Isles and parts of France to colonisation by the notorious tiger mosquito.

The southeast Asian "tiger mosquito" - Aedes (Stegomyia) albopictus - is a notorious vector of dengue and other potentially life-threatening or disabling human and animal diseases. During the past several years it has taken advantage of international commerce to spectacularly expand its distribution and is now well established in parts of North America and Europe.

Mosquitoes of the genus Aedes have adopted an effective strategy for survival through long unfavourable periods. Their eggs are laid in dried-out places and do not hatch until soaked by rising water levels, often many months later. This period of delayed development during the egg stage appears to be obligatory and eggs are never laid directly onto a water surface. Aedes species belonging to the subgenus Stegomyia exploit a particular larval niche, loosely termed the "container habitat". This type of habitat includes holes in trees as well as domestic and industrial items such as water storage containers, old discarded vessels, opened tins, and litter capable of holding water after rain or other source of flooding.

In addition to the tiger mosquito, the subgenus Stegomyia includes the "yellow fever mosquito" - Aedes aegypti - and other tropical species which together transmit many disease organisms. This has given them world-wide notoriety. Though both develop in containers, the habitats of Ae. aegypti and Ae. albopictus are not identical. While the former readily exploits indoor water collections and is common in urban areas in the tropics and subtropics, the latter invariably develops outside buildings and favours suburban or rural wooded situations. The tiger mosquito is an opportunistic feeder, willing to take blood from a variety of hosts, and is well suited to transmit to humans a number of viruses with mammal and bird reservoirs.

Before the advent of an effective vaccine, the control of the yellow fever mosquito was a major item in many tropical and sub-tropical public health budgets. Until decimated by the malaria eradication campaigns after the Second World War, populations of this mosquito could be found as far north as about the 10°C January isotherm, and the species was present in some southern European port areas. Although this mosquito is unable to withstand the cold winters experienced in most of Europe, there are records of summer outbreaks of yellow fever in temperate regions, including southern England and southern Wales, during the 18th and 19th centuries.(6) In all these episodes, the disease was transmitted by the mosquito developing in stored water on board ships arriving from tropical ports.

The yellow fever mosquito and the tiger mosquito both originated in the "old world". Because of its predilection for water storage containers, the yellow fever mosquito became increasingly cosmotropical with man's growing ability to cross previously unnavigable seas, and seems to have established itself in the "new world" at an early stage of settlement by people of European and African stock. It posed a major threat to the building of the Panama Canal and necessitated expensive mosquito control operations over a large area in the southern United States, Central America, the Caribbean and the northern half of South America. Though eradication campaigns restricted its distribution in the Americas, it is still well established in many places. It has not been recorded in southern Europe for many years, but public health departments throughout the Mediterranean are aware of the danger of its re-appearance.

In contrast, until comparatively recently, the distribution of the tiger mosquito remained restricted to a fairly well defined region in the Far East and islands of the Indian Ocean and the western Pacific. However, it tolerates lower minimum temperatures and is present in parts of northern China and Japan where the yellow fever mosquito is absent. The past two or three decades have seen the establishment and rapid growth of a lucrative international trade in used tyres and these are now routinely transported by ship between widely separated industrial regions of the world. Stacked tyres exposed to the elements before and after transportation constitute attractive egg-laying sites for the tiger mosquito. Tyres now play a major role in aiding the rapid spread of this mosquito throughout the world.

Larvae of the tiger mosquito were first discovered in tyres imported into the United States during the 1970s but it was not until 1985 that an established breeding population was found in Houston, Texas. This was soon followed by the discovery of breeding populations in other, widely separated localities. Established populations of the species are now known from 674 counties in 24 states, all east of the Rocky Mountains and as far north as Missouri, Illinois, Indiana and Ohio. The mosquito was found in tyres near Minneapolis (Minnesota) in 1991, 1996, 1997 and 1999. The first three infestations were successfully dealt with; information regarding the last is not yet available.

In Europe, larvae of the tiger mosquito were discovered in tyres at a number of widely separated sites in Albania between August and October, 1979. The initial infestation was probably at a rubber factory adjacent to the port of Durrës (Durazzo), from where it was sent in tyres to recapping plants in different parts of the country. At that time, Albania was diplomatically and commercially isolated from the rest of Europe (including Russia), the Americas and all countries trading with the western world. Thus, the species was probably introduced directly from China, the only country with which Albania had links during the 1970s.(1,2) Breeding populations of the same species were later discovered in several places in Italy, first in northern and then in western port areas. All of these Italian populations seem to have arrived in tyres imported from the United States.(3,4) The species is now established over a wide area in Italy.(7)

Elsewhere in the world the distribution of the tiger mosquito has expanded to include Hawaii and other Pacific islands south to New Zealand, and encompasses New Guinea, Indonesia and parts of continental Africa. In addition to eggs, live larvae, pupae and adults were discovered in shipments into New Zealand from Japan.

Could the tiger mosquito become established in Britain? Studies of cold hardiness and distributional data from Asia and North America indicate that the natural northerly limits of the distribution of this species are set by the -5°C cold month isotherm. This includes large parts of Europe, particularly where maritime influences prevail, which has led to the conclusion that the British Isles and the Atlantic and Channel coasts of France are susceptible to colonisation by this mosquito.(5)

Following the discovery of the tiger mosquito in Italy, a programme of surveillance was set up in the adjacent French Riviera in 1992. Though several indigenous nuisance mosquitoes were present this particular mosquito was not detected. However, in October 1999, larvae of the tiger mosquito were found at a used tyre depot near the village of Orne, some 30 miles south of the port of Caen in Normandy.(8) It is too early to ascertain whether or not the species is established in Normandy, but it is now a proven fact that it is capable of reaching there, with consequent implications about the vulnerability of this country. Caen is linked to Portsmouth by ferry. Nevertheless, consignments of tyres constitute the most probable means of entry into Britain, as has happened in other parts of the world.

It would seem only prudent, therefore, for local health and/or environmental authorities to make a register of companies participating in the international tyre trade and to arrange for monitoring of the presence of the tiger mosquito in and around tyre storage premises. Given that tyres are regularly transported throughout Britain, such a survey must include the whole of the country.

Concern about this dispersal goes beyond the addition of another mosquito to our native fauna. Since its arrival in North America, the tiger mosquito has been connected to the transmission of Potosi, Cache Valley, LaCrosse, Eastern Equine Encephalitis, Keystone and Tensaw viruses, as well as the infective stages of heart worm (Dirofilaria spp.). Further, experimental work indicates it is a competent vector of numerous Alphaviruses (Chikungunya, Eastern Equine Encephelitis, Mayaro, Ross River, Western Equine Encephalitis, Venezuelan Equine Encephalitis, Sindbis), Flaviviruses (Dengue serotypes 1, 2, 3, and 4, Japanese Encephalitis, West Nile, Yellow Fever) and Bunyaviruses (Jamestown Cañon, Keystone, LaCrosse, Oropouche, Potosi, Rift Valley Fever, San Angelo, Trivittatus).

This country is, so far, remarkably free of mosquito-borne arboviruses, but several are widespread in continental Europe, even as far north as Scandinavia. European mosquito-borne viruses are zoonoses and may be divided into bird associated (eg West Nile, Sindbis and the Bunyavirus Lednice) and animal associated (eg the Bunyaviruses Tahyna, Inkoo and Batai). Clinical symptoms of these viral infections range from merely flu-like to meningo-encephalitis and haemorrhagic fever, but are not virus specific and reliable diagnosis must be based on serological tests.

It is not suggested that devastating outbreaks of mosquito-borne viruses are imminent here. However, probabilities of arbovirus transmission in this country will increase following establishment and proliferation of the tiger mosquito and these probabilities might further increase should predicted climate change lead to generally warmer conditions.

References
1. Adhami, J and Murati, N (1987) "Presence of the mosquito Aedes albopictus in Albania". Revista Myekesore 1, 13-16. [In Albanian]
2. Adhami, J and Reiter P (1998) "Introduction and establishment of Aedes (Stegomyia) albopictus Skuse (Diptera, Culicidae) in Albania". Journal of the American Mosquito Control Association 14, 340-343.
3. Dalla Pozza, G and Majore, G (1992) "First record of Ae. albopictus establishment in Italy". Journal of the American Mosquito Control Association 8, 318-320.
4. Della Torre, A C, Bomboi, C and Cancrini, G (1992) "Estensione dell'areale di Aedes albopictus in Italia". Primo reperto della specie in Italia centrale. Parassitologia 34, 143-146.
5. Mitchell C J (1995) "Geographic spread of Aedes albopictus and potential for involvement in arbovirus cycles in the Mediterranean basin". Journal of Vector Ecology 20, 44-58.
6. Ramsdale, C D. and Snow, K.R. (1995) Mosquito control in Britain. University of East London Publications. Dagenham, 100 pp.
7. Romi, R, Luca, M di and Majori, D (1999) "Current status of Aedes albopictus and Ae. atropalpus in Italy". Journal of the American Mosquito Control Association 15, 425-427.
8. Schaffner, F and Karsh, S (1999) "Aedes albopictus discovered in France". Society for Vector Ecology Newsletter 30, 11.

Clement Ramsdale can be contacted at: Varndean Lodge, London Road, Brighton BN1 6YA or by e-mail at clem.ramsdale@claranet.co.uk
Keith Snow can be contacted at the Department of Environmental Sciences, University of East London, Romford Road, London E15 4LZ and by e-mail at k.r.snow@uel.ac.uk