Have disease, will travel

The world isn’t just getting smaller for us humans – disease carrying insects are exploring pastures new as well. Now the challenge is to keep track of them and prevent spread of disease says Owen Jones  

CLIMATE change and accelerating trade and travel allows arboviruses (virus particles carried and transmitted by insect vectors) into parts of the world experts thought they could never reach. Mosquitoes are reaching northern hemisphere countries to spread hitherto unknown virus diseases which may subsequently arrive on travellers from countries where they are endemic. Two such scenarios involving Aedes albopictus (Asian tiger mosquito) have created parallel disease problems in Western Europe and North America.

Ae. albopictus is an established though underrated vector of dengue fever/dengue haemorrhagic fever (DHF) and chikungunya virus fever (CHIKV) cause for concern on both sides of the North Atlantic Ocean. Ae. albopictus is a fast-moving and wide-spreading vector able to pick up and transmit ‘new’ diseases in Western Europe and North America.

Dengue is an entrenched problem in South and Central America carried and transmitted by the truly tropical Ae. aegyptii, the so called dengue/yellow fever mosquito. Ae. albopictus is also a dengue carrier albeit less known and appreciated. Northward movement of this climatically versatile mosquito currently threatens a potentially large area of the United States. Most immediately at risk are south western US states bordering Mexico and notably Texas and Arizona.

Detailed surveys and studies show large numbers of people in cross-border cities and towns of United States and Mexico, separated only by The Rio Grande, already exposed to the dengue virus. Typical are the so-called ‘sister’ cities of Brownsville Texas and Matamoros, Tamaulipas Mexico. Serological testing carried out by Mary Ramos and colleagues in December 2005 estimated the incidences of recent dengue infection to be 32% and 4% among Matamoros and Brownsville participants, respectively. Estimated prevalence of past dengue infection was 77% and 39%. With populations (2006) of 161,225 and 418,141 for, respectively, Matamoros and Brownsville there are clearly a lot of people involved. 
 
These are not recorded cases, but the dengue virus problem is obviously there. Dengue outbreaks have occurred every year in Mexico since 1978 with DHF first appearing in 1984. Twelve years later there was an outbreak along the border in the Mexican state of Tamaulipas. Subsequently, and as recently as 1999, outbreaks occurred on either side of the border with dengue in Mexico and Texas and DHF on the Mexican side. Autochthonous transmission of DHF was first documented in Brownsville Texas in 2005.

In 2006 Mary Hayden of the National Center for Atmospheric Research in Boulder Colorado said presence of the vector together with thousands of documented cases in Matamoros, including nearly one thousand of clinical DHF, suggested dengue could already be endemic across this border region. Infection has since moved further afield with Arizona and Florida recording, respectively, 40 and 10 actual cases of Dengue fever during 2007.

Ae. albopictus is already present across 25% of the continental United States mostly the south east region. With massive regular movement of people across the Mexican and US border - 3 million pedestrian and 22 million vehicle crossings between Brownsville and Matamoros in 2004 - experts claim the clock is ticking on wider dengue penetration into the US heartland.

Ae. albopictus is there and it is just a matter of time before the mosquito starts to pick up infection in the border areas of Texas then carry it up the Eastern Seaboard perhaps as far as New Jersey where Ae. albopictus already occurs. Alternatively an infected person from Texas could travel by air to an eastern seaboard state right to a vector sitting ready and waiting for something to vector.

The plans are already afoot for a nationwide survey to monitor and then mitigate Ae. albopictus infestation. A pilot programme involving Rutgers University is planned for two counties of New Jersey including Mercer County. The aim is to target an area of 4km2 to see if it is possible to reduce Ae. albopictus by 95% and if so roll out the programme over a much wider area. The project will use BG Sentinel traps loaded with the custom-designed AgriSense lure containing a dedicated profile of human sweat components. This combination of trap and lure is the only one that can reliably monitor Ae. albopictus populations.

Researchers are looking ahead because experience shows Ae. albopictus is extremely versatile in geographical range and spectrum of arbovirus pathogens taken on board. These include Japanese encephalitis, eastern equine encephalitis, Rift Valley Fever and chikungunya virus. This is a pre-emptive project carried out on the basis that Ae. albopictus along the Eastern Seaboard is not carrying anything at the moment but has the potential to carry all these virus diseases and more into the very heart of the United States.

North American experts are well aware of how chikungunya virus fever (CHIKV)

appeared suddenly in Northern Italy during July 2007 brought into the country on an infected airline passenger from India and spread by Ae. albopictus which has been resident in Italy for some years. Ae. albopictus now persists in many temperate areas of the eastern and western hemispheres including Europe and the United States so there is real risk of chikungunya being introduced into previously non-endemic areas by travellers with viremia. This could subsequently lead to local virus transmission especially in tropical or subtropical areas of the United States such as the Gulf Coast and Hawaii, or its territories such as Guam, Puerto Rico, and the U.S. Virgin Islands. Early recognition of local transmission followed by prompt, aggressive vector control and other public health measures might prevent long-term establishment of the virus in new areas.

Dengue is causing renewed problems in Caribbean island countries. It re-appeared in Trinidad during 2008 with dengue fever and DHF cases right across the island. Latest clutch of cases including one fatality was reported in December 2008 at Frederick Settlement in Caroni, Central Trinidad where fifty people tested positive for dengue in 2008. Local residents blame the authorities for failing to keep drains silt free, thus providing continuous breeding sites for the mosquitoes.

After much local pressure the Insect Vector Control Unit of the Ministry of Health

‘Lucky bamboo’ is sold as an ornament. If it is harbouring a disease carrying mosquito it may not be so lucky however

sprayed the area. But subsequent heavy rains and flooding across Trinidad, and its sister island of Tobago, has done nothing to alleviate community fears. According to the Trinidad Express there were 2500 cases of dengue fever with 65 cases of DHF and 35 deaths in 2008. Fatalities are in all age groups and death rate appears on the high side compared with figures from South East Asian countries. Population of Trinidad and Tobago is about 1.2million. 

Chikungunya virus fever (CHIKV) is an alphavirus indigenous to tropical Africa and Asia and transmitted to humans via bites of infected mosquitoes mostly of the genus Aedes. It is not a new disease being first recognised in epidemic status in East Africa (Tanzania) during 1952-1953. Chikungunya is a ‘Makonde’ word (a language in Tanzania) literally meaning ‘bends up’ and accurately describes the contorted postures of patients afflicted with severe joint pain associated with the disease. Ae. aegypti is the primary CHIKV vector in Asia but Ae. albopictus is increasingly implicated following the recent epidemics across one side of the Indian Ocean and culminating in an outbreak in Northern Italy.

The Italian outbreak in July 2007 was a surprise to say the least in spite of the virus and disease having already done the rounds across the Indian Ocean. Following an outbreak in Kenya during 2004 the virus spread rapidly to the Comoros and Seychelles.  Next port of call was particularly devastating, with almost half the population Réunion (255,000) contracting the disease in 2005, courtesy of Ae. albopictus.

During 2006/07 the virus hit Mauritius, Sri Lanka (37,000 cases) and then mainland India (at least 500,000 cases) before popping up in Italy on the Adriatic coast near Rimini and Bologna during the first week of July 2007. Eruption in Italy may seem surprising but it was a consequence of modern trade and travel, this time caused by an infected airline passenger.  That said, nothing could or would have happened without the Ae. albopictus vector being ready and waiting in Italy to pick up the pathogen and transfer infection to erstwhile healthy people. Ae. albopictus has spread throughout Italy having been first identified in 1990 after introduction as larvae in water contained in second hand tyres imported from Asia.

Speed of chikungunya spread was remarkable. The infected passenger carrier from India stepped off the plane at end of June and by 4th July first Italian cases were beginning to show. Between 4th July and 31st December 2007 there were 248 cases of chikungunya, mostly confined to two villages, Castiglione di Ravenna and Castiglione di Cervia.

Next concern was whether infected vector mosquitoes would survive and continue disease spread in 2008. Research in Rome showed how temperate Ae. albopictus populations can produce diapausing eggs that facilitate survival through cold winter conditions. The arrival of this virus in Northern Italy where the vector is only likely to survive as the egg stage was clearly preferable to southern Italy, where conditions support continuous insect breeding as in the tropics and sub-tropics. In the event Northern Italy experienced a particularly cold winter and spring (2007/8) affecting Ae. albopictus survival and any carryover of the virus. Experts said if the infection had occurred in southern Italy it would have been a completely different story.

So how far can the Ae. albopictus vector and its arboviruses (including chikungunya) spread northwards and westwards into Europe? It can certainly spread into the United Kingdom (UK) says Jolyon Medlock of HPA (Health Protection Agency) at Porton Down in Hampshire. Medlock has published a paper on the projected spread and distribution of Ae. albopictus within the UK should it arrive on the shores of the British Isles. Maps defining likely areas of establishment show huge swathes of southern and middle England including the capital London and second city Birmingham are at risk.

Experts in the UK like their counterparts in the US are saying it is just a matter of time before this highly versatile Ae. albopictus spreads hitherto unknown serious human diseases right across the country. UK fears were heightened following identification of Ae. albopictus in the Netherlands, originating in a consignment of ‘lucky bamboo’ (Dracaena sanderiana) imported from Asia. International trade in 1lucky bamboo’ is an established mode of spread for Ae. albopictus. This widely used ornamental is produced in large quantities in Asia and shipped around the world with its stems in standing water and clearly ideal for dissemination of mosquitoes. Luckily for the Netherlands and the rest of Western Europe the insect(s) involved was the tropical type Ae. albopictus which died off in the cold Netherlands winter climate. 

Jolyon Medlock believes the UK authorities should organise a National Survey to reveal just how important is the threat from Ae. albopictus and the range of arbovirus diseases it can carry and transmit. The story for Asian tiger mosquitoes entering the UK and spreading chikungunya is exactly the same as for Dengue carrying Ae. albopictus in the US – “It is only a matter of time.” A national survey would allow more immediate identification and pin-pointing of Ae. albopictus arrival in the UK. Furthermore it would allow the authorities to ‘snuff out’ any insect infestation potential for infection and transmission of chikungunya virus whether or not mosquitoes were infected before arrival.

Even if Ae. albopictus mosquitoes arrive virus free, increasing numbers of

Blocked drains and flooded roads are ideal breeding grounds for mosquitos

travellers arriving in the UK have displayed symptoms of CHIKV and subsequently tested positive for the virus. From January 2006 to March 20067 United Kingdom’s HPA reported 155 cases of chikungunya infection in travellers returning from various parts of Asia including India, Sri Lanka, Mauritius and Seychelles. Approximately one third were laboratory confirmed by PCR (polymerase chain reaction) and/or virus isolation.

The virus is constantly arriving in the UK on travellers, but fortunately there are yet no suitable mosquito vectors to pick up the virus and spread it throughout the wider human population. It is no use closing the stable door after the horse has bolted, so monitoring and surveying for Ae. albopictus should be top priority. To be completely effective it requires a dedicated trap/lure system like the BG Sentinel/AgriSense lure, already tried and tested throughout South America, Asia and Australia for Aedes vector mosquitoes. 

Ae. albopictus is currently found throughout Italy, in Spain from Barcelona down to Valencia and in the South of France. This mosquito is a day flier, as such it presents a much greater risk than night–flying mosquito species like Culex pipiens (the northern house mosquito and vector of West Nile Virus in United States) that is easier to avoid and protect against with screens, mosquito nets and precautionary behaviour patterns.

The Netherlands clearly had a lucky escape when Ae. albopictus was introduced as larvae in water contained in the petioles of ‘lucky bamboo’. It brought home the real potential for arrival and spread of a vector capable of picking up and spreading chikungunya or even dengue. Dengue is widespread and serious throughout East and Southeast Asia and responsible for many of deaths every year.

Ae. albopictus on your doorstep is having a vector ready and waiting to pick up, carry and transmit disease. This is exactly what happened on Reunion Island where half the population caught chikungunya in the space of one year. Experts believe there is a strong possibility of dengue arriving and establishing in Europe. 
Dengue is endemic in South and South East Asia and Sub Sahara Africa. Given the recent exponential increase in international travel, coupled with continued spread of Ae. albopictus, ably assisted by global warming, arrival and spread of dengue in Europe is not as fanciful as first seems.

There is a clear need to monitor this vector in those areas already infested and to survey further afield so that scientists are one step ahead of any future arrival and spread. Only a truly dedicated Aedes trap and lure system using a human profile lactic acid/ammonia lure can provide the degree of sensitivity required.