EHJ December 2004, pages 368-371

The emergence of the West Nile virus in the United States in 1999 sent shock waves through public health circles. Nick Warburton asks whether serious mosquito-borne viral infections could happen here and what precautions are being taken to protect public health

When Dr Tracy McNamara, a veterinary pathologist at New York's Bronx zoo, started receiving phone calls about dead crows, she had no idea what was happening or the havoc it would wreak. Little did she know that the birds had fallen victim to a foreign insect-borne virus, never before seen in the western hemisphere.

West Nile virus, a mosquito-borne viral disease that can lead to serious complications of the liver or nervous system, inflammation of the brain (encephalitis) or paralysis, had touched down in New York with a vengeance.

Barely four months on, the virus had killed seven people in the New York area, hospitalised a further 55 and infected nearly 2,000 residents in the Queens area, the centre of the human outbreak. Since then, the virus has spread like wildfire across the United States, and even set up home in Canada and the Caribbean. In 2002 alone, over 4,000 people in the United States were infected. Two hundred and eighty four people died as a result.

Transmitted to humans through mosquito bites, the virus originated in Uganda's West Nile province, where it was first isolated in 1937. Mosquitoes become infected when they feed on infected birds that have high levels of the virus in their blood. They then pass this on when they feed on animals and humans.

Fortunately, most humans that become infected do not become seriously ill. The United States Geological Survey reports that less than 1 per cent of infected people develop severe illnesses such as meningitis and encephalitis. Of those that develop the latter illness, a very small number die - less than 1 out of 1,000 infections. Also, in the 1999 outbreak, most fatalities took place in those over 70 and this age group is generally recognised as being the most at risk. Nevertheless, the fact that there is no specific treatment for West Nile virus infection or vaccine to prevent it, is a matter of concern.

Since its isolation in the 1930s, West Nile virus has spread globally with cases reported in places as far flung as Russia, South Africa and France, although the outbreaks have not always affected humans. The virus can also be transmitted to horses, which, according to Professor Mike Taylor at the Central Science Laboratory in York, is worrying - horses are one of the few animals that have a fairly free licence to be moved across the world and it's still not clear if they are dead-end hosts.

The Department of Health assesses the chances of the West Nile virus setting foot in the UK as "low", but has not ruled out the possibility that it could happen. Although the UK hasn't had any reported cases to date, a long list of mosquitoes that can transmit the virus are resident here.

Unlike in the United States, the UK has yet to witness any large unexplained bird deaths. This is important because experts believe dead birds are characteristic of the kind of outbreaks when you can expect infection. As Prof Taylor points out, this could be because the UK's bird populations have had previous exposure from repeated bird migration from Africa and consequently may have an element of natural flock immunity. He also says the UK's mosquito densities are generally considered to be low.

Even so, the government has taken the threat seriously enough to take action. In May this year, the chief medical officer, Liam Donaldson, took steps to put measures in place with the publication of a West Nile virus contingency plan. This calls on primary care trusts, strategic health authorities, the Health Protection Agency and local authorities to prepare local contingency arrangements to deal with a potential outbreak.

The HPA has even carried out a retrospective study of cases of encephalitis where the causative organism was not identified to see if there have been any previously undiagnosed cases of West Nile virus in the UK's population. They are also carrying out surveillance for infection in people but in both instances have not identified any positive cases.

While it is still unclear how the virus was introduced into New York, factors like climate change, long haul travel and changes in land-use have the potential to assist in the spread of infectious diseases and underline the importance of preparing for the unexpected. As Maggie Tomlinson, portfolio manager, emerging infections at the DoH, points out: "We have a climate that's different from other areas where outbreaks of disease have occurred and that's very, very important. Climatic factors are becoming increasingly recognised as a major determinant of whether the disease can occur. We can take some comfort from this."

She also says we can take comfort from the fact that mosquito-borne viral infections are virtually unknown in the UK. That, however, should not allow us to become complacent, she adds.

Manchester MDC's environmental health service carries out work at Manchester airport where it helps to control pests arriving from exotic destinations. David Oldbury, principal EHO and group manager at the council, argues that the mosquito is by far the most important insect pest vector in terms of a public health threat. Apart from West Nile virus, it also brings in malaria and the range of arboviruses (spread by blood sucking insects) that they can transmit.

Mr Oldbury points to figures published by the World Health Organisation which show an increase in the numbers of airport malaria cases reported in communities living in close proximity to major international airports. Between 1969 and 2000, there were 89 confirmed cases in Europe, including 14 reported in the UK. The fact that these people had never travelled outside the UK, suggests a strong link between aircraft coming into airports and mosquitoes escaping and transmitting malaria to the immediate population.

But airports are not the only entry points into the UK. Over the last two to three decades, there's been a huge growth in the lucrative and international trade in used tyres, brought into ports by container ships and stacked at dumps across the country. Experts fear this trade could introduce species such as the Asian tiger mosquito (Aedes albopictus), a notorious vector that can survive temperate climates and spreads diseases such as dengue and West Nile virus.

The Asian tigers are forest mosquitoes that breed in tree holes where water collects. The eggs are laid on the side of the tree. When there's sufficient water available and sufficient food in the water, the eggs hatch. Thanks to this effective breeding method, the mosquito has become highly adaptable and lays its eggs in a variety of man-made breeding sites - water storage containers, discarded tins and used tyres. The nature of this trade appears to particularly suit the Asian tiger.

"Used tyres aren't always stacked in nice lines where no water is going to collect. They're not always covered," says Jolyon Medlock who works for the Health Protection Agency at Porton Down. "Not all tyre dumps are legal or registered. So, because the inside of the tyres have a bit of a dip, this collects water and it also collects leaves, a nutrient for the mosquitoes once they've hatched to actually feed."

Mr Medlock says the most up-to-date figures, taken from 2001, show that over 5 million used tyres were imported into the UK during that year. While it is not clear if that number has increased over the last three years, what is clear is that tyres are coming in and coming in from endemic areas.

Prof Keith Snow, from the University of East London, is convinced that tyres play a major role in aiding the spread of this mosquito throughout the world (EHJ November 2000, pages 358-360). Patterns of movement certainly seem to support his claim. In 1985, the Asian tiger was first spotted in Houston, Texas and within 15 years spread across parts of the south and southeast of the United States. Experts believe the mosquitoes arrived on imported used tyres from Asia, and as Mr Medlock points out, "through the internal movement of these tyres, you can monitor the movement of these mosquitoes through the interstate highway systems, which is pretty cunning really."

Asian tiger activity has also been linked to the used tyre trade in Europe. "There's a strong suspicion that the Aedes albopictus mosquito went into Montenegro from tyres in Italy," says Jonathan Peck, managing director of Killgerm. "The problem is nowadays we move stuff around so much and so quickly that if you've got a container full of old tyres, the chances are that it will have mosquito larvae in them."

Tiger mosquito larvae were found at a used tyre dump near the village of Orne, south of Caen in Normandy in October 1999. This is worrying news considering the French port is linked to Portsmouth by ferry. And while it is not certain if the mosquito has actually established itself in the UK, Mr Snow argues that its most likely means of entry is through the tyre trade.

In November 2000, Mr Snow and Dr Clement Ramsdale, a consultant entomologist, recommended that all local authorities make a register of companies in the UK that are participating in the international tyre trade and start to monitor for the presence of the Asian tiger mosquito in and around tyre storage premises. Four years on, Mr Snow doubts any environmental health departments are looking for it or know what it looks like.

The Asian tiger mosquito is not the only member of the Aedes genera that presents a potential public health threat. Mr Medlock identifies the Anopheles, Aedes and the Culex genera as the main global vectors of infectious disease, each with its own unique breeding habits and sites. The Culex pipiens is the most important vector of West Nile virus and the most widespread and abundant in the UK.

Faced with these potential threats, mosquito surveillance is becoming a matter of necessity. The HPA, CIEH and Killgerm have recently set up a mosquito watch scheme that aims to document mosquito species throughout the UK. Environmental health departments and pest control operators are being encouraged, along with others, to send mosquitoes in their various stages to the HPA and Killgerm for identification. Once identified, details will be posted on the CIEH's national pest advisory panel website, which went live in October, creating a geographical map of where the species are. The information will be shared with the HPA to assist with strategic planning.

Professor Moray Anderson, technical director at Killgerm, believes that environmental health departments and pest control operators should become familiar with mosquitoes' taxonomy, habits and ability to transmit disease. Armed with this information, he argues, they will better equipped to plan suitable control strategies of breeding sites.

At the NPAP conference on emerging diseases in July, Ms Tomlinson cited the important role of environmental health departments, and raised the issue of pesticide usage. She said that only one or two local authorities had expertise in spraying mosquitoes to keep their biting nuisance at bay. In order to prepare for a potential outbreak, she says local authorities should ensure they have pest contractors and pesticides to go with them to undertake vector control.

Ms Tomlinson also recognises that there is a broader role for EHPs, providing advice to the public. "Local authority environmental health departments would be key locally in getting the right kind of advice out," she says. "The advice would be quite straightforward. It's about avoiding being bitten by mosquitoes, it's about using insect repellent, it's about using screens, mosquito nets, anything that is appropriate to prevent mosquitoes biting you and possibly passing on the infection to you."

It's also about educating people that any water that is lying around can act as a breeding site for mosquitoes. "In parts of Italy, these mosquitoes are breeding in people's gardens because they've got a vase or a container outside collecting water," says Mr Medlock. "All you have to do is empty it and turn it upside down if it's not being used. Suddenly, there's no mosquito breeding site."

Judging by past trends, public health experts believe that emerging diseases are most likely to be zoonotic pathogens. In 2001, the Royal Society published a comprehensive literature review by the Centre for Tropical Veterinary Medicine, which found that 61 per cent of the 1,415 species of infectious human pathogens identified were zoonotic. And as Ms Tomlinson explains, of those pathogens that had been associated in the past with emerging infections, 75 per cent were zoonotic.

Last month, the NPAP met to discuss the findings of the July conference. One of the issues on the agenda is a guidance document for environmental health departments faced with an emerging disease outbreak, caused by mosquitoes. The document, which should be available in the spring, will contain information advising local authorities on how to prepare for a potential outbreak, which mosquitoes are of public health concern and guidance on contingency plans and control methods. It will also provide details of the mosquito watch scheme, which EHPs are being invited to join.

Clearly, it's not a matter of if, but a matter of when new and emerging infections arrive. On the basis of this certainty, taking a precautionary approach would seem the most sensible option.

The NPAP website can be accessed at: www.cieh-npap.org.uk