EHJ June 2005, pages 20-21

Consultation on new guidance and contingency plans for EHPs to counter the threat of mosquito-borne diseases like West Nile virus takes place this month. EHJ reports

One question that's never far from scientists' minds at the Health Protection Agency and the Department of Health is, when is the next major pest-borne disease outbreak going to happen? It's an important question because despite the availability of technology to map and model disease transmission rates, scientists cannot predict when outbreaks are going to happen. And when they do happen, the effect can be truly devastating.

Take the West Nile virus outbreak in New York during 1999. Transmitted to humans by mosquitoes that feed on infected birds, the virus causes a mild influenza-like illness, which in some cases can develop and prove fatal. The virus soon grabbed the headlines. Within four months, it had killed seven people, hospitalised a further 55 and infected nearly 2,000 residents in the city's Queens area (EHJ December 2004, pages 368-371). Public health officials were in for another surprise, however, when the virus spread like wildfire across the United States. In 2003, nearly 10,000 cases were reported and over 240 deaths.

So far, the UK has had no reported cases. While the vectors and mosquitoes that can transmit it are here, as are the birds and the horses (the other hosts apart from humans), experts agree that mosquito-borne viral infections are virtually unknown in the UK. They also believe that climatic factors are a major factor in determining if the disease can occur. Fortunately, the UK's climate appears to differ from other areas where outbreaks of disease have occurred. Even so, the DoH is taking no chances and has already put in place measures to detect and control mosquitoes. EHPs are important players in this process and in the implementation of vector control programmes.

This month sees the publication of the CIEH's National Pest Advisory Panel's consultation document, Emerging diseases - preparing a contingency plan for mosquito-borne diseases. It contains a section focusing specifically on the virus, which informs EHPs on the method of transmission and provides a list of the mosquitoes present in the UK that are associated with West Nile virus transmission elsewhere. It also explains the importance of climatic conditions and provides a detailed assessment on the likelihood of West Nile virus already being here.

The consultation document has another section on problem mosquitoes and their relevance to the UK. Fortunately, very few species found here do, in fact, bite humans. People who believe they have been bitten by mosquitoes are more often than not victims of other insects such as black flies, buffalo gnats and midges. This misidentification, as the document, explains, can prove costly. Over 5,000 insect identifications are carried out on behalf of local authorities every year and quite often the samples sent in are of midges rather than mosquitoes. The confusion is understandable because midges have a similar breeding habit. But this is an important point that environmental health departments need to take on board because misidentifying insects does waste considerable time and money.

This is where the guidance is particularly useful. Its section on the biology and behaviour of UK mosquito species goes into great detail, covering the mosquitoes' distinguishing diagnostic features, including those of the sub families. It also provides valuable information about mosquitoes' breeding habits. As the guidance explains, environmental health departments need to become more familiar with mosquitoes' taxonomy, habits and ability to transmit disease because without this knowledge, they will find it difficult to plan effective control strategies, which target the vulnerable stages of the mosquito's life cycle.

Equally important is raising EHPs' awareness of potential breeding sites across the UK. The Health Protection Agency has already done a great deal of work using GIS in this area, working with collaborators to develop a model of what may occur in the UK and where relevant mosquitoes could become established. To do this, it is important to understand where surveillance is needed in the long term. As the document explains, there is no point in EHPs and entomologists looking in the wrong place for the mosquitoes and then concluding that they are not present in the district.

The document looks at the various locations where different species of mosquitoes are likely to breed, which range from tree holes to used tyres. The habits of the Culex pipiens is of particular importance. A ubiquitous mosquito, it has the ability to occupy a number of breeding sites, including tyres, which may explain why it's been so efficient in transmitting West Nile virus in other countries.

Tyre dumps are an important breeding site for mosquitoes. The UK imports thousands of tonnes of used tyres every year, which are recycled for a number of uses, for example, for re-treading, for use as fuel, in landfill sites and for coastal protection. Tyres are also regularly dumped illegally in wooded and other areas where mosquitoes can use them as breeding sites.

One important factor that increases the risk of disease transmission by mosquitoes, or other vectors, is human population growth and movement. The UK is already one of the most densely populated countries in the world, and as people move more and more into rural areas, there is a greater likelihood that they will be exposed to vector habitats. This in turn increases the potential for people to come into contact with infected or potentially infected vectors.

Another driving force is the huge growth in international travel, which has seen holidaymakers venturing to more exotic destinations and returning with a whole range of new and previously unseen diseases. Pet passports have allowed owners to take their pets abroad, further increasing the potential for exotic diseases to be introduced in to the UK.

As the UK's climate continues to change, the potential for invasive species to gain a foothold here increases, which could in turn allow indigenous species to increase and spread further a field. A knock-on effect is that this may also lead to an increase in the risk of contact and spread of disease.

Taking all of these factors, and others, into consideration, it is clear that the threat from emerging diseases is very real. The guidance document explains that it is important that the UK should not take a singular approach to tackling the threat and suggests that it should broaden its liaison with European and other countries to improve our understanding of the issue.

In the United States where the West Nile virus has caused havoc to people's lives, the University of Illinois has put together a detailed and wide-ranging contingency plan, outlining various approaches that can be taken to control mosquito populations. Elements of the plan can be applied to the UK and have been adapted by the NPAP for its guidance to EHPs.

As the document explains in its section on guidance and contingency plans, major mosquito problems can be attributed to two kinds of mosquitoes, those that deposit their eggs singly in areas that will be flooded later and those that deposit their eggs singly or in egg-raft clusters of 50-400 eggs directly on the surface of water in permanent or semi-permanent water bodies, such as artificial containers.

In the past, mosquito abatement measures have largely been directed against the adult mosquito populations rather than the elimination of breeding sites and the control of mosquito larvae through the proper use of larvicides. The guidance follows advice from the DoH, which recommends that local authorities should give priority to the control of mosquito larvae for mosquitoes like the Culex pipiens, which deposits its eggs in rafts directly on the water's surface.

The guidance provides detailed preventative measures for EHPs, who, with the assistance of the local community, can eliminate the standing water where these mosquitoes breed. This same section provides EHPs with information on the most effective non-chemical control methods and the use of insecticides.

NPAP is also currently developing a training course on mosquito control targeted at EHPs, particularly those working in local authorities. The training course is expected to be launched this year after discussions have taken place with other interested parties, such as the British Pest Control Association.

As the severe acute respiratory syndrome (Sars) outbreak in 2003 clearly underlined, infectious diseases that are a problem in one part of the world can rapidly develop into a global threat. Likewise, diseases that previously caused only a mild illness can suddenly become more virulent or serious.

Good detection and surveillance measures are essential to avoid the erosion of public confidence. EHPs need to be involved in the implementation of these control measures to ensure the UK doesn't succumb to mosquito-borne diseases and the headaches it can cause communities.

The consultation document and training course will be available this month at the NPAP website at: www.cieh-npap.org.uk/