EHJ April 2005, pages 22-24

As travel to exotic destinations becomes easier and more popular, the risk of pest disease vectors gaining a foothold in the UK increases. David Oldbury reports from Manchester Airport on why control measures should be implemented rigorously

Over successive years an impressive range of pest animal specimens have landed on my desk for identification. Found on board aircraft arriving at Manchester Airport, they have included rats, geckos, preying mantis, bush crickets, giant centipedes and tarantula spiders. Apart from the rodents, most of these animals are a nuisance and pose no significant public health threat.

But one uninvited pest that gives great cause for concern is the mosquito. On many occasions, this disease vector eludes detection. Once it arrives on board aircraft, it escapes into the environment during disembarkation to enjoy the vagaries of the British climate. And this pattern will become more commonplace as air travel is set to increase significantly in future years.

Currently 95 airlines and 300 tour operators serving 180 destinations worldwide operate in and out of Manchester Airport. Traffic is forecast to more than double from 19.5 million passengers in 2003 to around 42 million passengers by 2015. A corresponding increase in the amount of cargo handled is also envisaged from 125,548 tonnes in 2003 to 250,000 tonnes by 2015.

A recent DTI consultation paper on the future of air transport has projected that over 500 million air passengers will arrive in to the UK by 2030. The public's appetite for air travel appears to be insatiable, no doubt fuelled by the many exotic holiday destinations promoted on TV. The desire to fly to even more exotic locations for holidays is set to continue.

The problem is that travel to such destinations brings travellers increasingly in contact with the mosquito. This particular pest animal poses a significant public health risk through its ability to transmit diseases like malaria and arboviruses such as dengue fever, yellow fever and West Nile virus onto an unsuspecting British public.

Malaria is a common life-threatening disease in many tropical and subtropical areas throughout the world. It is currently endemic in over 100 countries, visited by more than 125 million international travellers every year. Research published in the Independent on 10 March 2005, reveals that more than half a billion people are suffering from malaria, twice as many as scientists thought. The malaria parasite is transmitted by various species of Anopheles mosquitoes, a mosquito which tends to bite humans in between sunset and sunrise.

The Health Protection Agency reports the largest number of imported malaria cases in Europe with around 2,000 cases (predominantly P. falciparum) being imported into the UK each year. Cases of "airport malaria" (cryptic malaria) occur in both the UK and other European countries in residents living near to international airports (EHJ December 2004, pages 368-371). These residents contract malaria even though they've never ventured abroad. Subsequent investigations carried out attribute the cause to malaria-infected mosquitoes escaping from untreated aircraft arriving from malaria-endemic countries.

Following the recent tsunami disaster in Asia, areas flooded by salt water made most of the stagnant water bodies unsuitable for malaria vectors. However, the World Health Organization has now warned that with the onset of monsoon rainfall, this is effectively desalinating the seawater making breeding sites more suitable for vectors of malaria and other diseases.

In 2003, a real "wakeup" call was issued worldwide with the Sars epidemic. This epidemic affected 26 countries and resulted in 8,000 Sars cases. The speed at which it spread was incredible. With direct long-haul flights it is possible for a passenger to contract an infectious disease in India or Cuba in the early morning and arrive back in the UK by evening of the same day before symptoms become apparent. Air travel has made the world a much smaller place.

Not all threats, however, lie at the door of the mosquito. Threats to public health can occur from other pests such as rodents on board aircraft, especially when the aircraft has arrived from a plague endemic area. These fears were heightened during the Indian plague outbreak in 1994 when all passengers, baggage and aircraft arriving at UK airports, including Manchester, from India required checking.

Cockroaches on board aircraft are another concern. While not attributed to vectoring major diseases, these insects can replicate in their gut pathogens present in the immediate environment. Physical contamination from their bodies and from their excrement can occur on surfaces the insect crawls over.

This can pose a problem for airport flight catering premises when food carts accidentally infested on board aircraft spread infestation to airport catering facilities via the infested food carts.

To combat such threats, the WHO introduced the International Health Regulations in 1969, which were enacted in the UK as the Public Health (Aircraft) Regulations 1979 (as amended). A similar set of regulations was introduced at the same time for ships to combat pest disease vectors. These regulations provide (among a number of other provisions), measures for deratting and disinsecting an aircraft arriving from a plague or malaria endemic area. The WHO has introduced three approved methods of disinsecting aircraft.

Blocks away

This procedure takes place prior to take off after passengers have boarded and the aircraft is taxiing. Cabin crew walk through the cabins and discharge aerosols containing 2 per cent d-phenothin, a quick-acting "knockdown" pyrethroid insecticide. All possible insect harbourages, including toilets, galleys, overhead stowage compartments etc are treated and the cans retained for inspection by the UK port health authority. Holds and the flight deck are sprayed prior to departure. This procedure has proved unpopular with passengers and airlines. Passengers often complain of respiratory problems and log claims for compensation against airlines. There are also marked inconsistencies in treatment application.

Pre-flight and top-of-descent spraying

This method is similar to "blocks away", except that the aircraft cabin is sprayed on the ground with an aerosol containing a residual insecticide prior to passengers boarding the aircraft. The timing of this spraying allows overhead lockers, wardrobes and toilets to be opened and properly sprayed with minimum inconvenience to passengers. Pre-flight spraying is followed by a further in-flight spray of quick-acting "knockdown" insecticide, carried out at "top-of-descent" as the aircraft starts its descent.

Though favoured by the Australian and New Zealand quarantine services, this treatment method is unpopular with passengers for the same reasons as "blocks away" because it involves discharging insecticide in the presence of passengers.

Residual treatment

With this method, the internal surfaces of the aircraft, excluding food preparation areas, are treated every eight weeks (during maintenance) with a residual insecticide with localised topping up on surfaces, which are subsequently deep cleaned or refurbished. At the end of the procedure, the aircraft is issued with a certificate of residual disinsection. It is the preferred disinsection procedure with a number of airlines, as passengers are not directly exposed to insecticide aerosol sprays. It also has an added advantage of controlling crawling public health pests such as cockroaches on board the aircraft, reducing for example potential contamination of food mobile trolleys and flight catering facilities.

Even so, many countries fail to adequately enforce the international health regulations when it comes to aircraft disinsection. In the United States, the Environmental Protection Agency withdrew clearance on insecticides registered for use in aircraft cabins etc in 1996. Those countries that do enforce control measures appear to do so on an ad hoc basis.

The risk of imported diseases vectors entering via UK airports and seaports is very real. Public health officials need to implement the measures available as robustly as possible to control these potential threats as is practised in New Zealand and Australia. These countries take the view, exercised through their border control procedures, to move the risk off-shore ie to leave pests and diseases in the country of origin by enforcing stringent standards for high-risk goods such as used tyres, machinery, ships/aircraft and seaborne containers.

Rigorous inspection of goods are carried out on their arrival in both countries and any contaminated goods either cleansed, fumigated or sent back to their port of origin. What's more, risk-based surveillance of potential habitat/breeding sites eg mosquitoes, focusing on international ports and airports with the highest volumes of passengers and goods arriving, is routinely carried out.

At last year's National Pest Advisory Panel seminar on emerging diseases, a paper on climate mapping focused on mosquito survival and seasonal activity modelling. It revealed that the UK's mild winter temperatures might allow mosquitoes such as Aedes albopictus to overwinter with possible activity occurring in the summer months. Aedes albopictus is a prolific man-biter that favours urban settings.

To counter the threat posed by Aedes albopictus and other mosquitoes, NPAP has worked in conjunction with the HPA to develop "mosquito watch", a dedicated database to record the incidences of mosquitoes on the NPAP website. EHPs are requested to assist in the database's development by reporting incidences of biting and nuisance mosquitoes that may occur throughout the UK.

Action needs to be taken now to ensure that all available control measures are rigorously implemented at air and seaports to protect public health and prevent pest disease vectors from gaining entry and establishing a viable foothold in the UK before it is too late

David J Oldbury is NPAP's secretary and group manager for environmental health services at Manchester CC. To contribute to mosquito watch, visit: www.cieh-npap.org.uk

A detailed protocol on carrying out the residual disinsection of aircraft has been developed by a working group set up by Manchester CC's environmental health, comprising Bayer Environmental Science, Airline Services and First Choice Airways. To obtain a copy, e-mail: d.oldbury@manchester.gov.uk

The author would like to thank Tina Murray, principal EHO at the airport group, for contributing to this article.