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Prevention of insect-borne diseases

Carolyn Driver
RGN RM RHV FPCert MSc(TravelMed) FFTM RCPS(Glas)
Independent Travel Health and Immunisation Specialist Nurse Cheshire

It is more than 100 years since Ronald Ross discovered the part that the mosquito played in the transmission of malaria. Around the same time Walter Reed proved that mosquitoes were also the vector for the yellow fever virus. Sadly, more than a century later, insect- (or arthropod-) borne diseases are still a major health problem around the world. While they are most commonly associated with tropical countries they can also occur seasonally in more temperate climates. Tickborne encephalitis, for example, can occur through Europe, Russia and North Eastern China in the late spring and summer months. West Nile virus occurs in Europe and the USA as well as in Africa.(1)
Despite major vector control programmes and the advent of vaccines and chemoprophylaxis for some of these infections there has been a worrying reemergence in recent years. Chikungunya virus, for example, has been responsible for a large epidemic in the Indian Subcontinent and South East Asia over the last two years, having been comparatively rare for the previous 30 years.(2) Insect bite avoidance measures thus remain the most important preventive strategy for the individual at risk.

Which insects carry disease?
Mosquitoes are probably the most infamous insects for disease transmission, but flies, bugs, fleas, lice, mites and ticks are also vectors for a huge variety of infections (see Table 1). Many of these infections also have intermediate hosts, such as animals or birds. Where there is an animal involved in the lifecycle of an organism, eradication is virtually impossible and thus control of the vector becomes the mainstay of control of spread of infection. Prompt diagnosis and isolation of cases can also help minimise outbreaks, but these measures are often insufficient in poorer countries.

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Advising the traveller
Many travellers will have heard of malaria and yellow fever, but they may not always know how these infections are transmitted. Most will be unaware of just how many other infections are transmitted by insects and that the only protection available is the use of bite avoidance measures. It is very important when administering vaccines such as yellow fever and Japanese encephalitis, or providing a prescription for malaria chemoprophylaxis that the travel health adviser reinforces the need for personal protection measures. Many of the infections listed in Table 1 constitute a small risk for short-term tourists, but malaria, yellow fever, Japanese encephalitis, dengue and chikungunya pose a more significant risk in many destinations that are popular with UK tourists.
It is important to carry out a thorough risk assessment when giving travel health advice and to have access to up-to-date, reliable resources.
New malaria guidelines were published by the Advisory Committee on Malaria Prevention in UK Travellers in December 2006 and can be downloaded from www.malaria-reference.co.uk. It is essential that anyone who advises travellers has access to a copy of these guidelines. They contain maps of many countries where the risk of malaria is variable and an extensive "Question and Answer" section that addresses many of the common travel-related situations.
The website, run by the National Travel Health Network and Centre (NaTHNaC - www.nathnac.org), provides a lot of information for both the adviser and the traveller. Yellow fever risk maps can be found on this site as well as important information about the yellow fever vaccine and registered yellow fever centres. NaTHNaC also runs a telephone helpline for healthcare professionals, which is useful for more complex travel-related questions. Details of this service and the phone number are displayed on the website.
The NaTHNaC website also highlights current outbreaks so that the adviser is able to highlight these to relevant travellers.
Japanese encephalitis
Japanese encephalitis is a viral infection spread by the culex mosquito that bites many animals as well as humans. The intermediate hosts are pigs and water fowl so the disease is found mainly in rural farming areas in South East Asia. Because the mosquito does not feed exclusively on humans the risk of transmission is low. Only 10% of those who are infected with the virus will develop symptomatic disease, but about one-third of these will develop encephalitis with resultant long-term sequelae.
Currently only an unlicensed inactivated vaccine is available in the UK. Vaccination is recommended for travellers who will be spending prolonged times in rural parts of infected countries and all travellers should be advised about bite avoidance measures. The vaccine is available from Sanofi Pasteur MSD or MASTA on a named patient basis. A course of three injections given over 28 days confers two to three years protection and can be used from one year of age. As the vaccine is unlicensed it cannot be given under a patient group direction, thus a private prescription should be issued by the prescribing doctor.

Dengue fever
Dengue fever is a viral infection transmitted by the aedes aegypti mosquito. There is widespread distribution of the four serotypes of this virus in the tropics. Infection with one serotype confers lifelong immunity to that specific serotype, but subsequent infection with any of the other serotypes may result in a higher chance of the severe form of the disease known as dengue haemorrhagic fever (DHF). Currently the only method of prevention is by bite avoidance. Because of the increase in frequency of travel tourists are now more at risk of developing DHF and thus it is very important to prevent any infection. Dengue infection produces a flu-like illness in which a profound myalgia can occur leading to its colloquial name of "breakbone" fever. The infection is self-limiting, but can result in a prolonged postviral syndrome in a small number of cases.

Chikungunya virus
Chikungunya virus is a viral infection transmitted by the culicine mosquito, eg, aedes aegypti and aedes albopictus. The word chikungunya is a Makonde word meaning "the one which bends up", relating to the posture of victims of this infection who suffer intense joint pain. Chikungunya occurs in Africa, India and South East Asia. It is primarily found in urban/periurban areas. There is no specific treatment for Chikungunya and it is usually self-limiting. Since January 2005 there has been a widespread epidemic sweeping from the island of Réunion across India and reaching Malaysia. Before this epidemic the virus had not been identified in India since 1973.(2) Bite avoidance measures remain the only preventive strategy for the individual.

Bite avoidance measures
Japanese encephalitis, dengue and chikungunya are all spread by mosquitoes, which bite mainly in the late afternoon rather than at night, whereas the anopheline mosquito, which transmits malaria, is predominantly a nighttime biter. It is important therefore that bite avoidance measures are used throughout the day and night. Dawn is another key feeding time for most species of mosquitoes.
Insect bites can be avoided by:

  • Insect repellents: should be applied to all exposed skin. It is important that the traveller is aware of the length of efficacy of the product they are using and reapplies it as often as necessary, especially after swimming or if perspiring significantly. Those products containing the chemical N,N-diethyl-methyl-toluamide (DEET) are most effective although there are some newer more naturally based products such as Mosi-guard, which contain eucalyptus oil, but these generally need to be applied more frequently and are not as effective in high-risk areas. Concentrations of 30-50% DEET may be applied to adult skin, but it is safest to keep to a maximum of 20% in children and pregnant women. It is important that the insect repellent is the last thing to be applied to the skin (ie, after sunscreens or moisturisers).
  • Clothing: trousers, socks and clothing with long sleeves should be worn outdoors at high-risk times (dusk and dawn for most species). Clothing can be sprayed with an insecticide such as permethrin. Ankle and wristbands that have been pretreated, may be worn. It is important to note that these measures should be in addition to and not instead of insect repellent on exposed skin.
  • Windows: sleeping accommodation should be in rooms with mesh screened windows and rooms should be sprayed with a "knockdown" insecticide each evening. Plug-in pyrethroid vaporisers can also be used. 
  • Bednets: a bednet, which has been pretreated with permethrin, should be used if there is no reliable air conditioning available. Nets need to be retreated every six months for maximum effectiveness and they should be regularly checked for holes. The net should be knotted up during the day to prevent insects from getting inside and they should be tucked in all around the mattress or sleeping bag when in use.

Returning travellers
All of the infections described in this article may present with flu-like symptoms. Travellers should be taught to present promptly to a doctor if they develop such problems while away or on their return and to mention their travel history. Fever in a returning traveller should be regarded as malaria until proven otherwise.(3) Such patients should be referred urgently for investigation as differential diagnosis for malaria can only be made by examination of thick and thin blood films. Malaria is treatable and the earlier the treatment is commenced the better the outcome for the patient. There is no specific treatment for the viral infections mentioned here. Care will be determined symptomatically and confirmation of diagnosis may be made by serology.

Conclusion
One of the most important aspects of the travel health consultation is empowerment of the traveller to enable them to look after their health as much as possible while abroad. Bite avoidance measures are a vital part of this process and they should be taught about the combination of methods that are required to ensure they reduce their risk of insect-borne infection as much as possible. These measures are important during the summer months even in the less exotic destinations in view of the reemergence in recent years of infections such as tickborne encephalitis and West Nile virus. Health promotion materials displayed in waiting rooms may help with this as travellers to European destinations or the USA may not necessarily attend for pretravel advice.

References

  1. Gubler DJ. Resurgent vector-borne disease as a global health problem. Emerg Infect Dis 1998;4:442-9.
  2. Lahariya C, Pradhan SK. Emergence of chikungunya virus in Indian subcontinent after 32 years: a review. J Vect Borne Dis 2006;43:151-60.
  3. Chiodini P, et al. Guidelines for malaria prevention in travellers from the United Kingdom. London; Health Protection Agency: 2007.

Resources

For the healthcare
professional:
TRAVAX
Run by Health Protection Scotland, the site gives country-specific profiles and information on all aspects of travel health. Free to practitioners in Scotland, Wales and Northern Ireland. Subscription charge for practitioners in England, but primary care trusts can subscribe on behalf of the whole trust.
W: www.travax.nhs.uk

MASTA
Country-specific profiles and multidestination trip recommendations can be created here. Professionals need to register, and there is a charge for printing the profiles.
W: www.masta.org

VIS online
Provided by Sanofi Pasteur MSD, this is a comprehensive database of vaccine information and country-specific information. Professionals can register for free. 
W: www.spmsd.co.uk

For the traveller:
NATHNAC
Provides disease information, outbreak reports, etc.
W: www.nathnac.org

Fit for Travel
Public access version of the TRAVAX database - not as detailed as the professional site, but useful nonetheless.
W: www.fitfortravel.scot.nhs.uk

Malaria Hotspots
Educational site for the public about risk areas, the disease and prevention.
W: www.malariahotspots.co.uk 

iFabric
Insect repellents
W: www.ifabric.info