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Travel: Mosquito-borne diseases

Key learning points

 - Factors such as increased international travel, migration and trade, along with climate change, have made mosquito-borne diseases a global public health threat

 - Mosquitoes can transmit potentially fatal diseases with just one bite

 - Mosquito bite avoidance is the first line of defence against mosquito-borne diseases

Increased international travel, migration and trade, along with climate change have led to the re-emergence and spread of vector-borne diseases.1 Mosquitoes are probably the most prominent disease vector, known to spread many potentially fatal diseases such as malaria, the biggest killer of all vector-borne diseases.1 Although traditionally regarded as a problem for tropical countries, over the past two decades some vector-borne diseases have spread to new parts of the world, increasing the threat to global public health in terms of geographical spread and the number of people affected.1,2 Dengue fever is the most rapidly spreading mosquito-borne disease in the world; currently endemic in over 125 countries, with serious outbreaks in continents that have had no prior occurrence, including some cases in Europe.3 Public Health England (PHE) recently published annual 2013 data for dengue fever cases in returning UK travellers; reporting a total of 541 cases which represents an overall increase of 58% in comparison to 2012.4 Like most mosquito-borne diseases, there is no specific chemoprophylactic drug or vaccine to prevent dengue fever; this makes mosquito bite avoidance the only defence against this disease.2,3 

There are in fact very few mosquito-borne diseases that are preventable with chemoprophylactic drugs or vaccinations, and as such mosquito bite avoidance should always be considered the first line of defence.5 Chemoprophylactic drugs are available to protect against malaria, however, efficacy is variable, depending on both geographical drug resistance and patient compliance with medication.6 Vaccinations are available to protect against yellow fever and Japanese encephalitis, however vaccination may not always lead to immunisation, highlighting the importance of educating travellers about disease transmission and mosquito bite avoidance.7

Mosquito Bite Avoidance

Fresh standing water plays a key role in the life cycle of mosquitoes. Seasonal outbreaks of mosquito-borne diseases occur as there is a relationship between rainfall and the existence of breeding sites.8 Temperature is another significant factor; mosquitoes prefer warm climates and are therefore less likely to be found at altitude or in colder climes. Additionally, different species of mosquitoes carrying different mosquito-borne diseases and bite at differing times of the day. 

The Advisory Committee on Malaria Prevention for UK Travellers (ACMP) recommend the use of N,N-diethyl-m-toluamide (DEET)-based insect repellents, asserting that few insect repellents are as effective.9 Duration of protection is one to three hours for 20%, up to six hours for 30% and up to 12 hours for 50% DEET. Duration of protection does not increase in concentrations beyond 50%.9 The interval between applications is dependent on humidity, perspiration rate and concentration of DEET used. DEET can be used on babies from the age of two months upwards (unless allergic).9 DEET should be applied after sunscreen.5,8,9

Clothing can act as a protective barrier against mosquito bites. 

Light coloured clothing, long sleeves, high necklines, and trousers or long skirts are preferable to clothing that leaves exposed areas of skin.5 Clothing can be impregnated with mosquito repelling permethrin, however, frequently washing impregnated clothing will reduce effectiveness.5,8 DEET should be applied to remaining exposed skin.

There is a variety of mosquito nets available and they vary in size, shape, weight and hanging method, and careful consideration should be given to choosing a net that best suits the travellers needs. Mosquito nets that are impregnated with insecticides such as permethrin greatly enhance the level of protection, however, efficacy is reduced over time and nets should be re-impregnated as per the manufacturer's instructions.5 Additionally, mosquito nets should be checked for holes prior to use, and any apparent holes mended with a repair kit or needle and thread.5

Mosquito-repelling coils are perhaps the best-known form of vapouriser that release insecticides as they burn, and insecticidal cones work in the same way. More sophisticated electronic vapourisers are available, however, plug-in vapourisers require a reliable power source, so battery operated vaporisers or the more basic coils and cones may be better suited to the requirements of some travellers.8 Door screens, windows or shutters should be closed from dusk to dawn, and checked for any holes that may allow mosquitoes to enter the room.5 Air conditioned rooms are considered sealed and highly effective at keeping mosquitoes out, nevertheless, windows and doors should still be checked for any gaps that may allow mosquitoes to enter.5,8 Aerosol =sprays are an effective way of killing any mosquitoes that may have entered the room before bedtime.8 Mosquitoes lay their eggs in fresh standing water, such as water left behind in plant pots after watering. Emptying any vessels that contain fresh standing water reduces the availability of mosquito breeding sites. Where possible, insecticidal spaying of breeding sites should also be practiced.5

Mosquito-borne Diseases

Malaria

Malaria is a potentially fatal, acute febrile illness caused by human protozoan parasites which are transmitted via the bite of the female Anopheles mosquito.10 The Anopheles mosquito primarily feeds between dusk and dawn; as such the highest risk of disease transmission occurs within this timeframe.10 Malaria is predominantly found in Africa, South and Central America, Asia and the Middle East. Disease risk is often seasonal and regionally variable, consequently up-to-date malaria prevention advice should always be obtained from a reputable source such as those listed in the resource section. There is currently no commercial vaccine available to prevent malaria. Various anti-malarial chemoprophylactic drugs are available to help protect travellers against the disease. However, choice of chemoprophylaxis requires careful consideration and is dependent on factors such as; patient age, weight, pre-existing medical conditions, previous adverse reactions, patient compliance and geographical drug resistance.6 Chemoprophylactic efficacy is variable, reliant on patient compliance and geographical drug resistance.6 Consequently, travellers to malaria risk areas should always be advised to practice mosquito bite avoidance alongside any chemoprophylactic regimen.

Chikungunya

Chikungunya is a viral disease spread via the day biting Aedes aegypti and Aedes albopictus mosquitoes.11 Chikungunya is an acute febrile disease that causes a sudden onset of fever and joint pain, particularly affecting the hands, wrists, ankles and feet.11 Other common symptoms include headaches, rash and muscle pains and while most patients recover within days, joint pains may persist for weeks and in some cases months later.11 Chikungunya occurs in sub-Saharan Africa, south-east Asia, tropical areas of the Indian subcontinent and islands in the south-western Indian ocean.11

There is currently no specific chemoprophylactic drug or vaccine to prevent chikungunya; this makes mosquito bite avoidance the only defence against this disease.2,3 

Dengue fever 

Dengue is a viral disease spread via the day biting Aedes aegypti mosquito.11 Dengue is an acute febrile illness with a sudden on-set of fever, sometimes accompanied by a macular skin rash, also known as 'break-bone' fever due to severe muscle, bone and joint pain symptoms it causes.11 Most patients recover after a few days, however, occasionally dengue progresses to a haemorrhagic form, potentially with shock, which can be fatal.11 Dengue fever is the most rapidly spreading mosquito-borne disease in the world; widespread in tropical and subtropical regions of central and South America, south-east Asia, Africa and Oceania.3,11 There is currently no specific chemoprophylactic drug or vaccine to prevent dengue fever; this makes mosquito bite avoidance the only defence against this disease.2,3 

Filariasis

Filariasis is a parasitic disease spread via the bite of infected Culex mosquito which predominantly feeds from dusk until dawn.12 The infected mosquitos transfer larvae of Wucheria bancrofti, a threadlike round worm, which settles in lymph nodes (principally groin and axilla).12 Fever is caused when adult female release microfilaria into blood to make available for another feeding mosquito.12 The worm does not multiply in the host, making severity of disease dependent on number of infected bites received.12 Consequently, the risk to travellers is generally low unless travel involves extensive exposure to infected mosquitoes.11 However, repeated infection can cause chronic lymphoedema usually of the limbs, or the genitalia, but early treatment can avoid severe complications.12 Filariasis occurs in tropical areas of Africa, India, parts of south-east Asia, and Central and South America.12 There is currently no specific chemoprophylactic drug or vaccine to prevent filariasis and mosquito bite avoidance is the only defence against this disease.12

Japanese encephalitis 

Japanese encephalitis a flavivirus infection of the central nervous system.13 The virus is harboured by pigs and birds, and transmitted to humans via the bite of infected Culex mosquitoes which predominantly bite between dusk and dawn.13 Most infections are asymptomatic, with approximately one in 200-500 of those infected developing clinical disease.13 However, symptomatic individuals have a high case fatality of 25-30% and approximately 30% of survivors will develop permanent neurological sequelae.13Japanese encephalitis is confined to the Indian sub-Continent, south-east and east Asia, and the Pacific.13 There are vaccines available to protect against Japanese encephalitis and vaccination should be considered for travellers that may be at risk of this disease.13 However, travellers should be advised to practice mosquito bite avoidance regardless of immunisation status. Disease risk is often seasonal and regionally variable and consequently up-to-date advice should always be obtained from a reputable source, such as those listed in the resource section.

Rift Valley fever 

Rift Valley fever is a viral disease that is harboured in livestock such as sheep and goats. Most infections are caused by direct contact with infected animal blood, tissue or organs, however, humans can also be infected by mosquito bite. Most human cases of the disease are mild and not long-lasting, however some go on to develop the severe haemorrhagic form of the disease which can be fatal. The first cases of Rift Valley fever where documented in Kenya in 1931. In 2000, cases were identified in Saudi Arabia and Yeman and there is concern that the disease may spread to Asia and Europe. There is currently no specific chemoprophylactic drug or vaccine to prevent Rift Valley fever making mosquito bite avoidance the only defence against this disease.5

West Nile fever

West Nile fever is a flavivirus infection, harboured in birds and other animals, spread to humans via the bite of infected Culex mosquitoes (which predominantly feeds between dusk and dawn).14 Most infections are asymptomatic and those that become ill experience Dengue-like symptoms, but very few cases progress to the potentially fatal meningoencephalitis stage.14 West Nile fever has worldwide distribution wherever mosquito vector is present.14 Cases are reported from Africa, Asia, Middle East, Europe and North America.14  There is currently no specific chemoprophylactic drug or vaccine to prevent West Nile fever; this makes mosquito bite avoidance the only defence against this disease.5

Yellow fever 

Yellow fever is a flavivirus infection of humans and monkeys, most commonly spread via the day biting Aedes aegypti mosquito.15 It is more common in rural areas where monkeys harbour the disease, however mosquitoes can transfer from human to human if an infected individual introduces the virus to a densely populated area.15 Most patients recover from Yellow Fever after three to five days, however, 15% of infected patients go on to develop the more toxic phase, of which approximately 50% will die within 10 to 14 days.15 Yellow fever transmission occurs in tropical areas of Africa, South America, Eastern Panama and Trinidad.15

There is a live vaccine available to protect against Yellow Fever and vaccination should be considered for travellers that may be at risk of this disease.15 Yellow Fever is one of the few diseases for which there is an internationally recognised certificate which may be required as a condition of entry to some countries.15 Travellers should be advised to practice mosquito bite avoidance regardless of immunisation status. Yellow Fever certificate requirements and transmission areas do change, and as such up-to-date advice should always be obtained from a reputable source, such as those listed in the resource section.

Zika virus

Zika virus is a flavivirus spread via day biting infected aedes aegypti mosquitoes.16 Zika virus causes a Dengue-like illness, it is usually mild and self limiting.16 Transmission of Zica Virus has been documented in Africa, Asia and Oceania.16 There is currently no specific chemoprophylactic drug or vaccine to prevent Zika virus which makes mosquito bite avoidance the only defence against this disease.5

Resources

For up-to-date information on regional disease protection, transmission and current outbreaks of mosquito-borne diseases please use the following resources:

TRAVAX
Travel health information for health professionals, funded by the Scottish Government Health Department
www.travax.nhs.uk

Fit for Travel
Free travel health information for the public, funded by the Scottish Government Health Department
www.fitfortravel.nhs.uk

NaTHNaC
Travel health information for both health professionals and the public, funded by the Department of Health for England
www.nathnac.org

Advisory Committee for Malaria Prevention
Latest information on malaria prevention and chemoprophylactic drugs
www.hpa.org.uk/infections/topics_az/malaria/ACMP.htm

Green Book
Latest information on vaccines and vaccination procedures, for vaccine preventable infectious diseases
www.gov.uk/government/collections/immunisation-against-infectious-diseas...

 

References

1. WHO. World Health Day. 2014.

2. Savioli L, Velayudhan R. Small bite, big threat: world health day 2014. East Mediterr Health J 2014;20:217-218.

3. European Centre for Disease Prevention and Control. Dengue outbreak in Madeira, Portugal. Stockholm: European Centre for Disease Prevention and Control; 2013. 

4. Gov.uk. PHE publish dengue fever and chikungunya annual data - Press releases - GOV.UK. 2014. 

5. Health Protection Scotland. 2012.

6. Nasci R, et al. Protection against mosquitos, ticks & other insects & arthropods. In: CDC Health Information for International Travel 2014. 1st ed. Oxford University Press USA; 2013.

7. Jong E, Zuckerman J. Travellers' Vaccines. 2nd ed. Connecticut: People's Medical Publishing House; 2010.

8. World Health Organization. Environmental Health Risks. In: International Travel and Health 2012. 1st ed. Geneva: WHO; 2012.

9. Advisory Committee on Malaria Prevention for UK Travellers. Guidelines for malaria prevention in travellers from the UK. London: PHE; 2013.

10. World Health Organization. Malaria. In: International Travel and Health 2012. 1st ed. Geneva: World Health Organization; 2012.

11. World Health Organization. Infectious Diseases of Potential Risk for Travellers. In: International Travel and Health 2012. 1st ed. Geneva: World Health Organization; 2012.

12. Health Protection Scotland. 2011.

13. Health Protection Scotland. 2013. 

14. Health Protection Scotland. 2012.

15. Health Protection Scotland. 2013. 

16. Health Protection Scotland. 2013.