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Dengue: a travel-related health hazard

Hilary Simons RGN RSCN MSc FFTM RCPS(Glasg)
Senior Specialist Nurse
National Travel Health Network and Centre
Liverpool School of Tropical Medicine

The pre-travel consultation provides an ideal opportunity to increase awareness of dengue and to discuss with the traveller risk management strategies for mosquito bite avoidance.

Dengue (also known as dengue fever) is a mosquito-borne viral illness that occurs widely in the tropics and is an emerging disease in other parts of the world. There is no vaccine and prevention is by mosquito control in endemic areas and personal protection by scrupulous mosquito bite avoidance measures. Travellers can be at risk of infection during both short and prolonged visits to dengue endemic regions, which include many tourist destinations.

Dengue is an acute viral illness caused by a flavivirus. The virus is transmitted to humans by the bite of an infected female Aedes spp. mosquito (e.g Aedes aegypti), which requires a blood meal to reproduce. This mosquito species is widely distributed throughout sub-tropical and tropical regions of the world. Populations of the Aedes spp. mosquito are becoming established in other parts of the world, including southern Europe, possibly because of the effects of climate change resulting in a more favourable habitat.

Aedes mosquitoes lay their eggs in fresh standing water, often in areas close to human habitation in receptacles such as vases, water butts, toilet pans or old tyres. Although most abundant inrban areas, some Aedes mosquitoes live in the jungle environ- ment, where they lay their eggs in rain water that collects in tree holes or in the leaf axils of plants. This mosquito species has a predominantly day time biting habit, with the most intensive activity taking place around dawn and dusk.¹

Outbreaks of dengue are reported from Africa, the Middle East, the islands in the Indian Ocean, South East Asia, the Western Pacific, the Americas, and the Caribbean; occasionally locally acquired cases are reported from southern Europe.² The World Health Organization estimates 50 million cases of dengue occur globally each year with 2.5 billion people living in dengue endemiccountries.³ There are four different serotypes of dengue virus (DEN-1, 2, 3 and 4). One or more serotypes can circulate in an area at the same time. Infection with one serotype only confers immunity to that serotype. A second infection with a different dengue virus serotype (particularly DEN-2 or DEN-3) has been associated with more severe illness.³,⁴
 

Following a bite from an infected mosquito, the incubation period is five to eight days. Dengue has a broad spectrum of symptoms from a mild, flu-like illness to a more debilitating illness with symptoms including sudden onset of high fever lasting one to five days, bone pain (dengue is commonly called 'break bonefever'), muscle pain, a typical 'dengue' macular rash and thrombocytopenia. Most cases are uncomplicated and recover within a few days of developing the rash; however, fatigue and depression can persist following recovery from the acute phase.

Uncomplicated dengue can progress to a severe and often life-threatening illness - Dengue Haemorrhagic Fever (DHF) with or without Dengue Shock Syndrome (DSS). DHF is more common in children and in people who become re-infected with a different dengue virus serotype. DHF is characterised by the presence of plasma leakage from capillaries, haemorrhage and major organ damage. In DSS, there is rapid deterioration with obvious signs of shock including hypotension, weak pulse and clammy cold skin. Without treatment, mortality rates can be high but with early recognition and intervention mortality rates for DHF and DSS are between 1-2%.¹,⁵ Treatment is supportive for both uncomplicated and severe dengue. Those with severe manifestations of the illness may require intensive medical management including intravenous fluid replacement.

Dengue prevention
Primary prevention in endemic areas focuses on mosquito control. Control measures involve disease surveillance to identifiy case numbers and managing the mosquito population through chemical and enviromental strategies. Chemical control (use of insecticides or larvicides) and/or biological control (introduction of natural larvae predators, eg, fish or crustaceans to breeding sites) are used in endemic areas. Large-scale 'space' spraying with insecticides may be initiated in an outbreak situation.Environmental management to minimise mosquito breeding sites can be effective in reducing the risk of dengue virus transmission (keeping water storage containers clean and sealed, changing water in animal drinking containers frequently and removing all non-essential receptacles where water could collect).

In the domestic environment, humans can minimise mosquito contact by the use of screened doors and windows, air condition-ing and if sleeping during the daytime, using a mosquito net which has been impregnated with insecticide.⁶,⁷

Personal protective measures against mosquito bites are the mainstay of secondary dengue prevention and should be discussed with all travellers to dengue endemic areas, whatever the length of their stay. Long-term travellers are at particular risk as they will be exposed to mosquito bites over a long period of time, may live in challenging environmental situations and may have a different perception of risk and become complacent regarding mosquito bite precautions.
 

There is currently no vaccine to prevent dengue. Development of a vaccine is challenging, not least because a vaccine would have to protect against all four serotypes. The safety of a number of potential dengue vaccines is being researched in pre-clinical trials and it is hoped that a vaccine will be eventually become available.

Dengue in the returned traveller
During 2010 there was an increase in the number of imported cases of dengue to the UK (449 in 2010, compared to 177 in 2009). Travel history of confirmed cases included visits to India, Indonesia, Thailand, the Americas and the Caribbean.¹¹

Fever in the recently returned traveller should be managed swiftly and guidance is provided for the clinician.¹² Malaria should always be excluded (malaria can occur up to one year following return from a malaria endemic region). Dengue should be considered in all recently returned travellers presenting with a fever who are malaria negative. Dengue is unlikely in travellers where the last exposure to mosquito bites was more than four weeks ago.

Conclusion
Travellers venturing to tropical regions of the world where dengue is endemic are at risk of infection.

Health professionals should consider dengue as a differential diagnosis in the traveller with fever and/or rash who has recently returned from endemic areas and be acquainted with the protocol for the management of the returned ill traveller.

References
1.    Field VF, Ford L, Hill DR, eds. Disease Guide. Dengue. Section 5. In: Health Information for Overseas Travel. London: National Travel Health Network and Centre; 2010.
2.    National Travel Health Network and Centre. Outbreak Surveillance Database. Available from: http://www.nathnac.org/countrysearch.aspx
3.    World Health Organization. Dengue: Guidelines for Diagnosis, Treatment, Prevention and Control. New Edn. Geneva: World Health Organization; 2009.
4.    Whitehorn J, Farrar J. Dengue. Br Med Bull 2010; 95:161-73. 5.    Centers for Disease Control and Prevention. Dengue and Dengue
Hemorrhagic Fever: Information for Healthcare Professionals. Available from: http://www.cdc.gov/dengue/epidemiology/index.html
6.    World Health Organization. Dengue control: Control Strategies. Available from: http://www.who.int/denguecontrol/control_strategies/en/
7.    National Travel Health Network and Centre. Health Information Sheet: Insect and tick bite avoidance. October 2010. Available from: http://www.nathnac.org/pro/factsheets/iba.htm
8.    Chen LH, Wilson ME, Davis X, Loutan L, Schwartz E, Keystone J et al. Illness in long-term travelers visiting GeoSentinel clinics. Emerg Infect Dis 2009;15(11):1773-82.
9.    Schmitz J, Roehrig J, Barett A, Hombach J. Next generation dengue vaccines: A review of candidates in preclinical development. Vaccine 2011;29(42):7272-84.
10.  Murphy ME, Montemarano AD, Debboun M, Gupta R. The effect of sunscreen on the efficacy of insect repellent: a clinical trial. J Am Acad Dermatol 2000;43:219-22
11.    Health Protection Agency. Dengue fever in England, Wales and Northern Ireland, 2009/10. Health Protection Report 2011; 5(18): travel health. Available from: http://www.hpa.org.uk/hpr/archives/2011/hpr1811.pdf
12.    Field VF, Ford L, Hill DR, eds. The Post-Travel Consultation. Section 4.2. The Ill Returned Traveller. In: Health Information for Overseas Travel. London: National Travel Health Network and Centre; 2010.

Resources
The National Travel Health Network and Centre www.nathnac.org
Health Protection Agency www.hpa.org.uk
Health Protection Scotland www.hps.scot.nhs.uk
TRAVAX www.travax.nhs.uk (subscription required)
World Health Organization www.who.int/en