Mosquitos can carry a number of serious diseases (Ton Rulkens)
Article Words : Dr Jane Wilson-Howarth | 19 October

Dealing with dengue and other mosquito-borne diseases

Dengue is a risk for 40% of the world’s population. Our expert advice will help ensure you’re not among them

Dengue fever is the world’s most common mosquito-borne virus. The first recorded outbreak was in Bangkok in 1953. Following that, the virus spread, becoming a huge problem in South-East and South Asia and the Western Pacific; now, there are also frequent epidemics in the tropical Americas.

December 2008 saw an outbreak in Cairns, Australia. About 40% of the world’s population is said to be at risk, with around 100 million cases worldwide annually.

Dengue is caused by one of four viral subtypes; illness confers immunity to further attacks of the same type, but not the other three.

The disease commonly strikes as ‘classical’ dengue, also know as break-bone fever due to the severe muscle and bone pain suffered during the illness. There is also an altogether nastier form of the disease – dengue haemorrhagic fever (DHF) – which affects at least two million people each year, causing 140,000 deaths.

Will I catch it?

Ordinary travellers are unlikely to contract the dangerous haemorrhagic form of dengue. They do, however, risk classical dengue fever.

Fewer than 200 laboratory-confirmed cases of dengue are reported in Britain each year – compared with about 2,000 cases of malaria – although, as dengue is usually benign in travellers and tends to settle without treatment in a week, it is likely that this figure hugely under-emphasises the number of travellers struck. One estimate is that dengue causes illness in one in 1,000 tourists visiting risk regions.

How is it transmitted?

The dengue virus is spread by day-biting Aedes mosquitoes. Sometimes called tiger mosquitoes, these stripy-legged insects breed in clean water, including drinking-water tanks.

They like to lay eggs in the rainwater caught in the bracts of big plants, common in tropical gardens and the grounds of international hotels. In Indonesia there are days when everyone in the community empties their water tanks in order to eradicate Aedes larvae. Mosquitoes transmit dengue from person to person; there is no animal host.

What are the symptoms?

A high fever (40°C/104°F) comes on suddenly two to eight days after the infective bite. Often there is a behind-the-eyes headache plus severe muscle and bone pains, and a measles-like rash. The illness rarely lasts more than a week and complete recovery is the norm.

Is it fatal?

Occasionally. People most at risk are those who were brought up in dengue-endemic countries – the haemorrhagic form seems to develop almost exclusively in people who have already had at least one attack of one of the four possible sub-types.

In people suffering DHF with shock, the fatality rate can be high. Left untreated, up to 60% of all victims of haemorrhagic shock die, but with good hospital care the chances of dying fall to 1-2%.

A British tour guide contracted DHF, but with good care she made a complete recovery in six weeks. Generally, Western travellers are at very low risk.

Where can I catch it?

Classical dengue (DF) usually occurs between latitudes 30° north and 40° south: it is endemic in parts of South and South-East Asia, the Pacific, Africa, Central America, the Caribbean and the tropical parts of South America. DF has spilt into Texas and south-eastern USA, Greece, Japan and the extreme north-eastern tip of Australia, but these outbreaks are sporadic and rare.

Dengue has become a big problem in India recently. New outbreaks are likely in burgeoning tropical cities.

The more serious DHF continues to be a big problem in South-East Asia; since 1996 there have also been many cases in Latin America and the Caribbean. It is present in the Pacific, but, so far, not in Africa.

The most likely time of day to be bitten by the day-active dengue mosquito is during the period of frenetic early-morning mosquito feeding (in the two or three hours after dawn) and around dusk.

Can I prevent it?

The only protection from dengue is to avoid being bitten. Aedes mosquitoes are more adventurous and persistent than malarial mosquitoes and will bite through thin clothing. To counter this, proof your clothes with permethrin spray – treatment for one outfit costs about £7 but lasts a fortnight and helps reduce the amount of DEET- or IR3535-based insecticide you need to apply directly to your skin.

Aedes mosquitoes breed in clean water. They can be discouraged by emptying vessels such as buckets and flowerpots, disposing of old car tyres, and putting fish in water tanks to eat the mosquito larvae.

Several vaccines are under development but none are commercially available yet. Field trials on one dengue vaccine in Thailand have proved very encouraging; there are plans to test another vaccine in Brazil in 2010.

Clinical trials need to be slow and meticulous. Sanofi hopes to submit for regulatory approval for one in 2012; GlaxoSmithKline (GSK) is working on a competing product. The World Health Organization is pushing for a low-cost dengue vaccine aimed at protecting locals.

What if I catch it?

Paracetamol helps the fever and pain. Aspirin is not given – it promotes bleeding and will make DHF worse. Generally, paracetamol is all you’ll need. Clinical research suggests that ribavirin and mycophenolic acid inhibit the virus and might therefore be of use in dangerous DHF with shock, but these treatments are yet to be tried.

How do I know if I've got dengue fever?

Head - All flavivirus infections cause fever; West Nile virus and Japanese encephalitis cause brain swelling

Eyes - Dengue causes a behind-the-eyes headache

Skin - A blotchy rash may appear

Lymph nodes - Chikungunya and West Nile virus are similar to dengue but cause more-swollen lymph nodes

Liver & kidneys - Yellow fever, another flavivirus, can cause liver and kidney failure

Elbows, knees and back - Dengue causes limbs to ache; hence the colloquial name 'back-bone fever'.

You give me fever

The mosquito-borne flavivirus to watch out for:

Chikungunya: Similar to dengue but causes more-swollen lymph nodes; sometimes causes bleeding and a blotchy skin rash. Only 4% of cases are fatal, not usually serious. Both day or night-biting mosquitoes carry the disease. It is found in the Indian Ocean region.

West Nile Virus: Brain swelling; can be similar to dengue but causes more-swollen lymph nodes. In 11% of cases the disease proves fatal. Again both day and night-biting mosquitoes carry this disease. It is found in Europe, Asia, Australia and the US. Birds and horses also carry this disease.

Yellow fever: Symptoms include fever, generalised pain, shock, bleeding, liver and kidney failure. This disease is fatal in 60% of cases. Only day-biting mosquitoes carry this disease although monkeys can be infected too. It is found in Africa, Central and South America.
A vaccine is advised if travelling to a country where the infection is prominent and is sometimes required to gain entry into others.

Japanese encephalitis: Symptoms include fever, vomiting, brain swelling, fits and unconsciousness. This disease is rarely fatal, but can leave the sufferer with serious, long-term effects. This disease is carried by night-biting mosquitoes and is mainly found in south-east Asia, western Pacific and north-east Australia. Again a vaccination is available for this virus, and is strongly advised. Pigs, bats and herons can also be infected by this disease.