Does dengue pose a threat to the WHO European Region?

Dengue is an arbovirus, one of a family of viruses that are transmitted by mosquitoes and other insects such as ticks. Only the Aedes species of mosquitoes carry the DENV. For a person to become infected with dengue, an infected female mosquito must bite a human host. The virus then passes from the infected human back to a mosquito, which in turn infects another human.

Up to 80% of dengue cases are asymptomatic, meaning an infected person will not be unwell, and therefore may be unaware that they are carrying the disease. However, some will experience fever, headaches, muscle aches, joint pain, nausea and vomiting. In severe cases, this can lead to haemorrhage, organ failure and death. 

It is not understood why some people develop severe disease. There are 4 types of DENV. If you get one of those strains and manage to fight it off, you will become immune to that strain. However, in the case of dengue, unlike many other diseases that produce protective antibodies, you can develop a second infection with a different strain of dengue, which can be more severe.

A person travelling from an area where dengue fever is endemic, such as Latin America, may become infected in that area and be diagnosed with dengue on their return home. These cases do not represent much of a public health risk because, if they are detected early, the infected person will typically manage their infection with rest and painkillers. Usually, the virus will not pass on via a local mosquito.  

However, since the 2010s, the WHO European Region has experienced autochthonous (locally acquired) cases in some countries, including Croatia, France, Italy, Portugal and Spain. This happens when DENV continues to be passed on by an infected mosquito to people who have not travelled, giving rise to locally acquired infections.  

Since dengue’s vector, mosquitoes, is only present in Europe in the summer months, autochthonous cases only occur seasonally in the Region.

WHO/Europe, together with partners such as the European Centre for Disease Prevention and Control (ECDC), is currently carrying out analysis and surveillance in the countries in the Region where autochthonous cases could occur. However, for the disease to become endemic, 2 criteria which are not currently fulfilled must be met.  

Firstly, the mosquito vector must be present – that is Aedes albopictus (sometimes called the tiger mosquito due to its stripes) or Aedes aegypti – and able to survive the winter. Secondly, the virus possesses its own temperature sensitivity, independent from the mosquito or human host. This means that even if the mosquito is present in a country, it does not necessarily lead to transmission of the virus and spread of dengue, because the virus only tolerates a specific temperature range.

Climate change is already affecting the distribution and seasonality of the mosquito vectors and the climatic conditions conducive to virus transmission. WHO has identified 30 countries, mainly along the Mediterranean coast, but stretching all the way into the Caucasus and beyond, that could potentially possess both the mosquito vector and temperature sensitivity for the virus. Enhanced surveillance of dengue is in place in these areas, especially during the mosquito season in the summer. This is crucial for early detection and appropriate disease control measures.

Unfortunately, there is no specific medication designed to treat the symptoms of dengue – most cases of dengue can be treated at home with pain medicine to relieve the mild symptoms. For people with a severe form of disease, hospitalization and fluid therapy or even blood replacement may be necessary.

A recently licensed vaccine, Dengvaxia ®, protects people who have previously been infected, as they are at risk of severe dengue if they acquire a second infection. However, prior infection is very difficult to establish because of unavailability of antibody tests in affected areas.

A second vaccine, Qdenga, has also been licensed and is not limited to the already immune. Qdenga provides protection against fever and hospitalization resulting from dengue disease caused by any of the 4 types of DENV.

Preventing mosquito bites is the best way to avoid getting dengue. It is important to be vigilant in the daytime, as this is when Aedes are most likely to feed. Using mosquito repellent and wearing long-sleeved clothes can help to deter mosquitoes.  

The best management approach is to reduce the likelihood of mosquito propagation by destroying breeding sites. Aedes mosquitoes thrive in areas close to human populations (specifically urban areas), where the mosquito lays its eggs in water-filled containers in dwellings and surrounding areas; this includes bottles, containers, discarded waste, tyres etc. in which water has accumulated. 

Community participation is also key to dengue prevention. If every household aims to reduce the breeding grounds for mosquitoes, the transmission rate will decrease or stop in the affected area.