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There’s no doubt about it: the tiger mosquito is at the door! A colony was spotted in a tyre plant close to the port of Antwerp; several dozen individuals, like some kind of vanguard. It was a group of researchers from Gembloux Agro-Bio Tech (Université de Liège) who discovered the first specimen, and sounded the action stations at the Institute of Tropical Medicine in Antwerp. Because this mosquito, originally from South-East Asia, is the vector of serious diseases such as dengue fever or chikungunya and it is... anthropophilic.

Slimane Boukraa smiled when he checked his mosquito trap on 7 July last year at the factory in Vrasene (Antwerp): he had a good catch, with hundreds of insects in the trap. Once back at the laboratory, the researcher from Gembloux Agro-BioTech/Université de Liège (Functional and Evolutionary Entomology Unit) embarked upon the slow and fastidious task of identifying each insect, one by one. With his sharp eye, Slimane can recognise the majority of mosquitoes with the naked eye, confirming them under the microscope. “When I was holding it with the tweezers, I immediately recognised it as the tiger mosquito”, he tells us.  “Its black and white stripy legs are very characteristic”. There was only one among the 500 mosquitoes caught; all the others belonged to the already well-established and known species in Belgium. Only one, but it was an important discovery. This tropical mosquito had already been spotted once before in the same factory in 2000, but hadn't reappeared since, despite a major detection campaign between 2007 and 2011. This second entry proves that its arrival in Belgium some 15 years ago wasn’t accidental.between 2007 and 2011. Thjgneagneeappe,  speciaitoes with the naked eye, confirming them with the Aedes albopictus, its scientific name, has clearly found a weak point...

The company in Vrasene imports used tyres essentially from Japan and the United States to recycle them. For reasons which still remain unknown, the mosquitoes like to lay their eggs in tyres. They often find a bit of stagnant water there, but there are undoubtedly also other elements that attract them: the colour black, a certain level of acidity, the absence of predators, etc. In fact, further investigations uncovered tiger mosquito larvae in small pools of stagnant water inside the tyres. "What’s more", adds S. Boukraa, "a genetic analysis of the adult specimen caught in the trap shows that it’s a strain 99% identical to the stain established in the USA. Our mosquito very probably comes from America. It probably arrived here via the port of Antwerp in a big cargo ship". Under the direction of Professor Frédéric Francis, Slimane Boukraa published his research in the magazine Parasite (1).

The researchers from ULg informed their colleagues at the Institute of Tropical Medicine (ITM) in Antwerp of their discovery. ITM is in charge of a programme to monitor exotic mosquitoes. Between last August and October, ITM researchers discovered several more dozens of tiger mosquitoes (adult and larvae) in the close surroundings of the tyre plant. And that’s not all: end November, a live larvae was found in the port of Antwerp in a cargo of bamboo shoots from China. Thus revealing a second method of entry into our country. Bamboo and tree holes are the natural tropical home of the tiger mosquito in South-East Asia. Large international ports, such as Antwerp, are obviously the ideal port of entry for invasive tropical species.

An invasion to worry about?

But should we be afraid of the tiger mosquito? Here in Europe, mosquitoes are, at most, accused of spoiling our pleasant summer nights with their mocking attacks, resulting in an exasperating itchiness in the early hours. Not the end of the world. But in tropical countries, some mosquitoes can transmit serious diseases. The tiger mosquito is a vector for dengue fever, chikungunya (see below) and even yellow fever. For the time being, scientists don’t want to cause panic. “There’s no reason for public health concerns at the moment", the researchers at ITM confirmed in a recent memo. “The risk of the imported tiger mosquitoes carrying viruses such as dengue fever or chikungunya is very low”. Furthermore, it is uncertain whether the current small colony will be able to survive the winter. The larvae of this tropical species have trouble withstanding the cold. But there is indeed a risk in the longer term because the intensification of international trade increases the probability of tropical species entering Europe and global warming favours their presence.

For a tropical disease to emerge (or re-emerge) in our latitudes, four factors would have to be present.

1.    A pathogen, i.e. an infectious parasite. For instance, dengue fever is caused by a virus. Symptoms appear several days after infection; they can range from a mild fever to a high fever accompanied by headaches and sharp muscular and articular pain. Dengue hemorrhagic fever (abdominal pains, vomiting, haemorrhaging) is a sometimes fatal complication of the disease. The disease is especially rife in Asia and Latin America. There is no specific treatment for dengue fever. Chikungunya, also caused by a virus, has comparable symptoms:  high fever and articular pain. In the majority of cases, the symptoms disappear after several days or weeks. Sometimes, although more rarely, the articular pains may persist for months or even years. There is no treatment. This disease in endemic in Africa, Asia and in the Indian sub-continent.

2.    Hosts and reservoirs. Reservoirs are animals (often birds or rodents) that carry the virus without being ill, while the hosts fall ill when they are contaminated by the virus. The West Nile Virus is a good example of the difference between a host and a reservoir. In the majority of cases, birds can live with this virus without developing any symptoms. They are reservoirs. However, the virus can cause a serious, sometimes fatal, neurological disease in humans or horses.  Humans are the host for the West Nile Virus as is the case for dengue fever or chikungunya. With the growth in travel and migration, it is clear that the number of people contaminated by one of these viruses will have a tendency to increase in countries that have hereto been free of them, such as Belgium.

3.    A vector. Haematophagous insects (which feed off blood) often play this role. By biting and sucking the blood of an infected host, the insect absorbs a certain quantity of the virus circulating in the blood. The majority of species of mosquitoes are nothing more than flying syringes. This means that the virus isn’t able to reproduce and develop inside the insect (because of some physical and immune barriers of these species). However, some vector species aren’t content with being simple flying syringes. In this case, the insect’s body serves as a “biological reactor” allowing the viral charge to be multiplied. The tiger mosquito is an excellent vector of dengue fever and Chikungunya.

4.    An environment favourable to this vector ecology, especially humidity and heat. Malaria gradually disappeared in our regions as authorities dried up the marshes whose humid terrain attracted the proliferation of certain mosquitoes.  Some vectors and some parasites cannot survive or proliferate if the temperature is too low. Aptly-named tropical diseases develop above all... in the Tropics.

A highly invasive tropical species

The least we can say about Belgium is that it certainly isn’t tropical! The last factor - a favourable environment - should therefore protect us against the emergence of tropical diseases such as dengue fever or chikungunya.  Furthermore, the ITM researchers seem to doubt that the colony of tiger mosquitoes observed this summer close to Antwerp would be able to survive the winter. Furthermore, the number of people who carry these viruses is very low in Belgium. It is therefore highly unlikely that one of these mosquitoes will come across someone contaminated by dengue fever or chikungunya, bite them and then transmit the virus to another healthy person. In Belgium, the risk of “autochthonous” transmission, as the scientists say, is almost zero.

For the moment... Because geographically, it appears that the risk is advancing. The vector, in any case, is spreading rapidly: leaving South-East Asia around the 1980s, the tiger mosquito invaded the five continents in 20 years: USA (1985), South Africa (1989), Italy (1990), New Zealand (1999), France (1999), Belgium (2000), Spain (2004), Germany (2007)… Before 1970, according to the WHO, only nine countries had had an epidemic of dengue fever. There are currently more than 100! Forty percent of the world’s population is now exposed to the risk. And Europe is no longer safe from an outbreak of dengue fever, the WHO believes. The first cases of “autochthonous" transmission of dengue fever were recorded in France in 2010. A first outbreak of chikungunya struck north-east Italy in 2007.

Faced with the clear increase in risk, the researchers from Gembloux Agro-Bio Tech recommend a long-term monitoring plan at the sites identified as import sites, especially tyre depots or companies that import bamboo. And treatment with biopesticides when the colonies are detected. “But this won’t be enough”, Slimane Boukraa believes. “We may also have to envisage the disinfection of the means of transport by which the incriminated goods – tyres and bamboo - were conveyed. A collective response to the problem must also be developed. Every one of us can take action in an effort to contain the invasion. How? By getting rid of unnecessary stagnant water reservoirs, which provide a site for the invasive mosquitoes to lay their eggs: the small wheelbarrow in the garden, an old flowerpot, a plastic toy, etc." According to the researcher, a special effort should also be made on farms as regards cattle troughs or by covering used tyres used for silage heaps.

But Slimane Boukraa is certain of one thing: we haven't heard the last of the tiger mosquito...