Dengue fever cases currently reported in Europe are almost exclusively represented by infections associated with travel abroad. However, since 2010 sporadic autochthonous (local) Dengue transmission has been reported in France, Spain and other countries. This year for the first time it is Italy’s turn, as reported by a recent article published in Eurosurveillance.

The epidemiological investigation

Everything seems to have started with a woman from the province of Vicenza, in the Veneto region. Shortly after returning to Italy in late July after a 16 months’ stay on a tropical island in Indonesia, she experienced fever, malaise, back pain and upper limb itchiness for a few days. She tested negative for COVID-19, and no further tests were made.

But around mid-August, five family members experienced the same symptoms. Four had never travelled abroad, while one visited the Indonesian island in January for only two weeks. However, he did not report any symptoms upon return at the time.

Given the similarity and the almost simultaneous appearance of symptoms, a more thorough investigation revealed the presence of Dengue serotype 1 (DENV-1) infection in all six people in the form of either viral RNA and/or anti-DENV-1 antibodies in the blood.

The role of tiger mosquitoes

In order for an arbovirus like Dengue to be transmitted locally, two conditions must be met. The first is the presence of people with a viremia (presence of the virus in the peripheral blood) sufficient to infect the insect vector. The second is obviously the presence of the latter: in the case of Dengue, a mosquito of the genus Aedes. In addition, the vector must survive at least 8-12 days for the virus to reach the salivary glands and be transmitted to a new host when the insect bites.

In summer, at temperate latitudes, the environmental conditions are particularly favorable for transmission: in fact, high temperatures not only favor the reproduction of mosquitoes, which reach high densities, but also accelerate the replication of the virus in the insect.

Frontal view of a tiger mosquito
The infamous tiger mosquito (Aedes albopictus). Note the single white line in the middle of the thorax, a distinguishing feature of this species [Credits: LiCheng Shih]

The Asian tiger mosquito (Aedes albopictus) is certainly responsible for what happened in Veneto. It is an invasive species from Asia which has colonized much of southern Europe, including Italy. It is a very adaptable insect, capable of reproducing in urban and semi-urban environments, and its eggs survive during the winter months. The tiger mosquito bites mainly during the day and can transmit numerous arboviruses in addition to Dengue (Chikungunya and Zika, just to name a few).

The importance of surveillance

Cases of local transmission are likely to occur again in Italy and Europe. For example, outbreaks of Chikungunya virus have been recently observed in other Italian regions such as Emilia Romagna, Lazio and Calabria.

A robust surveillance system capable of distinguish promptly the imported cases from the local ones, combined with mosquito monitoring and control interventions, becomes pivotal to manage emerging arboviruses.

You can also help by signing the petition to establish a World Dengue Day, to bring more attention to this important and emerging disease.

[This article is the english version of the original, in italian, which has been published here]

REFERENCES

Lazzarini L, Barzon L, Foglia F, Manfrin V, Pacenti M, Pavan G, Rassu M, Capelli G, Montarsi F, Martini S, Zanella F, Padovan MT, Russo F, Gobbi F. First autochthonous dengue outbreak in Italy, August 2020. Euro Surveill. 2020 Sep;25(36). doi: 10.2807/1560-7917.ES.2020.25.36.2001606.

Benelli G, Wilke ABB, Beier JC. Aedes albopictus (Asian Tiger Mosquito). Trends Parasitol. 2020 Feb 6:S1471-4922(20)30011-8. doi: 10.1016/j.pt.2020.01.001.

Rezza G. Chikungunya is back in Italy: 2007-2017. J Travel Med. 2018 Jan 1;25(1). doi: 10.1093/jtm/tay004. PMID: 29669058.